The vertical variability of ammonia in urban Beijing, China

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.