An improved TROPOMI tropospheric HCHO retrieval over China.

We present the improved retrieval of TROPOspheric Monitoring Instrument (TROPOMI) tropospheric 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. HCHO SCDs are retrieved using the basic optical differential spectroscopy (BOAS) technique, while AMFs are calculated with a priori HCHO profile from a higher resolution regional chemistry transport model. Compared to the operational product, the new TROPOMI HCHO retrieval shows better agreement with the ground based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in China. The operational product in general overestimates HCHO VCDs by 14.01 %, while the improved HCHO only shows an underestimation of 3.67 %. The improvements are mainly related to the AMF calculation with higher resolution a priori profile (61.11 %), while the SCD retrieval only shows a minor effect of 0.15 %. The improved HCHO is also used to investigate the spatial-temporal characteristic of HCHO over China. The result shows that HCHO VCDs reach maximum in summer and minimum in winter. High HCHO VCDs mainly located over populated areas, i.e., Sichuan Basin, Central and Eastern China, indicating a significant contribution of anthropogenic emissions. The result indicates the improved TROPOMI HCHO is more suitable for the analysis of regional and city scale pollution in China. [ABSTRACT FROM AUTHOR]