HCHO and NO2 profile characteristics under different synoptic patterns in Shanghai, China

Large-scale synoptic patterns significantly affect meteorological conditions and air pollution, yet their impacts on the vertical distribution of formaldehyde (HCHO) and nitrogen dioxide (NO₂) have been little studied. From 1 June 2020 to 31 December 2021, Multi-AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) was used to observe NO₂ and HCHO vertical profiles in three typical environments of Shanghai, China, representing urban, suburban and coastal rural environments, respectively. HCHO level is the highest at suburban site, NO₂ is the highest at urban site. HCHO is mainly distributed between 0 and 1 km in altitude, and NO₂ is concentrated near the ground. The ratio of HCHO to NO₂ is used to identify ozone formation regimes, ozone sensitivities vary with environmental area, season and altitude. The principal component analysis in the T-mode approach and typhoon “In-Fa” case is applied to analyze the effects of synoptic patterns on HCHO and NO₂ vertically. HCHO concentrations show a pattern of low-pressure type > uniform-pressure type > high-pressure type at each altitude layer, while NO₂ concentrations follow the opposite pattern. Meteorological factors (especially radiation, temperature, relative humidity, cloud cover and wind), external transport and initial emissions contribute to the differences in HCHO and NO₂ levels across synoptic types. The “In-Fa” case shows how this special synoptic pattern elevates HCHO and NO₂ levels by improving meteorological conditions, boosting biogenic precursors and shifting air mass directions. This study assesses the impacts of synoptic patterns on HCHO and NO₂ vertical distribution in Shanghai, offering insights into understanding causes of pollution.