We report a continuous record of surface ozone (O-3) in urban Beijing, China, from 2013 to 2019. A linear fit to the 7-year record shows that the annual MDA8-O-3 (the maximum daily average of 8-h O-3 concentration) and annual average O-3 increased by 2.30 and 1.91 ppbv yr(-1) (p < 0.05), respectively. Both the MDA8-O-3 level and the number of exceeding days are increased, demonstrating the surface O-3 pollution in Beijing is increasingly serious. An overall decrease in annual surface NO2 was observed at a rate of -1.21 ppbv yr(-1) (p < 0.01). The total oxidants (O-x, = NO2 + O-3) had an upward trend during 2013-2019 at a rate of 0.70 ppbv h(-1) (p = 0.168). The increasing O-3 and Or trends imply the atmospheric oxidation capacity is increasing in Beijing, even though the strict emission policies have been implemented. The periodical changes of surface O-3 in different time scales are studied. We found that the increases in O-3 are mainly at a high O-3 level with a threshold of 30 ppbv. The relative diurnal variability of surface O-3 is weakened, with a decrease in the diurnal amplitude variation. Both the extremely low and high 5% surface O-3 are increased, indicates an overall uplift of surface O-3. The weekday periodic trends showed an increment of weekend MDA8-O-3 (2.2 ppbv on average) and companies with a decrement of weekend NO2 (1.5 ppbv on average). The weekend effect provides a chance to look insights into reducing O-3 exceeding days during summertime and proposes the need for emission abatements of volatile organic compounds to the mitigation of ozone pollution in Beijing.

Heterogeneous reactivity of N2O5 on aerosols is a critical parameter in assessing NOx fate, nitrate production, and particulate chloride activation. Accurate measurement of its uptake coefficient (gamma N2O5) and representation in air quality models are challenging, especially in the polluted environment. With an in situ aerosol flow-tube system, the gamma N2O5 was directly measured on ambient aerosols at two rural sites in northern and southern China. The results were analyzed together with the gamma N2O5 derived from previous field studies in China to obtain a holistic picture of gamma N2O5 uptake and the influencing factors under various climatic and chemical conditions. The field-derived or measured gamma N2O5 was generally promoted by the aerosol water content and suppressed by particle nitrate. Significant discrepancies were found between the measured gamma N2O5 and that estimated from laboratory-determined parameterizations. An observation-based empirical parameterization was derived in the present work, which better reproduced the mean value and variability of the observed gamma N2O5. Incorporating this new parameterization into a regional air quality model (WRF-CMAQ) has improved the simulation of N2O5, nitrogen oxides, and secondary nitrate in the polluted regions of China.

Particulate nitrate (pNO3–) 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 pNO3–. Here, for the first time, we combine ground and tall-tower observations to quantify the chemical formation of pNO3– using observationally constrained model approach based on direct observations of OH and N2O5 for the urban air-shed. We find that the gas-phase oxidation pathway (OH+NO2) during the daytime is the dominant channel over the nocturnal uptake of N2O5 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 N2O5 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.