Abstract. Twelve months of measurements collected during the Two-Column Aerosol Project field campaign over Cape Cod, Massachusetts, which started in the summer of 2012, were used to investigate aerosol physical, optical, and chemical properties, and their influence on the dependence of cloud development on thermodynamic (lower tropospheric stability, LTS) conditions. Relationships between aerosol loading and cloud properties under different dominant air-mass conditions and the magnitude of the first indirect effect (FIE), as well as the sensitivity of the FIE to different aerosol compositions, are examined. The seasonal variation in aerosol number concentration (Na) was not consistent with variations in aerosol optical properties (scattering coefficient, σs, and columnar aerosol optical depth), which suggests that a greater number of smaller particles with less optical sensitivity were present. Strong surface winds generally resulted in smaller σs and a smaller contribution of fine particles to the total scattering extinction, but resulted in large Na, suggesting that strong surface winds transported more aerosols with small particle sizes and less optical sensitivity to the site. The large contribution of organics to small particle sizes was observed which decreased during the particle growth period. For low aerosol loading conditions, the liquid water path (LWP) and droplet effective radius (DER) significantly increase with increasing LTS, but for high aerosol loading conditions, LWP and DER changed little, indicating that aerosols significantly weaken the dependence of cloud development on LTS. The reduction in LWP and DER from low to high aerosol loading conditions was greater in stable environments, suggesing that clouds in a stable conditions are more influenced by aerosol perturbations than those in more unstable conditions. High aerosol loading weakened the increase in DER as LWP increased and strengthened the increase in COD with increasing LWP, resulting in the changes in the pattern of cloud properties each other. Under both continental and marine air-mass conditions, high aerosol loading can significantly made the shift in COD towards larger values, and in LWP and DER towards smaller values, and significantly narrowed the distribution of LWP and DER. Magnitudes of the FIE estimated under continental air-mass condition ranged from 0.07 ± 0.03 to 0.26 ± 0.09 with a mean value of 0.16 ± 0.03 and showed an increase trend as LWP increased. The calculated FIE values for aerosols with a low mass of organics dominated cases are larger than that for aerosols with a high mass of organics dominated cases, implying that clouds over regions dominated by aerosol particles containing mostly inorganics are more susceptible to aerosol perturbations, resulting in larger climate forcing, than clouds over regions dominated by aerosol particles containing mainly organics.
Aerosol Properties and Their Influences on Low Warm Clouds during the Two-Column Aerosol Project
