The contribution of relative humidity (RH) on Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) was investigated based on in-situ aircraft and ground observations of aerosol and RH from 2007 to 2010 in Beijing, China. The profiles of dry aerosol particle number and size distributions with diameter ranging from 0.1 to 3.0 μm as well as RH were observed during aircraft measurement. The calculated AOD based on aircraft observed dry aerosol particles is lower than MODIS AOD under high RH, but consistent with MODIS AOD after accounting for the aerosol hygroscopic growth effect. A comparison of ground PM2.5 and MODIS AOD further shows that the ratio of PM2.5 concentration to AOD decreases with RH. These results collectively suggest that a strong aerosol hygroscopic growth effect on AOD. The averaged contribution of RH (hygroscopic growth) on AOD are estimated as 37% and 22% respectively based on in-situ aircraft and ground measurements, and hence aerosol concentration would be overestimated if the aerosol hygroscopic growth effect is neglected. As a result, it should be cautious with the applications of AOD in studying the complex relationships between aerosol and its climatic effects.
