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

摘要

In Part 1 of this two-part paper, we present the results of extensive andcollocated measurements of the columnar and near-surface (in the well mixedregion) properties of atmospheric aerosol particles at a tropical coastallocation, Trivandrum (8.55° N; 76.97° E), located close to thesouthwest tip of Indian peninsula. These are used to evolve average,climatological pictures of the optical and microphysical properties and todelineate the distinct changes associated with the contrasting monsoonseasons as well as the transition from one season to the other. Ourobservations show a dramatic change in the columnar aerosol optical depth(AOD) spectra, being steep during winter monsoon season (WMS, months ofDecember through March) and becoming quite flat during summer monsoon season(SMS, June through September). The derived ?ngstr?m exponent(α) decreases from a mean value of 1.1±0.03 in WMS to 0.32±0.02in SMS, signifying a change in columnar aerosol size spectrumfrom an accumulation mode dominance in WMS to a coarse mode dominance inSMS. The composite aerosols near the surface follow suit with the share ofthe accumulation mode to the total mass concentration decreasing from ~70% to34% from WMS to SMS. The overall mass burden also decreasesin tandem. The changes in α are well correlated to those in theaccumulation fraction of the mass concentration. Long-term measurements ofthe concentration of aerosol black carbon (BC), show prominent annualvariations, with its mean value decreasing from as high as 6 μg m−3in WMS to 2 μg m−3 in SMS. Correspondingly, its massmixing ratio to the composite aerosols comes down from 11% to 4%.The changes in AOD and α are significantly positively correlated tothose of BC concentration. The columnar properties are, in general wellassociated with the features near the surface. The implications of thesechanges to the optical properties and single scattering albedo and theresulting impact on direct radiative forcing are examined in the companionpaper (Part 2).