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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]

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]

To prevent the spread of the COVID-19 epidemic, the Chinese megacity Wuhan has taken emergent lockdown measures starting on January 23, 2020. This provided a natural experiment to investigate the response of air quality to such emission reductions. Here, we decoupled the influence of meteorological and non-meteorological factors on main air pollutants using generalized additive models (GAMs), driven by data from the China National Environmental Monitoring Center (CNEMC) network. During the lockdown period (Jan. 23 – Apr. 8, 2020), PM 2.5 , PM 10 , NO 2 , SO 2 , and CO concentrations decreased significantly by 45 %, 49 %, 56 %, 39 %, and 18 % compared with the corresponding period in 2015–2019, with contributions by S(meteos) of 15 %, 17 %, 13 %, 10 %, and 6 %. This indicates an emission reduction of NO x at least 43 %. However, O 3 increased by 43 % with a contribution by S(meteos) of 6 %. In spite of the reduced volatile organic compound (VOC) emissions by 30 % during the strict lockdown period (Jan. 23 – Feb. 14, 2020), which likely reduced the production of O 3 , O 3 concentrations increased due to a weakening of the titration effect of NO. Our results suggest that conventional emission reduction (NO x reduction only) measures may not be sufficient to reduce (or even lead to an increase of) surface O 3 concentrations, even if reaching the limit, and VOC-specific measures should also be taken. [Display omitted] • COVID-19 lockdown caused a significant reduction in pollutant, except ozone. • GAMs model can explain nonlinear relationship between pollutants and meteorology. • Increasing of ozone mainly due to weakening of titration effect. • Ozone pollution needs to be controlled by a variety of pollutants. [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]