Trends in atmospheric ammonia at urban, rural, and remote sites across North America

Interannual variabilities in atmospheric ammonia (NH3) during the most recent 7-11 years were investigated at 14 sites across North America using the monitored data obtained from NAPS, CAPMoN and AMoN networks. The long-term average of atmospheric NH3 ranged from 0.8 to 2.6 ppb, depending on location, at four urban and two rural/agricultural sites in Canada. The annual average at these sites did not show any deceasing trend with largely decreasing anthropogenic NH3 emission. An increasing trend was actually identified from 2003 to 2014 at the downtown Toronto site using either the Mann-Kendall or the ensemble empirical mode decomposition method, but no or stable trends were identified at other sites. The similar to 20% increase during the 11-year period at the site was likely caused by changes in NH4+-NH3 partitioning and/or air-surface exchange process as a result of the decreased sulfur emission and increased ambient temperature. The long-term average from 2008 to 2015 was 1.6-4.9 ppb and 0.3-0.5 ppb at four rural/agricultural and at four remote US sites, respectively. A stable trend in NH3 mixing ratio was identified at one rural/agricultural site while increasing trends were identified at three rural/agricultural (0.6-2.6 ppb, 20-50% increase from 2008 to 2015) and four remote sites (0.3-0.5 ppb, 100-200% increase from 2008 to 2015). Increased ambient temperature was identified to be a cause for the increasing trends in NH3 mixing ratio at four out of the seven US sites, but what caused the increasing trends at other US sites needs further investigation.