Variability of surface aerosol properties at an urban site in Beijing based on two years of in-situ measurements

Abstract

Aerosol optical properties are important indices for monitoring atmospheric aerosol pollution. In this study, we report the seasonal and diurnal variability of major aerosol optical parameters, including scattering coefficient (σs), absorbing coefficient (σa), single scattering albedo (SSA), together with PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) and PM10 (particulate matter with aerodynamic diameter ≤ 10 μm) mass concentrations measured at an urban site in Beijing from September 2017 to August 2019. The relationships between these aerosol properties with meteorological conditions, in particular, the mixing layer height (MLH), wind speed, and wind direction, are further investigated. Results show that the annual average values of PM2.5 and PM10 mass concentrations, σs, and σa during the study period are 54 ± 54 μg/m3, 100 ± 72 μg/m3, 170 ± 180 Mm−1, 17 ± 15 Mm−1 respectively, which are much lower than those previously reported, indicating the effectiveness of strict pollution control strategies implemented in recent years. SSA is calculated with σs and σa, whose annual average value is 0.88 ± 0.07. Seasonally, σa exhibits dual peaks except in summer, PM2.5 and PM10 concentrations are the highest value in the spring, whereas σs has the highest value in the summer with a secondary peak in the spring. The diurnal cycle of σa is highly anti-correlated with that of the MLH. For σs, PM2.5 and PM10, their diurnal cycles often peak around noon and are in phase with the MLH in the spring and summer, which may be associated with the photochemical production of secondary aerosols. σs, σa, PM10, and PM2.5 concentrations are inversely related to wind speed, but PM10 starts to increase as wind speed exceeds 4 m/s, possibly caused by dust and catkins. The increase in wind speed also weakens the aerosol-MLH relationship. Back trajectory analysis indicates that high aerosol concentrations are mostly associated with southward and westward airmasses.