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Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a 50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO2), and organic peroxy (RO2) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (kOH, the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 × 106 cm−3 at noon, and median concentrations of both HO2 and RO2 radicals were 3 × 108 cm−3. The measured OH reactivity was between 4 and 18 s−1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (Ox= NO2 + O3) determined from NO2 and O3 concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO2 and RO2 radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h−1 for OH, HO2, and RO2. Total ROX (OH, HO2, and RO2) initiation and termination rates were below 3 ppbv h−1. The highest OH radical turnover rate of 13 ppbv h−1 was observed during a high-temperature (max. 40 ∘C) period in August. In this period, the highest HO2, RO2, and ROX turnover rates were around 11, 10, and 4 ppbv h−1, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO2 production and destruction rates were balanced, but unexplained RO2 and ROX production reactions with median rates of 2 and 0.4 ppbv h−1, respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO2 and RO2 measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO2 were found with maximum rates of 3.0 and 2.0 ppbv h−1, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO2 to OH conversion reaction (up to 2 ppbv h−1) and a missing primary radical source (0.5–1.4 ppbv h−1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO2 isomerization reactions from isoprene, HO2 uptake on aerosol, and RO2 production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.

Chlorinated cleaning products containing hypochlorous acid (HOCl) and hypochlorite ion (OCl−) act as sources of toxic chloramines (NH2Cl, NHCl2, NCl3) and atmospheric oxidant precursors (i.e. Cl2) which impact air quality.

Particulate nitrate ( pNO 3 − ) has long been considered a permanent sink for NO x (NO and NO 2 ), removing a gaseous pollutant that is central to air quality and that influences the global self-cleansing capacity of the atmosphere. Evidence is emerging that photolysis of pNO 3 − can recycle HONO and NO x back to the gas phase with potentially important implications for tropospheric ozone and OH budgets; however, there are substantial discrepancies in “renoxification” photolysis rate constants. Using aircraft and ground-based HONO observations in the remote Atlantic troposphere, we show evidence for renoxification occurring on mixed marine aerosols with an efficiency that increases with relative humidity and decreases with the concentration of pNO 3 − , thus largely reconciling the very large discrepancies in renoxification photolysis rate constants found across multiple laboratory and field studies. Active release of HONO from aerosol has important implications for atmospheric oxidants such as OH and O 3 in both polluted and clean environments.

Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a 50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO2), and organic peroxy (RO2) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (kOH, the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 × 106 cm−3 at noon, and median concentrations of both HO2 and RO2 radicals were 3 × 108 cm−3. The measured OH reactivity was between 4 and 18 s−1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (Ox= NO2 + O3) determined from NO2 and O3 concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO2 and RO2 radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h−1 for OH, HO2, and RO2. Total ROX (OH, HO2, and RO2) initiation and termination rates were below 3 ppbv h−1. The highest OH radical turnover rate of 13 ppbv h−1 was observed during a high-temperature (max. 40 ∘C) period in August. In this period, the highest HO2, RO2, and ROX turnover rates were around 11, 10, and 4 ppbv h−1, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO2 production and destruction rates were balanced, but unexplained RO2 and ROX production reactions with median rates of 2 and 0.4 ppbv h−1, respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO2 and RO2 measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO2 were found with maximum rates of 3.0 and 2.0 ppbv h−1, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO2 to OH conversion reaction (up to 2 ppbv h−1) and a missing primary radical source (0.5–1.4 ppbv h−1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO2 isomerization reactions from isoprene, HO2 uptake on aerosol, and RO2 production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.

Volatile organic compounds (VOCs) are important precursors to the formation of ozone (O3) and secondary organic aero-sols (SOA) and can also have direct human health impacts. Generally, given the range and number of VOC species, their emissions are poorly characterised. The VOC levels in Beijing during two campaigns (APHH) were investigated using a dispersion model (NAME), and a chemical box model (AtChem2) in order to understand how chemistry and transport affect the VOC concentrations in Beijing. Emissions of VOCs in Beijing and contributions from outside Beijing were modelled using the NAME dispersion model combined with the emission inventories and were used to initialize the AtChem2 box model. The modelled concentrations of VOCs from the NAME-AtChem2 combination were then compared to the output of a chemical transport model (GEOS-Chem). The results from the emission inventories and the NAME air mass pathways suggest that industrial sources to the south of Beijing and within Beijing both in summer and winter are very important in con-trolling the VOC levels in Beijing. A number of scenarios with different nitrogen oxides to ozone ratios (NOx / O3) and hydroxyl (OH) levels were simulated to determine the changes in VOC levels. In Beijing over 80 % of VOC are emitted locally during winter, while during summer about 35 % of VOC concentrations (greater for some individual species) are transported into Beijing from the surrounding regions. Most winter scenarios are in good agreement with daily GEOS-Chem simulations, with the best agreements seen for the modelled concentrations of ethanol, benzene and propane with correlation coefficients of 0.67, 0.63 and 0.64 respectively. Furthermore, the production of formaldehyde within 24 hours air travel from Beijing was investigated, and it was determined that 90 % of formaldehyde in the winter and 83 % in the summer in Beijing is secondary, produced from oxidation of non-methane volatile organic compounds (NMVOCs). The benzene / CO and toluene / CO ratios during the campaign is very similar to the ratio derived from literature for 2014 in Beijing, however more data are needed to enable investigation of more species over longer timeframes to determine whether this ratio can be applied to predicting VOCs in Beijing. The results suggest that VOC concentrations in Beijing are driven predominantly by sources within Beijing and by local atmospheric chemistry during the winter, and by a combination of transport and chemistry during the summer. Moreover, the relationship of the NOx / VOC and O3 during winter and summer shows the need for season-specific policy measures.

Ambient measurements of nitryl chloride (ClNO2) were performed at a rural site in Germany covering 3 periods in winter, summer, and autumn 2019 as part of the JULIAC campaign (Jülich Atmospheric Chemistry Project) that aimed for understanding the photochemical processes in air masses typical for mid-west Europe. Measurements were conducted at 50 m above ground, which was most located mainly at the nocturnal boundary layer and thus uncoupled from local surface emissions. ClNO2 is produced at nighttime by heterogeneous reaction of dinitrogen pentoxide (N2O5) on chloride ion (Cl-) containing aerosol. Its photolysis at day is of general interest as it produces chlorine (Cl) atoms that react with different atmospheric trace gases forming radicals. The highest observed ClNO2 mixing ratio was 1.6 ppbv (15-min average) in the middle of one night in September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO2 mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while values were higher ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N2O5 necessary for ClNO2 formation comes from the reaction of nitrate radicals (NO3) with nitrogen dioxide (NO2), where NO3 itself is formed by reaction of NO2 with ozone (O3). Measured concentrations of ClNO2, NO2 and O3 were used to quantify ClNO2 production efficiencies, i.e., the yield of ClNO2 formation per NO3 radical formed, and a box model was used to examine the idealized dependence of ClNO2 on the observed nocturnal O3 and NO2 concentrations. Results indicate that ClNO2 production efficiency was most sensitive to the availability of NO2 rather than that of O3 and increase with decreasing temperature. The average ClNO2 production efficiency was highest in February and September with values of 18 % and was lowest in December with values of 3 %. The average ClNO2 production efficiencies were in the range of 3 and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in literature indicating the importance of ClNO2 chemistry in rural environments in mid-west Europe.

We present an instrument for the measurement of total ozone reactivity – the reciprocal of the chemical lifetime of ozone (O3) – in the troposphere. The Total Ozone Reactivity System (TORS) was developed with the objective to study the role of biogenic volatile organic compounds (BVOCs) as chemical sinks of tropospheric ozone. The instrument was extensively characterized and tested in the laboratory using individual BVOCs and small plants (lemon thyme, Thymus citriodorus) in a Teflon bag and proved able to measure reactivities corresponding to >4.5×10-5 s−1 (at 5 min averaging time), with an estimated total uncertainty of ∼32%. Such reactivities correspond to >20 ppb of α-pinene or >150 ppb of isoprene in isolation – larger than typical ambient levels but observable in environmental chamber and enclosure experiments as well as in BVOC-rich environments. The functionality of TORS was demonstrated in quasi-ambient conditions with a deployment in a horticultural glasshouse containing a range of aromatic plants. The measurements of total ozone reactivity made in the glasshouse showed a clear diurnal pattern, following the emissions of BVOCs, and are consistent with mixing ratios of tens of parts per billion of monoterpenes and several parts per billion of sesquiterpenes.

Ambient measurements of nitryl chloride (ClNO2) were performed at a rural site in Germany, covering three periods in winter, summer, and autumn 2019, as part of the JULIAC campaign (Jülich Atmospheric Chemistry Project) that aimed to understand the photochemical processes in air masses typical of midwestern Europe. Measurements were conducted at 50 m aboveground, which was mainly located in the nocturnal boundary layer and thus uncoupled from local surface emissions. ClNO2 is produced at night by the heterogeneous reaction of dinitrogen pentoxide (N2O5) on chloride (Cl−) that contains aerosol. Its photolysis during the day is of general interest, as it produces chlorine (Cl) atoms that react with different atmospheric trace gases to form radicals. The highest-observed ClNO2 mixing ratio was 1.6 ppbv (parts per billion by volume; 15 min average) during the night of 20 September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO2 mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while the values were higher, ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N2O5 necessary for ClNO2 formation comes from the reaction of nitrate radicals (NO3) with nitrogen dioxide (NO2), where NO3 itself is formed by a reaction of NO2 with ozone (O3). Measured concentrations of ClNO2, NO2, and O3 were used to quantify ClNO2 production efficiencies, i.e., the yield of ClNO2 formation per NO3 radical formed, and a box model was used to examine the idealized dependence of ClNO2 on the observed nocturnal O3 and NO2 concentrations. Results indicate that ClNO2 production efficiency was most sensitive to the availability of NO2 rather than that of O3 and increased with decreasing temperature. The average ClNO2 production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO2 production efficiencies were in the range of 3 % and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in the literature, indicating the importance of ClNO2 chemistry in rural environments in midwestern Europe.

Nitrogen oxides (NO and NO2, collectively NOX) derived from vehicle exhausts are critical pollutants with significant implications for urban air quality and human and environmental health. In this study, we investigate trends in measured ambient nitrogen dioxide (NO2) and NOx mixing ratios at urban traffic - dominated monitoring sites in the UK for the period 2009–2016. We apply an oxidant analysis approach alongside a number of assumptions to the ambient data to determine trends in the inferred primary NO2/NOx ratio, and examine evidence for enhanced vehicle “cold start” effects upon these inferred emissions. Ambient NO2 and NOX mixing ratios have experienced an overall decrease of 17.2% and 11.3% respectively for the locations considered over this time period. The inferred primary NO2/NOx ratio for the majority of the study locations is found to have fallen by 29% (from 0.175 to 0.125) as a monthly mean from 2009 to 2016, with a statistically significant median decrease of 0.32 percentage points per year. However, during cold weather (temperatures less than or equal to 5 OC), the inferred primary NO2/NOx ratio averaged across all locations, when compared with normal conditions (temperatures higher than 5 OC) increased from 0.062 (±0.004) to 0.102 (±0.001) (64.5% higher) and from 0.056 (±0.004) to 0.098 (±0.001) (75% higher) for cold morning and evening rush hours, with substantially greater increases at some sites. This “cold start” result suggests that the combination of recent vehicle driving history and ambient weather conditions, in conjunction with technological constraints on the operating temperature range of emission control systems in some vehicles, affects NOx emissions and hence has a detrimental impact upon air quality in urban environments. Increased cold start emissions imply an increased NO2 - derived health burden from air pollution, under certain meteorological conditions, assessment of which should consider changes in vehicle use as a result of weather, and hence altered personal exposure.

The Indian megacity of Delhi suffers from some of the poorest air quality in the world. While ambient NO2 and particulate matter (PM) concentrations have received considerable attention in the city, high ground level ozone (O3) concentrations are an often overlooked component of pollution. O3 can lead to significant ecosystem damage, agricultural crop losses, and adversely affect human health. During October 2018, concentrations of speciated non-methane hydrocarbons volatile organic compounds (C2 – C13), oxygenated volatile organic compounds (o-VOCs), NO, NO2, HONO, CO, SO2, O3, and photolysis rates, were continuously measured at an urban site in Old Delhi. These observations were used to constrain a detailed chemical box model utilising the Master Chemical Mechanism v3.3.1. VOCs and NOx (NO + NO2) were varied in the model to test their impact on local O3 production rates, P(O3), which revealed a VOC-limited chemical regime. When only NOx concentrations were reduced, a significant increase in P(O3) was observed, thus VOC co-reduction approaches must also be considered in pollution abatement strategies. Of the VOCs examined in this work, mean morning P(O3) rates were most sensitive to monoaromatic compounds, followed by monoterpenes and alkenes, where halving their concentrations in the model led to a 15.6 %, 13.1 % and 12.9 % reduction in P(O3), respectively. P(O3) was not sensitive to direct changes in aerosol surface area but was very sensitive to changes in photolysis rates, which may be influenced by future changes in PM concentrations. VOC and NOx concentrations were divided into emission source sectors, as described by the EDGAR v5.0 Global Air Pollutant Emissions and EDGAR v4.3.2_VOC_spec inventories, allowing for the impact of individual emission sources on P(O3) to be investigated. Reducing road transport emissions only, a common strategy in air pollution abatement strategies worldwide, was found to increase P(O3), even when the source was removed in its entirety. Effective reduction in P(O3) was achieved by reducing road transport along with emissions from combustion for manufacturing and process emissions. Modelled P(O3) reduced by ~20 ppb h−1 when these combined sources were halved. This study highlights the importance of reducing VOCs in parallel with NOx and PM in future pollution abatement strategies in Delhi.

The Jülich Atmospheric Chemistry Project campaign (JULIAC) was performed using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich (FZJ), Germany. Ambient air was continuously drawn into the chamber through a 50m high inlet line for one month in each season throughout 2019. The residence time of air inside the chamber was one hour. As the sampling point is surrounded by a mixed deciduous forest and is located close to a small–size city (Jülich), the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements included hydroxyl radical (OH) achieved by laser induced fluorescence (LIF) instrument that implemented a newly implemented chemical modulation reactor (CMR) and by differential optical absorption spectroscopy (DOAS). Measurement of both instruments were in good agreement within about 10% and showed no evidence of unknown OH interferences. In addition to OH, hydroxyl and peroxy radicals (HO2 and RO2, respectively), and OH reactivity (kOH, inverse of the OH lifetime) were measured together with a comprehensive set of trace gases concentrations and aerosol properties, allowing for the investigation of the seasonal and diurnal variation of atmospheric oxidant concentrations and their roles in the degradation of volatile organic compounds (VOCs) and contribution to secondary pollutants (ozone and particles).The experimental budget analyses of OH, HO2, RO2, and ROx radical production and destruction rate will be presented for the campaigns in spring and summer (April and August). For most conditions, the concentrations of radicals were sustained by regeneration of HO2 and RO2 radicals via reactions with nitric oxide (NO). The highest radical turnover rates of up to 17 ppbv·hr-1 was observed during a heat wave period in August. For NO levels below 1ppbv, the budget shows a missing OH radical source up to 4 ppbv h-1, while HO2 and RO2 productionand destruction rates were balanced. Above 2 ppbv of NO, missing HO2 production and RO2 loss paths with rates of up to 5 ppbv h-1 were found. In addition, the dataset allows for a detail examination of the importance of radical production and destruction processes from isomerization reactions, HO2 uptake on aerosol, chlorine nitrate chemistry.

Which published papers have transformed our understanding of the chemical processes in the troposphere and shaped the field of atmospheric chemistry? By way of expert solicitation and interactive peer review, this paper explores the influence of the ideas in peer-reviewed articles based on input from our community of atmospheric scientists. We explore how these papers have shaped the development of the field of atmospheric chemistry and identify the major landmarks in the field of atmospheric chemistry through the lens of those papers' impact on science, legislation and environmental events. We also explore the ways in which one can identify the papers that have most impacted the field and discuss the advantages and disadvantages of the various approaches. Our work highlights the difficulty of creating a simple list, and we explore the reasons for this difficulty. The paper also provides a history of the development of our understanding of tropospheric chemistry and points some ways for the future.

Daytime atmospheric oxidation chemistry is conventionally considered to be driven primarily by the OH radical, formed via photolytic sources. In this paper we examine how, during winter when photolytic processes are slow, chlorine chemistry can have a significant impact on oxidative processes in the urban boundary layer. Photolysis of nitryl chloride (ClNO2) provides a significant source of chlorine atoms, which enhances the oxidation of volatile organic compounds (VOCs) and the production of atmospheric pollutants. We present a set of observations of ClNO2 and HONO made at urban locations in central England in December 2014 and February 2016. While direct emissions and in-situ chemical formation of HONO continue throughout the day, ClNO2 is only formed at night and is usually completely photolyzed by midday. Our data show that, during winter, ClNO2 often persists through the daylight hours at mixing ratios above 10–20 ppt (on average). In addition, relatively high mixing ratios of daytime HONO ( > 65 ppt) provide a strong source of OH radicals throughout the day. The combined effects of ClNO2 and HONO result in sustained sources of Cl and OH radicals from sunrise to sunset, which form additional ozone, PAN, oxygenated VOCs, and secondary organic aerosol. We show that radical sources such as ClNO2 and HONO can lead to a surprisingly photoactive urban atmosphere during winter and should therefore be included in atmospheric chemical models.

AtChem is an open source zero-dimensional box-model for atmospheric chemistry. Any general set of chemical reactions can be used with AtChem, but the model was designed specifically for use with the Master Chemical Mechanism (MCM, http://mcm.york.ac.uk/). AtChem was initially developed within the EUROCHAMP project as a web application (AtChem-online, https://atchem.leeds.ac.uk/webapp/) for modelling environmental chamber experiments; it was recently upgraded and further developed into a standalone offline version (AtChem2) which allows the user to run complex and long simulations, such as those needed for modelling of intensive field campaigns, as well as to perform batch model runs for sensitivity studies. AtChem is installed, set up and configured using semi-automated scripts and simple text configuration files, making it easy to use even for non-experienced users. A key feature of AtChem is that it can easily be constrained to observational data which may have different timescales, thus retaining all the information contained in the observations. Implementation of a continuous integration workflow, coupled with a comprehensive suite of tests and version control software, makes the AtChem codebase robust, reliable and traceable. The AtChem2 code and documentation are available at https://github.com/AtChem/, under the open source MIT license.

Vehicle use, as driver or passenger, is a key transport mode and important microenvironment for personal exposure to air pollutants. Here, the air pollution exposure of vehicle occupants, relative to external, ambient levels was explored under different vehicle ventilation settings and driving routes in an urban area. Four vehicles were driven on a consistent route encompassing three contrasting road types, measuring simultaneous within-vehicle and ambient levels of particulate matter (PM10, PM2.5, PM1), ultrafine particles number (UFP), lung surface deposited area (LSDA), nitric oxide (NO) and nitrogen dioxide (NO2). For the majority of ventilation settings available, the inside/outside (I/OVEH) ratio for PM10, PM2.5, PM1, UFP, LSDA and NO2 concentrations was below 1, with the exception of NO, for which the ratio was independent of ventilation settings, within uncertainty. The lowest within-vehicle exposure for particles and gases was observed under the conditions of windows closed, recirculation and air conditioning on. Vehicle occupants are exposed to and inhale more air pollution when traveling on urban roads, followed by ring-roads and sub-urban roads. However, through reduced within-vehicle concentrations and reduced physical activity and hence breathing rate, they inhale less air pollution than people cycling/walking on the same routes. Within-vehicle air pollution exposure displays significant dependence upon both the ventilation setting and route selected. Vehicle occupants can, therefore, modify their personal exposure through these choices. Finally, vehicle occupants inhale more mass of NO2 than PM2.5 with a trip-average inhalation dose ratio of 6.4 (NO2 dose/PM2.5 dose). These results may have significant health impacts upon highly exposed groups such as professional drivers.

©2015. The Authors. We have developed a chemical mechanism describing the tropospheric degradation of chlorine containing very short-lived substances (VSLS). The scheme was included in a global atmospheric model and used to quantify the stratospheric injection of chlorine from anthropogenic VSLS (ClyVSLS) between 2005 and 2013. By constraining the model with surface measurements of chloroform (CHCl3), dichloromethane (CH2Cl2), tetrachloroethene (C2Cl4), trichloroethene (C2HCl3), and 1,2-dichloroethane (CH2ClCH2Cl), we infer a 2013 ClyVSLS mixing ratio of 123 parts per trillion (ppt). Stratospheric injection of source gases dominates this supply, accounting for ∼83% of the total. The remainder comes from VSLS-derived organic products, phosgene (COCl2, 7%) and formyl chloride (CHClO, 2%), and also hydrogen chloride (HCl, 8%). Stratospheric ClyVSLS increased by ∼52% between 2005 and 2013, with a mean growth rate of 3.7 ppt Cl/yr. This increase is due to recent and ongoing growth in anthropogenic CH2Cl2 - the most abundant chlorinated VSLS not controlled by the Montreal Protocol.

The organic content of shale has become of commercial interest as a source of hydrocarbons, owing to the development of hydraulic fracturing ("fracking"). While the main focus is on the extraction of methane, shale also contains significant amounts of non-methane hydrocarbons (NMHCs). We describe the first real-time observations of the release of NMHCs from a fractured shale. Samples from the Bowland-Hodder formation (England) were analyzed under different conditions using mass spectrometry, with the objective of understanding the dynamic process of gas release upon fracturing of the shale. A wide range of NMHCs (alkanes, cycloalkanes, aromatics, and bicyclic hydrocarbons) are released at parts per million or parts per billion level with temperature- and humidity-dependent release rates, which can be rationalized in terms of the physicochemical characteristics of different hydrocarbon classes. Our results indicate that higher energy inputs (i.e., temperatures) significantly increase the amount of NMHCs released from shale, while humidity tends to suppress it; additionally, a large fraction of the gas is released within the first hour after the shale has been fractured. These findings suggest that other hydrocarbons of commercial interest may be extracted from shale and open the possibility to optimize the "fracking" process, improving gas yields and reducing environmental impacts.

We present a comprehensive chemical mechanism for gas-phase iodine, to be used for modelling tropospheric chemistry. The mechanism has been compiled from evaluated data and individual literature studies, where available; a number of key processes have not been studied experimentally or theoretically and in these cases estimations have been made. The uncertainty associated with these assumptions is evaluated. We analyze the mechanism using a box-model under a variety of boundary layer scenarios – representative of environments where iodine species have been observed – to study the response of the chemical system to changes in the kinetic parameters of selected reactions. We focus in particular on key species such as IO, OIO, INO 3 and I 2 O y and the impact of iodine chemistry on ozone formation and HO x levels. The results indicate that the chemical system is most sensitive to reactions leading to comparatively stable iodine compounds, which should be a focus of future laboratory studies.

Measurements of total peroxy radicals (HO2 + RO2) and nitrate radical (NO3) were made on the NOAA research vessel R/V Brown along the U.S. Gulf Coast during the TexAQS 2006 field campaign. The measurements were modelled using a constrained box-model based upon the Master Chemical Mechanism (MCM). The agreement between modelled and measured HO2 + RO2 was typically within ∼40% and, in the unpolluted regions, within 30%. The analysis of the model results suggests that the MCM might underestimate the concentrations of some acyl peroxy radicals and other small peroxy radicals. The model underestimated the measurements of NO3 by 60–70%, possibly because of rapid heterogeneous uptake of N2O5. The MCM model results were used to estimate the composition of the peroxy radical pool and to quantify the role of DMS, isoprene and alkenes in the formation of RO2 in the different regions. The measurements of HO2 + RO2 and NO3 were also used to calculate the gas-phase budget of NO3 and quantify the importance of organic peroxy radicals as NO3 sinks. RO2 accounted, on average, for 12–28% of the total gas-phase NO3 losses in the unpolluted regions and for 1–2% of the total gas-phase NO3 losses in the polluted regions.

Cl atoms in the marine atmosphere may significantly impact the lifetimes of methane and other hydrocarbons. However, the existing estimates of Cl atom levels in marine air are based on indirect evidence. Here we present measurements of the Cl precursors HOCl and Cl2 in the marine boundary layer during June of 2009 at the Cape Verde Atmospheric Observatory in the eastern tropical Atlantic. These are the first measurements of tropospheric HOCl. HOCl and Cl2 levels were low in air with open ocean back trajectories, with maximum levels always below 60 and 10 ppt (pmol/mol), respectively. In air with trajectories originating over Europe, HOCl and Cl2 levels were higher, with HOCl maxima exceeding 100 ppt each day and Cl2 reaching up to 35 ppt. The increased Cl cycling associated with long distance pollutant transport over the oceans likely impacts a wide geographic area and represents a mechanism by which human activities have increased the reactivity of the marine atmosphere. Data-constrained model simulations indicate that Cl atoms account for approximately 15 % of methane destruction on days when aged polluted air arrives at the site. A photochemical model does not adequately simulate the observed abundances of HOCl and Cl2, raising the possibility of an unknown HOCl source.

Photochemical processes inside urban plumes in the Northeast of the United States have been studied using a highly detailed chemical model, based upon the Master Chemical Mechanism (MCM). The model results have been compared to measurements of oxygenated VOCs (acetone, methyl ethyl ketone, acetaldehyde, acetic acid and methanol) obtained during several flights of the NOAA WP-3D aircraft, which sampled plumes from the New York City area during the ICARTT campaign in 2004. The agreement between the model and the measurements was within 40–60% for all species, except acetic acid. The model results have been used to study the formation and photochemical evolution of acetone, methyl ethyl ketone and acetaldehyde. Under the conditions encountered during the ICARTT campaign, acetone is produced from the oxidation of propane (24–28%) and i-propanol (C5 alkanes, propanal and MEK. Ethane and ethanol oxidation account, respectively, for 6–23% and 5–25% of acetaldehyde photochemical formation. The results highlight the importance of long-chain alkanes for the photochemical production of ketones and the role of hydroperoxides in sustaining their formation far from the emission sources.

During the Texas Air Quality Study II (TexAQS 2006) campaign, a PEroxy Radical Chemical Amplifier (PERCA) was deployed on the NOAA research vessel R/V Brown to measure total peroxy radicals (HO2+Σ RO2). Day-time mixing ratios of HO2+Σ RO2 between 25 and 110 ppt were observed throughout the study area – the Houston/Galveston region and the Gulf coast of the US – and analyzed in relation to measurements of nitrogen oxides, volatile organic compounds (VOC) and photolysis rates to assess radical sources and sinks in the region. The measurements of HO2+Σ RO2 were used to calculate the in-situ net photochemical formation of ozone. Measured median values ranged from 0.6 ppb/h in clean oceanic air masses up to several tens of ppb/h in the most polluted industrial areas. The results are consistent with previous studies and generally agree with observations made during the previous TexAQS 2000 field campaign. The net photochemical ozone formation rates determined at Barbours Cut, a site immediately south of the Houston Ship Channel, were analyzed in relation to local wind direction and VOC reactivity to understand the relationship between ozone formation and local VOC emissions. The measurements of HO2+Σ RO2 made during the R/V Brown TexAQS 2006 cruise indicate that ozone formation is NOx-limited in the Houston/Galveston region and influenced by highly reactive hydrocarbons, especially alkenes from urban and industrial sources and their photo-oxidation products, such as formaldehyde.

Measurements of nitryl chloride (ClNO2) and its precursors (O3, NO2, particulate chloride) were made in 2014–2016 at three contrasting locations in the United Kingdom: Leicester, Penlee Point and Weybourne. ClNO2 was observed at all sites and in every season, with the highest concentrations between 00:00 and 04:00 GMT. The median nocturnal concentration of ClNO2 ranged between the detection limit (4.2 ppt) and 139 ppt. A clear seasonal cycle, with maxima in spring and winter, and significant differences between locations in the same season were observed. The main source of particulate chloride was sea salt aerosol (including at Leicester, ∼200 km from the coast). In general, ClNO2 levels were controlled by the concentrations of O3 and NO2, rather than by the uptake and reaction of N2O5 with particulate chloride. Under these conditions, the seasonality and geographical distribution of ClNO2 can be explained in terms of O3-limited and NO2-limited regimes affecting the formation of the N2O5 precursor. A global version of the GEOS-Chem model at medium resolution (2° × 2.5°) was not able to fully capture the observed seasonality of ClNO2, mostly because the model overestimated the concentrations of the precursors, particularly of nocturnal O3. A higher-resolution (0.25° × 0.3125°) version of GEOS-Chem showed better agreement with the observations, although it still overestimated ClNO2 concentrations during summer.

This paper describes a modelling study of several HOx and NOx species (OH, HO2, organic peroxy radicals, NO3 and N2O5) in the marine boundary layer. A model based upon the Master Chemical Mechanism (MCM) was constrained to observations of chemical and physical parameters made onboard the NOAA ship R/V Brown as part of the New England Air Quality Study (NEAQS) in the summer of 2004. The model was used to calculate [OH] and to determine the composition of the peroxy radical pool. Modelled [NO3] and [N2O5] were compared to in-situ measurements by Cavity Ring-Down Spectroscopy. The comparison showed that the model generally overestimated the measurements by 30–50%, on average. The model results were analyzed with respect to several chemical and physical parameters, including uptake of NO3 and N2O5 on fog droplets and on aerosol, dry deposition of NO3 and N2O5, gas-phase hydrolysis of N2O5 and reactions of NO3 with NMHCs and peroxy radicals. The results suggest that fog, when present, is an important sink for N2O5 via rapid heterogeneous uptake. The comparison between the model and the measurements were consistent with values of the heterogeneous uptake coefficient of N2O5 (γN2O5)>1×10−2, independent of aerosol composition in this marine environment. The analysis of the different loss processes of the nitrate radical showed the important role of the organic peroxy radicals, which accounted for a significant fraction (median: 15%) of NO3 gas-phase removal, particularly in the presence of high concentrations of dimethyl sulphide (DMS).

Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemically during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO3 in regionally polluted areas with large NOx levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a region with large emissions of both isoprene and NOx. Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO3 and were the most important factor determining the lifetime of this radical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO3. A determination of the mass of isoprene oxidized in darkness showed it to be a large fraction (>20%) of emitted isoprene. Organic nitrates produced from the NO3+isoprene reaction, though not directly measured, were estimated to account for 2–9% of total reactive nitrogen. The mass of isoprene oxidized by NO3 was comparable to and correlated with the organic aerosol loading for flights with relatively low organic aerosol background. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1–17%, and isoprene SOA mass derived from NO3 was calculated to exceed that due to OH by approximately 50%.

Secondary organic chemistry inside a typical urban plume in the North-East of the United States has been studied using a highly detailed chemical model, based upon the Master Chemical Mechanism (MCM). The model results have been qualitatively compared to measurements taken during three flights of the NOAA WP-3D aircraft, which sampled a plume from the New York City area during the NEAQS 2004 campaign. The model has been used to study the formation processes and photochemical evolution of alkyl nitrates. While long-chain (C5) alkyl nitrates are produced for 90% or more from the oxidation of a single parent alkane, short-chain ( C 4 ) alkyl nitrates can be formed from several precursors. The relative importance of each production route has been quantitatively determined thanks to the high level of chemical detail provided by the MCM. These secondary routes to the formation of alkyl nitrates include the oxidation of longer-chain alkanes and oxygenated intermediates, like carbonyls, peroxides and carboxylic acids.

Nitryl chloride, an active halogen, can be produced through the night-time reaction of dinitrogen pentoxide with chloride-containing aerosol in the polluted marine boundary, and has been measured at levels that are sufficient to affect the photochemistry of oxidants off the southwestern US coast and near Houston, Texas.

Night-time chemistry in the Marine Boundary Layer has been modelled using a number of observationally constrained zero-dimensional box-models. The models were based upon the Master Chemical Mechanism (MCM) and the measurements were taken during the North Atlantic Marine Boundary Layer Experiment (NAMBLEX) campaign at Mace Head, Ireland in July–September 2002. The model could reproduce, within the combined uncertainties, the measured concentration of HO2 (within 30–40%) during the night 31 August–1 September and of HO2+RO2 (within 15–30%) during several nights of the campaign. The model always overestimated the NO3 measurements made by Differential Optical Absorption Spectroscopy (DOAS) by up to an order of magnitude or more, but agreed with the NO3 Cavity Ring-Down Spectroscopy (CRDS) measurements to within 30–50%. The most likely explanation of the discrepancy between the two instruments and the model is the reaction of the nitrate radical with inhomogeneously distributed NO, which was measured at concentrations of up to 10 ppt, even though this is not enough to fully explain the difference between the DOAS measurements and the model. A rate of production and destruction analysis showed that radicals were generated during the night mainly by the reaction of ozone with light alkenes. The cycling between HO2/RO2 and OH was maintained during the night by the low concentrations of NO and the overall radical concentration was limited by slow loss of peroxy radicals to form peroxides. A strong peak in [NO2] during the night 31 August–1 September allowed an insight into the radical fluxes and the connections between the HOx and the NO3 cycles.

Environmental context Secondary organic aerosols together with sea-salt aerosols are a major contribution to global aerosols and influence the release of reactive halogens, which affect air quality and human health. In this study, the loss of reactive halogen species from simulated salt aerosols due to three different types of secondary organic aerosols was quantified in chamber experiments and investigated with the help of a numerical model. The loss rate can be included into chemistry models of the atmosphere and help to quantify the halogen budget in nature. Abstract The interaction between secondary organic aerosols (SOAs) and reactive bromine species (e.g. BrO, Br2, HOBr) coexisting in the environment is not well understood and not included in current chemistry models. The present study quantifies the quenching of bromine release from an artificial salt aerosol caused by SOAs from ozonolysis of three precursors (α-pinene, catechol or guaiacol) in a Teflon smog chamber and incorporates it into a chemical box model. The model simulations perform very well for a blank experiment without SOA precursor, capturing BrO formation, as detected by differential optical absorption spectrometry. A first-order BrO loss rate of 0.001s–1 on the surface of SOA represents the overall effective Brx (total inorganic bromine) loss included in the model. Generally, the model agrees with the maximum BrO mixing ratio in time and magnitude, with some disagreements in the exact shape. Formation of reactive OClO was observed in the presence of organics but could not be reproduced by the model. According to current knowledge, most inorganic chlorine would be in the form of HCl in the presence of organics, as predicted by the model. In order to reproduce the net effects of the presence of SOA, the effective uptake coefficients of reactive bromine on the SOA surface are estimated to be 0.01, 0.01 and 0.004 for α-pinene, catechol and guaiacol respectively. The uptake coefficient can now be incorporated into box models and even global models, where sinks for bromine species are thought to be inadequately represented.

Several zero-dimensional box-models with different levels of chemical complexity, based on the Master Chemical Mechanism (MCM), have been used to study the chemistry of OH and HO2 in a coastal environment in the Northern Hemisphere. The models were constrained to and compared with measurements made during the NAMBLEX campaign (Mace Head, Ireland) in summer 2002. The base models, which were constrained to measured CO, CH4 and NMHCs, were able to reproduce [OH] within 25%, but overestimated [HO2] by about a factor of 2. Agreement was improved when the models were constrained to oxygenated compounds (acetaldehyde, methanol and acetone), highlighting their importance for the radical budget. When the models were constrained to measured halogen monoxides (IO, BrO) and used a more detailed, measurements-based, treatment to describe the heterogeneous uptake, modelled [OH] increased by up to 15% and [HO2] decreased by up to 30%. The actual impact of halogen monoxides on the modelled concentrations of HOx was dependant on the uptake coefficients used for HOI, HOBr and HO2. Better agreement, within the combined uncertainties of the measurements and of the model, was achieved when using high uptake coefficients for HO2 and HOI (γHO2=1, γHOI=0.6). A rate of production and destruction analysis of the models allowed a detailed study of OH and HO2 chemistry under the conditions encountered during NAMBLEX, showing the importance of oxygenates and of XO (where X=I, Br) as co-reactants for OH and HO2 and of HOX photolysis as a source for OH.

Model-measurement comparisons of HOx in extremely clean air ([NO]

A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH3I, CH2I2, CH2ClI, CH2BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved if an additional sea-air flux of inorganic iodine (e.g., I2) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.

We used a one-dimensional model to simulate the chemical evolution of air masses in the tropical Atlantic Ocean, with a focus on halogen chemistry. The model results were compared to the observations of inorganic halogen species made in this region. The model could largely reproduce the measurements of most chlorine species, especially under unpolluted conditions, but overestimated sea salt chloride, BrCl, and bromine species. Agreement with the measurements could be improved by taking into account the reactivity with aldehydes and the effects of dimethyl sulfide (DMS) and Saharan dust on aerosol pH; a hypothetical HOX → X(-) aqueous-phase reaction could also improve the agreement with measured Cl(2) and HOCl, especially under semipolluted conditions. The results also showed that halogens speciation and concentrations are very sensitive to cloud processing. The model was used to calculate the impact of the observed levels of halogens: Cl atoms accounted for 5.4-11.6% of total methane sinks and halogens (mostly bromine and iodine) accounted for 35-40% of total ozone destruction.

Clustering air mass trajectories is used to identify source regions of certain chemical species. Current clustering methods only use the trajectory coordinates as clustering variables, and as such, are unable to differentiate between similar shaped trajectories that have different source regions and/or seasonal differences. This can lead to a higher variance in the chemical composition within each cluster and loss of information. We propose an adaptive K-means clustering algorithm that uses both the trajectory variables and the associated chemical value. We show, using carbon monoxide data from the Cape Verde for 2007, that our method produces a far more informative clustering than the existing standard method, whilst achieving a lower level of subjectivity.

[1] Oceanic emissions of gaseous organic iodine-atom precursors have the potential to significantly affect atmospheric chemistry and climate, however there is currently considerable uncertainty associated with quantifying their sources. We present sea-air fluxes calculated from simultaneous air and seawater measurements of a comprehensive range of volatile organic iodine compounds (VOICs), including CH3I and the less commonly reported dihalomethanes CH2ICl, CH2IBr and CH2I2, made during two cruises in the Atlantic Ocean between 15–58°N. The combined dihalomethane flux provides a global iodine source (∼0.33 ± 0.19 Tg I y−1) comparable to that of CH3I, and a surface iodine atom source 3–4 times higher. However, a 1D atmospheric model reveals that, in the tropical east Atlantic Ocean in the vicinity of Cape Verde, even these combined VOIC fluxes are capable of supporting only ∼10–25% of the observed IO levels, and suggests that a substantial (340–640 nmol I m−2 d−1) additional photochemical source of iodine is required.

[1] Mixing ratios of dimethyl sulfide (DMS) and its nighttime oxidant, the nitrate radical (N0 3 ), were measured in the summertime marine boundary layer (MBL) of the Gulf of Maine during the New England Air Quality Study-International Transport and Chemical Transformation campaign in 2004. DMS fluxes from the ocean were derived from simultaneous measurements of the wind speed and DMS in seawater. Day and night DMS oxidation rates were determined from modeled OH and measured NO 3 concentrations. The average DMS lifetime with respect to oxidation by OH at noon was 13.5 ± 3.4 (1σ) h, while at night, DMS lifetimes with respect to N0 3 oxidation varied by sampling region from 11 min to 28 h. Oxidation by photochemically generated halogen species likely also played a role during the day, although the nature and extent of the halogen species is more difficult to predict due to lack of halogen measurements. Closure of the DMS budget in the MBL required a vertical entrainment velocity of ∼0.4 cm s ―1 . This study suggests that entrainment of DMS out of the MBL competes with daytime oxidation and that the presence of pollution in the form of NO x and 0 3 in near-coastal regions at night results in nearly complete DMS oxidation within the MBL via reaction with N0 3 , with a much smaller contribution from entrainment. One potential implication of near-complete DMS oxidation within the MBL is a reduction of the amount of sulfur available for aerosol formation and growth at higher altitudes in the atmosphere.

[1] Measurements of peroxycarboxylic nitric anhydrides (PANs) were made during the New England Air Quality Study 2002 cruise of the NOAA RV Ronald H Brown. The four compounds observed, PAN, peroxypropionic nitric anhydride (PPN), peroxymethacrylic nitric anhydride (MPAN), and peroxyisobutyric nitric anhydride (PiBN) were compared with results from other continental and Gulf of Maine sites. Systematic changes in PPN/PAN ratio, due to differential thermal decomposition rates, were related quantitatively to air mass aging. At least one early morning period was observed when O3 seemed to have been lost probably due to NO3 and N2O5 chemistry. The highest O3 episode was observed in the combined plume of isoprene sources and anthropogenic volatile organic compounds (VOCs) and NOx sources from the greater Boston area. A simple linear combination model showed that the organic precursors leading to elevated O3 were roughly half from the biogenic and half from anthropogenic VOC regimes. An explicit chemical box model confirmed that the chemistry in the Boston plume is well represented by the simple linear combination model. This degree of biogenic hydrocarbon involvement in the production of photochemical ozone has significant implications for air quality control strategies in this region.