Summertime urban ammonia emissions may be substantially underestimated in Beijing, China

Ammonia (NH₃) emission control has been advocated for its potential to mitigate PM_2.5 air pollution, yet emission quantifications at city levels are limited. Here we develop high-resolution (3 km) bottom-up emission inventories of agricultural NH₃ in the Beijing-Tianjin-Hebei (BTH) region and traffic NH₃ in Beijing for the year 2016. The resulting WRF-Chem simulated NH₃ and PM_2.5 are compared against ground-based and satellite observations. Our estimated annual BTH agricultural NH₃ emissions (625 Gg) and Beijing’s traffic emissions (7.8 Gg) are within the ranges of published inventories. However, simulated NH₃ concentrations are significantly lower than observations during August in urban Beijing, while wintertime underestimations are much more moderate. Further evaluation and sensitivity experiments show that such discrepancies cannot be attributed to biases in meteorology or regional transport. Using measurements as constraints, our inversed NH₃ inventory indicates both agricultural and non-agricultural NH₃ emissions in Beijing during August should increase by ~5 times to match NH₃ and PM_2.5 observations. Current underestimations may result from the missing power sector, urban green space emissions, the lack of representation of industrial hotspots, and uncertainties in traffic emissions. Our study highlights that denser and more frequent urban NH₃ observations are urgently needed to constrain and validate bottom-up inventories.