The vertical variability of ammonia in urban Beijing, China

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