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

Abstract

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 NO x from BB emission over South Asia that had undergone N2O5 + H2O processes enhanced the nitrate concentrations in the springtime over the HTP region.

Wildfire-Derived Nitrogen Aerosols Threaten the Fragile Ecosystem in Himalayas and Tibetan Plateau

“…Clearly, these strong correlations demonstrate that those specific nitrogen components in aerosols are also closely linked to BB emissions, likely excluding the possible impacts of fossil fuel combustion. This is in agreement with the findings of previous studies in other areas experiencing intensive forest/agricultural burning. , It can be explained by secondary reactions and gaseous precursors like NH 3 and NO x , released by biomass burning, ultimately forming inorganic ions in aerosols (i.e., NH 4 + and NO 3 – ) . Similarly, atmospheric reactions between NO x and volatile organic compounds followed by gas-to-particle condensation also produce high water-soluble secondary organic nitrogen compounds (i.e., WSON).…”

“…28,29 It can be explained by secondary reactions and gaseous precursors like NH 3 and NO x , released by biomass burning, ultimately forming inorganic ions in aerosols (i.e., NH 4 + and NO 3 − ). 30 Similarly, atmospheric reactions between NO x and volatile organic compounds followed by gas-toparticle condensation also produce high water-soluble secondary organic nitrogen compounds (i.e., WSON). Moreover, we have also quantified sixteen FAAs, important species of WSON.…”

Increasing Nonfossil Fuel Contributions to Atmospheric Nitrate in Urban China from Observation to Prediction

“…In this model, 797 observations in total of δ 15 N values of p NO 3 – in seven sites in China (including Guangzhou, Shanghai, Wuhan, Chengdu, Changchun, Beijing, and Qomolangma) were collected from this and previous studies for the use as target variables. ,, The site description is shown in the Supporting Information (Figure S1 and Table S1). Then, the machine learning process was performed using the RF algorithm in ranger packages (a C++ built RF package) in R, and the nonlinear relationship between nitrate δ 15 N and predictors was built.…”

Formation Mechanisms and Source Apportionments of Nitrate Aerosols in a Megacity of Eastern China Based On Multiple Isotope Observations

“…For instance, nocturnal chemistry was found to contribute equally with NO 2 + OH/H 2 O to nitrate formation near ground surface but dominate the production of nitrate at high altitude (∼260 m) in winter (Fan et al., 2021). The hydrolysis of N 2 O 5 was found to be an important mechanism of nitrate formation over the Himalayan–Tibetan Plateau (Lin et al., 2021).…”