Chemical characterization and stable nitrogen isotope composition of nitrogenous component of ambient aerosols from Kanpur in the Indo-Gangetic Plains

Abstract

Measurements of water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), water-soluble organic nitrogen (WSON) and ẟ15NTN (total N) was carried out on PM2.5 aerosol samples during wintertime to understand the major sources of ambient nitrogenous species at a heavily polluted location of Kanpur in north India. During the nighttime sampling campaign, WSON and NH4+_N contributed dominantly to the WSTN. Ammonium-rich condition persisted during sampling (NH4+/SO42− average equivalent mass ratio = 3.1 ± 0.7), suggesting complete neutralization of SO42− and formation of NH4NO3, which is stable in winter due to low temperature and high relative humidity (RH). Stagnant atmospheric conditions during wintertime enhanced concentrations of ionic species (SO42−, NH4+, and NO3−) at this location. Good correlations between NO3−_N, NH4+_N and biomass burning tracer K+BB (and also between NO3−_N, NH4+_N and SO42−) suggests a strong impact of biomass burning activities. Multi-linear regression (MLR) analysis shows a strong dependence of ẟ15N on NO3−_N, SO42− and WSON in night-1 (10:00 pm to 2:00 am) and on NO3−_N and SO42− in night-2 (2:00 am to 6:00 am) depicting different formation and removal mechanism of aerosols during both the time-periods. ẟ15NTN in PM2.5 varied from +8.8 to +15.5‰ (10.8 ± 1.3), similar to the variability observed for many urban locations in India and elsewhere. NH4+_N and WSON control the final ẟ15N value of nitrogenous aerosols. High relative humidity during nighttime enhanced the secondary organic aerosols formation due to aqueous-phase formation and gas to particle-phase partitioning. Isotopic fractionations associated with multi-phase reactions during gas to particle conversion of NH3 would result in an increase in ẟ15N by ~48‰ to 51‰ (at T of 5.4 °C to 15.4 °C) than that of the emission source(s), which indicates the most likely N-emission sources at Kanpur to be from agriculture activities and waste generation.