Radiative properties of Bay of Bengal aerosols: spatial distinctiveness and source impacts

Abstract

Simultaneous and collocated measurements of spectrally resolved scattering and absorption coefficients and mass concentration of near-surface composite aerosols in the marine atmosphere over the Bay of Bengal (BOB), along with incoming shortwave (0.3–3 μm) global solar radiation and columnar spectral aerosol optical depths (AOD), were made on a research cruise during the winter phase of the Integrated Campaign for Aerosols, Gases and Radiation Budget (W-ICARB). The aerosol radiative properties revealed distinct spatial features associated with the contrasting outflows from Indo-Gangetic Plain (IGP) and East Asia. Both scattering and absorption coefficients depicted very high values (>200 and >15 Mm⁻¹) over the northwestern and southeastern BOB and extremely low values (<50 and <10 Mm⁻¹) over the central BOB. The mean value of the total scattering coefficient at 550 nm (∼123.7 ± 85.3 Mm⁻¹) over the entire BOB during winter was higher than the mean values (∼94 ± 47 Mm⁻¹) reported for the premonsoon season. While SSA at 550 nm showed very low values (<0.8) over a very large region in the central BOB and moderately low values over the southern BOB (∼0.85–0.9), the columnar AOD varied from the least values of ∼0.1 over the northeastern BOB to the highest values of ∼0.8 over the northwestern BOB. While significant cooling was observed at the top of the atmosphere and surface over the northwestern BOB, the atmospheric forcing was found to be significantly high (∼15 W m⁻²) over the southern BOB, where the aerosol radiative forcing efficiency (ARFE) at the surface was also found to be high. Examination of the Moderate Resolution Imaging Spectroradiometer (MODIS)-derived fire count along with the advection pathways revealed a strong contribution from the emissions of biomass smoke from East Asia, which might be contributing to the enhanced aerosol induced warming over the southern BOB. However, the ARFE at the surface was low over the northwestern BOB, where the advection from IGP was predominant.