Evidence for global increases in urban ammonia pollution and their drivers

Ammonia (NH₃) affects air quality, human health, and life expectancy through its important role in forming fine particulate matter. However, the spatial patterns and trends of NH₃ concentrations in the urban environment remain unknown worldwide. Here we use satellite measurements to produce a global distribution of NH₃ at fine resolution, and then identify, categorize and quantify NH₃ 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 NH₃ concentrations between 2008 and 2019, with an annual increase of 1.2 % yr^-1. Our results show that the decline of acidic gas (NO_x/SO₂) explains the largest part of the increasing NH₃ concentrations (42 %), exceeding the contribution of local NH₃ emissions. Our results also show that increasing temperature can explain 20 % of the increase in urban NH₃ concentrations implying that efforts to reduce NH₃ emissions need to be greatly strengthened to compensate for increases in urban NH₃ pollution induced by global warming and so improve the urban environment.