Attributes of aerosol bound water soluble ions and carbon, and their relationships with AOD over the Brahmaputra Valley

The present study is a ground based investigation of chemical properties of aerosol as PM10 and its relationship with the upper air optical properties. A total of 161 aerosol samples collected during 2010–2014 were characterized for water soluble ions viz. SO42−, NO3−, Cl−, F−, NH4+, K+, Ca2+, Mg2+and Na+ and water soluble carbon factions – water soluble organic carbon (WSOC) and water soluble inorganic carbon (WSIC). The entire study period was subdivided into four distinct seasons -pre-monsoon, monsoon, post-monsoon and winter to assess the characteristics of chemical species in different times of the year contributing to the particulate loading over the study site. To understand the relationship between chemical species and optical properties, aerosol optical depth (AOD) data of the study area have been retrieved from MODIS satellite data at 550 nm. Mean mass concentration of PM10 was found to be 49.3 ± 37 μg/m3 for the whole study period with an explicit seasonal variation and winter maximum of mass concentration. Also, secondary ions have strong influence on the total aerosol loading in the region. Vivid seasonal variability was found in the concentrations of ions and carbons. The winter season showed maximum loading of ionic and carbonaceous species and the presence of crustal derived ions – Ca2+ and Mg2+ – remained uniform all through the seasons. The anions were found to be dominant over the cations during the study period. Interestingly, K+, originating mostly from biomass burning emissions, also play important neutralizing role together with NH4+. Significant relationships between AOD with PM10 and attributes were observed. Strong correlation of anthropogenic (SO42−, NO3−, NH4+), biomass burning (K+) and organic carbon fraction of PM10 with AOD was observed, which indicated the influence of these fractions on the attenuation of incoming light over the study region. HYSPLIT backward trajectories of air masses, which were computed for the study area, revealed the influence of long range transport of aerosol that carried characteristics of the areas they represented. •Characteristics of PM10, ions and carbon over Brahmaputra Valley assessed.•Low mixing heights, long range transport, wind gust– important functions of PM10.•Crustal dust a steady contributor of Ca2+ and Mg2+ of PM10 through seasons.•Anthropogenic fraction of PM10 have greater effect on AOD.•Biomass burning indicator, K+ plays important neutralizing role.