Evidence for global increases in urban ammonia pollution and their drivers.

Ammonia (NH) affects air quality, human health, and life expectancy through its important role in forming fine particulate matter. However, the spatial patterns and trends of NH concentrations in the urban environment remain unknown worldwide. Here we use satellite measurements to produce a global distribution of NH at fine resolution, and then identify, categorize and quantify NH air concentrations in urban clusters throughout the world, as well as explore associated trends and drivers. Based on satellite records, a significant increase is evident in global urban NH concentrations between 2008 and 2019, with an annual increase of 1.2 % yr. Our results show that the decline of acidic gas (NO/SO) explains the largest part of the increasing NH concentrations (42 %), exceeding the contribution of local NH emissions. Our results also show that increasing temperature can explain 20 % of the increase in urban NH concentrations implying that efforts to reduce NH emissions need to be greatly strengthened to compensate for increases in urban NH pollution induced by global warming and so improve the urban environment.