Ambient concentrations of atmospheric ammonia, nitrogen dioxide and nitric acid across a rural–urban–agricultural transect in southern Ontario, Canada

The spatial and temporal variation in ambient atmospheric concentrations of gaseous reactive nitrogen (Nr) species (ammonia [NH3], nitrogen dioxide [NO2] and nitric acid [HNO3]) were measured at four sites (spanning a transect in land use types: intensive agricultural, urban–agricultural transition, urban, and rural background) across southern Ontario, Canada. Atmospheric concentrations were measured with the Willems badge diffusive passive sampler for one year (April 2010–March 2011) at two-week exposure periods (biweekly integrated concentration measurements); dry deposition was estimated using the inferential method. There was high spatial and temporal variability in the ambient concentrations and estimated dry deposition of NH3, NO2 and HNO3 across the four study sites. Annual average Nr concentrations were greatest at the urban site owing to high NO2 concentrations (>25 μg m−3), while annual average ambient NH3 concentrations were greatest at the intensive agricultural site (>3 μg m−3) followed by the urban site (∼2 μg m−3) suggesting that outside of intensive agricultural areas, urban regions have the highest ambient NH3 concentrations in southern Ontario. The high ambient NH3 and NO2 concentrations resulted in high dry deposition of gaseous Nr (urban [>12 kg N ha−1 yr−1] and intensive agricultural [>8 kg N ha−1 yr−1]), accounting for 50–60% of estimated total Nr deposition (dry gaseous + wet ammonium and nitrate) at the study sites. Atmospheric NH3 and NO2 are significant components of total Nr deposition in southern Ontario and should be routinely monitored.

► Intensive agricultural and urban sites have the highest NH3 concentrations.
► Nr deposition was highest at the high-density urban site owing to NO2.
► NH3 and NO2 account for 50–60% of total N deposition at urban and agricultural sites.
► Gaseous Nr concentrations were variable across the rural–urban–agricultural transect.
► Routine monitoring of NH3 and NO2 would better quantify N budgets in southern Ontario.