Abstract
Reducing nitrogen oxide (NOx) emissions is key to limiting fine particulate pollution globally. Nitrogen isotopes are powerful tools for exploring sources of atmospheric nitrate and its precursor NOx. This work performed simultaneous vertical measurements of nitrogen isotopes of nitrate (δ15N(NO3–)) at 8 and 260 m heights on a meteorological tower in Beijing. Although δ15N(NO3–) of the two heights were close, a significant difference (P < 0.01) in the δ15N of nitrogen oxides (NOx) was found in winter after considering the nitrogen isotope fractionation between NOx and the nitrate constrained by the oxygen anomaly (Δ17O) of nitrate. Combining the estimated δ15N of NOx and a Bayesian model, it was found that the proportion of coal combustion to nitrate aerosols was higher at 260 m (54% ± 9%) than that at the ground surface (31% ± 10%) in winter, suggesting that the air masses carrying coal-derived aerosols reached the upper air of Beijing through regional transport. In summer, the vertical variations of nitrate sources were not obvious. Our findings suggest that conventional ground-based observations may underestimate the contribution of coal combustion to atmospheric aerosols in the urban boundary layer as air pollutants are influenced by a combination of local emissions, meteorology, and regional transport.
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