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By analyzing the data of urban air pollutant measurements from 2013 to 2015 in Nanjing, East China, we found that the correlation coefficients between major atmospheric compound pollutants PM 2.5 and O 3 were respectively 0.40 in hot season (June, July and August) and −0.16 in cold season (December, January and February) with both passing the confidence level of 99%. This provides evidence for the inverse relations of ambient PM 2.5 and O 3 between cold and hot seasons in an urban area of East China. To understand the interaction of PM 2.5 and O 3 in air compound pollution, the underlying mechanisms on the inversion relations between cold and hot seasons were investigated from the seasonal variations in atmospheric oxidation and radiative forcing of PM 2.5 based on three-year environmental and meteorological data. The analyses showed that the augmentation of atmospheric oxidation could strengthen the production of secondary particles with the contribution up to 26.76% to ambient PM 2.5 levels. High O 3 concentrations in a strong oxidative air condition during hot season promoted the formation of secondary particles, which could result in a positive correlation between PM 2.5 and O 3 in hot season. In cold season with weak atmospheric oxidation, the enhanced PM 2.5 levels suppressed surface solar radiation, which could weaken O 3 production for decreasing ambient O 3 level with the low diurnal peaks. Under the high PM 2.5 level exceeding 115 µg·m −3 , the surface O 3 concentration dropped to 12.7 µg·m −3 at noon with a significant inhibitory effect, leading to a negative correlation between PM 2.5 and O 3 in cold season. This observational study revealed the interaction of PM 2.5 and O 3 in air compound pollution for understanding the seasonal change of atmospheric environment.

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Abstract. Weekly vertical profiles of ammonia (NH3) were measured at 16 heights on the Beijing 325 m meteorological tower for 1 year from March 2016 to March 2017. The average NH3 concentrations exceeded 4 µg m−3 at all heights with an overall average (±1σ) value of 13.3 (±4.8) µg m−3. The highest NH3 concentrations along the vertical profiles mostly occurred from 32 to 63 m, decreasing both towards the surface and at higher altitudes. Significant decreases in NH3 concentrations were only found at the top two heights (280 and 320 m). These results suggest an NH3 rich atmosphere during all seasons in urban Beijing, from the ground to at least 320 m. The highest seasonal NH3 concentrations across the profile were observed in summer (18.2 µg m−3) with high temperature, followed by spring (13.4 µg m−3), autumn (12.1 µg m−3) and winter (8.3 µg m−3). A significant vertical variation in the NH3 concentration was only found in summer. Source region analyses suggest that air masses from intensive agricultural regions to the south contribute most to the high NH3 concentrations in Beijing. Local sources such as traffic emissions also appear to be important contributors to atmospheric NH3 in this urban environment.

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