Submicron-scale aerosol above the city canopy in Beijing in spring based on in-situ meteorological tower measurements

Abstract

Non-refractory submicron-scale aerosol (NR-PM1) chemical species above the city canopy in Beijing in spring were studied based on observations at the Beijing 325-m meteorological tower in combination with analyses of the meteorological field and mixing layer height (MLH). The parallel sampling showed that the NR-PM1 at 260 m (60 μg/m3) was ~70% of that at ground level. Organic aerosol (Org) was the species with the most significant vertical difference, as its average mass concentration at 260 m was half of that at ground level. In 91% of cases, the Org at ground level was higher than that at 260 m. Sulfate (SO4) and chloride had more regional transport characteristics. Nitrate (NO3) and Org were more enriched at ground level, as the online molar ratio of NO3 to SO4 and the mass ratio of Org to SNA (sulfate–nitrate–ammonium) were higher at ground level. Among all the five NR-PM1 chemical species, ammonium nitrate was the species with the most uniform vertical distribution. Pollution events in spring in urban Beijing were accompanied by a gradual increase in ground RH from 10% to more than 35%. Strong winds from the southwest polluted region could also maintain the NR-PM1 at a high level. The polluted air masses transported from the south accounted for 39%, which was about four times that of the clean air masses from the northwest (11%). Thus, atmospheric circulation in spring is generally conducive to the formation of pollution. The estimated dilution efficiency on NR-PM1 chemical species influenced by boundary layer variation in spring showed that, when the mix layer was elevated the dilution effect at higher altitude was stronger than that at ground level.