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A unified regional air-quality modelling system (AURAMS) was used toinvestigate the effects of reductions in ammonia emissions on regional airquality, with a focus on particulate-matter formation. Three simulations ofone-year duration were performed for a North American domain: (1) abase-case simulation using 2002 Canadian and US national emissionsinventories augmented by a more detailed Canadian emissions inventory foragricultural ammonia; (2) a 30% North-American-wide reduction inagricultural ammonia emissions; and (3) a 50% reduction in Canadianbeef-cattle ammonia emissions. The simulations show that a 30%continent-wide reduction in agricultural ammonia emissions lead toreductions in median hourly PM2.5 mass of <1 μg m−3 on anannual basis. The atmospheric response to these emission reductions displaysmarked seasonal variations, and on even shorter time scales, the impacts ofthe emissions reductions are highly episodic: 95th-percentile hourlyPM2.5 mass decreases can be up to a factor of six larger than themedian values. A key finding of the modelling work is the linkage between gas and aqueouschemistry and transport; reductions in ammonia emissions affect gaseousammonia concentrations close to the emissions site, but substantial impactson particulate matter and atmospheric deposition often occur at considerabledistances downwind, with particle nitrate being the main vector ofammonia/um transport. Ammonia emissions reductions therefore havetrans-boundary consequences downwind. Calculations of critical-loadexceedances for sensitive ecosystems in Canada suggest that ammonia emissionreductions will have a minimal impact on current ecosystem acidificationwithin Canada, but may have a substantial impact on future ecosystemacidification. The 50% Canadian beef-cattle ammonia emissions reductionscenario was used to examine model sensitivity to uncertainties in the newCanadian agricultural ammonia emissions inventory, and the simulationresults suggest that further work is needed to improve the emissionsinventory for this particular sector. It should be noted that the model inits current form neglects coarse mode base cation chemistry, so thepredicted effects of ammonia emissions reductions shown here should beconsidered upper limits

Modelling the impacts of ammonia emissions reductions on North American air quality