Field Determination of Nitrate Formation Pathway in Winter Beijing.

Particulate nitrate (pNO) has often been found to be the major component of fine particles in urban air-sheds in China, the United States, and Europe during winter haze episodes in recent years. However, there is a lack of knowledge regarding the experimentally determined contribution of different chemical pathways to the formation of pNO. Here, for the first time, we combine ground and tall-tower observations to quantify the chemical formation of pNO using observationally constrained model approach based on direct observations of OH and NO for the urban air-shed. We find that the gas-phase oxidation pathway (OH+NO) during the daytime is the dominant channel over the nocturnal uptake of NO during pollution episodes, with percentages of 74% in urban areas and 76% in suburban areas. This is quite different from previous studies in some regions of the US, in which the uptake of NO was concluded to account for a larger contribution in winter. These results indicate that the driving factor of nitrate pollution in Beijing and different regions of the US is different, as are the mitigation strategies for particulate nitrate.