OMI-observed HCHO in Shanghai, China during 2010–2019 and ozone sensitivity inferred by improved HCHO / NO<sub>2</sub> ratio

Abstract

Abstract. In recent years, satellite remote sensing has been increasingly used in the long-term observation of ozone (O3) precursors and its formation regime. In this work, formaldehyde (HCHO) data from Ozone Monitoring Instrument (OMI) were used to analyse the temporal and spatial distribution of HCHO vertical column densities (VCDs) in Shanghai from 2010 to 2019. HCHO VCDs exhibited the highest value in summer and the lowest in winter, the high-VCD concentrated in western Shanghai. Temperature largely influence HCHO by affecting the biogenic emissions and photochemical reactions, and industry was the major anthropogenic source. The satellite observed formaldehyde to nitrogen dioxide ratio (FNRSAT) reflects that the O3 formation regime had significant seasonal characteristics and gradually manifested as transitional ozone formation regime dominated in Shanghai. The uneven distribution in space was mainly reflected as the higher FNRSAT and surface O3 concentration in rural area. To compensate the shortcoming of FNRSAT that it can only characterize O3 formation around satellite overpass time, correction of FNRSAT was implemented with hourly surface FNR and O3 data. After correction, O3 formation regime showed the trend moving towards VOC-limited in both time and space, and regime indicated by FNRSAT can better reflect O3 formation for a day. This study can help us better understand HCHO characteristics and O3 formation regime in Shanghai, and also provide a method to improve FNRSAT for characterizing O3 formation in a day, which will be significant for developing O3 prevention and control strategies.