PM mass concentrations in comparison with aerosol optical depths over the Arabian Sea and Indian Ocean during winter monsoon

Abstract

Abstract An analysis of PM2.5 mass concentrations and 0.5 μm aerosol optical depths (AODs) during the Northeast winter monsoon seasons of 1996–2000 is performed and intercompared. AODs are found to show diurnal variations over Coastal India (CI) (west coast) while they are relatively smooth over the Arabian Sea (AS) (5–20°N) and tropical Indian Ocean (TIO) (5°N–20°S). PM2.5, PM10 and total mass concentrations show less variations in a day over these oceanic regions. Columnar AODs are found to increase with an increase in the marine boundary layer aerosol concentrations over CI and AS while an opposite trend is seen over TIO. The yearly-mean AODs and mass concentrations are found to increase over CI and AS, over TIO the mass concentrations increased while the AODs decreased during 1996–2000. It is found from the 7-days air back trajectory analyses that at different altitudes air masses can originate from different source regions leading to changes in chemical, physical and optical characteristics of the aerosol between the surface and column. The differences in the surface and columnar measurements could also occur due to changes in the meteorological conditions, wind patterns, in addition to changes in production and subsequently the transport of aerosols. Least-squares fits to the above intercomparison resulted in intercepts of 0.24 and 0.22 over CI and AS indicating that the background AODs over these oceanic regions are higher. An examination of the daily-mean wind speeds and PM2.5 mass concentrations yielded an index of wind dependence of 0.04 for AS and 0.07 for TIO. The background PM2.5 mass concentrations are also found to be high at 36 and 25 μg m−3 over AS and TIO, respectively, indicating a stronger influence from the continent. Frequency distribution figures show that 28% of the PM2.5 values over CI lie in the 60–80 μg m−3 range. Over AS the dominant mode of distribution is 40–60 μg m−3 with a peak value of 42%. Over TIO PM2.5 values are found to peak in the lower mass bin of 0–20 μg m−3 at 33%. A latitudinal gradient is seen in the peak bin value of PM2.5 as the ship moves away from the coast. About half the days over CI and 20% over AS and TIO, PM2.5 values are found unhealthy indicating the influence of anthropogenic pollution and long-range transport of pollutants from the surrounding continental locations across these oceanic regimes. AODs are found to peak in the 0.2–0.4 bin at 52% over CI and 47% over AS. Over AS 32% of the AODs are found to be α values are found to peak in the 1.5–2 range over CI at 55% while they peak in the lower range of 1–1.5 at 49% and 26% over AS and TIO, respectively. Over the Pacific and Atlantic Oceans in more than 90% of the cases α was ⩽1.5, indicating that the amount of smaller and larger particles are higher over the Indian Ocean when compared to the Pacific and Atlantic. The spread in PM2.5 and AOD indicates that it is a challenging task to obtain a good relation without additional inputs on the vertical distribution of aerosols as varied kinds of aerosols from different source regions contribute at different heights over these oceanic regions.