Analysis of influential factors for the relationship between PM_2.5 and AOD in Beijing

Abstract. The relationship between aerosol optical depth (AOD) and PM_2.5 is often investigated in order to obtain surface PM_2.5 from satellite observation of AOD with a broad area coverage. However, various factors could affect the AOD–PM_2.5 regressions. Using both ground and satellite observations in Beijing from 2011 to 2015, this study analyzes the influential factors including the aerosol type, relative humidity (RH), planetary boundary layer height (PBLH), wind speed and direction, and the vertical structure of aerosol distribution. The ratio of PM_2.5 to AOD, which is defined as η, and the square of their correlation coefficient (R²) have been examined. It shows that η varies from 54.32 to 183.14, 87.32 to 104.79, 95.13 to 163.52, and 1.23 to 235.08 µg m⁻³ with aerosol type in spring, summer, fall, and winter, respectively. η is smaller for scattering-dominant aerosols than for absorbing-dominant aerosols, and smaller for coarse-mode aerosols than for fine-mode aerosols. Both RH and PBLH affect the η value significantly. The higher the RH, the smaller the η, and the higher the PBLH, the smaller the η. For AOD and PM_2.5 data with the correction of RH and PBLH compared to those without, R² of monthly averaged PM_2.5 and AOD at 14:00 LT increases from 0.63 to 0.76, and R² of multi-year averaged PM_2.5 and AOD by time of day increases from 0.01 to 0.93, 0.24 to 0.84, 0.85 to 0.91, and 0.84 to 0.93 in four seasons respectively. Wind direction is a key factor for the transport and spatial–temporal distribution of aerosols originated from different sources with distinctive physicochemical characteristics. Similar to the variation in AOD and PM_2.5, η also decreases with the increasing surface wind speed, indicating that the contribution of surface PM_2.5 concentrations to AOD decreases with surface wind speed. The vertical structure of aerosol exhibits a remarkable change with seasons, with most particles concentrated within about 500 m in summer and within 150 m in winter. Compared to the AOD of the whole atmosphere, AOD below 500 m has a better correlation with PM_2.5, for which R² is 0.77. This study suggests that all the above influential factors should be considered when we investigate the AOD–PM_2.5 relationships.