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Due to the difference of industrialization degree and meteorological conditions, there are obvious differences in the composition of air pollution between islands and inland areas. With Zhoushan (ZS) and Nanjing (NJ) representing islands and inland cities in the Yangtze River Delta, the differences in vertical distribution of atmospheric components were investigated. A combination of multi-axial differential optical absorption spectroscopy (MAX-DOAS), weather research and forecasting (WRF), and potential source contribution function (PSCF) models were used to obtain vertical distribution data for aerosols, nitrogen dioxide (NO2) and formaldehyde (HCHO), meteorological factors, and pollution sources in summer 2019. The findings indicate that, except for the aerosol extinction coefficient (AE), the atmospheric composition at the ZS site was not significantly stratified. However, the AE, NO2, and HCHO at NJ all displayed a decreasing trend with altitude. Here is the interesting finding that the ZS site has a higher AE value than the NJ site, while NJ displays higher NO2 and HCHO columns than the ZS site. This discrepancy was primarily attributable to Zhoushan City's extremely low traffic emissions when compared to inland cities. In addition, HCHO in the YRD region was significantly affected by human activities. Analysis of potential pollution sources found that regional transport contributed to differences in atmospheric composition at different altitudes in different regions. Aerosols, NO2, and HCHO in Nanjing were significantly affected by transport in inland areas. Aerosols in Zhoushan were easily affected by transport in the Yellow Sea and East China Sea, and NO2 and HCHO were significantly affected by transport contributions from surrounding areas in inland areas. The study strongly suggests that land and sea breezes play an important role in the vertical distribution of aerosols over island regions. [ABSTRACT FROM AUTHOR]

Urban air pollution has become a regional environmental problem. In order to explore whether island areas were affected by the urban development of surrounding areas, in this paper, we systematically study the vertical distribution characteristics of atmospheric components, meteorological drivers, potential pollution sources, and the population health risks of fine particulate matter in island cities in China. The vertical profiles of three atmospheric pollutants (aerosols, NO2, and HCHO) in the lower troposphere of Huaniao Island in the East China Sea (ECS) were obtained using ground-based multi-axial differential optical absorption spectroscopy (MAX-DOAS). The results show that the aerosol extinction coefficients, NO2, and HCHO were primarily distributed at altitudes below 1 km, and the atmospheric pollutants in Zhoushan were obviously affected by high-altitude transfer. The main meteorological driving factors of aerosols, NO2, and HCHO were different at different altitudes. The key factor contributing to the high column concentrations of NO2 and HCHO in the upper air (greater than 400 m) was the transport of pollutants brought about by changes in wind speed. By exploring the main potential sources of atmospheric pollutants, it was found that the main sources of aerosols, NO2, and HCHO are coastal cities in the Yangtze River Delta, including southeast Zhejiang Province, southeast Fujian Province, Shanghai, ECS, and the Yellow Sea. Compared with aerosols and HCHO, local primary emissions are an important source of NO2, which are mainly related to industrial activities in Zhoushan Port. In addition, using the expose-response function model, the number of attributable cases of PM2.5 air pollution in Zhoushan City in 2019 accounted for 6.58% of the total population. This study enriches our understanding of the vertical distribution characteristics of atmospheric composition and health risk assessment on Chinese islands. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) events are an increasingly interesting topic in air quality and climate science. In this study, the particle number size distributions, and the frequency of NPF events over Hefei were investigated from November 2018 to February 2019. The proportions of the nucleation mode, Aitken mode, and accumulation mode were 24.59%, 53.10%, and 22.30%, respectively, which indicates the presence of abundant ultrafine particles in Hefei. Forty-six NPF events occurred during the observation days, accounting for 41.82% of the entire observation period. Moreover, the favorable meteorological conditions, potential precursor gases, and PM 2.5 range of the NPF events were analyzed. Compared to non-NPF days, the NPF events preferentially occurred on days with lower relative humidity, higher wind speeds, and higher temperatures. When the PM 2.5 was 15–20, 70–80, and 105–115 μg/m3, the frequency of the NPF events was higher. Nucleation mode particles were positively related to atmospheric oxidation indicated by ozone when PM 2.5 ranged from 15 to 20 μg/m3, and related to gaseous precursors like SO 2 and NO 2 when PM 2.5 was located at 70-80 and 105–115 μg/m3. On pollution days, NPF events did not directly contribute to the increase in the PM 2.5 in the daytime, however, NPF events would occur during the night and the growth of particulate matter contributes to the nighttime PM 2.5 contents. This could lead to pollution that lasted into the next day. These findings are significant to the improvement of our understanding of the effects of aerosols on air quality. [Display omitted] [ABSTRACT FROM AUTHOR]

Accurate measurement of air gases concentration are crucial for effective air quality management and minimizing the harm caused by pollution events, as well as providing guidance for healthy travel. We developed develop a physical informed deep learning model that combines a traditional atmospheric chemical transport model and a data-based deep learning model. The model uses multisource observation data to constrain and optimize the results of atmospheric pollution gases concentrations. The accuracy and spatial resolution in central and eastern China (20–45°N, 100–125°E) have been improved compared to the original Weather Research and Forecasting model coupled to Chemistry (WRF-CHEM) results by incorporating constraints from ground-based and satellite observations. Compared with the WRF-CHEM results, our model increases the verification Pearson coefficient (R) of NO2 and CNEMC sites from 0.56 to 0.8, and the RMSE decreases from 25.33 to 15.54 μg cm−3. For O3 results, the model increased the verification R of the CNEMC from 0.57 to 0.76, and the RMSE decreased from 24.55 to 20.22 cm−3. We employed independent MAXDOAS monitoring to authenticate the HCHO results. The R was 0.78, while the validation results for satellite HCHO and MAXDOAS exhibited a R of 0.63 and the validation results for WRF-CHEM and MAXDOAS demonstrated a R of 0.11. Additionally, the RMSE was 6.97E15 mole cm−2, whereas the validation results for satellite HCHO and MAX-DOAS had an RMSE of 7.16E15 mole cm−2. By utilizing satellite remote sensing to effectively capture the spatial distribution of pollutants, and by combining satellite data with other sources of monitoring data and optimizing our models, we have improved the accuracy of our results and have overcome the limitation of cloud coverage. As a result, we are now able to better understand the space-time distribution of pollutants in different regions. • By integrating multi-source data and constructing a PINN model, comprehensive coverage of O3, HCHO, and NO2 is achieved. • Satellite monitoring results can effectively improve the accuracy of the model and the spatial distribution of the results. • The method effectively filled in the results of pollutant gas concentration in rural areas without national control stations. [ABSTRACT FROM AUTHOR]

Organic acids are major components in marine organic aerosols. Many studies on the occurrence, sources and sinks of organic acids over oceans in the low and middle latitudes have been conducted. However, the understanding of relative contributions of specific sources to organic acids over oceans, especially in the high latitudes, is still inadequate. This study measured organic acids, including C 14:0 – C 32:0 saturated monocarboxylic acids (MCAs), C 16:1 , C 18:1 and C 18:2 unsaturated MCAs, and di-C 4 – di-C 10 dicarboxylic acids (DCAs), in the marine boundary layer from the East China Sea to the Arctic Ocean during the 3rd Chinese Arctic Research Expedition (CHINARE 08). The average concentrations were 18 ± 16 ng/m 3 and 11 ± 5.4 ng/m 3 for ΣMCA and ΣDCA, respectively. The levels of saturated MCAs were much higher than those of unsaturated DCAs, with peaks at C 16:0 , C 18:0 and C 14:0 . DCAs peaked at di-C 4 , followed by di-C 9 and di-C 8 . Concentrations of MCAs and DCAs generally decreased with increasing latitudes. Sources of MCAs and DCAs were further investigated using principal component analysis with a multiple linear regression (PCA-MLR) model. Overall, carboxylic acids originated from ocean emissions, continental input (including biomass burning, anthropogenic emissions and terrestrial plant emissions), and secondary formation. All the five sources contributed to MCAs with ocean emissions as the predominant source (48%), followed by biomass burning (20%). In contrast, only 3 sources (i.e., secondary formation (50%), anthropogenic emissions (41%) and biomass burning (9%)) contributed to DCAs. Furthermore, the sources varied with regions. Over the Arctic Ocean, only secondary formation and anthropogenic emissions contributed to MCAs and DCAs. [ABSTRACT FROM AUTHOR]

In China diesel vehicles dominate the primary emission of particulate matters from on-road vehicles, and they might also contribute substantially to the formation of secondary organic aerosols (SOA). In this study tailpipe exhaust of three typical in-use diesel vehicles under warm idling conditions was introduced directly into an indoor smog chamber with a 30 m 3 Teflon reactor to characterize primary emissions and SOA formation during photo-oxidation. The emission factors of primary organic aerosol (POA) and black carbon (BC) for the three types of Chinese diesel vehicles ranged 0.18–0.91 and 0.15–0.51 g kg-fuel − 1 , respectively; and the SOA production factors ranged 0.50–1.8 g kg-fuel − 1 and SOA/POA ratios ranged 0.7–3.7 with an average of 2.2. The fuel-based POA emission factors and SOA production factors from this study for idling diesel vehicle exhaust were 1–3 orders of magnitude higher than those reported in previous studies for idling gasoline vehicle exhaust. The emission factors for total particle numbers were 0.65–4.0 × 10 15 particles kg-fuel − 1 , and particles with diameters less than 50 nm dominated in total particle numbers. Traditional C 2 -C 12 precursor non-methane hydrocarbons (NMHCs) could only explain less than 3% of the SOA formed during aging and contribution from other precursors including intermediate volatile organic compounds (IVOC) needs further investigation. [ABSTRACT FROM AUTHOR]

Limitations in the current capability of monitoring PM2.5 adversely impact air quality management and health risk assessment of PM2.5 exposure. Commonly, ground-based monitoring networks are established to measure the PM2.5 concentrations in highly populated regions and protected areas such as national parks, yet large gaps exist in spatial coverage. Satellite-derived aerosol optical properties serve to complement the missing spatial information of ground-based monitoring networks. However, satellite remote sensing AODs are hampered under cloudy/hazy conditions or during nighttime. Here we strive to overcome the long-standing restriction that surface PM2.5 cannot be obtained with satellite remote sensing under cloudy/hazy conditions or during nighttime. In this work, we introduce a deep spatiotemporal neural network (ST-NN) and demonstrate that it can artfully fill these observational gaps. We quantified the quantitative impact of input variables on the results using sensitivity and visual analysis of the model. This technique provides ground-level PM2.5 concentrations with a high spatial resolution (0.01°) and 24-h temporal coverage, hour-by-hour, complete coverage. In central and eastern China, the 10-fold cross-validation results show that R2 is between 0.8 and 0.9, and RMSE is between 6 and 26 (µg m−3). The relative error varies in different concentration ranges and is generally less than 20%. Better constrained spatiotemporal distributions of PM2.5 concentrations will contribute to improving health effects studies, atmospheric emission estimates, and air quality predictions. [ABSTRACT FROM AUTHOR]

Formaldehyde (HCHO) is a toxic and hazardous air pollutant that widely exists in atmosphere. Insufficient spatial and temporal coverage of surface HCHO measurements is limiting studies on surface HCHO-related air quality management and health risk assessment. This study develops a method to derive global ground-level HCHO concentrations from satellite-based tropospheric HCHO columns using TM5-simulated surface-to-column conversion factor with coarse spatial resolution. The method improves the factor more representative in finer grids by constraining TM5-simulated vertical profile shapes with satellite HCHO columns. The surface HCHO concentrations derived by the Ozone Mapping and Profiler Suite (OMPS) show good correlation with in situ HCHO measurements (R = 0.59) from the U.S. Environmental Protection Agency surface network. We investigated how surface HCHO relates to urbanization and population aggregation over seven regions with high HCHO pollution. The results show urban HCHO increases as a power function with population size in China, India, and West Asia. HCHO concentrations in rural aeras also present strong log–log relationship with population aggregation in China, India, the United States, and Europe. Moreover, OMPS-derived ground-level HCHO concentrations were used to estimate global cancer burden caused by long-term outdoor HCHO exposure. The results show that up to 418188 more people worldwide will develop this cancer during the human life cycle. The global cancer burden is mainly from the South-East Asia region (33.11 %) and the Western Pacific region (22.95 %). This cancer occurrence in India and China is ranked 1st and 2nd in the world due to the large population size and serious HCHO pollution. Besides, global surface HCHO concentrations and cancer burden derived from the Environmental Trace Gases Monitoring Instrument which is China's first hyperspectral space-based spectrometer are found similar patterns with that from OMPS. Our results provide new insight into the impact of population urbanization on HCHO pollution and global outdoor HCHO-caused health risks. [ABSTRACT FROM AUTHOR]

Formaldehyde (HCHO) and glyoxal (CHOCHO) are important oxidization intermediates of most volatile organic compounds (VOCs), but their vertical evolution in urban areas is not well understood. Vertical profiles of HCHO, CHOCHO, and nitrogen dioxide (NO 2) were retrieved from ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations in Hefei, China. HCHO and CHOCHO vertical profiles prefer to occur at higher altitudes compared to NO 2 , which might be caused by the photochemistry-oxidation of longer-lived VOCs at higher altitudes. Monthly means of HCHO concentrations were higher in summer, while enhanced amounts of NO 2 were mainly observed in winter. CHOCHO exhibited a hump-like seasonal variation, with higher monthly-averaged values not only occurred in warm months (July-August) but also in cold months (November-December). Peak values mainly occurred during noon for HCHO but emerged in the morning for CHOCHO and NO 2 , suggesting that HCHO is stronger link to photochemistry than CHOCHO. We further use the glyoxal to formaldehyde ratio (GFR) to investigate the VOC sources at different altitudes. The lowest GFR value is almost found in the altitude from 0.2 to 0.4 km, and then rises rapidly as the altitude increases. The GFR results indicate that the largest contributor of the precursor VOC is biogenic VOCs at lower altitudes, while at higher altitudes is anthropogenic VOCs. Our findings provide a lot more insight into VOC sources at vertical direction, but more verification is recommended to be done in the future. [Display omitted] [ABSTRACT FROM AUTHOR]

Air pollutant transport plays an important role in local air quality, but field observations of transport fluxes, especially their vertical distributions, are very limited. We characterized the vertical structures of transport fluxes in central Luoyang, Fen-Wei Plain, China, in winter based on observations of vertical air pollutant and wind profiles using multi-axis differential optical absorption spectroscopy (MAX-DOAS) and Doppler wind lidar, respectively. The northwest and the northeast are the two privileged wind directions. The wind direction and total transport scenarios were dominantly the northwest during clear days, turning to the northeast during the polluted days. Increased transport flux intensities of aerosol were found at altitudes below 400 m on heavily polluted days from the northeast to the southwest over the city. Considering pollution dependence on wind directions and speeds, surface-dominated northeast transport may contribute to local haze events. Northwest winds transporting clean air masses were dominant during clean periods and flux profiles characterized by high altitudes between 200 and 600 m in Luoyang. During the COVID-19 lockdown period in late January and February, clear reductions in transport flux were found for NO2 from the northeast and for HCHO from the northwest, while the corresponding main transport altitude remained unchanged. Our findings provide better understandings of regional transport characteristics, especially at different altitudes. [ABSTRACT FROM AUTHOR]

Air quality is strongly influenced by both local emissions and regional transport. Atmospheric chemical transport models can distinguish between emissions and regional transport sources in air pollutant concentrations. However, quantifying model inventories is challenging due to emission changes caused by the recent strict control measures taken by the Chinese government. In this study, we use NO2 column observations from the Tropospheric Monitoring Instrument to retrieve top-down nitrogen oxide (NOX) emissions and quantify the contributions of local emissions and regional transport to NOx in Beijing (BJ), from 1 November 2018 to 28 February 2019 (W_2018) and 1 November 2019 to 29 February 2020 (W_2019). In W_2018 and W_2019, the BJ bottom-up NOX emissions from the multi-resolution emission inventory for China in 2017 were overestimated by 11.8% and 40.5%, respectively, and the input of NOX from other cities to BJ was overestimated by 10.9% and 51.6%, respectively. The simulation using our adjusted inventory exhibited a much higher spatial agreement (slope = 1.0, R2 = 0.79) and reduced a mean relative error by 45% compared to those of bottom-up NOX emissions. The top-down inventory indicated that (1) city boundary transport contributes approximately 40% of the NOX concentration in BJ; (2) in W_2019, NOX emissions and transport in BJ decreased by 20.4% and 17.2%, respectively, compared to those of W_2018; (3) in W_2019, NOX influx substantially decreased (−699 g/s) in BJ compared to that of W_2018 despite negative meteorological conditions that should have increased NOx influx by +503 g/s. Overall, the contribution of intercity input to NOx in BJ has declined with decreasing emissions in the surrounding cities due to regional cooperative control measures, and the role of local emissions in BJ NOx levels was more prominent. Our findings indicate that local emissions may play vital roles in regional center city air quality. [ABSTRACT FROM AUTHOR]

This study analyzed the differences in ozone (O3) sensitivity in four different urban areas in China from February 2019 to January 2020 based on data on various near-surface pollutants from passive multi-axis differential optical absorption spectroscopy (MAX-DOAS) sites and nearby China National Environmental Monitoring Center (CNEMC) sites. Across the four cities, the nitrogen dioxide (NO2) and formaldehyde (HCHO) concentrations varied seasonally. Xianghe consistently displayed the lowest NO2 levels, suggesting reduced emissions compared to other cities. Guangzhou, a city with a robust economy and a high level of vehicle ownership, exhibited higher concentrations in spring. Summer brought elevated HCHO levels in Guangzhou, Xianghe, and Shenyang due to intensified photochemical processes. Autumn and winter showed higher HCHO concentrations in Guangzhou and Xianghe compared to Lanzhou and Shenyang. Overall, Guangzhou recorded the highest annual averages, due to its developed economy, while Xianghe's lower NO2 levels were offset by the elevated HCHO due to higher O3 values. The analysis delved into primary and secondary HCHO sources across seasons and used carbon monoxide (CO) and O3 data. Xianghe showcased the dominance of secondary sources in summer and autumn, while Lanzhou was characterized by primary dominance throughout the year. Shenyang mirrored Xianghe's evolution due to industrial emissions. In Guangzhou, due to the high levels of vehicular traffic and sunlight conditions, secondary sources predominantly influenced HCHO concentrations. These findings highlight the interplay between primary and secondary emissions in diverse urban settings. This study explored O3 sensitivity variations across seasons. Xianghe exhibited a balanced distribution among volatile organic compound (VOC)-limited conditions, nitrogen oxide (NOx)-limited conditions, and transitional influences. Lanzhou was mainly affected by VOC-limited conditions in winter and NOx-limited conditions in other seasons. Shenyang's sensitivity varied with the seasons and was primarily influenced by transitions between VOCs and NOx in autumn and NOx-limited conditions otherwise. Guangzhou experienced varied influences. During periods of high O3 pollution, all regions were affected by NOx-limited conditions, indicating the necessity of NOx monitoring in these areas, especially during summer in all regions and during autumn in Xianghe and Guangzhou. [ABSTRACT FROM AUTHOR]

• The ratio between OH production from HONO and from (HONO+O3) lager than 90% before 12:00. • The source of HONO from combustion emission is ~26%. • NO 2 being converted to HONO under the conditions with high aerosol extinction coefficient and high relative humidity was a dominant factor for the secondary produce of HONO. • HONO mainly distributed near the surface and localized, and NO 2 mainly transmitted from north at ~ 0.6 km. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations were operated from 02 to 21 December 2018 in Leshan, southwest China, to measure HONO, NO 2 and aerosol extinction vertical distributions, and these were the first MAX-DOAS measurement results in Sichuan Basin. During the measurement period, characteristic ranges for surface concentration were found to be 0.26–4.58 km−1 and averaged at 0.93 km−1 for aerosol extinction, 0.49 to 35.2 ppb and averaged at 4.57 ppb for NO 2 and 0.03 to 7.38 ppb and averaged at 1.05 ppb for HONO. Moreover, vertical profiles of aerosol, NO 2 and HONO were retrieved from MAX-DOAS measurements using the Heidelberg Profile (HEIPRO) algorithm. By analysing the vertical gradients of pollutants and meteorological information, we found that aerosol and HONO are strongly localised, while NO 2 is mainly transmitted from the north direction (city center direction). Nitrogen oxides such as HONO and NO 2 are important for the production of hydroxyl radical (OH) and oxidative capacity in the troposphere. In this study, the averaged value of OH production rate from HONO is about 0.63 ppb/hr and maximum value of ratio between OH production from HONO and from (HONO+O 3) is > 93% before12:00 in Leshan. In addition, combustion emission contributes to 26% for the source of HONO in Leshan, and we found that more NO 2 being converted to HONO under the conditions with high aerosol extinction coefficient and high relative humidity is also a dominant factor for the secondary produce of HONO. [Display omitted] [ABSTRACT FROM AUTHOR]

Information on the vertical distribution of air pollutants is essential for understanding their spatiotemporal evolution underlying urban atmospheric environment. This paper presents the SO 2 profiles based on ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements from March 2018 to February 2019 in Hefei, East China. SO 2 decrease rapidly with increasing heights in the warm season, while lifted layers were observed in the cold season, indicating accumulation or long-range transport of SO 2 in different seasons might occur at different heights. The diurnal variations of SO 2 were roughly consistent for all four seasons, exhibiting the minimum at noon and higher values in the morning and late afternoon. Lifted layers of SO 2 were observed in the morning for fall and winter, implying the accumulation or transport of SO 2 in the morning mainly occurred at the top of the boundary layer. The bivariate polar plots showed that weighted SO 2 concentrations in the lower altitude were weakly dependent on wind, but in the middle and upper altitudes, higher weighted SO 2 concentrations were observed under conditions of middle-high wind speed. Concentration weighted trajectory (CWT) analysis suggested that potential sources of SO 2 in spring and summer were local and transported mainly occurred in the lower altitude from southern and eastern areas; while in fall and winter, SO 2 concentrations were deeply affected by long-range transport from northwestern and northern polluted regions in the middle and upper altitudes. Our findings provide new insight into the impacts of regional transport at different heights in the boundary layer on SO 2 pollution. [Display omitted] [ABSTRACT FROM AUTHOR]

Knowledge of vertical distribution of air pollutants is important for understanding the mechanisms underlying haze pollution. In this paper, we characterize the vertical structure of air pollutants based on ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in the wintertime in Hefei, East China. Both NO 2 and aerosol show exponentially decreasing profile with high concentrations concentrated close to the surface, suggesting the major emissions are at ground level. SO 2 profiles extend to slightly higher altitudes than NO 2 , which might be due to the SO 2 emissions from elevated point sources (e.g., power plants). Elevated HCHO was observed in the middle layer during noon time, probably indicating that the larger effect of the photochemical formation of atmospheric HCHO rather than direct HCHO emissions. Potential source contribution function (PSCF) and concentration-weighted trajectory (CWT) models were used to evaluate the potential sources of Hefei's air pollution in the lower, middle and upper boundary layer. The results revealed that the major regional source of NO 2 and aerosol were located in the northern and eastern part of Anhui and the transport mainly occured in the lower layer. Transported SO 2 from the northwestern regions in the middle layer made great contribution to SO 2 in Hefei. HCHO was mostly produced locally while the transport of HCHO from potential source areas was negligible. To further explore the potential source contributions affecting haze pollution, PSCF and CWT analysis for heavy haze episodes suggested that the potential source areas for aerosol were almost located in the northern region, the northwestern region, and the YRD region. Our findings provide a more comprehensive understanding for regional transport through the study on potential sources at different altitudes, which are useful for designing collaborative air pollution control strategies on a regional scale. • Vertical distribution of air pollutants is investigated to identify potential sources at different altitudes during winter. • Both NO 2 and aerosol show exponentially decreasing profile with high concentrations concentrated close to the surface. • SO 2 and HCHO profile typically extended to higher altitudes than NO 2 and aerosol did during winter. • The main potential sources for aerosol were located in the northern region, the northwestern region, and the YRD region. • This study is useful for designing collaborative air pollution control strategies on a regional scale. [ABSTRACT FROM AUTHOR]

Climate change may aggravate air pollution through altering emissions, ventilation, chemical production, and deposition. Under a warming climate, persistent weather patterns are likely to increase in both number and intensity, yet their implications on future air quality have been less explored. Here we use ground-level observations of air pollutants and meteorological reanalysis dataset to explore how persistence of weather conditions would affect extremes of wintertime PM 2.5 and summertime O 3 in China. Associated changes in the shapes of their distributions are discussed also with contrasting weather conditions. As air stagnation days persist, median summertime O 3 significantly increases, with 0.07, 0.12 and 0.19 standard deviation per day (about 1.97, 3.12 and 4.63 ppb per day) for the Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD) regions, respectively. As high temperature days continue, significant increases in median (about 0.31 ppb per day) and 90th percentile (about 0.78 ppb per day) of summertime O 3 is found only in the YRD region, although the highest probability of O 3 extreme (90th percentile) is identified on the last day of four-day high temperature events for both the BTH and YRD regions. In contrast, persistent stagnation causes a significant increase in the 90th percentile of PM 2.5 of about 17.87 and 10.88 μg/m3 per day for BTH and YRD regions, rather than the median. Stagnation events lasting for five, four and five days are the best indicators for PM 2.5 extreme (90th percentile) in the BTH, YRD and PRD regions, respectively. Analysis on contrasting monthly weather conditions suggests that summertime O 3 extreme is amplified by higher mean daily maximum air temperature (T) in the BTH region and co-occurrences of high T and stagnation in the YRD region, while wintertime PM 2.5 extreme is significantly amplified by stronger stagnation. These results suggest that megacities with relatively high emissions are more vulnerable to a hotter and more stable future. [Display omitted] • High temperature events are pivotal for high O 3 in BTH and YRD regions. • Persistent stagnation plays crucial role in high O 3 in PRD region. • Stagnation is of central importance for high PM 2.5 and amplifies its shape of distribution. [ABSTRACT FROM AUTHOR]

Recently, air pollution especially fine particulate matters (PM 2.5) and ozone (O 3) has become a severe issue in China. In this study, we first characterized the temporal trends of PM 2.5 and O 3 for Beijing, Guangzhou, Shanghai, and Wuhan respectively during 2018-2020. The annual mean PM 2.5 has decreased by 7.82%-33.92%, while O 3 concentration showed insignificant variations by -6.77%-4.65% during 2018-2020. The generalized additive models (GAMs) were implemented to quantify the contribution of individual meteorological factors and their gas precursors on PM 2.5 and O 3. On a short-term perspective, GAMs modeling shows that the daily variability of PM 2.5 concentration is largely related to the variation of precursor gases (R = 0.67-0.90), while meteorological conditions mainly affect the daily variability of O 3 concentration (R = 0.65-0.80) during 2018-2020. The impact of COVID-19 lockdown on PM 2.5 and O 3 concentrations were also quantified by using GAMs. During the 2020 lockdown, PM 2.5 decreased significantly for these megacities, yet the ozone concentration showed an increasing trend compared to 2019. The GAMs analysis indicated that the contribution of precursor gases to PM 2.5 and O 3 changes is 3-8 times higher than that of meteorological factors. In general, GAMs modeling on air quality is helpful to the understanding and control of PM 2.5 and O 3 pollution in China. [Display omitted] [ABSTRACT FROM AUTHOR]

Atmospheric oxidizing capacity (AOC) is an essential driving force of troposphere chemistry and self-cleaning, but the definition of AOC and its quantitative representation remain uncertain. Driven by national demand for air pollution control in recent years, Chinese scholars have carried out studies on theories of atmospheric chemistry and have made considerable progress in AOC research. This paper will give a brief review of these developments. First, AOC indexes were established that represent apparent atmospheric oxidizing ability (AOIe) and potential atmospheric oxidizing ability (AOIp) based on aspects of macrothermodynamics and microdynamics, respectively. A closed study refined the quantitative contributions of heterogeneous chemistry to AOC in Beijing, and these AOC methods were further applied in Beijing-Tianjin-Hebei and key areas across the country. In addition, the detection of ground or vertical profiles for atmospheric OH·, HO 2 ·, NO 3 · radicals and reservoir molecules can now be obtained with domestic instruments in diverse environments. Moreover, laboratory smoke chamber simulations revealed heterogeneous processes involving reactions of O 3 and NO 2 , which are typical oxidants in the surface/interface atmosphere, and the evolutionary and budgetary implications of atmospheric oxidants reacting under multispecies, multiphase and multi-interface conditions were obtained. Finally, based on the GRAPES-CUACE adjoint model improved by Chinese scholars, simulations of key substances affecting atmospheric oxidation and secondary organic and inorganic aerosol formation have been optimized. Normalized numerical simulations of AOIe and AOIp were performed, and regional coordination of AOC was adjusted. An optimized plan for controlling O 3 and PM 2.5 was analyzed by scenario simulation. [Display omitted] [ABSTRACT FROM AUTHOR]

In fall–winter, 2007–2013, visibility and light scattering coefficients ( b sp ) were measured along with PM 2.5 mass concentrations and chemical compositions at a background site in the Pearl River Delta (PRD) region. The daily average visibility increased significantly ( p < 0.01) at a rate of 1.1 km/year, yet its median stabilized at ~ 13 km. No haze days occurred when the 24-hr mean PM 2.5 mass concentration was below 75 μg/m 3 . By multiple linear regression on the chemical budget of particle scattering coefficient ( b sp ), we obtained site-specific mass scattering efficiency (MSE) values of 6.5 ± 0.2, 2.6 ± 0.3, 2.4 ± 0.7 and 7.3 ± 1.2 m 2 /g, respectively, for organic matter (OM), ammonium sulfate (AS), ammonium nitrate (AN) and sea salt (SS). The reconstructed light extinction coefficient ( b ext ) based on the Interagency Monitoring of Protected Visual Environments (IMPROVE) algorithm with our site-specific MSE revealed that OM, AS, AN, SS and light-absorbing carbon (LAC) on average contributed 45.9% ± 1.6%, 25.6% ± 1.2%, 12.0% ± 0.7%, 11.2% ± 0.9% and 5.4% ± 0.3% to light extinction, respectively. Averaged b ext displayed a significant reduction rate of 14.1/Mm·year ( p < 0.05); this rate would be 82% higher if it were not counteracted by increasing relative humidity (RH) and hygroscopic growth factor ( f (RH)) at rates of 2.5% and 0.16/year − 1 ( p < 0.01), respectively, during the fall–winter, 2007–2013. This growth of RH and f (RH) partly offsets the positive effects of lowered AS in improving visibility, and aggravated the negative effects of increasing AN to impair visibility. [ABSTRACT FROM AUTHOR]

Traditional ground-based air sampling measurements of air quality have blind monitoring areas in the junctions between provinces, cities and urban and rural areas, and they lack the ability of vertical monitoring. Stereoscopic hyperspectral remote sensing techniques provide a promising strategy for improving our understanding of air pollution. Satellite and ground based hyperspectral remote sensing techniques have been demonstrated to have unparalleled technical advantages in monitoring the horizontal and vertical distributions of air pollutants compared to other monitoring techniques. However, to unveil the complex evolutions and processes of the atmospheric environment, the current stereoscopic hyperspectral remote sensing techniques still face several technical bottlenecks, such as a limited temporal resolution in horizontal space, a limited stereoscopic spatial resolution, the limited types of trace gases, the impact of cloud coverage, and the difficulty in nighttime monitoring. The new technical requirements mainly include the following changes: (1) from horizontal and vertical to grid-stereoscopic monitoring; (2) from kilometer to meter resolutions; and (3) from once a day to full-time monitoring with a high temporal resolution. In this article, we systematically review the recent advances in satellite- and ground-based hyperspectral remote sensing techniques, including China's first hyperspectral satellite GF-5, hardware, algorithms, and applications. Moreover, we discuss the broad application prospects of the unmanned aerial vehicle hyperspectral remote sensing monitoring system, the active hyperspectral remote sensing monitoring system, and machine learning in air pollution monitoring in the future. We recommend using the expected multi-means joint hyperspectral stereoscopic remote sensing monitoring mode to assist the effective monitoring and regulation of air pollution in the future. [ABSTRACT FROM AUTHOR]

Atmospheric formaldehyde (HCHO) is considered as one of significant oxidization intermediate of most volatile organic compounds (VOCs). Previous studies using HCHO/NO 2 ratio for ozone formation sensitivity are based on the hypothesis that HCHO is a reliable indicator for total VOC reactivity (VOCR), while few studies focused on evaluating this hypothesis. To reassess this assumption, three fitting methods have been proposed for correlation analysis between the HCHO and VOCR, showing the Pearson correlation coefficient increased in the following order: logarithmic fitting < linear fitting < exponential fitting. Near-surface HCHO VMRs and the exponent-transformed VOCR showed good agreement during the entire daytime, indicating that HCHO concentrations primarily depend on VOCR, especially at midday and afternoon. High correlation coefficients are not only found between the HCHO and VOCR, but also between the HCHO and ozone formation potential (OFP), implying that HCHO is strongly influenced by the photochemical oxidation of VOCs and closely linked to ozone formation. Furthermore, good consistencies were exhibited between TROPOMI HCHO VCDs and the calculated photochemical property parameters of NMHCs (e.g., Total NMHCs, VOCR, and OFP), suggesting that satellite-based HCHO columns can infer the VOCs emissions over large spatial areas. We also make an attempt to convert the concentration of HCHO to total Prop-equiv concentration of NMHCs based on the linear regression analysis method, which overestimates the measured values by about 16%. Our findings imply that HCHO has the feasibility being a proxy for VOCs, which also indirectly proves the reasonability of HCHO/NO 2 ratio to indicate ozone sensitivity. [Display omitted] • HCHO concentrations primarily depend on VOCR especially at midday and afternoon. • Good consistency is found between the HCHO and ozone formation potential. • Ambient HCHO has the ability being a proxy for total NMHCs. • TROPOMI HCHO VCDs has the ability to infer the VOCs emissions. [ABSTRACT FROM AUTHOR]

Abstract: The total protein associated with bioaerosol particulate matter (PM) is generally measured as an all-inclusive indicator of airborne biological material, which may enhance the effects of allergens, allergic and asthmatic responses. To investigate the level and seasonal variations of biological loading, PM10 were collected in a metropolitan area of Hefei, central China from June 2008 to February 2009 and analyzed for total protein mass, trace elements, and water-soluble ions. The protein concentration in PM10 ranged from 2.08 to 36.71 μg m−3 with an average of 11.42 μg m−3. This was the highest value reported so far in the literature. The total protein was found to have a significant correlation with the air pollution index (API) and mean visibility (VV), indicating the potential influence of anthropogenic sources and/or crustal sources. The protein content displayed an obvious seasonal variation with respect to weather conditions. In the rainy season the level of protein was low, while in the dry season and foggy weather the level of protein was relatively high. A correlation analysis revealed that the relationship between total protein concentration and water-soluble ions K+ and in PM10 during the dry season is 0.92 and 0.66 (P < 0.05), respectively, suggesting that anthropogenic pollution and biomass burning are main contributors during this period. [Copyright &y& Elsevier]

Persistent heavy haze episodes have repeatedly shrouded North China in recent years. Besides anthropogenic emissions, natural dust also contributes to the aerosols in this region. Through continuous observation by a dual-wavelength Raman lidar, the primary aerosol types and their contributions to air pollution in North China were determined. The following three aerosol types can be classified: natural dust, anthropogenic aerosols, and the mixture of anthropogenic aerosols and dust (polluted dust). The classification results are basically consistent with the classification results from the cloud–aerosol lidar and infrared pathfinder satellite observations (CALIPSO) satellite measurements. The relative bias of the lidar ratio between the Raman lidar and CALIPSO is less than 25% over 90% of the cases, indicating that the CALIPSO lidar ratio selection algorithm is reasonable. The classification results show that approximately 45% of aerosols below 1.8 km are contributed by polluted dust during our one year observations. The contribution of dust increased with height, from 6% at 500 m to 28% at 1,800 m, while the contribution of anthropogenic aerosols decreased from 49% to 25%. In addition, polluted dust is the major aerosol subtype below 1.0 km in spring (over 60%) and autumn (over 70%). Anthropogenic aerosols contribute more than 75% of air pollution in summer. In winter, anthropogenic aerosols prevailed (over 80%) in the lower layer, while polluted dust (around 60%) dominated the upper layer. Our results identified the primarily aerosol types to assess the contributions of anthropogenic and natural sources to air pollution in North China, and highlight that natural dust plays a crucial role in lower-layer air pollution in spring and autumn, while controlling anthropogenic aerosols will significantly improve air quality in winter. [ABSTRACT FROM AUTHOR]

Multiaxis differential absorption spectroscopy (MAX-DOAS) is a newly developed advanced vertical profile detection method, but the vertical nitrogen dioxide (NO2) profiles measured by MAX-DOAS have not yet been fully verified. In this study, we perform MAX-DOAS and tower gradient observations to simultaneously acquire tropospheric NO2 observations in the Beijing urban area from 1 April to 31 May 2019. The average values of the tropospheric NO2 vertical column densities measured by MAX-DOAS and the tropospheric monitoring instrument are 15.8 × 1015 and 12.4 × 1015 molecules cm−2, respectively, and the correlation coefficient R reaches 0.87. The MAX-DOAS measurements are highly consistent with the tower-based in situ measurements, and the correlation coefficients R from the ground to the upper air are 0.89 (60 m), 0.87 (160 m), and 0.76 (280 m). MAX-DOAS accurately measures the trend of NO2 vertical profile changes, although a large underestimation occurs by a factor of two. By analyzing the NO2 vertical profile, the NO2 concentration reveals an exponential decrease with height. The NO2 vertical profile also coincides with the evolution of the boundary layer height. The study shows that the NO2 over Beijing mainly originates from local sources and occurs in the boundary layer, and its vertical evolution pattern has an important guiding significance to better understand nitrate production and ozone pollution. [ABSTRACT FROM AUTHOR]

Restrictions on human activities remarkably reduced emissions of air pollutants in China during the COVID-19 lockdown periods. However, distinct responses of O3 concentrations were observed across China. In the Beijing–Tianjin–Hebei (BTH) and Yangtze River Delta (YRD) regions, O3 concentrations were enhanced by 90.21 and 71.79% from pre-lockdown to lockdown periods in 2020, significantly greater than the equivalent concentrations for the same periods over 2015–2019 (69.99 and 43.62%, p < 0.001). In contrast, a decline was detected (−1.1%) in the Pearl River Delta (PRD) region. To better understand the underlying causes for these inconsistent responses across China, we adopted the least absolute shrinkage and selection operator (Lasso) and ordinary linear squares (OLS) methods in this study. Statistical analysis indicated that a sharp decline in nitrogen dioxide (NO2) was the major driver of enhanced O3 in the BTH region as it is a NOx-saturated region. In the YRD region, season-shift induced changes in the temperature/shortwave radiative flux, while lockdown induced declines in NO2, attributable to the rise in O3. In the PRD region, the slight drop in O3 is attributed to the decreased intensity of radiation. The distinct regimes of the O3 response to the COVID-19 lockdown in China offer important insights into different O3 control strategies across China. [ABSTRACT FROM AUTHOR]

We present an improved TROPOspheric Monitoring Instrument (TROPOMI) retrieval of formaldehyde (HCHO) over China. The new retrieval optimizes the slant column density (SCD) retrieval and air mass factor (AMF) calculation for TROPOMI observations of HCHO over China. Retrieval of HCHO differential SCDs (DSCDs) is improved using the basic optical differential spectroscopy (BOAS) technique resulting in lower noise and smaller random error, while AMFs are improved with a priori HCHO profiles from a higher resolution regional chemistry transport model. Compared to the operational product, the new TROPOMI HCHO retrieval shows better agreement with ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in Beijing. The improvements are mainly related to the AMF calculation with more precise a priori profiles in winter. Using more precise a priori profiles in general reduces HCHO vertical column densities (VCDs) by 52.37 % (± 27.09 %) in winter. Considering the aerosol effect in AMF calculation reduces the operational product by 11.46 % (± 1.48 %) and our retrieval by 17.61 % (± 1.92 %) in winter. The improved and operational HCHO are also used to investigate the spatial–temporal characteristics of HCHO over China. The result shows that both improved and operational HCHO VCDs reach maximum in summer and minimum in winter. High HCHO VCDs mainly located over populated areas, i.e., Sichuan Basin and central and eastern China, indicate a significant contribution of anthropogenic emissions. The hotspots are more obvious on the map of the improved HCHO retrieval than the operational product. The result indicates that the improved TROPOMI HCHO retrieval is more suitable for the analysis of regional- and city-scale pollution in China. [ABSTRACT FROM AUTHOR]

Driving conditions are among the important factors determining gasoline vehicle emissions, yet their relation with exhaust-derived secondary pollutants is poorly understood. Here, we introduced exhaust from a gasoline vehicle under hot idling and cruising conditions into an indoor smog chamber by using a chassis dynamometer and investigated the formation of secondary organic aerosols (SOA) during photochemical ageing under light after characterizing the primary emission of non-methane hydrocarbons (NMHCs), nitrogen oxide (NO x) and primary organic aerosol (POA) in the dark. When compared to emission factors (EFs) at idling, during cruising at 20 km h−1 or 40 km h−1, the EFs of NMHCs decreased by more than an order of magnitude, while the EFs of NO x were more than doubled, resulting in a large drop in the NMHC-to-NO x ratios. The percentages of reactive alkenes and aromatic hydrocarbons also decreased from idling to cruising at 20 km h−1 to that at 40 km h−1. The emission factor of benzene, a carcinogenic compound, decreased more than 10 times from ~0.35 g kg-fuel−1 at idling to ~0.03 g kg-fuel−1 during cruising. During photochemical ageing of exhaust, substantial SOA was formed, and the SOA/POA ratios decreased from 52 to 92 at idling to 4–14 during cruising. Traditional aromatics could explain 30–64% of the measured SOA at idling but less than 15% of the measured SOA during cruising. Our results highlight that traffic congestion would greatly promote the emission of reactive volatile organic compounds and carcinogenic benzene from gasoline vehicles and also show that NMHCs as a target in gasoline vehicle emission tests cannot effectively represent the SOA and ozone formation potentials of the partially oxidized hydrocarbons from poorly functioning converters. Unlabelled Image • Primary emissions and SOA formation from hot idling and cruising gasoline vehicle were compared. • Emissions at idling and cruising varied greatly in the NMHC/NOx ratios and NMHC compositions. • Aromatics could explain 30–64% of measured SOA at idling, but less than 15% of that at cruising. [ABSTRACT FROM AUTHOR]

Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations were performed from 27 December 2018 to 16 January 2019 in Changshou, one of subdistricts of Chongqing, China. Primary atmospheric pollutant in Changshou during wintertime was PM 2.5 , whose contribution averaged about 70.15% ± 9.5% of PM 10. The ratio of PM 2.5 /PM 10 decreased when PM 2.5 pollution became worse, and it should attribute to biomass burning and the contribution of hygroscopic growth and enhanced heterogeneous chemistry under high relative humidity condition. Moreover, nitrogen dioxide (NO 2), formaldehyde (HCHO) and glyoxal (CHOCHO) vertical profiles during the campaign period were retrieved separately. TROPOMI HCHO vertical column densities (VCDs) and MAX-DOAS HCHO VCDs were correlated well (R = 0.93). In order to identify the sources of volatile organic compound (VOC) in Changshou, the ratio of CHOCHO to HCHO (R GF) in five different layers were estimated. The estimated daily averaged R GF were 0.0205 ± 0.0077, 0.0727 ± 0.0286, 0.0864 ± 0.0296, 0.0770 ± 0.0275 and 0.0746 ± 0.0263 in 0–100 m, 100–200 m, 300–400 m, 500–600 m and 700–800 m layers, respectively. The estimated R GF will increase when biomass burnings were dominated. Using NO 2 as a tracer of anthropogenic emissions, we found the R GF values gradually decrease with the increase of NO 2 levels. R GF values in 0–100 m layer and all the other upper layers are 0.015–0.025 and 0.06–0.14, and that means the dominant sources of VOCs in 0–100 m layer and all the other upper layers are biogenic emission and anthropogenic emission (especially biomass burning), respectively. In addition, we found that R GF has site dependence which is in compliance with several previous studies. Unlabelled Image • The ratio of PM 2.5 /PM 10 decreased when PM 2.5 pollution became worse • HCHO and CHCOHO mainly distributed in 0-100 m and 100-200 m, respectively. • The sources of VOCs above 100 m are mainly from biomass burning. [ABSTRACT FROM AUTHOR]

We present the improved retrieval of TROPOspheric Monitoring Instrument (TROPOMI) tropospheric formaldehyde (HCHO) over China. The new retrieval optimizes the slant column density (SCD) retrieval and air mass factor (AMF) calculation for TROPOMI observations of HCHO over China. HCHO SCDs are retrieved using the basic optical differential spectroscopy (BOAS) technique, while AMFs are calculated with a priori HCHO profile from a higher resolution regional chemistry transport model. Compared to the operational product, the new TROPOMI HCHO retrieval shows better agreement with the ground based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in China. The operational product in general overestimates HCHO VCDs by 14.01 %, while the improved HCHO only shows an underestimation of 3.67 %. The improvements are mainly related to the AMF calculation with higher resolution a priori profile (61.11 %), while the SCD retrieval only shows a minor effect of 0.15 %. The improved HCHO is also used to investigate the spatial-temporal characteristic of HCHO over China. The result shows that HCHO VCDs reach maximum in summer and minimum in winter. High HCHO VCDs mainly located over populated areas, i.e., Sichuan Basin, Central and Eastern China, indicating a significant contribution of anthropogenic emissions. The result indicates the improved TROPOMI HCHO is more suitable for the analysis of regional and city scale pollution in China. [ABSTRACT FROM AUTHOR]

Abstract Carbon monoxide (CO) is regarded as a useful tracer of biomass burning and anthropogenic pollution, so CO measurements can provide valuable information about the intensity of various anthropogenic activities. However, the emission estimates of CO based on inventories are associated with high uncertainties, especially in China. As CO is co-emitted with CO 2 in the combustion of carbonaceous fuels, the relationship between CO and CO 2 is often used to estimate regional CO emissions. Hefei is located in the area of eastern central China, which is one of the most industrialized regions in China, with severe regional air pollution. The enhancement slopes of ∆CO to ∆CO 2 were calculated and compared from ground-based remote sensing observations, surface in-situ measurements, satellite and emission inventory data at the Hefei site during the period from September 2015 to August 2017. Both inventory based ratios of ΔCO to ΔCO 2 are significantly larger than the ratios based on the observation data, including Fourier Transform Spectrometer (FTS) data, in-situ data, and satellite data. Further the CO emissions in the central China were estimated from the enhancement slopes of ∆CO/∆CO 2 combined with the CO 2 emission inventory. The CO emission estimated from the ground-based FTS observations and the Peking University (PKU) inventory based CO 2 emission is about 10.96 ± 0.88 and 11.95 ± 0.71 Tg CO yr−1 during the 2015–2016 and the 2016–2017 period, respectively. The CO emission estimated from the ground-based FTS observations and the Emission Database for Global Atmospheric Research (EDGAR) inventory based CO 2 emission is about 11.27 ± 0.91 and 12.35 ± 0.74 Tg CO yr−1, respectively. So the CO emissions estimated from the ground-based FTS data and the different inventory based CO 2 emission show a good agreement. However, CO emissions derived from FTS data are substantially lower than those calculated directly from the inventories, i.e. there is a large difference between CO emissions derived from FTS and CO emissions directly derived from the two inventories. The phenomenon suggests that the emission inventories greatly overestimate the actual CO emission in the study area. This study estimates the regional CO emissions from ground-based remote sensing observations and investigates how much the difference is between the emissions from inventories and ground-based measurements. Highlights • The enhancement slopes of ∆CO to ∆CO2 were calculated and compared from ground-based remote sensing observations, surface in-situ measurements, satellite and emission inventory data at the Hefei sit. • The CO emissions estimated from the ground-based FTS data and the different inventory based CO 2 emission show a good agreement. • The CO emissions derived from ground-based remote sensing are substantially lower than those calculated directly from the inventories. [ABSTRACT FROM AUTHOR]

Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observation was carried out from November 2016 to February 2017 in Beijing, China, to measure the O4 absorptions in UV and visible bands and further to illustrate its relationship with aerosol optical properties (AOPs) under different weather types. According to relative humidity, visibility, and PM 2.5 , we classified the observation periods into clear, light-haze, haze, heavy-haze, fog, and rainy weather conditions. There are obvious differences for measured AOPs under different weather conditions, especially scattering coefficient (σsca) and absorption coefficient (σsca). It was also found that both the O4 differential slant column densities (DSCDs) at the UV and visible bands varied in the order of clear days > light-haze days > haze days > heavy-haze days > fog days. The correlation coefficients (R2) between O4 DSCDs at 360.8 and 477.1 nm mainly varied in the order of clear days > light-haze days > haze days > heavy-haze days. Based on the statistics of O4 DSCDs at an elevation angle 1 ∘ with the corresponding linear regression between UV and visible bands of segmental periods, the relationships between O4 DSCDs and AOPs were established. It should mainly be clear or light-haze days when the correlation slope is greater than 1.0, with a correlation coefficient (R2) greater than 0.9, and O4 DSCDs mainly greater than 2.5×1043 molec. cm -2. Meanwhile, σsca and σabs are less than 45 and 12 Mm -1 , respectively. For haze or heavy-haze days, the correlation slope is less than 0.6, with an R2 less than 0.8, and O4 DSCDs mainly less than 1.3×1043 molec. cm -2 , under which σsca and σabs are mainly located at 200–900 and 20–60 Mm -1. Additionally, the determination method was well validated based on another MAX-DOAS measurement at Gucheng from 19 to 27 November 2016. For more precise and accurate inversion of AOPs, more detailed look-up tables for O4 multiple-wavelength absorptions need to be developed. Since the ground surface AOPs were determined using MAX-DOAS observation at a 1 ∘ elevation in this study, we hope to highlight the potential of retrieved vertical spatially resolved AOPs being expected when multiple elevation angles of MAX-DOAS observation are used together. [ABSTRACT FROM AUTHOR]

Abstract Beijing–Tianjin–Hebei area is suffering from atmospheric pollution from a long time. The understanding of the air pollution mechanism is of great importance for officials to design strategies for the environmental governance. Mixing layer height (MLH) is a key factor influencing the diffusion of air pollutants. It plays an important role on the evolution of heavy pollution events. Light detection and ranging (lidar), is an effective remote-sensing tool, which can retrieve high spatial and temporal evolution process within mixing layer (ML), especially the variation of MLH. There are many methods to retrieve MLH, but each method has its own applicable limitations. The Mie-lidar data in Beijing was firstly used to compare three different algorithms which are widely used under different pollution levels. We find that the multi-layer structure near surface may cause errors in the detection of mixing layer. The MLH retrieved based on image edge detection was better than another two methods especially under heavy polluted episode. Then we applied this method to investigate the evolution of the mixing layer height during a pollution episode in December 2016. MLH at Gucheng county showed the positive correlation with the concentration of particulate matters during the start of this pollution episode. The elevated pollution level in Gucheng was not associated with MLH's decrease, and the significantly increased particulate matters raised the boundary layer, which trapped the pollutants near the surface. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

In recent years, China has experienced heavy air pollution, especially haze caused by particulate matter (PM). The compositions, horizontal distributions, transport, and chemical formation mechanisms of PM and its precursors have been widely investigated in China based on near-ground measurements. However, the understanding of the distributions and physical and chemical processes of PM in the vertical direction remains limited. In this study, an elastic lidar was employed to investigate the vertical profiles of aerosols along the Yangtze River during the Yangtze River Campaign of winter 2015. Some typical aerosols were identified and some events were analyzed in three cases. Dust aerosols can be transported from the Gobi Desert to the Yangtze River basin across a long distance at both low and high altitudes in early December. The transport route was perpendicular to the ship track, suggesting that the dust aerosols may have affected a large area. Moreover, during transport, some dust was also affected by the areas below its transport route since some anthropogenic pollutants were mixed with the dust and changed some of its optical properties. Biomass-burning aerosols covering a distant range along the Yangtze River were identified. This result directly shows the impact areas of biomass-burning aerosols in some agricultural fields. Some directly emitted aerosol plumes were observed, and direct effects of such plumes were limited both temporally and spatially. In addition, an aerosol plume with very low linear depolarization ratios, probably formed through secondary processes, was also observed. These results can help us better understand aerosols in large spatial scales in China and can be useful to regional haze studies. [ABSTRACT FROM AUTHOR]

This study presents the characterization of X CH4 (averaged-column dry air mole fraction) variability and trends using the near-infrared (NIR) solar spectra recorded by the ground-based high-resolution Fourier transform spectrometer (FTS) over Hefei, China. An annual increase of 10.55 ± 2.87 ppb yr −1 (0.56 ± 0.15% yr −1 ) in CH 4 concentrations was deduced from the three years’ worth of FTS measurements. Seasonal cycles and three typical diurnal variation modes were captured. The maximum CH 4 concentration occurred in late summer or early autumn, while the minimum CH 4 concentration occurred in winter. CH 4 concentrations in August were 30–55 ppb higher than those in January. In most cases, the diurnal variability exhibited an increasing trend in summer or early autumn, while it exhibited a decreasing trend or remained relatively constant in other seasons. We compared the FTS data with the GOSAT data and the GEOS-Chem model data. The average difference between the GOSAT and FTS data (GOSAT minus FTS) was 1.60 ppb (0.09%) ±13.0 ppb (0.70%), and between the smoothed GEOS-Chem model data and FTS data (GEOS-Chem minus FTS) was −8.09 ppb (0.45%) ±17.83 ppb (0.95%). The differences between GOSAT and FTS were season-independent; however, obvious season-dependent differences were observed between GEOS-Chem and FTS. The GOSAT and GEOS-Chem models could reproduce the seasonal cycle observed by FTS with correlation coefficients (r) of 0.77 and 0.86, respectively. [ABSTRACT FROM AUTHOR]

Chamber studies on the formation of secondary aerosols are mostly performed with purified air as matrix, it is of wide concern in what extent they might be different from the situations in ambient air, where a variety of gaseous and particulate components preexist. Here we compared the photo-oxidation of “toluene + NO x + SO 2 ” combinations in a smog chamber in real urban ambient air matrix with that in purified air matrix. The secondary organic aerosols (SOA) mass concentrations and yields from toluene in the ambient air matrix, after subtracted ambient air background primary and secondary organic aerosols, were 9.0–34.0 and 5.6–12.9 times, respectively, greater than those in purified air matrix. Both homogeneous and heterogeneous oxidation of SO 2 were enhanced in ambient air matrix experiments with observed 2.0–7.5 times higher SO 2 degradation rates and 2.6–6.8 times faster sulfate formation than that in purified air matrix, resulting in higher in-situ particle acidity and consequently promoting acid-catalyzed SOA formation. In the ambient air experiments although averaged OH radical levels were elevated probably due to heterogeneous formation of OH on particle surface and/or ozonolysis of alkenes, non-OH oxidation pathways of SO 2 became even more dominating. Under the same organic aerosol mass concentration, the SOA yields of toluene in purified air matrix experiments matched very well with the two-product model curve by Ng et al. (2007), yet the yields in ambient air on average was over two times larger. The results however were much near the best fit curve by Hildebrandt et al. (2009) with the volatility basis set (VBS) approach. [ABSTRACT FROM AUTHOR]

Isoprene is important to the formation of secondary organic aerosols and can change the atmospheric oxidation capacity in the remote marine environment. However, the influencing factors of marine atmospheric isoprene are still unclear. Here, we report observed atmospheric isoprene in ambient air along three cruises path from the Arctic Ocean to the Southern Ocean. The levels of isoprene ranged from not detected (ND) to 452 pptv, with an average value of 48 ± 81 pptv, with large variability. A negative correlation was found between isoprene and latitude (r = −0.40, p < 0.01). The spatial distributions of isoprene flux from oceanic phytoplankton by the modelled results were different from those of observed atmospheric isoprene. The observed isoprene concentration was 2–3 orders of magnitude higher than the model estimation. Environmental variables such as temperature, wind speed (WS), sea surface temperature (SST), salinity and atmospheric removal capacity can influence the distribution of isoprene. At latitudes north of 60°S, the marine emission capacity was relatively important and contributed 45.06%. Atmospheric removal capacity was the most important factor for atmospheric isoprene, contributing 55.05% to the concentration of atmospheric isoprene in latitudes south of 60°S. Low atmospheric oxidation capacity and wind speed cause high atmospheric isoprene in the Southern Ocean in summer and will eventually affect secondary organic aerosol concentrations. • Maine atmospheric isoprene has a negative correlation with latitude. • Observed marine atmospheric isoprene concentration 2–3 orders of magnitude higher than the model estimated. • Atmospheric removal capacity is an important factor for marine atmospheric isoprene in latitudes south of 60°S. [ABSTRACT FROM AUTHOR]

Tropospheric ozone (O 3) profiles, especially within the boundary layer, are essential for studying the vertical, temporal, spatial variations, as well as the formation sensitivity and regional transport of O 3. However, it is rare to find continuous tropospheric O 3 profiles with high temporal and spatial resolutions without blind areas using current remote sensing technologies, with issues such as low near-surface sensitivity or systematic blind areas from satellite and LiDAR observations, respectively, being encountered. In this study, multi-source data including stratospheric O 3 profiles from external datasets and local monthly dependent a priori profiles were fused in the retrieval algorithm, then vertical O 3 profiles from the near-surface to the free troposphere were retrieved from multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements. With the aim of deriving a set of best practice recommendations for applying a profile inversion algorithm to long-term observations, we comprehensively investigated the influence of various settings on profile retrieval, with emphasis on the selection of a Fraunhofer reference spectrum and appropriate a priori profiles in the upper troposphere. These recommendations are essential for applying the algorithm to real long-term measurements. In this study, tropospheric O 3 profiles were retrieved for operational MAX-DOAS observations in Beijing, and the results, especially for the boundary layer, were evaluated in detail with respect to well-established independent O 3 datasets, including one-year ozonesonde profiles and tower-based in-situ measurements at different altitudes. A good level of agreement was found for both near-surface and elevated-altitude results, and the MAX-DOAS O 3 profiles were able to reproduce the vertical distributions measured by ozonesonde. However, MAX-DOAS measurements are less sensitive to the upper troposphere and converge to the a priori profile, and the smoothing effect induced by the optimal estimation method makes it difficult to reproduce steep vertical gradients in real atmosphere. • Developing a novel remote sensing algorithm for O 3 profiles without blind area. • Capturing O 3 near-surface and vertical distributions as indicated by validations. • Deriving best practice recommendations for applying inversion algorithm. • Indicating stable algorithm performance between different instruments' measurements. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) have attracted an increasing concern in China's megacities. However, rare information is available on the spatial and seasonal variations of inhalation cancer risk (ICR) due to PAH exposure and their relations to specific sources. In this study, year-round PM 2.5 samples were collected from 2013 to 2014 by high-volume samplers at four sites (one urban, two rural and one roadside station) in Guangzhou in the highly industrialized and densely populated Pearl River Delta (PRD) region and analyzed for 26 polycyclic aromatic hydrocarbons (PAHs) together with molecular tracers including levoglucosan, hopanes and elemental carbon. Higher molecular weight PAHs (5-ring or above) accounted for 64.3–87.5% of total PAHs. Estimated annual averages of benzo( a )pyrene-equivalent carcinogenic potency (BaP eq ) were 1.37, 2.31 and 1.56 ng/m 3 at urban SZ, rural JL and rural WQS, respectively, much higher than that at the roadside station YJ in an urban street canyon. ICR of PAHs in wintertime reached 2.2 × 10 −4 , nearly 3 times that in summer; and cancer risk of PAHs was surprisingly higher at the rural site than at other sites. Source contributions by positive matrix factorization (PMF) in the aid of molecular tracers revealed that overall coal combustion and biomass burning altogether contributed 73.8% of total PAHs and 85.2% of BaP eq , and particularly in winter biomass burning became the most significant source of total PAHs and BaP eq (51.8% and 52.5%), followed by coal combustion (32.0% and 39.1%) and vehicle emission (16.2% and 8.4%). The findings of this work suggest that even in China's megacities like Guangzhou, limiting biomass burning may benefit PAHs pollution control and cancer risk reduction. [ABSTRACT FROM AUTHOR]

Atmospheric ammonia (NH 3) plays an important role in the formation of fine particulate matter, leading to severe environmental degradation and human health issues. In this work, ground-based Fourier transform infrared spectrometry (FTIR) observations are used to obtain the total columns of atmospheric NH 3 at Hefei, China, from December 2016 to December 2020. After the presentation of the retrieval algorithm and uncertainty budget, we perform a spatio-temporal analysis of the dataset. Over the four years, NH 3 columns have been increasing by 15.82% (2017–2018), 3.83% (2018–2019) and 3.68% (2019–2020). A clear seasonal cycle is observed, with the largest surface concentrations (12.93 ± 6.40 ppb) observed in June to August, and the lowest (4.08 ± 2.66 ppb) in November to January. The diurnal cycles of NH 3 exhibit increased morning and afternoon concentrations. Interpretation of the diurnal cycles is difficult, however, the absence of a peak during rush hours, and the absence of correlation with CO and NO 2 suggest that agriculture and not traffic is the main source of NH 3 at Hefei. The polar plots of NH 3 columns with wind and back trajectories of air masses calculated by the HYSPLIT model confirmed that agriculture was the dominant source of ammonia in four seasons, while urban anthropogenic emissions contributed to the high level of NH 3 in summer over the Hefei site. We end this paper with a short validation exercise of NH 3 columns retrieved from measurements of the IASI satellite data with the FTIR measurements over Hefei. Correlation coefficients (R) between the two datasets are 0.79 and 0.75 for IASI-A and IASI-B, with the slope of 0.96 and 1.10, respectively. The mean difference is −3.44 × 1015 and −3.96 × 1015 molec cm−2, with standard deviation of 7.16 × 1015 and 8.10 × 1015 molec cm−2, respectively. These results demonstrate the IASI and FTIR data, over Hefei, are in broad agreement. In order to know about the temporal and spatial distribution of atmospheric ammonia, and what influence ammonia columns over Hefei, China, we observed the total columns of atmospheric ammonia by ground-based FTIR remote sensing from December 2016 to December 2020. The time series of the ammonia column observed at the Hefei site are plotted. The green dots are the individual measurements of NH 3 ; the red diamonds represent the daily averaged NH 3 ; the blue line is the fitting curve of the individual NH3 columns. We discussed the seasonal trend, annual variability, diurnal variation of NH 3 , and the main factors that influence gaseous NH 3 concentrations in Hefei. [Display omitted] • FTIR measurements of NH 3 column and comparison with satellite data are reported. • FTIR observations captured the spatio-temporal variation of NH 3 columns. • Agriculture was the dominant source of ammonia in four seasons over Hefei. • IASI satellite data are in broad agreement with FTIR data over Hefei. [ABSTRACT FROM AUTHOR]

This study investigated the wintertime vertical distributions and source areas of aerosols, NO 2 , and HCHO in a coastal city of Dongying from December 2020 to March 2021, using ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) and a potential source contribution function (PSCF) model, respectively. Moreover, the chemical production sensitivity of O 3 at different height layers was analyzed using HCHO/NO 2 ratios. The results revealed that the wintertime averaged highest concentrations of aerosol (1.25 km−1), NO 2 (14.81 ppb), and HCHO (2.32 ppb) were mainly distributed at the surface layer, 100–200 m layer, and 200–300 m layer, respectively. Regarding the diurnal cycles, high concentrations of aerosol (>1.4 km−1) and NO 2 (>16.0 ppb) usually appeared in the early morning and late afternoon, while high concentrations of HCHO (>2.5 ppb) usually occurred during 12:00–15:00. The PSCF model revealed that the wintertime aerosol mainly originated from Shandong, northern Jiangsu, Korea, and the northwestern Mongolian Plateau. Below 200 m, NO 2 was mainly from western Shandong, whereas above 600 m, it was mainly from northern Shandong and the Beijing-Tianjin-Hebei (BTH) region. The corresponding sources for HCHO were central and southern Shandong (below 200 m) and northern Shandong, northern Jiangsu, and southeastern BTH (above 600 m). In addition, the chemical production sensitivity of O 3 below 100 m was observed only in the VOC-limited regime. The percentages of O 3 production under the NO x -limited, NO x -VOC-limited, and VOC-limited regimes were 10.75% (31.18%), 4.30% (19.35%), and 84.95% (49.47%) at the 500–600 m (900–1000 m) layer. This study has guiding significance for the coordinated control of PM 2.5 and O 3 , and can assist in the implementation of regional joint prevention and control strategies for air pollutants. • The averaged wintertime NO 2 and HCHO profiles all exhibited a Gaussian shape. • The vertical potential sources of aerosol, NO 2 and HCHO were analyzed using PSCF. • O 3 chemical production sensitivity under 100 m was only VOC-limited. • The percentages of VOC-limited O 3 production at 500–600 were 84.95%. [ABSTRACT FROM AUTHOR]

Since NOx has decreased significantly, ozone formation sensitivity in urban areas has gradually shifted from VOC-limited to NOx-limited. Ozone is usually formed at high altitudes. However, due to a lack of vertical gradient observation data support, the evidence of high-altitude sensitivity change is still not conclusive. To confirm this issue, ozone and NOx were observed at three heights (8 m, 160 m, and 280 m) from a 325 m tower in Beijing from 2019 to 2020. Combined with the NO 2 and HCHO column concentrations, the ozone formation sensitivity transition was determined, and it was proposed that NOx control could effectively deal with high ozone concentrations. By comparing the observations from 2009 to 2010, near-surface NOx and ozone concentrations were found to have changed. A large decrease in NOx (−20 ppbv) and increase in ozone (19 ppbv) indicated that the weakening of the NO titration effect was the main cause of the increase. To remove the influence of the titration effect, the change in Ox (Ox = O 3 +NO 2) was analyzed and found to have increased by 6 ppbv. Local formation was not the main reason for Ox growth. After strict emission standards, the proportion of primary NO 2 and southward short-distance transport increased, which were two key reasons for the Ox increase. In addition, the boundary layer height increased by 208 m, and the surface relative humidity decreased by 6%. Meteorological conditions have become more conducive to ozone formation. This study shows that NOx emission reduction is conducive to controlling high-level ozone pollution. With the in-depth development of NOx emission reduction, the near-surface ozone will also significantly decrease. This study also highlights the importance of strict controls on diesel vehicles and motor vehicle numbers. • Meteorological conditions have become more favorable for ozone formation. • Primary NO 2 and southward short-distance transport increased. • Continuous NOx reduction can control ozone pollution. • VOCs should be controlled throughout the year, and NOx in summer. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) are key precursors of PM 2.5 and ozone. Glyoxal can be utilized as an indicator of VOC emission sources and secondary organic aerosol (SOA) formation. In this study, the high-resolution satellite observations of glyoxal were utilized to identify anthropogenic VOC sources, which can be utilized to formulate pollution control policies. Glyoxal vertical column densities were firstly retrieved by the TROPOspheric monitoring instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor satellite. Glyoxal retrieval is sensitive to wavelength; therefore, four different areas were studied, and the optimum wavelength range of 435–462 nm was determined and applied in the wavelength sensitivity test. Glyoxal slant column density was corrected by utilizing the daily stripe-correction in the Sahara region which reduces the error by approximately 2.2 × 1014 molec/cm2. Glyoxal vertical column densities computed using TROPOMI were validated via multi-axis differential optical absorption spectroscopy measurements obtained at three sites; the daily normalized mean bias and standard deviation for the three sites were -27.5 % ± 6.5%, 1.8% ± 7.5%, and −6.6 % ± 17.3%, respectively. Monthly averaged data shows good correlation with Pearson correlation coefficients (R) of 0.90, 0.78 and 0.92, at three MAX-DOSA sites, respectively. The global spatial distribution of glyoxal showed large areas of high-concentration glyoxal distributed over tropical rainforests near the equator, which were attributed to biomass combustion and biogenic processes. Numerous urban hotspots were identified to be mainly concentrated in developing countries, especially those in East Asia and the Middle East. The highest glyoxal vertical column densities were observed over eastern China, and the high-glyoxal concentration areas were mainly located over urban and industrial areas, indicating a significant anthropogenic glyoxal source. Kilometer-level TROPOMI can identify factory emission sources that are undetectable by Ozone Monitoring Instrument due to the higher resolution of TROPOMI. • Realizing the global kilometer-level glyoxal observation for the first time. • Indicating of volatile organic compounds emission sources and secondary organic aerosol formation. • Utilizing glyoxal results to realize identification of factory sources. [ABSTRACT FROM AUTHOR]

Considerable efforts have been made to extend temperature records beyond the instrumental period through proxy reconstructions, in order to further understand the mechanisms of past climate variability. Yet, the global coverage of existing temperature records is still limited, especially for some key regions like the Tibetan Plateau and for earlier times including the Medieval Warm Period (MWP). Here we present decadally-resolved, alkenone-based, temperature records from two lakes on the northern Tibetan Plateau. Characterized by marked temperature variability, our records provide evidence that temperatures during the MWP were slightly higher than the modern period in this region. Further, our temperature reconstructions, within age uncertainty, can be well correlated with solar irradiance changes, suggesting a possible link between solar forcing and natural climate variability, at least on the northern Tibetan Plateau. [ABSTRACT FROM AUTHOR]