Long-term observation of aerosol–cloud relationships in the Mid-Atlantic of the United States

Abstract. Long-term ground-based observations (2006 to 2010) of aerosol and cloud properties derived from passive radiometric sensors deployed at an atmospheric measurement field station in the Baltimore–Washington corridor operated by Howard University were used to examine aerosol indirect effect on cloud optical depth (COD), liquid water path (LWP), cloud droplets effective radius (R_e) and cloud droplets number concentration (N_d). A higher frequency of clouds with large COD (> 20) and small R_e (< 7 m) was found during summer of 2006 and 2007 along with higher frequency of abundant aerosol loading. The five-year data are screened for summer months only and are separated into clean and polluted cases based on aerosol particulate matter with aerodynamic diameter ≤ 2.5 m (PM_2.5) value. Evidence of aerosol indirect effect is found where for polluted cases the mean and median values of COD and N_d distributions were elevated while the mean and median values of R_e were decreased. Further reinforcing this conclusion is the result that the mean and median values of LWP distributions did not show prominent difference between clean and polluted cases, this implies that differences between the two cases of influential factors on cloud properties were relatively controlled. Moreover aerosol indirect effects were found insignificant when LWP was small but significant when LWP was large through the analysis of sensitivity of N_d to LWP under different aerosol loading and the measurements of aerosol size distribution.