Formation Mechanisms and Source Apportionments of Airborne Nitrate Aerosols at a Himalayan-Tibetan Plateau Site: Insights from Nitrogen and Oxygen Isotopic Compositions.

Formation pathways and sources of atmosphere nitrate (NO3-) have attracted much attention as NO3- had detrimental effects on Earth's ecosystem and climate change. Here, we measured nitrogen (δ15N-NO3-) and oxygen (δ18O-NO3- and Δ17O-NO3-) isotope compositions in nitrate aerosols at the Qomolangma station (QOMS) over the Himalayan-Tibetan Plateau (HTP) to quantify the formation mechanisms and emission sources of nitrate at the background site. At QOMS, the enhanced NO3- concentrations were observed in the springtime. The average δ15N-NO3-, δ18O-NO3-, and Δ17O-NO3- values were 0.4 ± 4.9, 64.7 ± 11.5 and 27.6 ± 6.9‰, respectively. Seasonal variations of isotope ratios at QOMS can be explained by the different emissions and formation pathways to nitrate. The average fractions of NO2 + OH and N2O5 + H2O to nitrate production were estimated to be 43 and 52%, respectively, when the NO3 + hydrocarbon (HC)/dimethyl sulfide (DMS) (NO3 + HC/DMS) pathway was assumed to be 5%. Using stable isotope analysis in the R (SIAR) model, the relative contributions of biomass burning (BB), biogenic soil emission, traffic, and coal combustion to nitrate were estimated to be 28, 25, 24, and 23%, respectively, on yearly basis. By FLEXible PARTicle (FLEXPART) dispersion model, we highlighted that NOx from BB emission over South Asia that had undergone N2O5 + H2O processes enhanced the nitrate concentrations in the springtime over the HTP region.