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Abstract In this paper, we present a quantitative determination of the NO x emission in Hefei using mobile Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements. The measurements were carried out during winter time from December 2016 to February 2017. The measured NO 2 vertical column densities (VCDs) show a strong spatial gradient with higher values within the city center, indicating the majority of NO x emission sources are located at the city center. A significant "holiday effect" was found by comparing measurements taken during weekdays and weekend. The weekend reduction is more significant in the suburban (∼28%) compared to the city center (∼13%). Mobile measurements of NO 2 vertical column densities (VCDs) were analyzed together with meteorological data to determine the NO x emission by applying the loop-integral method. Detailed error analysis of the NO x emission shows the variation of wind field and large measurement gap dominated the total error of NO x emission calculation. The result shows the NO x emission in Hefei during winter time varies in a wide range from 10 × 1024 to 40 × 1024molecs−1 with an average of 18.44 × 1024molecs−1. Our estimation is about 43% lower than the number reported in the previous emission inventory in 2012. The reduction of NO x emission reflects the successful implementation of emission control measures in recent years. Our result also shows about 73% of the total NO x in the city were transported from outside of the city during winter. The impacts of transported NO x are especially large when air masses originated from heavily polluted regions, i.e. North China Plain and Yangtze River Delta. The result presented provides a better understanding of the impacts of local emissions and transportation of pollutants on the local air quality of Hefei. Highlights • NO x emission in Hefei was determined using mobile MAX-DOAS measurements in 2017. • NO x emission in Hefei varies from 10 to 40 × 1024 molec/s during winter. • Our estimation is ∼43% lower compared to previous emission inventory in 2012. • About 73% of NO x measured in the city were transported from outside. [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]

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]