Coal and biomass burning as major emissions of NOx in Northeast China: Implication from dual isotopes analysis of fine nitrate aerosols

In recent years, there are still huge amounts of NOx emissions in the Northeast, and this inevitably increases the concentration of aerosol nitrate (NO3-), which plays an important role in atmospheric pollution. Because of the mixed sources of atmospheric NO3-, it is difficult to quantify their contribution, and the use of certain means to identify their sources and pathways is critical to developing effective control measures. Since different sources of NOx have different ranges of delta N-15 values, delta N-15 is considered to be a useful tool for identifying the source of aerosol NO3-. But isotope fractionation occurs during the conversion of NOx to NO3-, and delta O-18 can be used to estimate its isotope fractionation value. In this study, daily PM2.5 samples were collected in four seasons from Northeast China, and their water-soluble ionic components (WSIs), delta N-15-NO3- and delta O-18-NO3- were analyzed. The isotope fractionation value of delta N-15 in which NOx was converted to NO3- was estimated and the contribution of different sources was quantified in combination with the Bayesian model. The results showed that NO3- was the most important inorganic ion component in the WSIs with the highest annual average ratio of 21.1%. Both delta N-15 and delta O-18 showed higher in winter (delta N-15: 13.79 parts per thousand +/- 2.17 parts per thousand; delta O-18: 70.50 parts per thousand +/- 10.02%0) than in summer (delta N-15: 2.69 parts per thousand +/- 2.95 parts per thousand; delta O-18: 58.67 parts per thousand +/- 4.52 parts per thousand). The daytime center dot OH pathway was considered to play a leading role in nitrate formation, with the annual average contribution of 61.0 +/- 18.8%. NOx was mainly from the contribution of coal combustion (34.5%) and biomass burning (34.3%) followed by traffic (19.5%) and biological soil (11.7%). During heating periods, NOx was dominated by coal combustion with the average contribution of 46.9% whereas biomass burning was the most important contributor during non-heating periods (39.5%). Therefore, controlling coal consumption and biomass burning can drastically reduce concentration of aerosol NO3- in Northeast China.