Processes Driving Diurnal and Seasonal Variations of Formaldehyde in an Urban Environment

This study aims to elucidate the physicochemical processes driving the persistently elevated levels of formaldehyde (HCHO) in Shanghai, the largest megacity in China. Field measurements combined with statistical analyses revealed that primary sources remained consistently prominent throughout the year, while secondary production exhibited pronounced seasonal variation. A process-level box model was employed to deconvolute the contributions of secondary production pathways to the temporal variability of HCHO. A sequence of physical removal schemes was integrated into the model framework to resolve the overestimation of HCHO across all seasons in the base configuration. Observation-constrained dilution and entrainment substantially reduced simulated HCHO levels, but failed to capture the observed diurnal patterns. Incorporating reactive uptake onto aerosol liquid water effectively matched the observed secondary production component during colder months, but remained insufficient to fully address the overestimation in summer. Further analysis identified isoprene photochemistry as the major secondary source of summertime HCHO, with its yield highly sensitive to subppb levels of nitric oxides (NO), where the recently discovered isoprene peroxy interconversion competitively regulates the carbon flow that leads to HCHO. Scaling NO measurements notably improved model performance, underscoring the importance of subppb NO data and refined chemical mechanisms in representing HCHO dynamics in urban centers with steadily improved air quality.