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. Ammonia (NH3) plays a central role in the chemistry of inorganic secondaryaerosols in the atmosphere. The largest emission sector for NH3 is agriculture,where NH3 is volatilized from livestock wastes and fertilized soils. Althoughthe NH3 volatilization from soils is driven by the soil temperature and moisture,many atmospheric chemistry models prescribe the emission using yearly emission inventoriesand climatological seasonal variations. Here we evaluate an alternative approach where theNH3 emissions from agriculture are simulated interactively using the process modelFANv2 (Flow of Agricultural Nitrogen, version 2) coupled to the Community AtmosphericModel with Chemistry (CAM-chem). We run a set of 6-year global simulations usingthe NH3 emission from FANv2 and three global emission inventories (EDGAR, CEDS andHTAP) and evaluate the model performance using a global set of multi-component(atmospheric NH3 and NH4+, and NH4+ wet deposition) in situobservations. Over East Asia, Europe and North America, the simulations with differentemissions perform similarly when compared with the observed geographical patterns. Theseasonal distributions of NH3 emissions differ between the inventories, and thecomparison to observations suggests that both FANv2 and the inventories would benefit frommore realistic timing of fertilizer applications. The largest differences between thesimulations occur over data-scarce regions. In Africa, the emissions simulated by FANv2are 200 %–300 % higher than in the inventories, and the available in situ observationsfrom western and central Africa, as well as NH3 retrievals from the Infrared Atmospheric Sounding Interferometer (IASI)instrument, are consistent with the higher NH3 emissions as simulated byFANv2. Overall, in simulating ammonia and ammonium concentrations over regions withdetailed regional emission inventories, the inventories based on these details (HTAP,CEDS) capture the atmospheric concentrations and their seasonal variability thebest. However these inventories cannot capture the impact of meteorological variabilityon the emissions, nor can these inventories couple the emissions to the biogeochemicalcycles and their changes with climate drivers. Finally, we show with sensitivityexperiments that the simulated time-averaged nitrate concentration in air is sensitive tothe temporal resolution of the NH3 emissions. Over the CASTNET monitoring networkcovering the US, resolving the NH3 emissions hourly instead monthly reduced thepositive model bias from approximately 80 % to 60 % of the observed yearly mean nitrateconcentration. This suggests that some of the commonly reported overestimation of aerosolnitrate over the US may be related to unresolved temporal variability in the NH3emissions.

Evaluation of interactive and prescribed agricultural ammonia emissions for simulating atmospheric composition in CAM-chem