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

摘要

The spatial and temporal variation in ambient atmospheric concentrations of gaseous reactive nitrogen (Nr) species (ammonia [NH_3], nitrogen dioxide [NO_2] and nitric acid [HNO_3]) 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, NO_2 and HNO_3 across the four study sites. Annual average Nr concentrations were greatest at the urban site owing to high NO_2 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 NO_2 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 NO_2 are significant components of total Nr deposition in southern Ontario and should be routinely monitored.