Spatial and temporal concentration of ambient atmospheric ammonia in southern Ontario, Canada

The spatial and temporal variability in ambient atmospheric ammonia (NH3) concentration was evaluated across southern Ontario, Canada, during the period August 2007–December 2010 using the Willems badge diffusive passive sampler (under a two week measurement frequency). A network of 11 monitoring sites was established across a land use transect reflecting low to high agricultural activity based on agricultural statistics and reported emissions. Ambient two-week average NH3 concentrations were lowest during the winter and highest during the spring consistent with seasonal agricultural activity; the largest range in observed NH3 concentrations at an individual site was 0.4–15.8 μg m−3 (winter–spring; intensive agricultural site). Annual average ambient NH3 concentrations ranged from 0.3 μg m−3 at low agricultural intensity (background) sites (n = 4) to 2.8 μg m−3 at intensive agricultural sites (n = 3); the largest range in biweekly NH3 concentrations across the 11 monitoring sites (during one exposure period) was ∼16 μg m−3 (spring season). Annual average concentrations ranged from 0.1 μg m−3 to 3.0 μg m−3 across sites; nonetheless, there was strong temporal coherence among all sites suggesting a common regional ‘footprint’ from agricultural emissions. Regional agricultural statistics correlated well with NH3 concentrations, cattle and pig numbers being the best predictors of annual average NH3 concentrations across southern Ontario.