Continuous Measurements of Ambient Ammonia at a Rural Site in Ontario

As part of the ongoing efforts of the Canadian Air and Precipitation Monitoring Network (CAPMoN) to characterize the ambient concentrations of different nitrogen compounds in rural/remote locations, a Thermo Model 42C trace level NOy analyzer was modified to facilitate continuous ammonia measurements. The measurement technique is based on the difference in responses of various nitrogen species converted to NO over stainless steel and molybdenum converters at elevated temperatures. The detection limit for ammonia using this difference method has been determined to be approximately 100 ppt with a time resolution of one minute. This modified instrument has been in operation since April 2006 at the Centre for Atmospheric Research Experiments (CARE), a rural site located approximately 80 km north of Toronto, ON, Canada. Typical ammonia mixing ratios in this environment were found to be less then 3 ppb with periodic elevated levels attributed to agricultural activity. The collected data have been compared to other co-located ammonia measurement techniques including various passive samplers, citric acid coated filters, citric acid denuders and commercially available instruments. The comparability of the various methods is presented and the seasonal and diurnal variations of the ambient ammonia mixing ratios are investigated.