Decadal variations in natural and anthropogenic aerosol optical depth over the Bay of Bengal: the influence of pollutants from Indo-GangeticPlain

Perennial increase in atmospheric pollution over the Bay of Bengal (BoB) and South China Sea is reported due to increase in human population and industrial activity in South and Southeast Asia. Based on total aerosol optical depth (AOD) derived from MODIS (moderate resolution imaging resolution imaging spectroradiometer), natural and anthropogenic fractions were derived. The seasonality and spatial variability in rate of increase in total, natural, and anthropogenic AOD fractions were examined over the BoB using data collected between 2001 and 2019. Both total and anthropogenic AOD displayed statistically significant rate of increase in the northwest BoB (NWB) and western coastal BoB (WCB) regions during 2001 to 2019 whereas the long-term changes are insignificant in the other regions of BoB. Significant increase in AOD in the NWB and WCB regions is mainly contributed by dominant outflow of anthropogenic emissions from Indo-Gangetic Plain (IGP) area of Indian subcontinent. The magnitude of AOD decreased by half from northern BoB to equatorial region due to increase in distance from the source region. The contribution of anthropogenic AOD was >70% to total AOD with higher contribution during winter and lower during summer. The rate of increase in both total and anthropogenic AOD was close to 0.104 and 0.099 per decade in the NWB and 0.069 and 0.059 per decade in the WCB region between 2001 and 2019. The rate of increase in total and anthropogenic AOD decreased from 2001-2009 (0.164 and 0.115 per decade respectively) to 2010-2019 (0.068 and 0.076 per decade respectively) in the NWB region. Significant increase in anthropogenic AOD by 50 and 30% was observed during El Niño and La Niña periods respectively than normal year in both northwest BoB (NWB) and western coastal (WCB) regions due to change in strength and direction of winds. Although some fraction of anthropogenic AOD is found over the entire BoB, significant rate of increase in anthropogenic AOD is found only about 23% of the area of BoB than hitherto reported as entire BoB. The impact of atmospheric deposition of anthropogenic aerosols on biogeochemical processes, such as primary production and ocean acidification, needs further evaluation.

Investigation of Black Carbon characteristics over southern ocean: Contribution of fossil fuel and biomass burning

Black Carbon (BC) is an absorbing aerosol which has significant impact on the Earth - Atmosphere radiation balance and hence on climate. The variation of BC mass concentration and contribution of fossil fuel and biomass burning have been investigated over the Indian ocean sector of the Southern Ocean during austral summer. BC mass was in the range of 300-500 ng m^-3 between 23.3^oS to 24.5^oS followed by decrease in BC to 150 ng m^-3 as moving to higher southern latitudes till 41^oS latitude. An increase in BC mass from 250 to 450 ng m^-3 was found between 41 and 50^oS due to trap of air masses by cyclonic wind and transport of aerosols from the southern part of African and eastern Madagascar regions. Higher BC concentration (250-350 ng m^-3) was observed in the latitude range of 57-60^oS which can be attributed to convergence of north-westerly and south-easterly winds. The dominant contributor to BC was fossil fuel, which was > 80% during half of the total observations, while > 20% biomass burning contributed to one fifth of observations. The coastal Antarctic region showed higher BC mass concentration with mixed type of contributions of biomass and fossil fuel. Such accumulation of BC near the Antarctic coast can have a crucial impact on the sea-ice albedo which significantly affect the Antarctic climate system locally and global climate in general.

Investigation of PM2.5 mass concentration over India using a regional climate model

Seasonal variation of PM_2.5 (Particulate Matter <2.5 μm) mass concentration simulated from WRF-Chem (Weather Research and Forecasting coupled with Chemistry) over Indian sub-continent are studied. The simulated PM_2.5 are also compared with the observations during winter, pre-monsoon, monsoon and post-monsoon seasons of 2008. Higher value of simulated PM_2.5 is observed during winter followed by post-monsoon, while lower values are found during monsoon. Indo-Gangetic Basin (IGB) exhibits high amount of PM_2.5 (60- 200 μg m^-3) throughout the year. The percentage differences between model simulated and observed PM_2.5 are found higher (40- 60%) during winter, while lower (< 30%) during pre-monsoon and monsoon over most of the study locations. The weighted correlation coefficient between model simulated and observed PM_2.5 is 0.81 at the significance of 98%. Associated RMSE (Root Mean Square Error) is 0.91 μg m^-3. Large variability in vertically distributed PM_2.5 are also found during pre-monsoon and monsoon. The study reveals that, model is able to capture the variabilities in spatial, seasonal and vertical distributions of PM_2.5 over Indian region, however significant bias is observed in the model. PM_2.5 mass concentrations are highest over West Bengal (82± 33 μg m^-3) and the lowest in Jammu & Kashmir (14± 11 μg m^-3). Annual mean of simulated PM_2.5 mass over the Indian region is found to be 35± 9 μg m^-3. Higher values of PM_2.5 are found over the states, where the reported respiratory disorders are high. WRF-Chem simulated PM_2.5 mass concentration gives a clear perspective of seasonal and spatial distribution of fine aerosols over the Indian region. The outcomes of the study have significant impacts on environment, human health and climate.

Winter time chemical characteristics of aerosols over the Bay of Bengal: continental influence

As part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) conducted under the Geosphere Biosphere Programme of Indian Space Research Organisation, ship-based aerosol sampling was carried out over the marine environment of Bay of Bengal (BoB) during the northern winter months of December 2008 to January 2009. About 101 aerosol samples were collected, covering the region from 3.4° to 21° N latitude and 76° to 98° E longitude-the largest area covered-including the south east (SE) BoB for the first time. These samples were subjected to gravimetric and chemical analysis and the total aerosol loading as well the mass concentration of the ionic species namely F(-), Cl(-), Br(-), NO2 (-), NO3 (-), PO4 (2-), SO4 (2-), NH4 (+), etc. and the metallic species, Na, Mg, Ca, K, Al, Fe, Mn, Zn, and Pb were estimated for each sample. Based on the spatial distribution of individual chemical species, the air flow pattern, and airmass back trajectory analysis, the source characteristics of aerosols for different regions of BoB were identified. Significant level of continental pollution was noticed over BoB during winter. While transport of pollution from Indo-Gangetic Plain (IGP) contributed to aerosols over north BoB, those over SE BoB were influenced by SE Asia. A quantitative study on the wind-induced production of sea salt aerosols and a case study on the species dependent effect of rainfall are also presented in this paper.