In situ ship cruise measurements of mass concentration and size distribution of aerosols over Bay of Bengal and their radiative impacts

Abstract

Simultaneous measurements of surface level aerosol mass concentrations, their size distribution, and aerosol optical depth (AOD) were made during a ship cruise study conducted over the Bay of Bengal (BoB) between 19 and 28 February 2003, when the prevailing surface level wind flow is predominantly from the continent toward the ocean, using a ten-stage QCM cascade impactor and Microtops Sun photometer. On all cruise days, air parcels at different altitude levels were coming either from west or from northwest directions, crossing a significant portion of the Indian subcontinent before finally reaching over BoB. Average value of surface level aerosol mass concentration is found to be around 50, 37, and 13 µg/m³ for coarse mode (>1 µm), accumulation mode (between 1 µm and 0.1 µm), and nucleation mode (<0.1 µm) particles, respectively. Size distribution of aerosols measured during the cruise showed the presence of four distinct modes, all of which could be fitted using lognormal distribution. Mode radii for the distributions lie in the range of 0.025-0.036 µm for mode 1, between 0.15 and 0.165 for mode 2, between 0.39 and 0.55 for mode 3, and between 2.2 and 3.5 for mode 4. Over the study region, daily mean AOD values at 380 nm were in the range of 0.34 to 0.75 while those at 1020 nm varied from 0.09 to 0.25. The mean value of Angstrom wavelength exponent a is found to be 1.19 ± 0.12. Regression analysis for the scatterplots between AOD values and surface mass concentrations showed good correlation between them over the entire cruise region. Aerosol optical depths, as well as extinction coefficients calculated from surface level aerosol number concentrations, show higher values over northern and coastal areas of BoB. An estimate of aerosol scale height has been made from the ratio of columnar AOD values and surface extinction coefficients. Columnar aerosol size distributions were derived using King's inversion technique, and the results are found to be less sensitive to the choice of aerosol refractive index values used. Independently, aerosol scale height is also estimated by taking the ratio of derived columnar number distribution and measured surface level aerosol number concentrations. AOD values observed over BoB are used in estimating the aerosol radiative forcing over this region. The Average value of the net direct radiative forcing for the region is found to be in the range of -30.6 to -26.6 W/m² at the surface and -8.4 to -10.6 W/m² at the TOA. Magnitude of LW forcing is found to be about 10% of its SW counterpart, at both surface and TOA. Results of a sensitivity test showed that the magnitude of LW aerosol forcing at the surface level decreases with the increase in column water vapor.