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Measuring ammonia (NH 3) is important for understanding the role of NH 3 in secondary aerosol formation and the atmospheric deposition of reactive N. In this study, NH 3 was measured in an urban area, a background region, and a tunnel in Beijing. The average NH 3 concentrations between September 2017 and August 2018 were 24.8 ± 14.8 ppb and 11.6 ± 10.3 ppb in the urban area and background region, respectively. Higher NH 3 concentrations at both the urban and background sites, relative to some earlier measurements indicated a likely increase in the NH 3 concentrations in these regions. The urban NH 3 level in Beijing was much higher than that typically observed at urban and industrial sites in other domestic and foreign cities, suggesting that the Beijing urban area was affected by greater NH 3 emissions than other regions. Based on the relationship among NH 3 , wind direction, and wind speed, the urban area was affected by both local emissions and air transported from North China Plain (NCP). Potential source contribution function analyses suggested that regional transport from the NCP could greatly affect local concentrations of NH 3 in both urban and background areas in spring and autumn; however, in addition to the NCP, urban emissions could also affect NH 3 levels in the background region in summer and winter. The average NH 3 concentration at the Fenshuiling Tunnel was 8.5 ± 7.7 ppb from December 2017 to February 2018. The NH 3 :CO emission ratio measured in the tunnel test was 0.022 ± 0.038 ppb/ppb, which was lower than values in the USA and South Korea. The contribution of traffic to NH 3 in Beijing did not agree well with the available emission inventories, suggesting that vehicular emissions were underestimated and further evaluation is necessary. Image 1 • The NH 3 levels of urban Beijing were much higher than those in the other megacities. • Urban area was affected by both local sources and regional transported emissions. • Background region was mainly influenced by regional transported emissions. • Transportation-related NH 3 in the emission inventory of Beijing is underestimated. [ABSTRACT FROM AUTHOR]

The ozone (O 3) concentration usually exhibits a secondary maximum at night in Beijing. To investigate the characteristics and reasons behind this nocturnal O 3 increase (NOI), surface O 3 and meteorological parameters, combined with wind profile data at the Haidian and Daxing stations were utilized and analyzed in this study. We identified 35 city-wide NOI episodes during the summertime (May to September) from 2014 to 2015. A total of 88.9% of these NOI cases occurred after midnight, and more than half of the nocturnal secondary O 3 maxima (NSOM) ranged from 45 to 85 μg/m3. When compared with the previous daytime maximum O 3 concentration, 72.2% of the NSOM could account for more than 40.0% of the daytime O 3 maxima. The nocturnal O 3 concentration in northern suburban areas was 18.7% higher than that in urban areas before a nocturnal O 3 increase, and this contrast reduced under the effect of vertical transport of O 3 during the NOI period. The reasons for such a downward transport of O 3 involved two mechanisms: downdrafts in convective storms and boundary layer low-level jets. • 88.9% of the NOI cases occur after midnight during the summertime in Beijing. • Nocturnal O 3 contrast between urban and suburban areas decreased with vertical transport effect. • Nocturnal O 3 maxima were typically caused by convective storms and LLJs. [ABSTRACT FROM AUTHOR]