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Ship-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were conducted along the marginal seas of China from 19 April to 16 May 2018 to measure the vertical profiles of aerosol, nitrogen dioxide (NO 2), and nitrous acid (HONO). Along the cruise route, we found five hot spots with enhanced tropospheric NO 2 vertical column densities (VCDs) in the Yangtze River Delta, Taiwan Strait, Guangzhou–Hong Kong–Macau Greater Bay Area, port of Zhanjiang, and port of Qingdao. Enhanced HONO concentrations could usually be observed under high-level aerosol and NO 2 conditions, whereas the reverse was not always the case. To understand the impacts of relative humidity (RH), temperature, and aerosol on the heterogeneous reaction of NO 2 to form HONO in different scenarios, the Chinese Academy of Meteorological Sciences (CAMS) and Southern University of Science and Technology (SUST) MAX-DOAS stations were selected as the inland and coastal cases, respectively. The RH turning points in CAMS and SUST cases were both ∼ 65 % (60 %–70 %), whereas two turning peaks (∼ 60 % and ∼ 85 %) of RH were found in the sea cases. As temperature increased, the HONO / NO 2 ratio decreased with peak values appearing at ∼ 12.5 ∘ C in CAMS, whereas the HONO / NO 2 gradually increased and reached peak values at ∼ 31.5 ∘ C in SUST. In the sea cases, when the temperature exceeded 18.0 ∘ C, the HONO / NO 2 ratio rose with increasing temperature and achieved its peak at ∼ 25.0 ∘ C. This indicated that high temperature can contribute to the secondary formation of HONO in the sea atmosphere. In the inland cases, the correlation analysis between HONO and aerosol in the near-surface layer showed that the ground surface is more crucial to the formation of HONO via the heterogeneous reaction of NO 2 ; however, in the coastal and sea cases, the aerosol surface contributed more. Furthermore, we discovered that the conversion rate of NO 2 to HONO through heterogeneous reactions in the sea cases is larger than that in the inland cases in higher atmospheric layers (> 600 m). Three typical events were selected to demonstrate three potential contributing factors of HONO production under marine conditions (i.e., transport, NO 2 heterogeneous reaction, and unknown HONO source). This study elucidates the sea–land and vertical differences in the forming mechanism of HONO via the NO 2 heterogeneous reaction and provides deep insights into tropospheric HONO distribution, transforming process, and environmental effects. [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]

The influence of regional transport on aerosol pollution has been explored in previous studies based on numerical simulation or surface observation. Nevertheless, owing to inhomogeneous vertical distribution of air pollutants, vertical observations should be conducted for a comprehensive understanding of regional transport. Here we obtained the vertical profiles of aerosol and its precursors using ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) at the Nancheng site in suburban Beijing on the southwest transport pathway of the Beijing-Tianjin-Hebei (BTH) region, China, and then estimated the vertical profiles of transport fluxes in the southwest-northeast direction. The maximum net transport fluxes per unit cross-sectional area, calculated as pollutant concentration multiply by wind speed, of aerosol extinction coefficient (AEC), NO 2 , SO 2 and HCHO were 0.98 km−1 m s−1, 24, 14 and 8.0 μg m−2 s−1 from southwest to northeast, which occurred in the 200–300 m, 100–200 m, 500–600 m and 500–600 m layers, respectively, due to much higher pollutant concentrations during southwest transport than during northeast transport in these layers. The average net column transport fluxes were 1200 km−1 m2 s−1, 38, 26 and 15 mg m−1 s−1 from southwest to northeast for AEC, NO 2 , SO 2 and HCHO, respectively, in which the fluxes in the surface layer (0–100 m) accounted for only 2.3%–4.2%. Evaluation only based on surface observation would underestimate the influence of the transport from southwest cities to Beijing. Northeast or weak southwest transports dominated in clean conditions with PM 2.5 <75 μg m−3 and intense southwest transport dominated in polluted conditions with PM 2.5 >75 μg m−3. Southwest transport through the middle boundary layer was a trigger factor for aerosol pollution events in urban Beijing, because it not only directly bringing air pollutants, but also induced an inverse structure of aerosols, which resulted in stronger atmospheric stability and aggravated air pollution in urban Beijing. [Display omitted] • Vertical profiles of pollutant transport flux were obtained via MAX-DOAS observation. • Average net column transport of air pollutants was from southwest cities to Beijing. • Regional transport on the southwest pathway was mainly via middle boundary layer. • Southwest transport would induce an inverse structure of aerosols in urban Beijing. [ABSTRACT FROM AUTHOR]

Satellite remote sensing is an important technique providing long-term and large-scale information of formaldehyde (HCHO), which plays a crucial role in atmospheric chemistry. Low signal-to-noise ratio and poor stability of the Environmental Trace Gases Monitoring Instrument (EMI) On board Gaofen-5 satellite, the first Chinese space-borne spectrometer, make HCHO retrieval extremely difficult. Here we firstly retrieved HCHO vertical column densities (VCDs) from EMI through in-flight spectral calibration, retrieval setting optimization and stripe correction. Retrieved EMI HCHO VCDs correlate well with those measured by Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) with normalize mean bias (NMB) below 25%. EMI HCHO VCDs are comparable with those observed by Ozone Monitoring Instrument (OMI) and TROPOspheric Monitoring Instrument (TROPOMI). This study reveals that HCHO can be observed successfully by algorithm optimization despite of poor performance of space-borne spectrometer. The retrieved EMI HCHO VCDs are applied to locate emission sources of volatile organic compounds (VOCs). • First retrieval of HCHO from Environmental Trace Gases Monitoring Instrument (EMI). • Algorithm optimization is proposed suitable for EMI HCHO retrieval. • HCHO vertical column density (VCD) correlates well with ground-based measurements. • EMI HCHO measurements are improved to a comparable level with OMI measurements. • HCHO hotspots and source of volatile organic compounds are captured successfully by EMI. [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]

Assessing the vertical distribution of aerosols in the atmosphere can elucidate the regional sources of accumulated pollutants at different altitudes. Although studies in Hefei have mainly focused on aerosols in the near-surface atmosphere, few studies have assessed the vertical variability in aerosol concentrations. In this study, the vertical aerosol distribution in Hefei was analyzed continuously for one year (from March 2018 to February 2019) using multi-axis differential optical absorption spectroscopy. The aerosol extinction coefficient was the highest in winter (0.94 km−1), followed by autumn (0.63 km−1), spring (0.57 km−1), and summer (0.47 km−1). Local pollution caused the accumulation of aerosols near the surface. The rate of decrease of the aerosol extinction coefficient was higher in autumn and winter than in spring and summer. The diurnal variations of aerosol extinction coefficient in the different seasons were affected by the planetary boundary layer height, and we observed notable high-altitude transport of aerosols in autumn. We assessed the vertical distribution of the aerosol extinction coefficient under different pollution concentrations and identified peak values mainly below a height of 0.5 km. With increasing height, the extinction coefficient decreased linearly and slowly under low PM 2.5 concentrations, whereas decreased more rapidly under high PM 2.5 concentrations. Clustering analysis, the weight potential source concentration function, and the concentration weighted trajectory model inferred different seasonal characteristics in the potential source areas of aerosols at different heights. In addition to local pollution sources, Jiangsu Province was found to be the main source of pollution in Hefei at a height of 0.4 km, whereas Hubei Province was the main source of pollution at a height of 2 km. [Display omitted] • The vertical decline rate of the aerosol extinction coefficient is highest in winter. • The aerosols in autumn exhibit obvious high-altitude area transmission. • Aerosol extinction coefficient decreased rapidly with height under high pollution. • Jiangsu (Hubei) Province was the main source of pollution at 0.4 km (0.2 km) height. [ABSTRACT FROM AUTHOR]

Water vapor vertical profiles are important in numerical weather prediction, moisture transport, and vertical flux calculation. This study presents the Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) retrieval algorithm for water vapor vertical profiles and the retrieved results are validated with corresponding independent datasets under clear sky. The retrieved Vertical Column Densities (VCDs) and surface concentrations are validated with the Aerosol Robotic Network (AERONET) and National Climatic Data Centre (NCDC) datasets, achieving good correlation coefficients (R) of 0.922 and 0.876, respectively. The retrieved vertical profiles agree well with weekly balloon-borne radiosonde measurements. Furthermore, the retrieved water vapor concentrations at different altitudes (100–2000 m) are validated with the corresponding European Centre for Medium-range Weather Forecasts (ECMWF) ERA-interim datasets, achieving a correlation coefficient (R) varying from 0.695 to 0.857. The total error budgets for the surface concentrations and VCDs are 31% and 38%, respectively. Finally, the retrieval performance of the MAX-DOAS algorithm under different aerosol loads is evaluated. High aerosol loads obstruct the retrieval of surface concentrations and VCDs, with surface concentrations more liable to severe interference from such aerosol loads. To summarize, the feasibility of detecting water vapor profiles using MAX-DOAS under clear sky is confirmed in this work. [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]

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]

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]

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]

In this study, the spatiotemporal distribution of the NO 2 and SO 2 Vertical Columns Densities (VCDs) in the North China Plain (NCP) region was measured by long-distance mobile measurements using a mobile Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument. The mobile observations were performed in both summer (July 2017) and winter (January and February 2018) and the total driving mileage exceeded 3000 km. The concentrations of NO 2 and SO 2 in different seasons and areas were significantly different. During winter, serious NO 2 and SO 2 pollution was observed in northern Anhui Province, central Shandong Province, and the Beijing-Tianjin-Hebei Region. The evolution and transportation processes of three typical heavy pollution cases were discussed in detail. Combined with the Weather Research and Forecasting with Chemistry model simulated wind field information, the NO 2 transportation flux from northern Jiangsu Province to northern Anhui Province is quantified as 7.12 kg s−1. Finally, we estimated the NO 2 and SO 2 emissions from the Dezhou and Hengshui power plants by plume cross section scanning and encircled observations, respectively. The NO 2 and SO 2 emission fluxes of the Dezhou Power Plant are 0.79 kg s−1 and 1.11 kg s−1, respectively, while the NO 2 and SO 2 emission fluxes of the Hengshui Power Plant are 0.12 kg s−1 and 0.36 kg s−1, respectively. This study quantitatively analyzed the transportation of atmospheric pollutants and power plant emissions, which is helpful for understanding the occurrence and evolution of pollution and is also useful for the government to introduce policies to protect and control the atmospheric environment. • Large-scale and long-distance mobile MAX-DOAS measurements were performed. • The spatiotemporal patterns of the NO 2 and SO 2 VCDs in NCP was measured. • A transportation calculation method was used to estimate the pollutant transportation. • Pollutant emission reduction measures significantly reduce emissions of power plants. [ABSTRACT FROM AUTHOR]