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• We developed an offline method to measure HO 2 uptake kinetics onto ambient aerosols. • γ HO 2 is strongly RH and pH dependent and inversely correlated with [HO2] 0. • Different ambient aerosol components have collective effects to γ HO 2. • HO 2 self-reaction and transition metal-catalyzed reactions both contributed to γ HO 2. The heterogeneous loss of HO 2 radicals onto ambient aerosols plays an important role in tropospheric chemistry. However, sparse investigation of the dominating parameters controlling the HO 2 uptake coefficients onto ambient aerosols (γ HO 2) has largely hindered the application of the measured γ HO 2 to the global spatial prediction. Here we induced an offline method using LFP-LIF technique to measure the kinetics of HO 2 uptake onto ambient aerosols collected in summertime and wintertime in Yokohama city, a regional urban site near Tokyo, Japan. By controlling the dominating parameters which influence γ HO 2 , we were able to investigate the detailed HO 2 uptake mechanism. We characterized the chemical composition of the collected ambient aerosols, including organics, inorganics, transition metals ions, etc. and modeled γ HO 2 using different mechanisms. Results show that γ HO 2 increased with the increase in RH, and the aerosol states ("dry" or wet/aqueous) have large effects on γ HO 2. With fixed RH and aerosol chemical composition, γ HO 2 was highly dependent on pH and inversely correlated with [HO 2 ] 0. By combing the measured γ HO 2 values with the modeled ones, we found that both the HO 2 self-reaction and transition metal-catalyzed reactions should be accounted for to yield a single parameterization to predict γ HO 2 , and different chemical compositions may have collective effects on γ HO 2. Results may serve for extending the γ HO 2 values measured at one observation site to different environmental conditions, which will help us to achieve more accurate modeling results concerning secondary pollutant formation (i.e., ozone). [ABSTRACT FROM AUTHOR]

Bromine radicals released from sea ice, snow, and marine sources play a critical role in the atmospheric chemistry of polar regions. The Chemical and Mesoscale Mechanisms of Polar Cell Aerosol Nucleation (CAMMPCAN) ship campaign conducted in coastal East Antarctica over two 6-month periods in 2017–18 and 2018–19 provides a unique dataset to identify the environmental drivers of bromine variability in Antarctic spring and summer. In this study, we used CAMMPCAN chemical and meteorological observations combined with reanalysis data from the Modern Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) and satellite-based sea ice data from the National Snow and Ice Data Center to select variables that showed statistically significant correlation with bromine monoxide (BrO) partial columns measured during CAMMPCAN. We then used those variables in principal component analysis and subsequent principal component regression to identify dominant modes of Antarctic environmental variability and their impacts on lower tropospheric BrO. Comparing our three dominant Antarctic principal components to those from a similar analysis conducted previously for the Arctic (Swanson et al., 2020), we found only one mode with clear overlap, representing a vertical mixing mode in which low-pressure systems mix BrO and its precursors into the lower troposphere. We also identified an Antarctic mode describing conditions favourable for blowing snow, similar to the combined effect of two modes from the Arctic analysis but more clearly disambiguated here due to the inclusion of sea ice data in our analysis. The third Antarctic mode, attributed to an ocean source (biological activity and/or sea salt aerosol), was particularly important in summer. The principal component regression model developed from these modes showed moderate skill in predicting BrO partial columns in the lowest 2 km of the troposphere (R = 0.51), a significant improvement over the Arctic-based regression model (R = 0.08). Neither model could reproduce the observed variability in BrO in the lowest 200 m. Finally, we applied the same analysis to coincident CAMMPCAN observations of gaseous elemental mercury and found regression of our three dominant modes could explain nearly 50% of observed mercury variability (R = 0.69). Our results reinforce the importance of sea ice and ocean processes in bromine cycling in coastal East Antarctica and highlight the need to consider Antarctic-specific processes in mechanistic models of atmospheric bromine chemistry. • Three modes dominate chemical-meteorological variability in coastal east Antarctica. • The modes represent vertical mixing, blowing snow, and ocean sources. • The three modes explain significant variability in tropospheric bromine and mercury. [ABSTRACT FROM AUTHOR]

Due to their pivotal role in atmospheric processes, hydrogen abstraction reactions are vital in the study of tropospheric chemistry. In this work, we present an algorithm that generates a large number of chemically sound geometries for the optimization of transition states (TSs) of tropospheric bimolecular H abstraction reactions. The code, which was developed in the early stages with the help of artificial intelligence (AI), automatically detects the active H atoms of the molecule and can be used with the OH, Cl, and NO 3 oxidants. Given the ubiquity of H transfer reactions in various fields, the algorithm was designed in general terms to facilitate easy adaptation to other oxidants, thus allowing for a broader range of applications. As a result of its use, a much greater number of TSs is predicted when compared with previous theoretical studies of six oxidation reactions. In addition to improving our understanding of the H-abstraction process, obtaining an increased number of TSs is also fundamentally important in calculating more accurate rate constants and atmospheric lifetimes for volatile organic compounds. The simplicity and significance of such a tool in the context of environmental chemistry should make it appealing to researchers of all backgrounds. [Display omitted] • An algorithm for transition state sampling of H abstraction reactions is presented. • The Fortran code is provided and is easy to adapt to other oxidants. • The algorithm enhances the calculation of atmospheric oxidation rate constants. [ABSTRACT FROM AUTHOR]

Open biomass burning (BB) frequently occurs in spring over Southeast Asia (SEA). The concentration of fine particulate (PM 2.5) produced by BB and its impacts on air quality in downwind aeras exhibit significant interannual variations, which are controlled by emission intensity and meteorological circulation. In this study, a regional air quality model system (RAQMS) was applied to investigate the contributions of BB emissions in SEA to atmospheric PM 2.5 concentrations over South China in March (a spring month) during 2009–2018 and their interannual variations. Higher near-surface BB-PM 2.5 (refers to the PM 2.5 produced by BB in SEA) concentrations and contributions were mainly distributed in the source region and its vicinity, while the higher tropospheric BB-PM 2.5 concentrations and contributions were characterized by a long belt extending from BB source regions to Yangtze River Delta (YRD) in South China. Combined with the vertical distribution of BB-PM 2.5 on the transported path profile, it indicated that higher BB-PM 2.5 concentrations were mainly concentrated in 0∼1500 m above the ground over source regions, while the maximum BB-PM 2.5 concentrations in the downwind area were distributed in 1500∼3500 m above the ground. The monthly mean tropospheric BB-PM 2.5 concentrations were higher in March 2009, 2010, 2012, and 2013, with evident correlation with interannual variation of BB emissions. Sensitivity experiments suggested that the near-surface and tropospheric BB-PM 2.5 concentrations over South China had changed by-48.1%∼190.5% and −48.0%∼213.8%, respectively in March 2009–2018 by fixing BB emissions unchanged at the mean of the 10 years, being greater than the changes caused by meteorological conditions and other affecting factors (∼20%). In addition, the relative contributions of BB emission from various regions in SEA on the mean near-surface and tropospheric BB-PM 2.5 concentrations over South China were further identified. The spatial distribution for monthly mean tropospheric BB-PM 2.5 concentrations (fill color, mg m−2) and contributions (grey contour, %) of BB-PM 2.5 to total tropospheric PM 2.5 concentrations in March of 2009–2018. [Display omitted] • Interannual variation of the impacts of BB on PM 2.5 in South China was investigated. • Contributions of BB in Southeast Asia to PM 2.5 over South China are quantified. • Dominant factors affecting the transboundary transport are explored. • BB in west Myanmar had a dominant contribution to tropospheric PM 2.5 in South China. [ABSTRACT FROM AUTHOR]

The released carbon monoxide (CO) into the atmosphere is a threat to human life and environmental safety. CO storage in surface and underground seawater/water may be viewed as a potential scenario to decrease the concentration of this dangerous gas in the atmosphere. A reliable tool to calculate CO solubility in aqueous media is a prerequisite for accomplishing such a process. Since the least-squares support vector regression (LSVR), CatBoost, extreme gradient boosting, light gradient boosting, random forest, and extra tree regression can extract even the most complex relationships among a series of independent-dependent variables, they are also potential candidates for modeling CO solubility in pure and saline water as a function of temperature and salt concentration. The present work performs relevancy tests, model construction, the best model selection, accuracy assessment, and trend monitoring using 232 literature records of CO solubility in aquatic solutions containing different salt concentrations. Relevancy analysis by the multiple linear regression as well as Pearson's method approve that CO solubility in water decreases by increasing the temperature and salinity. Moreover, trial and error justified that the LSVR with the Gaussian kernel function has the highest accuracy among the six checked models to estimate CO solubility in aqueous solutions. The acceptable agreement between literature and calculated CO solubility in aquatic solutions is also approved by comprehensive numerical and graphical investigations. According to the results, the LSVR predictions for the CO-water and CO-brine equilibrium behavior correspond well with the literature records (mean square error = 6.18 × 10−8, summation of absolute error = 0.02581 cm3 CO/mL H 2 O, correlation coefficient = 0.99844, and mean absolute percentage error = 0.48 %). • It is the first ML-based modeling task to predict CO solubility in pure/salty water. • Intelligent model is suggested to calculate CO solubility in pure/salty water. • The model simulates 232 CO solubilities with MSE = 6.18 × 10−8 and R = 0.99844 • CO solubility in salty water increases by decreasing either temperature or salinity. • Temperature effect on the CO solubility in salty water is stronger than salinity. [ABSTRACT FROM AUTHOR]

Understanding the formation mechanisms of secondary air pollution is very important for the formulation of air pollution control countermeasures in China. Thus, a large-scale outdoor atmospheric simulation smog chamber was constructed at Chinese Research Academy of Environmental Sciences (the CRAES Chamber), which was designed for simulating the atmospheric photochemical processes under the conditions close to the real atmospheric environment. The chamber consisted of a 56-m3 fluorinated ethylene propylene (FEP) Teflon film reactor, an electrically-driven stainless steel alloy shield, an auxiliary system, and multiple detection instrumentations. By performing a series of characterization experiments, we obtained basic parameters of the CRAES chamber, such as the mixing ability, the background reactivity, and the wall loss rates of gaseous compounds (propene, NO, NO 2 , ozone) and aerosols (ammonium sulfate). Oxidation experiments were also performed to study the formation of ozone and secondary organic aerosol (SOA), including α-pinene ozonolysis, propene and 1,3,5-trimethylbenzene photooxidation. Temperature and seed effects on the vapor wall loss and SOA yields were obtained in this work: higher temperature and the presence of seed could reduce the vapor wall loss; SOA yield was found to depend inversely on temperature, and the presence of seed could increase SOA yield. The seed was suggested to be used in the chamber to reduce the interaction between the gas phase and chamber walls. The results above showed that the CRAES chamber was reliable and could meet the demands for investigating tropospheric chemistry. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The detailed atmospheric processes responsible for altitude-dependent SP 17 sp O(SO SB 4 sb ) remain an open question at present because global SP 35 sp S(SO SB 4 sb )- SP 17 sp O(SO SB 4 sb ) measurements are limited [[11], [14]]. Given the removal lifetime of sulfate in the free troposphere (>1 week) [[7], [11]] is longer than the time scale of the eastward transport of air masses from the Himalayas to Southwest China through the prevailing westerlies (<1 week) (Fig. Overall, using cosmogenic SP 35 sp S, we have shown that the Himalayas and its extensive downwind areas are more affected by stratospheric air than other regions, and these stratospheric influences are also likely recorded by triple oxygen isotopes in atmospheric sulfates. Downward transport of stratospheric air significantly modifies the chemical and radiation budget of the Earth's atmosphere. [Extracted from the article]

Tropospheric ozone (O 3) plays predominant role in atmospheric chemistry, exacerbates the air quality, and contributes to the climate change. Near the Earth's surface, O 3 levels exhibit large heterogeneity due to diverse emissions of precursors and effects of meteorological conditions. Nevertheless, the understanding of key dynamical and photochemical processes governing O 3 remains limited in the Western Ghats, a tropical region of immense significance in context of the geosphere-biosphere processes. In this regard, we combined surface O 3 measurements at Ponmudi (8.758° N, 77.114° E, ∼1 km above mean sea level–amsl), a high-altitude site in the Western Ghats, with ground- and balloon-borne O 3 measurements at western coast of India (Thumba: 8.542° N, 76.858° E, ∼3 m amsl) and photochemical box model simulations. O 3 diurnal variation exhibits a small amplitude (3–9 ppbv) with lower values during the daytime, in contrast to substantial urban O 3 build-up (25–37 ppbv) at Thumba. The influence of regional pollution from the urban boundary layer, downdraft of O 3 -rich air during night, and dry deposition processes are suggested to govern the O 3 diurnal pattern over Ponmudi. Lower O 3 /CO and O 3 /NO 2 ratios indicating lower O 3 production efficiency, and dominant role of dynamics are consistently seen over the study region. O 3 variability between Ponmudi and Thumba are found to be well correlated (r = 0.7–0.8) during pre-monsoon and post-monsoon seasons as the sea breeze circulation transport the airmass from coast to the Western Ghats during the daytime. Mean O 3 levels at Ponmudi are typically lower than those above Thumba showing an effective O 3 loss towards Ponmudi. Chemistry involving strong natural emissions (e.g., isoprene) as simulated by photochemical box model, and greater dry deposition tend to supress O 3 build up over this densely vegetated tropical region. Unlike other high-altitude sites, O 3 seasonal cycle at Ponmudi is associated closely with synoptic wind changes, and peaks during the winter owing to the transport from Indo Gangetic Plain/northern India. Our study highlights the roles of regional pollution as well as natural processes including biogenic emissions in governing the surface O 3 variability over the Western Ghats. [Display omitted] • O 3 over the Western Ghats (Ponmudi) is well correlated (r = 0.7–0.8) with O 3 at western coastal station (Thumba). • Lower levels of O 3 is attributed to the higher dry deposition loss and biogenic emission of isoprene over the Western Ghats. • Unlike other high-altitude sites, Ponmudi-O 3 peaks during winter due to transport from Indo Gangetic Plain/northern India. [ABSTRACT FROM AUTHOR]

The nocturnal, seasonal and intra-annual variation of vertical distribution of tropospheric aerosols over two nearby stations Gadanki (13.5oN, 79.2oE) and Tirupati (13.6oN, 79.4oE) is investigated using ground-based Micro Pulse Lidar (MPL) and space-borne Lidar (CALIPSO) systems during 2010–2017. The nocturnal variation of aerosol extinction (AE) coefficient reveal high AE below ∼2 km in midnight hours and aerosols are slowly descending towards the surface during early morning hours. From the seasonal variation, AE values are found to be higher at lower altitudes (<2 km) during winter and post-monsoon seasons, a sharp decrease with increasing altitude is found in tandem with boundary layer and low wind speeds. Interestingly, during monsoon season, significant aerosol loading is found in the altitude range of ∼2–5.5 km mainly due to the influence of strong Low Level Jet (LLJ). The clean environment observed below ∼2 km during this season is attributed to the wet scavenging, downward vertical winds and existance of no strong local source. The seasonal mean AE profile derived from CALIPSO matches well with the MPL in all the seasons except in monsoon season where a large bias is noticed below 2 km. The intra-annual variation revealed more than 80% of aerosols existing above (below) the boundary layer during monsoon (winter) months contribute to the total Aerosol Optical Depth (AOD). The depolarization ratio (>0.2) in the month of July shows the dominance of dust particles which includes long-range transport over this locations. Back trajectories reveals that potential sources are changing from season to season at different altitudes and confirms that the aerosols observed at higher altitudes are advected from other land and oceanic regions. Thus, aerosol vertical distribution is mainly controlled by meteorology and dynamics over this region. Further, the reasonably good correlation found between MPL and MODIS AODs suggests that MODIS could provide reliable AOD over land region also. • Lidar observed aerosol vertical distribution over Indian tropical stations show strong nocturnal and seasonal variation. • Aerosol extinction over Gadanki/Tirupati region is higher at altitudes <2 km during winter and post-monsoon seasons. • Significant aerosol loading is noticed around ∼2–5.5 km and clean environment below ∼2 km during monsoon. • The contribution of aerosols above (below) the boundary layer during monsoon (winter) is found to be around 80% to the total AOD. • Back trajectory analyses confirms the advection of aerosols from other land and oceanic regions to the observation site. [ABSTRACT FROM AUTHOR]

The reduced representation of isoprene degradation in the Common Representative Intermediates (CRI) mechanism has been systematically updated, using the Master Chemical Mechanism (MCM v3.3.1) as a reference benchmark, with the updated mechanism being released as CRI v2.2. The complete isoprene degradation mechanism in CRI v2.2 consists of 186 reactions of 56 closed shell and free radical species, this being an order of magnitude reduction in size compared with MCM v3.3.1. The chemistry initiated by reaction with OH radicals, NO 3 radicals and ozone (O 3) is treated. An overview of the updates is provided, within the context of reported kinetic and mechanistic information. The revisions mainly relate to the OH-initiated chemistry, which tends to dominate under atmospheric conditions, although these include updates to the chemistry of products that are also generated from the O 3 - and NO 3 -initiated oxidation. The revisions have impacts in a number of key areas, including recycling of HO x and NO x. The performance of the CRI v2.2 isoprene mechanism has been compared with those of the preceding version (CRI v2.1) and the reference MCM v3.3.1 over a range of relevant conditions, using a box model of the tropical forested boundary layer. In addition, tests are carried out to ensure that the performance of MCM v3.3.1 remains robust to more recently reported information. CRI v2.2 has also been implemented into the STOCHEM chemistry-transport model, with a customized close-variant of CRI v2.2 implemented into the EMEP MSC-W chemistry-transport model. The results of these studies are presented and used to illustrate the global-scale impacts of the mechanistic updates on HO x radical concentrations. • Isoprene chemistry in a reduced (lumped chemistry) scheme is updated. • Detailed MCM v3.3.1 isoprene chemistry is used as a reference benchmark. • An order of magnitude reduction in species and reaction numbers is achieved. • Performance of MCM v3.3.1 is tested in relation to recently reported information. • Impacts of updates on global-scale [HO x ] are illustrated using two global models. [ABSTRACT FROM AUTHOR]

The number of ground-based GNSS reference stations has increased significantly in recent years, which provides very favorable conditions for ZTD/ZWD retrieval. The methods of estimating ZTD/ZWD based on three a-priori troposphere values are discussed using GNSS reference stations without meteorological equipments. GPS observation data for two weeks in different seasons from 22 global IGS stations were processed using online GAPS software with static and kinematic PPP mode based on ECMWF grid product and GPT2.1w/UNB3m blind tropospheric models. Vienna Mapping Function 1 (VMF1) is used for all the a-priori models. The numerical results show that: compared with the IGS ZTD products, mm-level and about 2 cm-level ZTD products can be obtained based on the three a-priori troposphere values using static PPP and kinematic PPP respectively. About 1 cm-level and 2 cm-level ZWD products can be obtained based on ECMWF grid product/GPT2.1w model using static PPP and kinematic PPP respectively. The discrepancy between the ZTD estimates based on the three a-priori troposphere values is no more than 3 mm, and the differences of the averaged ZTD Mbias, RMSE and Std of the 22 IGS stations based on the three a-priori troposphere values are no more than 0.3 mm. While there is obvious differences in ZWD estimates based on the three a-priori troposphere values. The differences of the averaged ZWD Mbias/RMSE between ECMWF grid product and GPT2.1w model is about 0.2 cm, and the differences of the averaged ZWD Mbias/RMSE between ECMWF grid product and UNB3m model are about 0.5–1 cm. There is a systematic bias of about ±1 cm between the ZWD estimates based on GPT2.1w and UNB3m. This study shows that GNSS PPP technology based on ECMWF grid product and blind tropospheric model has great potential for ZTD/ZWD retrieval using ground-based GNSS reference stations. • ZTD/ZWD retrieval based on GPT2.1w and UNB3m models using GPS PPP was investigated. • ZTD/ZWD retrieval based on ray-traced ECMWF grid product using PPP was investigated. • Static and kinematic PPP with IGS final and rapid orbits & clocks products were used. • Mm-level ZTD products and 1 cm-level ZWD products can be obtained based on static PPP. • About 2 cm-level ZTD/ZWD products can be obtained based on kinematic PPP. [ABSTRACT FROM AUTHOR]

Through scattering or absorbing radiation, aerosols have important impacts on tropospheric photochemistry and oxidation capacity. In this study, we systematically evaluate the radiative impacts of aerosols on the tropospheric actinic flux, photolysis rates (J(NO 2) and J(O 3)) and oxidants such as OH radicals and ozone (O 3) from a global and seasonal perspective. Sensitivity simulations for year 2013 are conducted by the global chemical transport model GEOS-Chem to quantify the photochemical effects of aerosols, including sulfate-nitrate-ammonium (SNA), organic carbon (OC), black carbon (BC), dust and sea salt. The results suggest that aerosols can enhance or reduce the actinic flux by scattering and absorbing ultraviolet (UV) radiation. It is found that the surface actinic flux can be reduced by over 30% in January over eastern China due to aerosol radiative effects, which leads to maximum reductions of approximately 30% in photolysis rates and OH production. The largest regional-averaged reduction of surface actinic flux is found over eastern China (12%), followed by India (9%) and North Africa (8%). The impacts over other regions are generally less than 5%. The aerosols have a slight positive impact on surface O 3 over some regions, possibly due to the dependence of O 3 production on NO x /VOCs ratio. The impacts on surface O 3 vary from −16% to 2%, with the largest reduction occurring in China during wintertime. The effects of aerosols on tropospheric chemistry are sensitive not only to Aerosol optical depth (AOD) but also to the vertical structure and chemical components of aerosols. The vertical profiles of the impacts over eastern China indicate that the radiative effect of aerosols can reduce the actinic flux in the lower troposphere by approximately 18%, while above the boundary layer, the actinic flux is increased (maximum of 7% approximately 2 km) because of scattering aerosols. In turn, changes in the actinic flux induce changes in photolysis rates (ranging from −17% to 8%), OH production (ranging from −15% to 6%) and O 3 concentrations (ranging from −3% to 1%) over eastern China. This study highlights the different roles of each aerosol type in atmospheric photochemistry. The scattering aerosols (SNA and OC) weaken photochemistry in the lower layer and enhance in the upper troposphere, while the absorbing aerosols (BC and dust) slightly weaken the photochemistry throughout the troposphere. Globally, the impact induced by SNA is the most significant, followed by dust and BC. • Surface O 3 tends to be reduced in VOCs-limited areas while enhanced in NO x -limited areas due to aerosol radiative effect. • The largest reduction in surface actinic flux is found over eastern China, followed by India and North Africa. • Over eastern China, the impact induced by SNA (Sulfate-Nitrate-Ammonia) is the most significant, followed by BC and dust. • Scattering aerosols weaken photochemistry in the lower troposphere and enhance in the upper troposphere. • Absorbing aerosols slightly weaken the photochemistry throughout the troposphere. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract In this work, a systematic investigation on the mechanism and kinetics of the reactions of t -amyl methyl ether (TAME) and t- amyl ethyl ether (TAEE) with OH radicals in the gas phase has been performed within the Density Functional Theory (DFT) framework. The potential energy surfaces were obtained at the M06-2X/6-311++G(d,p) level. Kinetic data for the initial step in the oxidative degradation of TAME and TAEE were obtained, as well as branching ratios between the possible reaction channels. The calculated total rate constants are 3.9 × 10−11 cm3 molecules−1 s−1 and 1.3 × 10−12 cm3 molecules−1 s−1, for TAME and TAEE, respectively. Contrary to what occurs in the case of the MTBE and ETBE oxidation initiated by OH radicals, for TAME and TAEE the H-abstraction from α methoxy or ethoxy groups is not favored. For both molecules, the contribution of the reaction channels involving H-abstraction from the t -amyl group amounts to about 70% of the total rate coefficients. Using our calculated rate constants and a chosen OH tropospheric concentration of 1 × 106 molecule cm−3, lifetimes of ∼43 and ∼128 min were obtained for TAME and TAEE, respectively. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The UV absorption cross-sections of a series unsaturated aldehydes have been measured. • This work provides the first UV cross-section measurements for trans-2-pentenal and 2-methyl-2-pentenal. • The obtained cross-section values enabled us to estimate the tropospheric photolysis. • lifetimes of these compounds. • The results suggest that photolysis could be an important removal process for these species in the troposphere. Abstract C 5 and C 6 unsaturated aldehydes are a category of BVOCs released by plants in response to their wounds and stress. In this work, the gas phase UV absorption spectra of trans-2-pentenal (t-2P), trans-2-hexenal (t-2H) and 2-methyl-2-pentenal (2M2P) are determined between 240 and 400 nm, at room temperature and at pressures (1–9) torr. The reported UV absorption cross sections are (cm2molecule−1): σ t-2P, 330 = (4.86 ± 0.53) × 10−20, σ t-2H, 330 = (5.43 ± 0.57) × 10−20 and σ 2M2P, 320 = (6.62 ± 0.50) × 10−20. Their photolysis constants as well as their atmospheric lifetimes toward photolysis were then estimated. [ABSTRACT FROM AUTHOR]

This work explored the heterogeneous photochemical conversions of chlorobenzene on γ-Al 2 O 3 particulates, unveiling their reaction pathways and potential PCDD/F generation, and providing an insight into the possible origin of PCDD/Fs on atmospheric particulates. [Display omitted] • Photochemical conversion of chlorobenzene (CB) on γ-Al 2 O 3 was evaluated. • Atmospheric CB can be considerably degraded by γ-Al 2 O 3 under light irradiation. • Photochemical CB conversion is markedly suppressed by adding SO 2. • Photochemical CB conversion is distinctly facilitated by adding NH 3. • CB can be ultimately converted into PCDD/Fs under light irradiation. Atmospheric photochemical conversion of chlorinated volatile organic compounds (CVOCs) is crucial for secondary aerosol formation and tropospheric chlorine chemistry. However, their corresponding conversion pathways and the reaction products are lack of sufficient exploration. Herein, the photochemical conversion of chlorobenzene (CB) on the mineral γ-Al 2 O 3 particulates was chosen as a model reaction to unveil the atmospheric chemistry of CVOCs. Chamber experiments were performed to determine the decay rates of CB with/without SO 2 or NH 3 under light irradiation. Reaction products were evaluated, along with the in situ diffuse reflectance infrared Fourier transform spectroscopy measurements, to explore the CB conversion pathways under different conditions. We showed that the CB could be greatly degraded by γ-Al 2 O 3 under light irradiation, and the added SO 2 significantly inhibited this conversion owing to surface acidification. However, the presence of NH 3 was shown to facilitate the CB conversion by immobilizing Cl− as NH 4 Cl, and recovering surface oxygen vacancies for oxygen activation. In particular, we found that polychlorinated dibenzo- p -dioxins and dibenzofurans (PCDD/Fs) could be generated from the photochemical CB conversion over γ-Al 2 O 3 , regardless of SO 2 or NH 3 coexistence, indicating a potential origin of PCDD/Fs on the atmospheric particulates from the heterogeneous photochemical CVOC conversion. [ABSTRACT FROM AUTHOR]

Bromine chemistry plays an important role in tropospheric ozone depletion events in polar regions. Autocatalytic reactions lead to bromine explosion events, causing ozone depletion to near-zero levels in the polar troposphere. Bromine chemistry over Antarctica is not fully understood, and ground-based observations are scarce. This work presents year-long observations of bromine oxide (BrO) over the Bharati station (69.41°S, 76.19°E) using Multi-axis Differential Optical Absorption Spectroscopy (MAX-DOAS) from December 2018 to February 2020. The results show that elevated BrO mixing ratios were found during spring (September), with a maximum value of 10.21 ± 4.38 pptv for clear sky conditions and 33.15 ± 2.23 pptv for cloudy conditions. BrO was not observed above the detection limit (∼3 × 1013 molecule cm−2) outside spring on clear days. In general, lower mixing ratios were observed on clear days over Bharati compared to stations in West Antarctica. This indicates a different source strength over East Antarctica compared to West Antarctica. BrO vertical column densities were high during spring, with a maximum value of 1.34 ± 0.35 × 1014 molecule cm−2. The vertical profiles of the BrO mixing ratios show a peak at the surface during spring (average of 6.5 ± 1.91 pptv), decreasing sharply with altitude. Back trajectories show that air masses passing over the first year ice showed higher BrO, although factors such as meteorology play an important role in determining the absolute levels. Using a box model, we show that bromine chemistry can deplete as much as 2.15 ppb of ozone in a day at the Bharati Station on clear days, which shows that it does not lead to complete ozone depletion events over Bharati. • Year-long observations over Bharati, Antarctica, show elevated BrO only during spring, unlike other Antarctic Stations. • BrO values at Bharati are lower compared to stations in West Antarctic. • BrO levels were not sufficient to cause severe ozone depletion. [ABSTRACT FROM AUTHOR]

Air pollution is a major concern for human health and the environment. Consequently, environmental standards have become stricter to improve air quality. Thanks to this, the ambient levels of O 3 precursors such as VOCs and NO X have decreased. However, O 3 levels in Europe, especially during winter, have increased, potentially impacting on atmospheric oxidation capacity and the associated chemistry of tropospheric oxidants. In this work, we focus on recent changes in the oxidation capacity of urban atmospheres. The study is conducted with the results of the CMAQ modelling system with a regional resolution with 12 × 12 km2 across the entire European continent for the winter (January) and summer (July) of 2007 and 2015. The 2015 meteorological data is used for both years to emphasise emission changes during the studied period. We scrutinise the changes in ambient concentration levels of the main tropospheric oxidants (O 3 and HO X radicals) in five representative cities, Valencia, Madrid, Milan, Berlin, and The Hague. The enhanced O 3 formation in winter seems to be due to the low VOC/NO X ratio, while the opposite trend in summer may be related to a relatively high ratio. Additionally, photooxidation experiments are carried out in the EUPHORE chambers to study the effect of changes in NO X concentration and NO/NO 2 ratio on the variation of the given oxidants at constant VOCs concentrations. For the baseline experiments, two scenarios are selected based on the model results of 2015: two representative winter and summer days of low and high pollution in Berlin and Madrid, respectively. The role of VOC/NO X and NO/NO 2 ratios on atmospheric reactivity is discussed. As a result, it is first suggested that further decreases in ambient NO X levels are required to reduce ambient O 3 levels. Moreover, additional factors should be considered when designing local-specific emission abatement strategies. [Display omitted] • CMAQ simulations and EUPHORE experiments are coupled to study oxidation capacity • Diurnal O 3 changes varies along with VOC/NO X ratio depending on the time of the day • Reduction in VOC and NO X levels can lead to increases in HO X levels • The NO/NO 2 ratio can be a determining factor for variations in oxidation capacity • By further reducing NO X , cites can remain in NO X -limited, and eventually reduce O 3 [ABSTRACT FROM AUTHOR]

Middle atmospheric carbon monoxide (CO) has been considered as a dynamical tracer to study the transport in the stratosphere and mesosphere because of its long lifetime. However, most trend studies of CO are limited to troposphere and the Upper Troposphere Lower Stratosphere (UTLS). Here, we investigate the seasonal, annual and long-term changes of CO in the tropical (30° N–30° S) middle atmosphere (20–80 km) for the period of 2005–2021. CO shows a semi-annual oscillation in the higher altitudes of both stratosphere and mesosphere. The response of solar cycle to CO increases with altitude and is largest in the mesosphere. On the other hand, Quasi Biennial Oscillation (QBO) response to CO change is highest in the lower stratosphere. The CO trends are statistically insignificant at all altitudes, although they are positive in the height range 20–25 km and 32–40 km, at about 0.01–0.035/yr, depending on altitude. The observed changes in the middle atmospheric CO in the tropics are well reproduced by the Whole Atmospheric Chemical Climate Model (WACCM). Therefore, this study provides new insights on the long-term changes in the tropical middle atmospheric CO and its connection to dynamics of the region. • Spatial and temporal trends in the middle atmospheric CO are estimated for the tropics. • Seasonal variation of CO shows maxima around 45 50 km and 75 80 km. • There are insignificant positive trends in the stratosphere and lower mesosphere. • Global climate model simulations of CO agree well with satellite CO observations. [ABSTRACT FROM AUTHOR]

Abstract Nitrous acid (HONO) stands as one of the main species in tropospheric chemistry, primarily in polluted, urban regions. Due to its fast photodissociation, it is considered as one the main sources of the hydroxyl radical (OH), the most relevant oxidant in the atmosphere. Therefore, the evaluation of HONO concentration profiles and their temporal evolution is important for urban atmospheric chemistry. In this study, we report a year-round measurement of HONO vertical concentration profiles, as well as their diurnal and seasonal evolution during 2016 in Madrid. Making use of the Multi-AXis Differential Absorption Spectroscopy (MAX-DOAS) technique in addition to inversion algorithms, we retrieved the aerosol extinction and trace gas concentrations. Our results show HONO maximum values of 3.5–4 ppbv in the early morning and late afternoon, and minima around noon, when the lifetime of HONO against photolysis is shortest. On average, there is a pronounced HONO concentration gradient across different seasons, being higher during the autumn and winter months. Finally, we estimate and discuss the production rate of OH radicals from HONO photolysis, along with its variability throughout the year. Graphical abstract Unlabelled Image Highlights • Vertical concentration profiles of aerosols and HONO retrieved in Madrid. • HONO shows a maximum value of around 3.5 – 4 ppbv in the early morning and in the late evening. • Seasonal monthly averages of HONO present higher values in autumn and winter than in spring and summer. • OH radical production from HONO shows high values at around noon and the maximum production rate appears in autumn. [ABSTRACT FROM AUTHOR]

Abstract It's essential to find out the delay caused by the troposphere layer to be able to get more accurate results. The precise positioning is very essential for accurate geodetic applications e.g., Crustal deformation and military purposes. The delay is usually modeled as a zenith value and a mapping function to map it to the observed angle. The current research develops a new mapping function model for Egypt through a least square analysis using ten radiosonde stations. The proposed model is a development of Herring continued fraction equation for dry case and wet case separately. The developed model coefficients depend on latitude and day of year. The produced model as well as Niell, Vienna and Chao models are all evaluated using Precise Tropospheric Delay database (PTD). The test data are 36 days over the year 2015 representing winter, spring, summer and fall. The obtained results clearly showed the proposed model is more accurate for Egypt than other models for both dry and wet cases. [ABSTRACT FROM AUTHOR]

Highlights • First defense mechanisms to O 3 were examined in three Chinese ornamental species. • O 3 -induced stomatal closure did not limit sufficiently stomatal O 3 flux. • Species-specific stomatal traits and ascorbates played a pivotal role in O 3 defense. • O 3 -tolerant Rosa chinensis increased leaf ascorbates to cope with oxidative stress due to O 3. • O 3 -sensitive Tagetes erecta and Cotinus coggygria exhibited impairment of ascorbate regulation. Abstract High concentrations of tropospheric ozone (O 3) represent a serious problem in the Northern Hemisphere, especially in hot spots as urban areas of China. In this study, the first defense mechanisms (i.e., stomatal behavior and morphology, ascorbate regulations) of three widely-used ornamental plant species (Cotinus coggygria Scop., Rosa chinensis Jacq., Tagetes erecta L.) to O 3 were evaluated to test the suitability of these species for utilization in urban green spaces (UGS). Seedlings were exposed to (i) ambient air (AA), (ii) AA + 60 ppb O 3 (AA + 60), and (iii) AA + 120 ppb O 3 (AA + 120) for 60 (C. coggygria and R. chinensis) or 32 days (T. erecta), 9 h d−1. Considering the different physiological responses and O 3 sensitivity among the species, data of the present study reveal that: (i) O 3 -induced stomatal closure observed in the three species was not an avoidance-strategy given the lack of a significant relation between g s and O 3 fluxes into the leaves, a feature that confirm the strong negative effects of O 3 on the photosynthetic metabolism previously reported; and (ii) stomatal morphology, as well as ascorbate regulations, seem to play a pivotal role in the responses of the investigated species to O 3 ". Overall, our study highlights the importance of investigating the first defense mechanisms adopted by plants to cope with urban environmental stresses, in order to direct a correct management of UGS. [ABSTRACT FROM AUTHOR]

Abstract Benzene, toluene and xylene (BTX) belong to an important group of aromatic volatile organic compounds (VOCs) that are usually emitted from various sources. BTX play a vital role in the tropospheric chemistry as well as pose health hazard to human beings. Thus, an investigation of ambient benzene, toluene and xylene (BTX) was conducted at urban and rural sites of Gorakhpur for a span of one year in order to ascertain the contamination levels. The sampling of BTX was performed by using a low-flow SKC Model 220 sampling pump equipped with activated coconut shell charcoal tubes with a flow rate of 250 ml/min for 20–24 h. The analysis was in accordance with NIOSH method 1501. The efficiency of pump was checked weekly using regulated rotameters with an accuracy of ±1%. The samples were extracted with CS 2 with occasional agitation and analyzed by GC-FID. The total BTX concentration ranged from 3.4 μg m−3 to 45.4 μg m−3 with mean value 30.95 μg m−3 and median 24.8 μg m−3. The mean concentration of total BTX was maximum during winter (39.3 μg m−3), followed by summer (28.4 μg m−3) and monsoon season (25.1 μg m−3). The mean concentration of BTX at urban site (11.8 μg m−3) was higher than that at rural site (8.8 μg m−3). At both the sites, T/B and X/B ratios were highest in monsoon and lowest in winters. Toluene against benzene plot shows R2 value of 0.96 and 0.49 at urban and rural sites respectively. Higher R2 value at urban site clearly indicates similar sources of emission for benzene and toluene. At both the sites, the estimated integrated lifetime cancer risk (ILTCR) for benzene exceeded the threshold value of 1E-06 whereas the individual hazard quotients (HQ) for BTX did not exceed unity at any of the sites. Graphical abstract Image Highlights • An investigation of ambient BTX was conducted at urban and rural sites of Gorakhpur for a span of one year. • The mean levels of B, T & X were 12.1, 22.1 and 1.3 µg m−3 at urban site whereas 7.4, 17.9 and 1.1 µg m−3 at rural site. • The interspecies ratios T/B and X/B were highest in monsoon season and lowest during winters. • At urban site, T vs. B plot was substantially linear. However at rural site, a considerably non-linear plot was observed. • ILTCR for benzene at both the sites was higher than the threshold value 1E-06 whereas HQ for BTX did not exceed unity. [ABSTRACT FROM AUTHOR]

Alkyl nitrates (RONO 2 ) are an important class of nitrogen oxides reservoirs in the atmosphere and play a key role in tropospheric photochemistry. Despite the increasing concern for photochemical air pollution over China, the knowledge of characteristics and formation mechanisms of alkyl nitrates in this region is limited. We analyzed C 1 -C 5 alkyl nitrates measured in Beijing at a polluted urban site in summer 2008 and at a downwind rural site in summers of both 2005 and 2008. Although the abundances of NO x and hydrocarbons were much lower at the rural site, the mixing ratios of RONO 2 were comparable between both sites, emphasizing the regional nature of alkyl nitrate pollution. Regional transport of urban plumes governed the elevated RONO 2 levels at the rural site. The concentrations of C 1 -C 2 RONO 2 were significantly higher at the rural site in 2008 compared to 2005 despite a decline in NO x and anthropogenic VOCs, mainly owing to enhanced contributions from biogenic VOCs. The photochemical formation regimes of RONO 2 were evaluated by both a simplified sequential reaction model and a detailed master chemical mechanism box model. The observed C 4 -C 5 RONO 2 levels can be well explained by the photochemical degradation of n -butane and n -pentane, while the sources of C 1 -C 3 RONO 2 were rather complex. In addition to the C 1 -C 3 alkanes, biogenic VOCs and reactive aromatics were also important precursors of methyl nitrate, and alkenes and long-chain alkanes contributed to the formation of C 2 -C 3 RONO 2 . This study provides insights into the spatial distribution, inter-annual variation and photochemical formation mechanisms of alkyl nitrate pollution over the Beijing area. [ABSTRACT FROM AUTHOR]

This work reports the first ground-based atmospheric measurements of 26 halocarbons in Singapore, an urban-industrial city-state in Southeast (SE) Asia. A total of 166 whole air canister samples collected during two intensive 7 Southeast Asian Studies (7SEAS) campaigns (August–October 2011 and 2012) were analyzed for C 1 -C 2 halocarbons using gas chromatography-electron capture/mass spectrometric detection. The halocarbon dataset was supplemented with measurements of selected non-methane hydrocarbons (NMHCs), C 1 -C 5 alkyl nitrates, sulfur gases and carbon monoxide to better understand sources and atmospheric processes. The median observed atmospheric mixing ratios of CFCs, halons, CCl 4 and CH 3 CCl 3 were close to global tropospheric background levels, with enhancements in the 1–17% range. This provided the first measurement evidence from SE Asia of the effectiveness of Montreal Protocol and related national-scale regulations instituted in the 1990s to phase-out ozone depleting substances (ODS). First- and second-generation CFC replacements (HCFCs and HFCs) dominated the atmospheric halocarbon burden with HFC-134a, HCFC-22 and HCFC-141b exhibiting enhancements of 39–67%. By combining near-source measurements in Indonesia with receptor data in Singapore, regionally transported peat-forest burning smoke was found to impact levels of several NMHCs (ethane, ethyne, benzene, and propane) and short-lived halocarbons (CH 3 I, CH 3 Cl, and CH 3 Br) in a subset of the receptor samples. The strong signatures of these species near peat-forest fires were potentially affected by atmospheric dilution/mixing during transport and by mixing with substantial urban/regional backgrounds at the receptor. Quantitative source apportionment was carried out using positive matrix factorization (PMF), which identified industrial emissions related to refrigeration, foam blowing, and solvent use in chemical, pharmaceutical and electronics industries as the major source of halocarbons (34%) in Singapore. This was followed by marine and terrestrial biogenic activity (28%), residual levels of ODS from pre-Montreal Protocol operations (16%), seasonal incidences of peat-forest smoke (13%), and fumigation related to quarantine and pre-shipment (QPS) applications (7%). [ABSTRACT FROM AUTHOR]

Guaiacol is the main representative of methoxyphenols generated from the pyrolysis of ligin. The gas-phase reaction mechanism of guaiacol with NO 3 radical was researched by using density functional theory at M06-2X/6-31+g(d,p)//M06-2X/6-311+g(3df,2p) level in this work, and the reaction rate constants were calculated by using RRKM theory. The relative complete gas phase reaction mechanism of guaiacol with nitrate radicals was obtained. The calculated results showed that, NO 3 addition and H abstraction are two dominant progresses for the initial reaction of guaiacol with NO 3 radicals. The total reaction rate constant is 3.11 × 10 −11  cm 3  molecule −1  s −1 at 298 K and 1 atm. The atmospheric lifetime is 64 s under typical tropospheric concentration of NO 3 radicals. The possibility of products (methoxybenzoquinone and nitro-guaiacol) is verified and new products (dialdehyde and diketone) are predicted. [ABSTRACT FROM AUTHOR]

Ambient carbon monoxide (CO) and meteorological parameters measured at the Egham (EGH) semi-rural site in SW London during 2000–2015 have permitted wind sector analysis of diurnal and seasonal cycles, and interpretation of long-term trends. CO daily amplitudes are used as a proxy for anthropogenic emissions. At EGH, morning and evening peaks in CO arise from the dominant contribution of road transport sources. Smaller amplitudes are observed during weekends than weekdays due to lower combustion emissions, and for mornings compared to evenings due to the timing of the development and break-up of the nocturnal inversion layer or planetary boundary layer (PBL). A wavelet transform revealed that the dominant mode of CO variability is the annual cycle, with apparent winter maxima likely due to increased CO emissions from domestic heating with summer minima ascribed to enhanced dispersion and dilution during the annual maximum of PBL mixing heights. Over the last two decades, both mitigation measures to reduce CO emissions and also a major switch to diesel cars, have accompanied a change at EGH from the dominance of local diurnal sources to a site measuring close to Atlantic background levels in summer months. CO observed in the S and SW wind sectors has declined by 4.7 and 5.9 ppb yr −1 respectively. The EGH CO record shows the highest levels in the early 2000s, with levels in E and calm winds comparable to those recorded at background stations in Greater London. However, since 2012, levels in S-SW sector have become more comparable with Mace Head background except during rush-hour periods. Marked declines in CO are observed during 2000–2008 for the NE, E, SE (London) and calm wind sectors, with the smallest declines observed for the S, SW and W (background) sectors. For the majority of wind sectors, the decline in CO is less noticeable since 2008, with an apparent stabilisation for NE, E and SE after 2009. The EGH CO data record exhibits a similar but slower exponential decay, but from a much lower starting concentration, than do CO data recorded at selected monitoring sites in urban areas in SE England. CO/CO 2 residuals determined using hourly averaged datain the diurnal cycle demonstrate a clear decline in CO from 2000 to 2015 during daily periods of increased vehicle traffic, which is consistent with a sustained reduction in CO emissions from the road transport sector. [ABSTRACT FROM AUTHOR]

An analysis of reaction products for the reaction of E,E-2,4-hexadienal with chlorine atoms (Cl) and OH and NO 3 radicals has been carried out at the first time with the aim of obtaining a better understanding of the tropospheric reactivity of α,β-unsaturated carbonyl compounds. Fourier Transform Infrared (FTIR) spectroscopy and Gas Chromatography-Mass Spectrometry with a Time of Flight detector (GC-TOFMS) were used to carry out the qualitative and/or quantitative analyses. Reaction products in gas and particulate phase were observed from the reactions of E,E-2,4- hexadienal with all oxidants. E/Z-Butenedial and maleic anhydride were the main products identified in gas phase. E-butenedial calculated molar yield ranging from 4 to 10%. A significant amount of multifunctional compounds (chloro and hydroxy carbonyls) was identified. These compounds could be formed in particulate phase explaining the ∼90% of unaccounted carbon in gas phase. The reaction with Cl atoms in the presence of NOx with a long reaction time gave Peroxy Acetyl Nitrate (PAN) as an additional product, which is known for being an important specie in the generation of the photochemical smog. Nitrated compounds were the major organic products from the reaction with the NO 3 radical. Based on the identified products, the reaction mechanisms have been proposed. In these mechanisms a double bond addition of the atmospheric oxidant at C4/C5 of E,E-2,4-hexadienal is the first step for tropospheric degradation. [ABSTRACT FROM AUTHOR]

The regional climate model RegCM4.4, including the surface model CLM4.5, was offline coupled to the chemistry transport model CAMx version 6.30 in order to investigate the impact of the urban canopy induced meteorological changes on the longterm summer photochemistry over central Europe for the 2001–2005 period. First, the urban canopy impact on the meteorological conditions was calculated performing a reference experiment without urban landsurface considered and an experiment with urban surfaces modeled with the urban parameterization within the CLM4.5 model. In accordance with expectations, strong increases of urban surface temperatures (up to 2–3 K), decreases of wind speed (up to −1 ms −1 ) and increases of vertical turbulent diffusion coefficient (up to 60–70 m 2 s -1 ) were found. For the impact on chemistry, these three components were considered. Additionally, we accounted for the effect of temperature enhanced biogenic emission increase. Several experiments were performed by adding these effects one-by-one to the total impact: i.e., first, only the urban temperature impact was considered driving the chemistry model; secondly, the wind impact was added and so on. We found that the impact on biogenic emission account for minor changes in the concentrations of ozone (O 3 ), oxides of nitrogen NOx = NO + NO 2 and nitric acid (HNO 3 ). On the other hand, the dominating component acting is the increased vertical mixing, resulting in up to 5 ppbv increase of urban ozone concentrations while causing −2 to −3 ppbv decreases and around 1 ppbv increases of NOx and HNO 3 surface concentrations, respectively. The temperature impact alone results in reduction of ozone, increase in NO, decrease in NO 2 and increases of HNO 3 . The wind impact leads, over urban areas, to ozone decreases, increases of NOx and a slight increase in HNO 3 . The overall impact is similar to the impact of increased vertical mixing alone. The Process Analysis (PA) technique implemented in CAMx was adopted to investigate the causes of the modeled impacts in more details. It showed that the main process contributing to the temperature impact on ozone is a dry-deposition enhancement, while the dominating process controlling the wind impact on ozone over cities is the advection reduction. In case of the impact of enhanced turbulence, PA suggests that ozone increases are, again as assumed, the result of increased downward vertical mixing supported by reduced chemical loss. Comparing the model concentrations with measurements over urban areas, a slight improvement of the model performance was achieved during afternoon hours if urban canopy forcing on chemistry via meteorology was accounted for. The study demonstrates that disregarding the urban canopy induced meteorological effects in air-quality oriented modeling studies can lead to erroneous results in the calculated species concentrations. However, it also shows that the individual components are not equally important: urban canopy induced turbulence effects dominate while the wind-speed and temperature related ones are of considerably smaller magnitude. [ABSTRACT FROM AUTHOR]

• Lightning radiometry is extended to the infrared portion of the optical spectrum. • Optical power around the H-alpha line scales with peak current to the power 1.25. • The derived power-law scaling is attributed to negative differential resistance. • Non-uniform detector response may affect band-averaged source power estimates. Calibrated measurements of lightning optical emissions are critical for both quantifying the impacts of lightning in our atmosphere and devising detection instruments with sufficient dynamic range capable of yielding close to 100% detection efficiency. However, to date, there is only a limited number of investigations that have attempted to take such calibrated measurements. In this work, we report the power radiated by lightning in both visible and infrared bands, assuming isotropic emission, and accounting for atmospheric absorption. More precisely, we report peak radiated power and total radiated energy in the combined visible plus near-infrared range (VNIR, 0.34–1.1 μm), around the H α line (652–667 nm), and for the 2–2.5 μm infrared band. The estimated peak power and total energy radiated by negative cloud-to-ground return strokes in the VNIR range is 130 MW and 20 kJ, respectively. Additionally, we detected peak radiated powers of 12 and 0.19 MW in the H α and infrared bands, respectively. We cross-reference the optical data set with peak current reported by a lightning detection network. The resulting trend is that optical power emitted around the H α line scales with peak return stroke current according to a power law with exponent equal to 1.25. This trend, which should be approximately true across the entire visible spectrum, can be attributed to the plasma negative differential resistance of the lightning return stroke channel. We conclude by discussing the challenges in performing calibrated measurements of lightning optical power in different bands and comparing the results with previously-collected data with different experimental setups, observation conditions, and calibration methods. [ABSTRACT FROM AUTHOR]

The El Niño-Southern Oscillation (ENSO) represents the dominant mode of global climate variability and is inherently nonlinear such that the linearity of the atmospheric response remains an area of ongoing research. The phase of North Atlantic Oscillation (NAO) and Pacific North American (PNA) patterns of intra-annual climate variability are favored to be the same as the phase of ENSO resulting in important climate impacts across Europe and North America. Advanced understanding of the symmetry of this response at global scale using monthly composite analyses of anomalous horizontal sensible and latent heat fluxes at various ENSO event magnitudes quantified from ERA-Interim output (January 1979 through June 2016) will advance impact predictability. A linear relationship between ENSO, PNA, and NAO patterns was identified, particularly for strong ENSO events. The nonlinear component indicated general eastward (westward) shifts in anomalous heat fluxes during El Niño (La Niña) events such that the greatest impacts were implied across North America during Decembers and Januarys of strong El Niño and weak La Niña events. Analyses of anomalous latent heat fluxes indicated spatial patterns consistent with more frequent atmospheric river phenomena, especially during Decembers and Januarys of strong El Niño events. This work demonstrates that the symmetric component of anomalous horizontal, tropospheric-mean heat fluxes corresponding to ENSO events are effective for identifying north-south dipoles of anomalous circulations consistent with PNA or NAO patterns and connections between tropical heat source regions and the PNA and NAO regions. This work also demonstrates the asymmetric component identified differences in anomalous circulation position and whether El Niño or La Niña resulted in larger heat flux anomalies. Therefore, this work provides insight into impacts associated with future ENSO events, especially across North America during strong El Niño and weak La Niña. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons and their oxidized derivatives are ubiquitous environmental pollutants that are toxic to different degrees. The NO 3 radical is known to be an important actor in fostering nighttime atmospheric chemistry; hence it could elicit some nocturnal PAH-loss processes. In this study the gas phase pathways open to the initial anthracene and phenanthrene NO 3 π−radical adducts in the presence of O 2 , NO and NO 2 are examined by Density Functional Theory to ascertain the mechanistic features of their NO 3 -initiated oxidative degradation. Unimolecular steps involving the initial adducts (ring closures or fragmentation) present rather high free energy barriers and seem unlikely. Regarding bimolecular reactions, any radical present in the tropospheric environment can give an intrinsically fast radical coupling with the initial adducts, thus producing bifunctional closed shell species. The addition of ground state dioxygen is a relatively fast step that is reversible in some cases, less so in others. It entails subsequent reaction steps, which involve the peroxyl radical intermediate and addition and loss of small species as NO, NO 3 , NO 2 and O 2 . These steps can either trigger a β−fragmentation with formation of a closed shell dialdehyde or formation of a closed shell nitroxy ketone, bifunctional species that appear to be the most likely products when typical tropospheric concentrations are taken into account. [ABSTRACT FROM AUTHOR]

A single water molecule is known to significantly lower the potential energy barrier in the atmospheric reactions of hydroxyl radical with volatile organic compounds but except at extremely low temperatures, it has never been observed to accelerate the reactions investigated so far. This quantum–mechanical computational work reveals a probable case of observing a single water molecule accelerating the atmospheric reactions of Trifluoroacetic acid and Trifluoroacetaldehyde at temperatures near 200 K under tropospheric conditions. [ABSTRACT FROM AUTHOR]

The Photochemical Ozone Creation Potential (POCP) scale quantifies the relative abilities of volatile organic compounds (VOCs) to produce ground level ozone. POCP values are usually calculated using atmospheric boundary layer models containing detailed representations of atmospheric VOC degradation chemistry. The sensitivity of POCP values to variation of a number of kinetic and mechanistic parameters has been investigated here. It is shown that POCP values for VOCs can be rationalized in terms of their molecular structure and OH reactivity. As a result, a simple method has been developed and optimized that allows POCP values for north-west European and USA urban reference conditions to be estimated for alkanes, alkenes, aromatic hydrocarbons, and several oxygenated VOC classes without the requirement to construct a detailed chemical mechanism or run an atmospheric model. The procedure for determining the estimated POCP value (POCP E ) is described, and the results are presented and discussed. [ABSTRACT FROM AUTHOR]

Unsaturated ethers are oxygenated volatile organic compounds (OVOCs) emitted by anthropogenic sources. Potential removal processes in the troposphere are initiated by hydroxyl (OH) radicals and photochemistry. In this work, we report for the first time the rate coefficients of the gas-phase reaction with OH radicals ( k OH ) of 2-chloroethyl vinyl ether (2ClEVE), allyl ether (AE), and allyl ethyl ether (AEE) as a function of temperature in the 263–358 K range, measured by the pulsed laser photolysis–laser induced fluorescence technique. No pressure dependence of k OH was observed in the 50–500 Torr range in He as bath gas, while a slightly negative T -dependence was observed. The temperature dependent expressions for the rate coefficients determined in this work are: k OH , 2 ClEVE ( T ) = ( 9.0 ± 2.0 ) × 10 − 12 exp ( 478 ± 72 T ) c m 3 molecul e − 1 s − 1 , k OH , AE ( T ) = ( 1.3 ± 0.4 ) × 10 − 11 exp ( 442 ± 91 T ) c m 3 molecul e − 1 s − 1 , k OH , AEE ( T ) = ( 5.8 ± 1.3 ) × 10 − 12 exp ( 563 ± 62 T ) cm 3 molecule − 1 s − 1 , The estimated atmospheric lifetimes (τ OH ) assuming k OH at 288 K were 3, 2, and 4 h for 2ClEVE, AE and AEE, respectively. The kinetic results are discussed in terms of the chemical structure of the unsaturated ethers by comparison with similar compounds. We also report ultraviolet (UV) and infrared (IR) absorption cross sections (σ λ and σ ( ν ˜ ) , respectively). We estimate the photolysis rate coefficients in the solar UV actinic region to be less than 10 −7 s −1 , implying that these compounds are not removed from the atmosphere by this process. In addition, from σ ( ν ˜ ) and τ OH , the global warming potential of each unsaturated ether was calculated to be almost zero. A discussion on the atmospheric implications of the titled compounds is presented. [ABSTRACT FROM AUTHOR]

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with two biogenic volatile organic compounds as (E) -2-hexenyl acetate and 4-methyl-3-penten-2-one have been determined at 298 K and atmospheric pressure. The decay of the organics was followed using a chromatograph with a flame ionization detector (GC-FID) and the rate constants were determined using a relative rate method. Rate coefficients are found to be (in cm 3 molecule −1 s −1 ): k 1 (OH + (E) -2-hexenyl acetate) = (6.88 ± 1.41) × 10 −11 , k 2 (Cl + (E) -2-hexenyl acetate) = (3.10 ± 1.13) × 10 −10 , k 3 (OH + 4-methyl-3-penten-2-one) = (1.02 ± 0.20) × 10 −10 and k 4 (Cl + 4-methyl-3-penten-2-one) = (2.66 ± 0.90) × 10 −10 at 298 K. This is the first kinetic experimental study for these reactions studied under atmospheric pressure. The rate coefficients are compared with previous determinations for other unsaturated and oxygenated compounds and reactivity trends are presented. Products identification studies were performed using solid-phase microextraction (SPME) method employing on-fiber products derivatization with o -(2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride using gas chromatograph with a mass spectrometer detector (GC-MS) for the reactions studied. In addition, atmospheric lifetimes of the unsaturated compounds studied are estimated and compared with other tropospheric sinks for these compounds. [ABSTRACT FROM AUTHOR]

An ozonesonde launched near electrically active convection in Houston, TX on 5 September 2013 during the NASA DISCOVER-AQ project measured a large enhancement of ozone throughout the troposphere. A separate ozonesonde was launched from Smith Point, TX (∼58 km southeast of the Houston site) at approximately the same time as the launch from Houston and did not measure that enhancement. Furthermore, ozone profiles for the descent of both sondes agreed well with the ascending Smith Point profile, suggesting a highly localized event in both space and time in which an anomalously large enhancement of 70–100 ppbv appeared in the ascending Houston ozonesonde data. Compared to literature values, such an enhancement appears to be the largest observed to date. Potential sources of the localized ozone enhancement such as entrainment of urban or biomass burning emissions, downward transport from the stratosphere, photochemical production from lightning NO x , and direct ozone production from corona discharges were investigated using model simulations. We conclude that the most likely explanation for the large ozone enhancement is direct ozone production by corona discharges. Integrating the enhancement seen in the Houston ozone profile and using the number of electrical discharges detected by the NLDN (or HLMA), we estimate a production of 2.48 × 10 28 molecules of ozone per flash which falls within the range of previously recorded values (9.89 × 10 26 –9.82 × 10 28 molecules of ozone per flash). Since there is currently no parameterization for the direct production of ozone from corona discharges we propose the implementation of an equation into a chemical transport model. Ultimately, additional work is needed to further understand the occurrence and impact of corona discharges on tropospheric chemistry on short and long timescales. [ABSTRACT FROM AUTHOR]

Southern Europe (Italy and the surrounding countries) experienced an unusual wet summer in 2014. The monthly rainfall in July 2014 was 84% above (more than three standard deviation) normal with respect to the 1982–2013 July climatology. The heavy rainfall damaged agriculture, and affected tourism and overall economy of the region. In this study, we tried to understand the physical mechanisms responsible for such abnormal weather by using model and observed datasets. The anomalously high precipitation over Italy is found to be associated with the positive sea surface temperature (SST) and convective anomalies in the tropical Pacific through the atmospheric teleconnection. Rossby wave activity flux at upper levels shows an anomalous tropospheric quasi-stationary Rossby wave from the Pacific with an anomalous cyclonic phase over southern Europe. This anomalous cyclonic circulation is barotropic in nature and seen extending to lower atmospheric levels, weakening the seasonal high and causing heavy precipitation over the Southern Europe. The hypothesis is verified using the National Centers for Environmental Prediction (NCEP) coupled forecast system model (CFSv2) seasonal forecasts. It is found that two-month lead forecast of CFSv2 was able to capture the wet summer event of 2014 over Southern Europe. The teleconnection pattern from Pacific to Southern Europe was also forecasted realistically by the CFSv2 system. [ABSTRACT FROM AUTHOR]

Photochemical smog, characterized by high concentrations of ozone (O 3 ) and fine particles (PM 2.5 ) in the atmosphere, has become one of the top environmental concerns in China. Volatile organic compounds (VOCs), one of the key precursors of O 3 and secondary organic aerosol (SOA) (an important component of PM 2.5 ), have a critical influence on atmospheric chemistry and subsequently affect regional and global climate. Thus, VOCs have been extensively studied in many cities and regions in China, especially in the North China Plain, the Yangtze River Delta and the Pearl River Delta regions where photochemical smog pollution has become increasingly worse over recent decades. This paper reviews the main studies conducted in China on the characteristics and sources of VOCs, their relationship with O 3 and SOA, and their removal technology. This paper also provides an integrated literature review on the formulation and implementation of effective control strategies of VOCs and photochemical smog, as well as suggestions for future directions of VOCs study in China. [ABSTRACT FROM AUTHOR]

We have developed an approach for retrieving HCFC-142b (CH 3 CClF 2 ) from ground-based high-resolution infrared solar spectra, using its ν 7 band Q branch in the 900–906 cm −1 interval. Interferences by HNO 3 , CO 2 and H 2 O have to be accounted for. Application of this approach to observations recorded within the framework of long-term monitoring activities carried out at the northern mid-latitude, high-altitude Jungfraujoch station in Switzerland (46.5°N, 8.0°E, 3580 m above sea level) has provided a total column times series spanning the 1989 to mid-2015 time period. A fit to the HCFC-142b daily mean total column time series shows a statistically-significant long-term trend of (1.23±0.08×10 13 molec cm −2 ) per year from 2000 to 2010, at the 2-σ confidence level. This corresponds to a significant atmospheric accumulation of (0.94±0.06) ppt (1 ppt=1/10 12 ) per year for the mean tropospheric mixing ratio, at the 2− σ confidence level. Over the subsequent time period (2010–2014), we note a significant slowing down in the HCFC-142b buildup. Our ground-based FTIR (Fourier Transform Infrared) results are compared with relevant data sets derived from surface in situ measurements at the Mace Head and Jungfraujoch sites of the AGAGE (Advanced Global Atmospheric Gases Experiment) network and from occultation measurements by the ACE-FTS (Atmospheric Chemistry Experiment-Fourier Transform Spectrometer) instrument on-board the SCISAT satellite. [ABSTRACT FROM AUTHOR]

TEMPO was selected in 2012 by NASA as the first Earth Venture Instrument, for launch between 2018 and 2021. It will measure atmospheric pollution for greater North America from space using ultraviolet and visible spectroscopy. TEMPO observes from Mexico City, Cuba, and the Bahamas to the Canadian oil sands, and from the Atlantic to the Pacific, hourly and at high spatial resolution (~2.1 km N/S×4.4 km E/W at 36.5°N, 100°W). TEMPO provides a tropospheric measurement suite that includes the key elements of tropospheric air pollution chemistry, as well as contributing to carbon cycle knowledge. Measurements are made hourly from geostationary (GEO) orbit, to capture the high variability present in the diurnal cycle of emissions and chemistry that are unobservable from current low-Earth orbit (LEO) satellites that measure once per day. The small product spatial footprint resolves pollution sources at sub-urban scale. Together, this temporal and spatial resolution improves emission inventories, monitors population exposure, and enables effective emission-control strategies. TEMPO takes advantage of a commercial GEO host spacecraft to provide a modest cost mission that measures the spectra required to retrieve ozone (O 3 ), nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), formaldehyde (H 2 CO), glyoxal (C 2 H 2 O 2 ), bromine monoxide (BrO), IO (iodine monoxide), water vapor, aerosols, cloud parameters, ultraviolet radiation, and foliage properties. TEMPO thus measures the major elements, directly or by proxy, in the tropospheric O 3 chemistry cycle. Multi-spectral observations provide sensitivity to O 3 in the lowermost troposphere, substantially reducing uncertainty in air quality predictions. TEMPO quantifies and tracks the evolution of aerosol loading. It provides these near-real-time air quality products that will be made publicly available. TEMPO will launch at a prime time to be the North American component of the global geostationary constellation of pollution monitoring together with the European Sentinel-4 (S4) and Korean Geostationary Environment Monitoring Spectrometer (GEMS) instruments. [ABSTRACT FROM AUTHOR]

Oil exploitation can release a large amount of volatile organic compounds (VOCs), which play an important role in tropospheric chemistry and contribute to ozone (O 3) and secondary organic aerosols (SOA) formation in its surrounding environment. To evaluate the impact of oil exploitation on tropospheric O 3 formation, a comprehensive field campaign on O 3 and its precursors (VOC, NOx, etc) was conducted during May–September 2019 in a site nearby oil fields of Shandong province, China. The average mixing ratio of the total VOC (TVOC) was 59.2 ± 30.2 ppbv, and alkanes contributed 78% of TVOC (with C 2 –C 5 alkanes as the main contributors). A 0-D box model incorporating the latest Master Chemical Mechanism (MCMv3.3.1) was applied using observations as constraints, and we found that the O 3 formation was mainly under transitional and VOC-limited regime for high-O 3 and low-O 3 periods, respectively. The relative incremental reactivity (RIR) values from individual AVOC species were basically consistent with those calculated from OH reactivity, and both metrics suggested that C 3 –C 5 alkanes, C 2 –C 5 alkenes, and C 7 –C 9 aromatics were the most sensitive VOC species to O 3 formation. The results from empirical kinetic modeling approach (EKMA) indicated large day-to-day variations of photochemical regimes, which covered VOC-limited, transitional, and NOx-limited regimes on a daily basis during the five-month campaign. The emission reduction modeling with a large number of scenarios further suggested that the reduction of anthropogenic VOC and NOx at a ratio of 1:3 would be a more practical strategy for mitigation of the local O 3 pollution. These results could provide a basis for understanding the O 3 -precursor relationship nearby the oil field area, and the derived policy-relevant emission control strategy could provide guidance for other oil field in Shandong and other regions to take effective O 3 control measures. [Display omitted] • O 3 and its precursors were investigated in a receptor site nearby oil fields of China. • Most sensitive VOC species to O 3 formation were identified based on different metrics. • 0-D box model showed large day-to-day variations of O 3 formation regime for the site. • A synergic reduction plan to NOx and AVOC was developed for local area O 3 mitigation. [ABSTRACT FROM AUTHOR]

The wildfire season in the Western United States (U.S.) was anomalously large in 2020, with a majority of burned area due to lightning ignitions resulting in overall fire emissions of carbon monoxide (CO) in the Western region almost 3 times the 2001–2019 average. We used the Community Atmosphere Model version 6 with Chemistry (CAM-chem) to investigate how the 2020 fires in the Western U.S. affected air quality locally as well as in surrounding regions that received transported pollution. Simulations with and without fire emissions over the Western U.S. (32.5–49°N, 115–125°W) in July–December 2020 were used to determine average changes in atmospheric composition across the country. Comparisons against satellite and ground-based column CO observations show that the model generally underestimated CO from fires but adequately reproduced spatial and temporal variability. Simulations showed the 2020 fire season contributed 14.5% to atmospheric CO over the Contiguous United States in September, and ∼ 3% to CO averaged across the Northern Hemisphere; these enhancements lasted several months. Fire emissions in 2020 continued later into the year than usual, resulting in sustained air pollution over the Western U.S. region, with noticeable meridional transport of ozone (O 3) and fine particulate matter (PM 2.5). Finally, we use the model to identify two transported fire pollution events at Boulder, Colorado. • The 2020 Western U.S. wildfires produced CO emissions 3 times the 2001–2019 average. • The fires contributed 0.5 – 14.5% to modeled CO averaged over the United States. • Enhancements in surface O 3 and PM 2.5 across the U.S. lasted through the fire season. [ABSTRACT FROM AUTHOR]

Tropospheric delay is a major limiting factor in ground displacement retrieval using interferometric synthetic aperture radar (InSAR). Such delays are caused by the variations of pressure, temperature, and humidity of the troposphere, potentially leading to elevation-dependent phases in interferograms. The atmospheric heterogeneity makes the phase-elevation dependence vary in both space and time, posing challenges on empirical models that estimated the elevation-topography relationship in empirically predefined windows interferogram by interferogram. Moreover, the portion of the deformation that is correlated with topography can be mistaken as tropospheric delays and therefore degrade the performance of linear regression between the tropospheric phase and the elevation. The effectiveness of these methods is further limited by the requisite knowledge presumed of the user, particularly with regard to how to divide the scene and which observation model is suitable. These limitations make reliable isolation of tropospheric delay challenging. We present here a novel approach that combines tropospheric delay, deformation and possible topographic error to a joint model, enabling a precise separation of tropospheric delay. The rationale is rooted in the distinct temporal dependencies of these parameters. To avoid empirical setting of segmentation windows, we employ quadtree to adaptively control the spatial variability of tropospheric properties. We validate the proposed method at two volcanic areas, Bali and Hawaii, where tropospheric delay presents notable variability coupled with elevation-correlated deformation signals. Both ascending and descending Sentinel-1 data over the island of Bali show that the proposed method outperforms the conventional ones. The misfit standard deviation (STD) is reduced by ∼50% at the island of Bali, where elevation-correlated deformation is simulated according to a predefined model. More impressively, at the island of Hawaii, the misfit STD between InSAR and ground truth (i.e., GPS displacements) decreases from 25.1 to 4.5 mm. The experimental results demonstrate that the proposed method is effective in reducing the spatially variable tropospheric delay while preserving the elevation-correlated deformation signal. • More accurate mitigation of tropospheric delays by spatiotemporal InSAR modelling. • Adaptive generation of spatial patches by quadtree according to model residuals. • Comparison with GPS at Hawaii island reveals the accuracy was improved by 5 times. [ABSTRACT FROM AUTHOR]

• A new BVOC Plant Specific Emission Model (PSEM) was developed • New algorithms used to describe seasonal emissions from plants and cultivars • The Plant Functional Type (PFT) approach can lead to high isoprene emissions • better agreement with observed BVOC concentrations using PSEM • PSEM has been used to derive biogenic emissions used in VEG-GAP Life Project Biogenic Volatile Organic Compounds (BVOCs) emitted from vegetation are precursors of ozone (O 3), photochemical oxidants and secondary organic aerosols (SOA) in the lower troposphere. The interaction between urban polluted air plumes originated along the coasts of the Mediterranean basin with biogenic emission occurring inland contributes to the occurrence of exceedances of the air quality standards for O 3 and fine suspended particles (PM 10 , PM 2.5). This interaction is favoured by prevailing sea-land breeze circulation during persistent high-pressure conditions. The actual contribution of BVOC to photochemical pollution is still uncertain because the approaches used to assess the emissions from terrestrial vegetation are quite different. There is some evidence in literature that BVOC emissions from Mediterranean vegetation is not accurately estimated by models based on the plant functional types (PFT) approach. To investigate these issues, a Plant Specific Emission Model (PSEM) was developed and applied to the Campania region in Southern Italy, for which a detailed vegetation inventory has been built. BVOC emission maps estimated by PSEM were compared with those generated using a PFT methodology, evidencing significant differences. BVOC emissions from the two models were then used to predict the concentrations of precursors and products of photochemical smog pollution over the Gulf of Naples (Italy) using a chemical-transport model. Simulations were performed during a period characterised by high pressure conditions that favour an enhanced O 3 production under a sea-breeze circulation regime. VOCs concentration profiles predicted by the two models were compared with field data collected over Mount Vesuvius using a tethered balloon coupled with a PTR-MS. The results proved the better capability of PSEM to predict the concentrations of many BVOCs, including isoprene and some of its primary oxidation products over the measuring site, and suggested the significant potential of BVOC emission to produce SOA over the gulf of Naples. [Display omitted] [ABSTRACT FROM AUTHOR]

Rate coefficients for the reactions of NO 3 with n -butyl methacrylate (k 1 ), iso -butyl methacrylate (k 2 ) and tert -butyl methacrylate (k 3 ) have been determined at 298 K and atmospheric pressure using the relative rate method. The following rate coefficients (×10 −15 cm 3 molecule −1 s −1 ) were obtained for the first time: k 1 = (5.5 ± 2.6), k 2 = (5.8 ± 2.8) and k 3 = (5.6 ± 2.5). The NO 3 reactions of these compounds could contribute to the removal of NO x and as NO y reservoirs. The potential importance for the tropospheric nitrogen budget of these reactions is discussed and atmospheric lifetimes for the butyl esters are calculated. [ABSTRACT FROM AUTHOR]

This study investigates the impact of ENSO on the CO 2 variability over the Indian subcontinent for the period 2003–2011 based on the relationships between NINO indices derived from the sea surface temperature (SST) and AIRS-retrieved mid-tropospheric CO 2 concentrations. The NINO4 region exhibits positive influence on the variability of CO 2 almost during the entire year except for the post-monsoon/winter months (October through December; OND). Significant positive relationship (correlation coefficient r = +0.68) between NINO4 index and CO 2 levels is observed for the month of June, while negative relationship (r = −0.73) for the month of October, and the negative relationship tends to continue till November with decreasing magnitudes (r = −0.41). The spatial distribution of mid-tropospheric CO 2 concentrations during El Niño and La Niña periods also indicate large-scale impact over the Indian subcontinent with positive (negative) anomalies of about 1–2 ppm during El Niño (La Niña). [ABSTRACT FROM AUTHOR]

The UV absorption spectrum of pyridine and its gas phase reactions with OH radicals and O 3 were investigated. UV absorption cross-sections were determined by using a D 2 -lamp system in the range 200–350 nm. The kinetic studies were carried out at room temperature and atmospheric pressure of purified air. The rate coefficient for the reaction of pyridine with OH was determined relative to that with acetone while that with O 3 was measured under pseudo first order conditions. The rate coefficients obtained are (in cm 3 molecule − 1 s − 1 ): k(OH + pyridine) = (5.40 ± 0.80) × 10 −13 and k(O 3 + pyridine) = (3.28 ± 1.70) × 10 −20 . [ABSTRACT FROM AUTHOR]

Surface ozone is an environmental issue occurring at several scales, ranging from local to continental. One of the most developed regions in China, the Yangtze River Delta (YRD), experiences severe tropospheric ozone problem. Hence, quantifying the contributions from various geographical source regions is helpful for better understanding the regional ozone problem. Ozone source apportionment studies can provide relevant information for designing suitable air pollution protection strategies. In the present work, the WRF-Chem model coupled with an online ozone tagging method is applied to a case study, with the objective of exploring the ozone contributions to the surface ozone from different source regions over the YRD region, during a frequent wind-shifting period. Our results show that the YRD was highly affected by the upwind source regions bearing high values both ozone and its precursors. The contribution from the source region outside the main air pollution zones in the Central Eastern China (super regional contribution) was also important, accounting for more than 30 ppb of daytime maximum mean ozone concentrations. Ozone arising from increased local and regional emissions during high-concentration events was more significant than super regional contribution. It reveals that the ozone from Anhui region was transported through vertical mixing and horizontal advection to receptor areas in the YRD during the study time focus. Chemical process contributed significantly at ground and high altitude levels of 500 and 1000 m. However, most of the ozone from the remote regions of Henan and Hubei provinces was transported to the receptor of Nanjing through physical processes. The vertical mixing process played a crucial positive role at super regional scales, with regard to the formation of surface ozone over the YRD region during the addressed time interval. [ABSTRACT FROM AUTHOR]

Ozone source–receptor relationships over East Asia have been quantitatively investigated using a chemical transport model including an on-line tracer-tagged procedure, with a particular focus on the source regions of different daily ozone mixing ratios. Comparison with observations showed that the model reproduced surface ozone and tropospheric nitrogen dioxide column densities. Long-range transport from outside East Asia contributed the greatest fraction to annual surface ozone over remote regions, the Korean peninsula, and Japan, reaching 50%–80% of total ozone. Self-contributions accounted for 5%–20% ozone in the Korean peninsula and Japan, whereas the contribution of trans-boundary transport from photochemical production in China was less than 5%–10%. At extra-high ozone levels, self-contributions reached 50%–60% in the Korean peninsula. Ozone source–receptor relationships showed high seasonal variability over East Asia. Significant transport was also found between sub-regions in China, which presents a great challenge to policy-makers because most current control strategies are confined to specific regions. [ABSTRACT FROM AUTHOR]

Regional Chemical transport models (CTM) are used extensively for modeling of Ozone concentration. WRF/Chem is one such CTM that includes various chemical mechanisms and is used for simulation of Ozone and other pollutant concentration at desired time step. This study focuses on the robustness of WRF/Chem simulated Ozone concentrations over a sub tropical urban airshed of megacity Delhi. Detailed analysis has been presented for the veracity of two different chemical mechanisms namely; Carbon Bond Mechanism (CBMZ) and Regional Atmospheric Chemical Model (RACM). It was observed that simulated Ozone concentrations are better predicted with CBMZ mechanism and is highly sensitive to the rate constants. The Ozone concentrations are analyzed for precursors such as Oxides of Nitrogen (NOx) and Carbon-monoxide (CO) as well as temperature considering their strong dependence on Ozone formation. A consistent positive correlation between Ozone concentration and temperature is noted whereas; NOx and CO show inverse relationship with Ozone. Further, Ozone concentration range vis-à-vis model performance is scrutinized. A poor model for low Ozone concentration levels is observed and a highly satisfactory for moderate Ozone concentration levels while satisfactory for higher Ozone levels. Despite of the limitations observed during model evaluation of Ozone predictions for low Ozone levels, it is concluded that WRF/Chem could effectively be applied for understanding its trends, tropospheric chemistry and air quality assessment for regulatory purposes at moderate Ozone concentration levels. Further, it is recommended that model implementation shall be made for policy decisions cautiously with due consideration to the magnitudes of Ozone levels present in the study domain and the performance measures in the specific concentration range. [ABSTRACT FROM AUTHOR]