Summertime Urban Ammonia Emissions May Be Substantially Underestimated in Beijing, China

Ammonia (NH₃) is critical to the nitrogen cycle and PM_2.5 formation, yet a great deal of uncertainty exists in its urban emission quantifications. Model-underestimated NH₃ concentrations have been reported for cities, yet few studies have provided an explanation. Here, we explore reasons for severe WRF-Chem model underestimations of NH₃ concentrations in Beijing in August 2018, including simulated gas-particle partitioning, meteorology, regional transport, and emissions, using spatially refined (3 km resolution) NH₃ emission estimates in the agricultural sector for Beijing-Tianjin-Hebei and in the traffic sector for Beijing. We find that simulated NH₃ concentrations are significantly lower than ground-based and satellite observations during August in Beijing, while wintertime underestimations are much more moderate. Further analyses and sensitivity experiments show that such discrepancies cannot be attributed to factors other than biases in NH₃ emissions. Using site measurements as constraints, we estimate that both agricultural and non-agricultural NH₃ emission totals in Beijing shall increase by ∼5 times to match the observations. Future research should be performed to allocate underestimations to urban fertilizer, power, traffic, or residential sources. Dense and regular urban NH₃ observations are necessary to constrain and validate bottom-up inventories and NH_x simulation.