Analysis of passive-sampler monitored atmospheric ammonia at 74 sites across southern Ontario, Canada

Abstract. Weekly/biweekly concentrations of atmospheric NH₃ were collected using passive samplers at 74 sites across southern Ontario, Canada, during the period from June 2006 to March 2007 with eighteen sites running as early as March 2006. The annual average of NH₃ (AAN) at all the sites across southwestern Ontario was over 1 μg m⁻³, a value that was recently proposed as the new critical level for protecting vegetation. High AAN values (3.6–6.1 μg m⁻³) were observed at eight sites located inside the intensive livestock production zones. AAN values at the sites across southeastern Ontario were generally lower than 1 μg m⁻³ and the values were less than 0.4 μg m⁻³ at non-agricultural sites. Agricultural sites generally showed the highest concentrations in spring to summer and the lowest in winter, but the occurrence time of the peak concentration varied among the sites, which could be due to their different land use and thus different fertilizer application intensities or periods. Seasonal trends of NH₃ concentration at some non-agricultural sites were somewhat similar to those at the agricultural sites, but the peak concentration appeared a few weeks later than at the agricultural sites. Surprisingly, at several remote non-agricultural sites, NH₃ concentrations were not decreasing in winter and were sometimes increasing to a level higher than those in summer, which could be caused by different transport mechanisms in different seasons. A sharp increase in NH₃ concentration was also surprisingly observed at 20 out of the 74 sites during the coldest two weeks when ambient temperature was lower than −7 °C, and cannot be explained by known sources or with existing knowledge. The NH₃ emission inventory for the region of southern Ontario was evaluated using the measurement data and emissions within two small zones in this region were identified to be potentially underestimated.