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. Weekly vertical profiles of ammonia ( NH3 ) were measured at16 heights on the Beijing 325 m meteorological tower for 1 year fromMarch 2016 to March 2017. The average NH3 concentrations exceeded4  µ g m −3 at all heights with an overallaverage ( ±1σ ) value of 13.3 ( ±4.8 )  µ g m −3 . Thehighest NH3 concentrations along the vertical profiles mostlyoccurred from 32 to 63 m, decreasing both towards the surface and at higheraltitudes. Significant decreases in NH3 concentrations were onlyfound at the top two heights (280 and 320 m). These results suggest an NH3 rich atmosphere during all seasons in urban Beijing, from theground to at least 320 m. The highest seasonal NH3 concentrationsacross the profile were observed in summer (18.2  µ g m −3 ) withhigh 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 NH3 concentration was only foundin summer. Source region analyses suggest that air masses from intensiveagricultural regions to the south contribute most to the high NH3 concentrations in Beijing. Local sources such as traffic emissions alsoappear to be important contributors to atmospheric NH3 in thisurban environment.

The vertical variability of ammonia in urban Beijing, China