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With the increasing concerns on summertime atmospheric photochemical pollution, the diagnosis and prevention of ozone pollution have been paid close attention. Both formaldehyde (HCHO) and glyoxal (CHOCHO) are ubiquitous oxidation intermediates of volatile organic compounds (VOCs). The ratio of glyoxal to formaldehyde (R GF) is used as a metric for VOCs emission sources. In this study, the mixing ratios of HCHO and CHOCHO have been measured by the active differential optical absorption spectroscopy (DOAS) method in the urban area of Shanghai during summertime in 2018, as well as other trace gases. The average levels of HCHO and CHOCHO are 3.31 ± 1.43 ppbv and 0.164 ± 0.073 ppbv, respectively. The similar diurnal patterns and high correlation between HCHO, CHOCHO and ozone levels implied that daytime photochemical processes are the dominant formation pathway for these trace gases. We find that with increased NO x levels, HCHO shows higher ozone formation potential relative to glyoxal. The R GF ratio increases with temperature and decreases with NO 2 levels. By investigating the coupling of typical VOCs species such as acetylene, toluene and isoprene with HCHO and CHOCHO, R GF is found to be strongly impacted by the ambient VOCs profiles, suggesting that R GF should be used with caution when linking it to a given VOC precursor source. Finally, the R GF variations with ozone pollution episodes and weather processes are also discussed. • Ambient HCHO and CHOCHO were observed by active DOAS technique. • HCHO and CHOCHO show the dominant pathway of photochemical formation during daytime. • With increased NOx levels, ozone formation potential of HCHO higher than CHOCHO. • CHOCHO to HCHO Ratio (R GF) increases with temperature and decreases with NO 2 levels. • R GF is strongly impacted by the VOCs profiles and should be used with caution. [ABSTRACT FROM AUTHOR]

Abstract Multi-AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed to remote sensing the aerosol vertical profiles in suburb area of Shanghai, China from April to December 2017. The retrieval of aerosol extinction was based on the optimal estimation method combing the measured oxygen dimer O 4 absorption with simulation of forward radiative transfer model. It suggests that the employment of O 4 correction factor (CF O4) for daily aerosol retrieval should be adjusted, which includes three typical cases that a fixed CF O4 , no usage of CF O4 and different CF O4 in the morning and afternoon. In cases of using CF O4 , elevation angle dependent CF O 4 (α) were further proposed. Moreover, the retrieval results can be improved if the local sounding data of atmospheric temperature and pressure profiles were introduced into the forward radiative transfer model without utilizing additional CF O4. Afterwards, the retrieved Aerosol Optical Depth (AOD) and the surface Aerosol Extinction Coefficient (AEC) was in a good agreement with the AOD obtained from sun photometer observations and in-suit PM 2.5 concentrations, with a correlation coefficient R of 0.866 and 0.833, respectively. Besides, the vertical AEC profiles retrieved by MAX-DOAS were also validated well by the co-located lidar measurement. It can be found that the particles were mainly distributed below 1 km, and the maximum AEC usually appeared in the lowest 300 m and decreased with the altitudes. The averaged AEC within 1 km varied from 0.20 to 0.75 km−1, 0.25 to 1.08 km−1, 0.55 to 2.45 km−1 and 0.70 to 2.75 km−1 under different air quality of Grade II, III, IV and V. The sounding meteorological parameters and backward trajectories of air masses were integrated to diagnose the influencing factor of AEC at different altitudes, which illustrated that the AEC levels at lowest layer were impacted by the long distance transportation of air pollutants from north-northwest and regional local air pollution nearby during the winter time. The study shows that ground-based MAX-DOAS observation is powerful remote sensing technique to provide better understanding of aerosol properties at both ground surface and higher altitudes. Highlights • Aerosol vertical distribution were obtained by MAX-DOAS measurements in Shanghai. • O 4 correction factor (CF O4) need to be adjusted in the aerosol retrieval scheme. • Using local temperature and pressure profiles can improve the aerosol retrieval. • MAX-DOAS remote sensing are well validated by other auxiliary datasets. • High aerosol extinction usually occurred at the lowest layer below 0.3 km in winter. [ABSTRACT FROM AUTHOR]

To investigate on the daytime OH and its implication to fine particle, the long-path differential optical absorption spectroscopy (LP-DOAS) system was employed to observe the main OH precursors of O 3 , HCHO and HONO, as well as NO 2 and NO 3 radical from April to August 2013 over Shanghai, China. The main OH production paths from HONO, HCHO and O 3 were estimated to be occupied around 57.6%, 30.5% and 11.9% during daytime. The daytime OH radical concentration under steady-state was averaged at 1.02 × 10 7 molec cm −3 , which was significantly impacted by the photolytic processes. The relationship between photolysis frequency j(O 1 D) and OH radical suggests that heavy fine particle loads can make the photolytic reactions less efficiently and decrease the OH production and concentration. Utilizing CO as the indicator, the part of PM 2.5 mass related to primary emitted sources was found less impacted by the OH levels. The contribution of secondary organic aerosol with metrics of O 3 was enhanced with the increases of the OH levels, while secondary inorganic part of PM 2.5 was favor of the condition that smaller OH concentrations that 5 × 10 5 –5 × 10 6 molec cm −3 . Meantime, a simplified multivariate model was employed to evaluate the influences of OH levels on different parts of fine particles related to different emission and sources. Normalized by solar radiation, this part of OH unrelated to radiation was found to be inversely related to the PM 2.5 , which indicates the self-cleansing capability of the atmosphere. [ABSTRACT FROM AUTHOR]

Formaldehyde (HCHO) provides a proxy to reveal the isoprene and biogenic volatile organic compounds emission which plays important roles in atmospheric chemical process and climate change. The ground-based observation with zenith-sky DOAS is carried out in order to validate the HCHO columns from OMI. It has a good correlation of 0.71678 between the HCHO columns from two sources. Then we use the OMI HCHO columns from January 2006 to December 2015 to indicate the interannual variation and spatial distribution in Xishuangbanna. The HCHO concentration peaks appeared in March or April for each year significantly corresponding to the intensive fire counts at the same time, which illustrate that the high HCHO columns are strongly influenced by the biomass burning in spring. Temperature and precipitation are also the important influence factors in the seasonal variation when there is nearly no biomass burning. The spatial patterns over the past ten years strengthen the deduction from the temporal variation and show the relationship with land cover and land use, elevation and population density. It is concluded that the biogenic activity plays a role in controlling the background level of HCHO in Xishuangbanna, while biomass burning is the main driving force of high HCHO concentration. And forests are greater contributor to HCHO rather than rubber trees which cover over 20% of the land in the region. Moreover, uncertainties from HCHO slant column retrieval and AMFs calculation are discussed in detail. [ABSTRACT FROM AUTHOR]

In the nocturnal boundary layer, nitrate radical (NO 3) has an important contribution to atmospheric chemistry through oxidation of nitrogen oxides and hydrocarbons. Vertical distributions of NO 2 , O 3 and NO 3 were measured by four differential optical absorption spectroscopy instruments at meteorological tower in Beijing from June 1 to July 22, 2019. The results show the mean diurnal variations of NO 2 , O 3 , and NO 3 display a single peak (up to 65.0 ppbv, 196.8 ppbv and 317.5 pptv, respectively) in time. O 3 and NO 3 mixing ratios generally increased against heights, which is opposite to NO 2 , suggesting the contribution of O 3 to NO 3 production at higher altitude. According to the correlation coefficients between NO 3 production rates (P NO3) and NO 2 or O 3 levels, P NO3 was sensitive to NO 2 mixing ratio at higher altitude but to O 3 near the ground. Averaged NO 3 lifetimes (τ NO3) of lowest, middle, upper and highest layer intervals were 104, 118, 164 and 213 s, respectively, which indicates τ NO3 increase against height and explains why NO 3 mixing ratios are larger at higher altitude to some extent. Main control factors of NO 3 removal changed from gas-phase reactions to N 2 O 5 hydrolysis with height increase. When relative humidity (RH) exceeded 70% or PM 2.5 level exceeded 50 μg·m−3, τ NO3 was almost less than 300 s with mixing ratio lower than 70 pptv. The clear negative dependence of τ NO3 on RH and PM 2.5 reveals the influencing factors on indirect loss. Under polluted conditions, vertical profiles of NO 2 , O 3 and NO 3 varied drastically. Stable atmosphere (low nocturnal boundary layer height and thermal inversion), RH level and RH gradient are the main reason for the evident difference in NO 3 gradient. Vertically increased NO 3 radicals may imply the formation of nitrate aerosols and further increase the nitrate content in high- altitude particulate matter. Unlabelled Image • Vertically increased NO 3 radical in the nocturnal boundary layer. • NO 3 removal mainly from gas-phase reactions to N 2 O 5 hydrolysis with height increase. • Stable atmosphere, low RH and negative RH gradient facilitate large NO 3 gradient. [ABSTRACT FROM AUTHOR]