Morphology and state of mixture of atmospheric soot aggregates during the winter season over Southern Asia-a quantitative approach

The atmospheric brown cloud phenomena characterized by a high content of soot and a large impact on the solar radiative heating especially affects the tropical Indian Ocean during the winter season. The present study focuses on morphological characteristics and state of mixture of soot aggregates during the winter season over India. Given are quantitative measures of size, morphology and texture on aggregates collected in air at two different sites: Sinhagad near Pune in India and Hanimaadhoo in Maldives. For the latter site two different synoptic patterns prevailed: advection of air from the Arabian region and from the Indian subcontinent, respectively. Aggregates collected at Sinhagad, were associated with open branched structures, characteristic of fresh emission and diameters between 220 and 460 nm. The Hanimaadhoo aggregates were associated with aged closed structures, smaller sizes (130-360 nm) and frequently contained inorganic inclusions. Those arriving from the Indian subcontinent were characterized by the presence of an additional organic layer that covered the aggregate structure. These organic coatings might be a reasonable explanation of the low average wash-out ratios of soot two to seven times lower than that of nss-SO(4)2- that have been reported for air flow arriving at Hanimaadhoo from the Indian subcontinent in winter.