Ambient concentrations of atmospheric ammonia, nitrogen dioxide and nitric acid in an intensive agricultural region

The spatial and temporal distribution of ambient atmospheric gaseous reactive nitrogen (Nr) species concentrations (ammonia [NH3], nitrogen dioxide [NO2] and nitric acid [HNO3]) were measured at the field scale in an intensive agricultural region in southern Ontario, Canada. Atmospheric concentrations were measured with the Willems badge diffusive passive sampler (18 sites for NH3, 9 sites for NO2 and HNO3) for one year (April 2010-March 2011; under a two week measurement frequency) within a 15 km × 15 km area. Dry deposition was calculated using the inferential method and estimated across the entire study area. The spatial distribution of emission sources associated with agricultural activity resulted in high spatial variability in annual average ambient NH3 concentrations (<3->8 μg m−3 within a 2 km distance, coefficient of variation ∼50%) and estimated dry deposition (4-13 kg N ha−1 yr−1) between sample sites. In contrast, ambient concentrations and deposition of both NO2 (∼5.2->6.5 μg m−3; 1.0-1.5 kg N ha−1 yr−1) and HNO3 (0.6-0.7 μg m−3; 0.5-1 kg N ha−1 yr−1) had low variability (coefficient of variation <10%). The observed NH3 concentrations accounted for ∼70% of gaseous Nr dry deposition. High NH3 concentrations suggest that reduced nitrogen species (NHx) will continue to make up an increasing fraction of Nr deposition within intensive agricultural regions in southern Ontario under legislated nitrogen oxide emission reductions. Further, estimated total inorganic Nr deposition (15-28 kg N ha−1 yr−1) may lead to potential changes in soil processes, nutrient imbalance and altered composition of mycorrhiza and ground vegetation within adjacent semi-natural ecosystems (estimated at ∼10% of the study area).

► Ambient NH3 concentrations and deposition were more variable than NO2 and HNO3. ► NH3 was responsible for ∼70% of gaseous reactive nitrogen dry deposition. ► Spatial variation in NO2 concentrations was associated with agricultural traffic. ► Future policy designed to reduce N deposition should focus on reducing NH3 emissions.