Atmospheric wet deposition of major ionic constituents and inorganic nitrogen in Bangladesh: Implications for spatiotemporal variation and source apportionment

Abstract

The quantitative and qualitative characterization of ions and inorganic nitrogen in precipitation assists in understanding the accompanying sources and chemistry of regional precipitation. A total of 212 event-based precipitation samples were collected from four sites in Bangladesh in 2017 to investigate the physicochemical characteristics, sources, and deposition of atmospheric ionic constituents and inorganic nitrogen. During the entire monitoring period, 5.7% of the total samples were acidic (i.e., pH < 5.6), indicating that Bangladesh does not suffer severely from acid rain. The electrical conductivity (EC) and total dissolved solids (TDS) values indicate the inverse relationship between the amount of precipitation on the spatiotemporal scale. Ca2+ was the dominant neutralizing factor at four sites, which was demonstrated by the equivalent ratio of the neutralization potential to the acidifying potential and fractional acidity. Moreover, it had the highest volume-weighted mean concentration of all the sites, validating the calcareous nature of the soil aerosolized into the atmosphere. Distinct seasonality was observed for the majority of the ions with higher concentrations in the non-monsoon climate. Sea salt species displayed the pattern of Satkhira > Cox's Bazar > Dinajpur > Sylhet, whereas the anthropogenic species exhibited the order of Dinajpur > Satkhira > Sylhet > Cox's Bazar, underlining the local and regional impacts of these species in Bangladesh. Based on the source apportionment, the sources were categorized as marine (Na+ and Cl−), terrigenous (Ca2+, Mg2+, and HCO3−), fossil fuel combustion (NO3− and SO42−), agriculture (NH4+), and biomass burning (K+). The Cl− in Sylhet and Satkhira suggests additional sources associated with anthropogenic activities. The back-trajectory analyses and the National Centers for Environmental Prediction's final (NCEP FNL) datasets illustrate the presence of significantly diverse air masses with contributions from various sources in the monsoon and non-monsoon climates. Both the amount of precipitation and the ionic quantity governs the fluxes in Bangladesh. The Na+ % and SAR lie under the safe category suggesting a good precipitation water quality for agriculture and soil in Bangladesh, while the deposition of inorganic nitrogen has resulted in a value above the threshold line (10 kg ha−1 y−1). Thus, this study conveys a comprehensive picture of the ionic composition, providing a baseline dataset to assess the atmospheric environment in this lowland region.