Long‐Term Evolution of Particulate Nitrate Pollution in North China: Isotopic Evidence From 10 Offshore Cruises in the Bohai Sea From 2014 to 2019

Atmospheric nitrate (NO 3 − ) pollution has become an obstacle to efforts to further reduce fine particulate (PM 2.5 ) concentration in North China. However, there have been limited long‐term measurements of NO 3 − and isotopic knowledge (δ 15 N, δ 18 O) on the driving factors during NO 3 − changes. Here, we report observations of 10 voyages from 2014 to 2019 conducted in the Bohai Sea, a typical background area in North China. The results show that the average proportion of NO 3 − in PM 2.5 increased from 0.08 to 0.16 over the study period. The δ 15 N–NO 3 − ranged from −4.1‰ to +20.5‰, with a significant annual decline ( p < 0.01), especially in winter. The average δ 18 O–NO 3 − was +72.6 ± 13.5‰, and a Monte Carlo calculation revealed that the contribution of the •OH pathway in the NO 3 − formation declined by 27.4% in winter, implying an increase in O 3 pollution. Coal combustion remained the most important contributor to NO 3 − (46.6 ± 15.9%), but its contribution showed a significant downward trend ( p < 0.01), consistent with the control of disperse coal use in North China. Enhancement of atmospheric oxidation and the unexpected large increase in contribution of microbial processes were found to be the main causes of the increasingly serious NO 3 − pollution in North China. In addition, a spike in the contribution of coal combustion in 2018 indicates that the coal‐control policy needs to be reinforced.