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 (NO) have attracted much attention as NO had detrimental effects on Earth's ecosystem and climate change. Here, we measured nitrogen (δN-NO) and oxygen (δO-NO and ΔO-NO) 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 NO concentrations were observed in the springtime. The average δN-NO, δO-NO, and ΔO-NO 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 NO + OH and NO + HO to nitrate production were estimated to be 43 and 52%, respectively, when the NO + hydrocarbon (HC)/dimethyl sulfide (DMS) (NO + 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 NO from BB emission over South Asia that had undergone NO + HO processes enhanced the nitrate concentrations in the springtime over the HTP region.