# Temporal heterogeneity in aerosol characteristics and the resulting radiative impact at a tropical coastal station – Part 1: Microphysical and optical properties

Moorthy, Krishna K and Babu, Suresh S and Satheesh, SK (2007) Temporal heterogeneity in aerosol characteristics and the resulting radiative impact at a tropical coastal station – Part 1: Microphysical and optical properties. In: Annales Geophysicae, 25 (11). pp. 2293-2308.

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## Abstract

In Part 1 of this two-part paper, we present the results of extensive and collocated measurements of the columnar and near-surface (in the well mixed region) properties of atmospheric aerosol particles at a tropical coastal location, Trivandrum $(8.55^0 N; 76.97^0 E)$, located close to the southwest tip of Indian peninsula. These are used to evolve average, climatological pictures of the optical and microphysical properties and to delineate the distinct changes associated with the contrasting monsoon seasons as well as the transition from one season to the other. Our observations show a dramatic change in the columnar aerosol optical depth (AOD) spectra, being steep during winter monsoon season (WMS, months of December through March) and becoming quite flat during summer monsoon season (SMS, June through September). The derived Angstrom exponent (\alpha) decreases from a mean value of $1.1 \pm 0.03$ in WMS to $0.32 \pm 0.02$ in SMS, signifying a change in columnar aerosol size spectrum from an accumulation mode dominance in WMS to a coarse mode dominance in SMS. The composite aerosols near the surface follow suit with the share of the accumulation mode to the total mass concentration decreasing from \sim 70% to 34% from WMS to SMS. The overall mass burden also decreases in tandem. The changes in \alpha are well correlated to those in the accumulation fraction of the mass concentration. Long-term measurements of the concentration of aerosol black carbon (BC), show prominent annual variations, with its mean value decreasing from as high as $6 \mu gm^{-3}$ in WMS to $2 \mu gm^{-3}$ in SMS. Correspondingly, its mass mixing ratio to the composite aerosols comes down from 11% to 4%. The changes in AOD and \alpha are significantly positively correlated to those of BC concentration. The columnar properties are, in general well associated with the features near the surface. The implications of these changes to the optical properties and single scattering albedo and the resulting impact on direct radiative forcing are examined in the companion paper (Part 2).

Item Type: Journal Article Copyright of this article belongs to European Geosciences Union. Atmospheric composition and structure (Aerosols and particles;Biosphere-atmosphere interactions;Transmission and scattering of radiation). Division of Mechanical Sciences > Centre for Atmospheric & Oceanic Sciences Dipti Chaurasia 11 Aug 2008 19 Sep 2010 04:48 http://eprints.iisc.ac.in/id/eprint/15514