Characteristics of sub-micron aerosols above the urban canopy in Beijing during warm seasons

To understand the characteristics of atmospheric pollution above the urban canopy in warm seasons, the characteristics of sub-micron aerosol (PM₁) was studied based on high-altitude observations at the Beijing 325 m meteorological tower. The PM₁ at 260 m was 34, 29 and 21 μg m⁻³ in May 2015, June 2015, and June 2017, respectively, indicating a reduction in PM₁ pollution above the urban canopy. Meanwhile, an overall decrease was also observed in the concentrations of all PM₁ chemical species (excluding Chl and BC) and organic aerosol (OA) factors. Previous instances of heavy haze in Beijing often coincided with high humidity and stagnant weather conditions. However, the heightened pollution episodes in June 2017 were accompanied by high wind speeds and low relative humidity. Compared to May 2015, the contribution of secondary components to PM₁ in June 2017 was more prominent, with the total proportion of SNA (sulfate, nitrate, and ammonium) and more-oxidized oxygenated OA (MO-OOA) to PM₁ increased by approximately 10 %. Secondary species of NH₄NO₃, (NH₄)₂SO₄, and MO-OOA, as well as black carbon, collectively contributed the vast majority of aerosol extinction coefficient (b_ext), with the four species contributing a total of ≥96 % to b_ext at 260 m. Hydrocarbon-like OA, cooking OA, and less-oxidized oxygenated OA have undergone significant reductions, so continued emphasis on controlling local sources to reduce these three aerosol species and addressing regional sources to further mitigate overall aerosol species is imperative. In lower pollution situation, the diurnal variation of PM was smoother, and its pollution sources were more regionally uniform, which might be attributed to the reduced diversity and complexity in the physical and chemical processes in air pollution.