Aerosol characteristics and radiative impacts over the Arabian sea during the intermonsoon season: results from ARMEX field campaign

Abstract

During the second phase of the Arabian Sea Monsoon Experiment (ARMEX-II), extensive measurements of spectral aerosol optical depth, mass concentration, and mass size distribution of ambient aerosols as well as mass concentration of aerosol black carbon (BC) were made onboard a research vessel during the intermonsoon period (i.e., when the monsoon winds are in transition from northeasterlies to westerlies/southwesterlies) over the Arabian Sea (AS) adjoining the Indian Peninsula. Simultaneous measurements of spectral aerosol optical depths (AODs) were made at different regions over the adjoining Indian landmass. Mean AODs (at 500-nm wavelength) over the ocean (~0.44) were comparable to those over the coastal land (~0.47), but were lower than the values observed over the plateau regions of central Indian Peninsula (~0.61). The aerosol properties were found to respond distinctly with respect to change in the trajectories, with higher optical depths and flatter AOD spectra associated with trajectories indicating advection from west Asia, and northwest and west-coastal India. On average, BC constituted only ~2.2% to total aerosol mass compared to the climatological values of 6% over the coastal land during the same season. These data are used to characterize the physical properties of aerosols and to assess the resulting short-wave direct aerosol forcing. The mean values were -27 W m^-2 at the surface and -12 W m^-2 at the top of the atmosphere (TOA), resulting in a net atmospheric forcing of +15 W m^-2. The forcing also depended on the region from where the advection predominates. The surface and atmospheric forcing were in the range -40 to -57 W m^-2 and +27 to +39 W m^-2, respectively, corresponding to advection from the west Asian and western coastal India where they were as low as -19 and +10 W m^-2, respectively, when the advection was mainly from the Bay of Bengal and from central/peninsular India. In all these cases, the net atmospheric forcing (heating) efficiency was lower than the values reported for northern Indian Ocean during northern winter, which is attributed to the reduced BC mass fraction.