Source oxygen contributions of primary nitrate emitted from biomass burning

Abstract

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 δ18Obb-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 δ18Obb-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 δ18Obb-NO3- values with the biomass N contents and relatively lower δ18Obb-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.