The use of stable oxygen and nitrogen isotopic signatures to reveal variations in the nitrate formation pathways and sources in different seasons and regions in China

Nitrate (NO3-) is one of the most important inorganic ions in fine particulate (PM2.5) and drives regional haze formation; however, the NO3- sources and formation mechanisms in different seasons and regions are still debated. Here, PM2.5 samples were collected from Kunming and Nanning in southwestern China from September 1, 2017, to February 28, 2018 (spanning warm and cold months). We measured the daily O and N isotopic compositions of NO3-(delta O-18-NO3- and delta N-15-NO3-), estimated the delta O-18-HNO3 values produced by different oxidation pathways, and quantified the NO3- formation pathways based on the isotope mass-balance equation. Our results showed that the delta O-18-NO3- values in Kunming (65.3 +/- 7.6 parts per thousand) and Nanning (67.7 +/- 10.1 parts per thousand) are close to the delta O-18-HNO3- values arising from the OH radical pathway (POH, 54.7 +/- 1.2 parts per thousand to 61.2 +/- 1.8 parts per thousand), suggesting that the delta O-18-NO3- values are mainly influenced by POH, which showed a contribution greater than 74%. Stronger surface solar radiation and higher air temperatures in low-latitude regions and warm months increased the amount of HNO3 produced by POH and reduced the amount of HNO3 produced by PN2O5, which produced low delta O-18-NO3- values. Increased air pollution emissions decreased the contribution from POH and increased the contribution from N2O5 and NO3 pathways (PN2O5+NO3). The delta N-15-NO3- values of PM2.5 in Kunming (7.3 +/- 2.8 parts per thousand) were slightly higher than those in Nanning (2.8 +/- 2.7 parts per thousand). The increased NOx emissions with positive isotopic values led to high delta N-15-NO3- values in northern China and during cold months. A higher f(NO2) (f(NO2) = NO2/(NO + NO2), temperature, and contribution of POH produced lower N isotope fractionation between NOx and delta N-15-NO3-, which was found to further decrease the delta N-15-NO3- values in southwestern China and during warm months.