Analysis of influential factors for the relationship between PM2.5 and AOD in Beijing

摘要

The relationship between aerosol optical depth (AOD) and PM isoften investigated in order to obtain surface PM from satelliteobservation of AOD with a broad area coverage. However, various factors couldaffect the AOD–PM regressions. Using both ground and satelliteobservations in Beijing from 2011 to 2015, this study analyzes theinfluential factors including the aerosol type, relative humidity (RH),planetary boundary layer height (PBLH), wind speed and direction, and thevertical structure of aerosol distribution. The ratio of PM to AOD,which is defined as , and the square of their correlation coefficient() have been examined. It shows that varies from 54.32 to183.14, 87.32 to 104.79, 95.13 to 163.52, and 1.23 to 235.08 µg mwith aerosol type in spring, summer, fall, and winter, respectively. issmaller for scattering-dominant aerosols than for absorbing-dominantaerosols, and smaller for coarse-mode aerosols than for fine-mode aerosols.Both RH and PBLH affect the value significantly. The higher the RH,the smaller the , and the higher the PBLH, the smaller the .For AOD and PM data with the correction of RH and PBLH compared tothose without, of monthly averaged PM and AOD at 14:00 LTincreases from 0.63 to 0.76, and of multi-year averaged PMand AOD by time of day increases from 0.01 to 0.93, 0.24 to 0.84, 0.85 to0.91, and 0.84 to 0.93 in four seasons respectively. Wind direction is a keyfactor for the transport and spatial–temporal distribution of aerosolsoriginated from different sources with distinctive physicochemicalcharacteristics. Similar to the variation in AOD and PM, alsodecreases with the increasing surface wind speed, indicating that thecontribution of surface PM concentrations to AOD decreases withsurface wind speed. The vertical structure of aerosol exhibits a remarkablechange with seasons, with most particles concentrated within about 500 m insummer and within 150 m in winter. Compared to the AOD of the wholeatmosphere, AOD below 500 m has a better correlation with PM, forwhich is 0.77. This study suggests that all the above influentialfactors should be considered when we investigate the AOD–PMrelationships.