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- (δ18O-NO3- and δ15N-NO3-), estimated the δ18O-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 δ18O-NO3- values in Kunming (65.3 ± 7.6‰) and Nanning (67.7 ± 10.1‰) are close to the δ18O-HNO3 values arising from the OH radical pathway (POH, 54.7 ± 1.2‰ to 61.2 ± 1.8‰), suggesting that the δ18O-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 δ18O-NO3- values. Increased air pollution emissions decreased the contribution from POH and increased the contribution from N2O5 and NO3 pathways (PN2O5+NO3). The δ15N-NO3- values of PM2.5 in Kunming (7.3 ± 2.8‰) were slightly higher than those in Nanning (2.8 ± 2.7‰). The increased NOx emissions with positive isotopic values led to high δ15N-NO3- values in northern China and during cold months. A higher fNO2 (fNO2 = NO2/(NO + NO2), temperature, and contribution of POH produced lower N isotope fractionation between NOx and δ15N-NO3-, which was found to further decrease the δ15N-NO3- values in southwestern China and during warm months.