Aerosol optical depth studies during INDOEX: comparison of the spectral features over coastal India with the pristine southern hemispheric environment over Mauritius

Abstract

Aerosol spectral optical depths, estimated using a ground-based network of multi wavelength radiometers (MWR) along the west coast of India [Trivandrum (TVM; 8.5°N, 77°E), and Minicoy (MCY; 8.3°N, 73.04°E)] and the pristine southern hemispheric environment at Mauritius (MRU; 20.26°S, 57.54°E) during the period January to June 1998 along with those obtained over the Arabian Sea and Indian Ocean during the INDOEX FFP-98 cruise (SK133) of ORV Sagar Kanya, are used to study the inter-hemispheric features of aerosols. Results indicate that there is a significant hemispherical difference for aerosol spectral optical depth (AOD) at shorter wavelengths (λ≤ 650 nm), while at the longer wavelengths (λ > 650 nm), AOD does not show any appreciable variation with location. The spectral variation of AOD at TVM and MCY (for March 1998) depicted a similar pattern with the AOD values between 0.5 and 0.6 at shorter wavelengths and between 0.2 and 0.4 at longer wavelengths. In contrast to this, the AOD at MRU are very low, lying in the range 0.1 to 0.2 in the shorter wavelengths, whereas at the longer wavelengths the AOD values are more or less comparable (in the range 0.2 to 0.4) with the northern hemispheric stations. The cruise data clearly showed that the transition occurs generally across the ITCZ. The increased AOD at shorter wavelengths in the northern hemisphere indicates higher concentration of sub-micron aerosols in these environments arising mainly due to anthropogenic activities, while the AOD at the longer wavelengths is attributed mainly to be of marine origin. In the post-cruise period, the spectral optical depths showed a gradual increase from March to June at MRU, while at TVM, the pattern followed more or less the climatological mean. By May the AOD at shorter wavelengths decreased at TVM (due to increased rainfall) and by June, the AOD at TVM are very much comparable with those seen at MRU, indicating a dominating marine aerosol influence at both these locations. The implications are discussed.