The importance of coal combustion and heterogeneous reaction for atmospheric nitrate pollution in a cold metropolis in China: Insights from isotope fractionation and Bayesian mixing model

Haze pollution in Harbin has been gradually alleviated in recent years, but it is still far from eliminated. Thus, precise and effective emission abatement of main chemical components like nitrate is urgently needed for further haze pollution control. This study aimed to estimate nitrate formation pathways, and the contributions of the emission sources of nitrogen oxide in urban Harbin, based on the measurement of isotope signatures in nitrate and the utilization of an improved Bayesian mixing model. The results showed that nitrate significantly aggravated particulate pollution in the heating season, and its driving role was enhanced as the pollution level increased. This study suggested that homogeneous reaction and heterogeneous reaction were the dominant conversion pathways of nitrate formation in non-heating season and heating season respectively, and the contribution of heterogeneous reaction increased as pollution levels increased. By considering isotope fractionation value (delta N-15), this paper emphasized coal combustion as the dominant contributor of nitrogen oxide in urban Harbin, while biomass burning, mobile sources, and biogenic soil emissions played relatively weak roles in nitrate pollution. This study has critical importance to provide scientific theoretical bases for nitrogen oxide reduction in urban Harbin and air quality improvement reference in other severely polluted regions in China.