Mesoscale variations of the chemical composition of submicron aerosols and its influence on the cloud condensation nuclei activation

Mesoscale variation of the chemical composition of submicron aerosol concentrations and their influence on cloud condensation nuclei (CCN) activation have been examined over a tropical coastal location in peninsular India during the winter season. The land-sea breeze circulation, boundary layer evolution, and photochemical processes significantly influence the diurnal variation of the aerosol chemical composition over the region. Broadly, the organic aerosols (mean similar to 37.50 +/- 27.00 mu gm(-3)) dominated the submicron mass loading with a mass fraction (MF) of 0.66, followed by sulfate (similar to 10.92 +/- 6.13 mu gm(-3); MF similar to 0.19), ammonium (3.46 +/- 1.71 mu g m(-3); MF similar to 0.06) and a small contribution from nitrate (1.7 +/- 1.5 mu gm(-3); MF similar to 0.03). The average mass concentration of organic carbon (OC) is 8.84 +/- 6.42 mu gm(-3) with a mean organics/OC ratio of similar to 3, indicating the presence of aged, highly oxidized organics. The diurnal variation of all the aerosol species, except sulfate, depicted daytime-low and nighttime-high. The photochemical formation of sulfate aerosols overwhelms the reduction in concentration due to boundary layer mixing and sea-land breeze circulation during the daytime. The decrease in the organics/sulfate ratio during the daytime has significant implications on the CCN concentration and its activation properties. The average CCN number concentration and activation ratio (ratio of CCN to total aerosol number concentration) at 0.38% supersaturation during the study period are similar to 3756 +/- 1609 cm(-3) and 0.52 +/- 0.17, respectively. The diurnal variation of CCN concentrations and activation ratio is distinctly different. The change in the mode diameter of the number size distribution between sea-land breeze was found to be less (similar to 8 nm) compared to the large variation in chemical composition. This suggested the importance of mesoscale variations of aerosols, especially the contrasting diurnal variation of organics and sulfate aerosols in CCN activation.