Spatial characteristics of the nighttime oxidation capacity in the Yangtze River Delta, China

Abstract Nighttime chemistry plays an important role in NO x and volatile organic compounds (VOCs) removal. A box model was used to study nighttime nitrate radical (NO 3 ), O 3 oxidation and nitrate products in Yangtze River Delta (YRD), China. The model input data were collected from five representative monitoring sites in YRD regions from 20 August to 11 September 2016. The production rate of NO 3 was found very fast with the nocturnal average of 1–3 ppbv h −1 in different sites. Large directly NO 3 loss frequency was determined from VOCs measurements, with the average of 0.01–0.03 s −1 , which represented a lower limit due to the lack of monoterpene measurement. The model results show NO 3 governs nocturnal oxidation of hydrocarbons (HCs) in YRD region (80%) and O 3 contributes to 20% in average. The oxidation percentage attributable to NO 3 increased against the NO x concentration, with 3.5–7.7 ppbv HCs removal by NO 3 per night in average, suggesting that the production of organic nitrate by NO 3 oxidation may be significant. As to products, the integrated inorganic nitrate production in the nighttime was considerable around 62% ± 20% of the whole day in urban sites, but the percentage was small in rural sites. More inorganic nitrate was produced with the increase of NO x , since the HCs oxidation by NO 3 has already been saturated at night. This study highlights nocturnal fast oxidation by NO 3 in YRD region, and the variability of oxidation is large in different NO x regime.