Abstract

Abstract. A unified regional air-quality modelling system (AURAMS) was used to investigate the effects of reductions in ammonia emissions on regional air quality, with a focus on particulate-matter formation. Three simulations of one-year duration were performed for a North American domain: (1) a base-case simulation using 2002 Canadian and US national emissions inventories augmented by a more detailed Canadian emissions inventory for agricultural ammonia; (2) a 30% North-American-wide reduction in agricultural ammonia emissions; and (3) a 50% reduction in Canadian beef-cattle ammonia emissions. The simulations show that a 30% continent-wide reduction in agricultural ammonia emissions lead to reductions in median hourly PM2.5 mass of <1 μg m−3 on an annual basis. The atmospheric response to these emission reductions displays marked seasonal variations, and on even shorter time scales, the impacts of the emissions reductions are highly episodic: 95th-percentile hourly PM2.5 mass decreases can be up to a factor of six larger than the median values. A key finding of the modelling work is the linkage between gas and aqueous chemistry and transport; reductions in ammonia emissions affect gaseous ammonia concentrations close to the emissions site, but substantial impacts on particulate matter and atmospheric deposition often occur at considerable distances downwind, with particle nitrate being the main vector of ammonia/um transport. Ammonia emissions reductions therefore have trans-boundary consequences downwind. Calculations of critical-load exceedances for sensitive ecosystems in Canada suggest that ammonia emission reductions will have a minimal impact on current ecosystem acidification within Canada, but may have a substantial impact on future ecosystem acidification. The 50% Canadian beef-cattle ammonia emissions reduction scenario was used to examine model sensitivity to uncertainties in the new Canadian agricultural ammonia emissions inventory, and the simulation results suggest that further work is needed to improve the emissions inventory for this particular sector. It should be noted that the model in its current form neglects coarse mode base cation chemistry, so the predicted effects of ammonia emissions reductions shown here should be considered upper limits.

Highlights

  • The chemistry describing the interactions of atmospheric ammonia (NH3) with other atmospheric constituents has been well established through field and laboratory studies

  • We describe the application of a comprehensive regional air-quality model for the entire North American continent, to predict the likely effects of reductions in North American emissions of agricultural ammonia on the mass and composition of atmospheric particulate matter (PM), and on the amount of acid deposition to sensitive ecosystems

  • Three simulations of one-year duration were performed for a North American domain for different sets of NH3 emissions

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Summary

Introduction

The chemistry describing the interactions of atmospheric ammonia (NH3) with other atmospheric constituents has been well established through field and laboratory studies. Equation (2a) describes a strongly ammonia-limited chemical regime, in which small perturbations in the gaseous ammonia concentration will likely result in changes to PM2.5 mass, due to the size separation of sulphate from coarse mode cations. Equation (2b) describes a weakly ammonialimited chemical regime, in which the excess total ammonia subsequent to sulphate charge-balancing is still less than that required to charge balance the remaining ions of the system The advantage of this two-level definition of ammonialimitation is that it captures the potential impact of ammonia on fine mode particle growth due to the presence of sulphate in the smaller particle sizes. We describe the application of a comprehensive regional air-quality model for the entire North American continent, to predict the likely effects of reductions in North American emissions of agricultural ammonia on the mass and composition of atmospheric PM, and on the amount of acid deposition to sensitive ecosystems.

Modelling system description
Description of emissions scenarios
Metrics and diagnostic fields for scenario analysis
Model performance evaluation for the base case
Analysis of the base case
Analysis of continental agricultural NH3 emission reduction scenario
NH3 concentrations
Total deposition
Annual critical load exceedances for sensitive ecosystems
A conceptual model
Findings
Conclusions and recommendations for future research

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