Source oxygen contributions of primary nitrate emitted from biomass burning

Atmospheric nitrate (NO₃^-) produced by photochemical oxidation in the atmosphere has high oxygen isotope ratios (δ¹⁸O values). Recently, the primary NO₃^- emitted from combustion sources was found to have much lower δ¹⁸O values. However, it is unclear how and to what extents the low δ¹⁸O signatures were controlled by major O sources during the primary NO₃^- formation of combustion processes. Here, we first measured concentrations and δ¹⁸O values of NO₃^- from burning five biomass materials (bb-NO₃^- and δ¹⁸O_bb-NO3-, respectively) in China. Distinctly higher concentration levels of the bb-NO₃^- emissions (42.1 ± 8.1 μmol m^-3) than ambient NO₃^- suggest it is a potential source of atmospheric NO₃^- pollution. Much lower δ¹⁸O_bb-NO3- signatures (27.6 ± 2.7 ‰) than ambient NO₃^- support it as a primary emission source with different O sources and formation mechanism from secondary NO₃^-. Isotope mass-balance modeling revealed that atmospheric O₂ and the biomass O dominated the O of bb-NO₃^- (53 ± 7 % and 40 ± 4 %, respectively) over the aqueous vapor (7 ± 3 %). Besides, we found increasing δ¹⁸O_bb-NO3- values with the biomass N contents and relatively lower δ¹⁸O_bb-NO3- values for biomasses with higher carbon (C) and lower O contents, indicating that biomass C, N, and O contents may influence the source O contributions of the bb-NO₃^-. This work provides a novel isotope analysis on the O source contribution of the bb-NO₃^-, which is useful for understanding the formation mechanism of combustion-related NO₃^- sources and evaluating the primary NO₃^- emissions.