Atmospheric particulate matter (PM2.5), EC, OC, WSOC and PAHs from NE–Himalaya: abundances and chemical characteristics

Abstract

Atmospheric concentrations of elemental, organic and water–soluble organic carbon (EC, OC and WSOC) and polycyclic aromatic hydrocarbons (PAHs) have been studied in PM2.5 (particulate matter of aerodynamic diameter ≤2.5 μm) from a site (Barapani: 25.7 °N; 91.9 °E; 1 064 m amsl) in the foot–hills of NE–Himalaya (NE–H). Under favorable wind–regimes, during the wintertime (January–March), study region is influenced by the long–range transport of aerosols from the Indo–Gangetic Plain (IGP). For rest of the year, ambient atmosphere over the NE–H is relatively clean due to frequent precipitation events associated with the SW– and NE–monsoon. The concentration of PM2.5 over NE–H, during the wintertime, varied from 39–348 μg m–3, with average contribution of OC and EC as 36±8% (AVG±SD) and 6±3%, respectively. For the OC/EC ratio as high as 10–15 (relatively high compared to fossil–fuel source) associated with WSOC/OC ratio exceeding 0.5 in NE–H, it can be inferred that dominant source of carbonaceous aerosols is attributable to biomass burning emissions and/or contributions from secondary organic aerosols (SOA). The OC/PM2.5 ratio from NE–H is somewhat higher compared to upwind regions in the IGP (Range: 0.16–0.24). The abundance of ΣPAHs show large variability, ranging from 4–46 ng m–3, and the ratio of sum of 4– to 6–ring PAHs (Σ(4– to 6–) PAHs) to EC is 2.4 mg g–1; similar to that in the upwind IGP and is about a factor of two higher than that from the fossil–fuel combustion sources. The cross-plot of PAH isomers [FLA/(FLA+PYR) vs. ANTH/(ANTH+PHEN), BaA/(BaA+CHRY+TRIPH), BaP/(BaP+B[b,j,k]FLA) and IcdP/(IcdP+BghiP)] reaffirms the dominant impact of biomass burning emissions. These results have implications to large temporal variability in aerosol radiative forcing and environmental change over the NE–Himalaya.