Concurrent measurements of nitrate at urban and suburban sites identify local nitrate formation as a driver for urban episodic PM 2.5 pollution.

Nitrate (NO ₃ ^- ) is often among the leading components of urban particulate matter (PM) during PM pollution episodes. However, the factors controlling its prevalence remain inadequately understood. In this work, we analyzed concurrent hourly monitoring data of NO ₃ ^- in PM _2.5 at a pair of urban and suburban locations (28 km apart) in Hong Kong for a period of two months. The concentration gradient in PM _2.5 NO ₃ ^- was 3.0 ± 2.9 (urban) vs. 1.3 ± 0.9 μg m ^-3 (suburban) while that for its precursors nitrogen oxides (NO _x ) was 38.1 vs 4.1 ppb. NO ₃ ^- accounted for 45 % of the difference in PM _2.5 between the sites. Both sites were characterized to have more available NH ₃ than HNO ₃ . Urban nitrate episodes, defined as periods of urban-suburban NO ₃ ^- difference exceeding 2 μg m ^-3 , constituted 21 % of the total measurement hours, with an hourly NO ₃ ^- average gradient of 4.2 and a peak value of 23.6 μg m ^-3 . Our comparative analysis, together with 3-D air quality model simulations, indicates that the high NO _x levels largely explain the excessive NO ₃ ^- concentrations in our urban site, with the gas phase HNO ₃ formation reaction contributing significantly during the daytime and the N ₂ O ₅ hydrolysis pathway playing a prominent role during nighttime. This study presents a first quantitative analysis that unambiguously shows local formation of NO ₃ ^- in urban environments as a driver for urban episodic PM _2.5 pollution, suggesting effective benefits of lowering urban NO _x .
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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