Trends of monitored nitrogen species at monitoring sites in North America

Leiming Zhang1, Irene Cheng1, Xiaohong Yao2

1 Air Quality Research Division, Environment and Climate Change Canada, Toronto (

2 Ocean University of China, Qingdao, China


Long-term (1983-2011) air concentrations and annual wet deposition of ammonium and nitrate at 30 Canadian sites were analyzed.  Long-term median atmospheric NH4+ and NO3 ranged from 0.1-1.7 and 0.03-2.0 µg/m3 among the sites, respectively.  Median annual wet deposition varied from 0.2-5.8 and 0.8-23.3 kg/ha for NH4+ and NO3, respectively.  Long-term decline in atmospheric NH4+ from 1994-2010 was observed, whereas atmospheric NO3 increased from 1991-2001 and then declined from 2001-2010.  Annual wet deposition of NO3 decreased at most sites by 0.07-1.0 kg/ha/a. Average gaseous HNO3 and particulate NO3wet scavenging contributions to nitrate wet deposition were 72±23% and 28±23%, respectively. Gaseous NH3 and particulate NH4+ contributed 30±19% and 70±19% to wet NH4+ deposition.

Interannual variabilities in atmospheric ammonia during the most recent seven to eleven years were investigated at fourteen sites across North America.  The long-term average of NH3 ranged from 0.8 to 2.6 ppb among the four urban and two rural/agriculture 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 one urban site.  In the U.S., average NH3 from 2008 to 2015 was 2.2-4.9 ppb at three rural/agriculture sites and was 0.3-0.5 ppb at four remote sites.  A stable trend at one and increasing trends at three rural/agricultural sites were identified.  Increasing trends at the four remote sites were also identified.  Changes in NH4+/NH3 partitioning and/or air-surface exchange process as a result of the decreased sulfur emission and increased ambient temperature were believed to be the causes of NH3 at some of the sites