Fossil-driven secondary inorganic PM enhancement in the North China Plain: Evidence from carbon and nitrogen isotopes.

Measuring isotopic ratios in aerosol particles is a powerful tool for identifying major sources, particularly in separating fossil from non-fossil sources and investigating aerosol formation processes. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM in Beijing (BJ) and Changdao (CD) in the North China Plain (NCP) from May to mid-June 2016. The mean PM concentrations were 48.6 ± 28.2 μg m and 71.2 ± 29.0 μg m in BJ and CD, respectively, with a high contribution (∼66%) from secondary inorganic aerosol (SIA; NO, NH, and SO). The mean δC of total carbon (TC) and δN of total nitrogen (TN) values differed significantly between the two sites (p-value of <0.001): -25.1 ± 0.3‰ in BJ and -24.5 ± 0.4‰ in CD and 10.6 ± 1.8‰ in BJ and 5.0 ± 3.1‰ in CD, respectively. In BJ, the average δN (NH) and δN (NO) values were 12.9 ± 2.3‰ and 5.2 ± 3.5‰, respectively. The ionic molar ratios and isotopic ratios suggest that NO in BJ was formed through the phase-equilibrium reaction of NHNO under sufficient NH conditions, promoted by fossil-derived NH transported with southerly winds. In BJ, fossil fuel sources comprised 52 ± 7% of TC and 45 ± 28% of NH on average, estimated from radiocarbon (C) analysis and the δN and isotope mixing model, respectively. These multiple-isotopic composition results emphasize that PM enhancement is derived from fossil sources, in which vehicle emissions are a key contributor. The impact of the coal source was sporadically noticeable. Under regional influences, the fossil fuel-driven SIA led to the PM enhancements. Our findings demonstrate that the multiple-isotope approach is highly advantageous to elucidate the key sources and limiting factors of secondary inorganic PM aerosols.