Spatiotemporal mapping of atmospheric aldehydes over Beijing in summer during 2019-2021 via their source apportionment study

Atmospheric aldehydes have the important impacts on human health and air pollutions, however, their surface concentrations are never captured due to the sparse monitoring works and need to be studied. Here, we tried to map the aldehyde pollutions over Beijing in summer of recent years, via monitoring their levels in one urban site, simulating their chemical formations based on box chemical model, and developing their prediction formulae by using CO and O-3 as explanatory variables based on statistical technique. Our monitoring showed that HCHO and CH3CHO were the dominant components respectively at the level of 4.5-27.2 and 0.9-10.78 ppb, and explained 86.9% of total aldehydes detected. Their mixing ratios to CO were higher by a factor of 2.5-4.0 than the corresponding emission ratios from combustion sources, which implied a big contribution of photochemistry to secondary aldehyde formation. Hence, we used box model coupled with MCM to simulate the evolutions of HCHO, CH3CHO and O-3 from the emission plumes of this city, and found the production ratio of 0.14 for HCHO: O-3 and 0.07 for CH3CHO:O-3 on daytime average. Afterwards, we developed the prediction formulae separately for the two aldehydes based on multiple linear regression by using CO and O-3 as explanatory variables, and obtained the regression coefficients of similar to 0.15 (HCHO) and similar to 0.04 (CH3CHO) with O-3, which were well consistent with the corresponding simulated ratios by box model. Thus, these prediction formulae were applied to map the spatiotemporal concentrations of the two aldehydes in summer over this city, combining CO and O-3 monitoring data. In result, the area means of HCHO and CH3CHO over Beijing (except north rural areas) reached 11.8 and 3.6 ppb in 2021 summer, decreasing by 11.9% and 9.3% relative to 2019 respectively; and their spatial difference became smaller apparently within three years. Such spatiotemporal variations were approved by HCHO satellite observations. Overall, the aldehyde pollutions in this city are still severe and deserve more studies in future.