Primary nitrate from combustion-related sources biases the Δ17O differentiation of formation pathway contributions of atmospheric secondary nitrate

Total nitrate in atmospheric particulates (p-NO3-) has long been regarded as purely secondary NO3− (s-NO3-), and its oxygen isotope (Δ17O values) has been used to differentiate relative contributions between the OH radical (OH·) and ozone (O3) oxidization pathways to the s-NO3- formation. Recently, combustion-related sources were found emitting primary NO3− (c-NO3-) directly, but its influences on O isotopic signatures and the pathway differentiation of s-NO3- remain unclear. Here, we measured Δ17O in the NO3− from burning five biomass materials in northern China (bb-NO3-). The uniform Δ17Obb-NO3- values of 0‰ verified that the bb-NO3- was a primary source and differed in O sources from s-NO3-. By combining Δ17O values and emission amounts of c-NO3- with Δ17O values and deposition amounts of p-NO3- in China, we found that the c-NO3- contributed 6 ± 8% to the p-NO3- and neglecting the c-NO3- had underestimated Δ17Os–NO3- by 1.7 ± 0.2‰ at site level. When using Δ17Op-NO3- values to differentiate the pathway contributions, the fractional contributions of the OH· and O3 pathways to the s-NO3- would be overestimated and underestimated by 14 ± 11% and 14 ± 12%, respectively. These results clarified the influences of the c-NO3- on both Δ17O signals and pathway differentiation of s-NO3-, which should be considered in tracing atmospheric NO3− sources and processes.