Search

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 (σ s), absorbing coefficient (σ a), single scattering albedo (SSA), together with PM 2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) and PM 10 (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 PM 2.5 and PM 10 mass concentrations, σ s , and σ a 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 σ s and σ a , whose annual average value is 0.88 ± 0.07. Seasonally, σ a exhibits dual peaks except in summer, PM 2.5 and PM 10 concentrations are the highest value in the spring, whereas σ s has the highest value in the summer with a secondary peak in the spring. The diurnal cycle of σ a is highly anti-correlated with that of the MLH. For σ s , PM 2.5 and PM 10 , 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. σ s , σ a , PM 10 , and PM 2.5 concentrations are inversely related to wind speed, but PM 10 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. • Seasonal and diurnal variability of aerosol properties in Beijing was analyzed. • Aerosol properties follow an inverse relationship with mixing layer height (MLH). • Increase of wind speed tends to weaken the aerosol-MLH inverse relationship. • South and west airmasses correspond to higher aerosol loading in Beijing. [ABSTRACT FROM AUTHOR]

The planetary boundary layer height (PBLH) is critical in affecting the concentration of air pollutants at surface. The relationship between PBLH and surface ozone concentration can be complicated but not entirely clear. In this study, by using observations and WRF-Chem simulations, we explore the relationship between the surface ozone and the PBLH in the summer daytime in Beijing, a major global ozone pollution hotspot. It is found that the surface ozone-PBLH relationship exhibits a three-stage evolution with ozone: first, increasing with PBLH, then keeping steady, and finally falling. The two turning points are around 1000 and 2000 m respectively. This process is likely controlled by the different contributions of chemical production and vertical mixing. Specifically, when PBLH is below ∼1000 m, as PBLH increases, chemical production increases owing to the light enhancement whereas the ozone contribution from vertical mixing remains weak due to the decreased ozone concentration gradient between the surface and the upper layer, resulting in a net gain of surface ozone concentration. When the PBL continues to rise, chemical production contributes less mainly due to the decrease of ozone precursors at the surface, whereas vertical mixing becomes more vigorous that further diluting surface ozone, resulting in a net loss of surface ozone concentration, especially when PBLH reaches above ∼2000 m. Our results contribute to understanding the variability of surface ozone in Beijing area and provide insights for local ozone air quality control. [Display omitted] • Surface ozone first increases with PBLH, then remains stable and finally decreases. • This relationship is jointly determined by vertical mixing and chemical processes. • Vertical mixing of ozone shifts from down to up when PBLH rises. • Chemistry contribution of ozone first increases then decreases when PBLH rises. [ABSTRACT FROM AUTHOR]