A Chemical Ionization Mass Spectrometer for ambient measurements of Ammonia

This study presents a chemical ionization mass spectrometer (CIMS) for fast response, in-situ measurements of gas phase ammonia (NH3). The NH3 background level detected with the CIMS ranged between 0.3-1 ppbv, with an uncertainty of 30 pptv under optimized conditions. The instrument sensitivity varied from 4-25 Hz/pptv for > 1 MHz of reagent ion signals (protonated ethanol ions), with a 30% uncertainty estimated based on variability in calibration signals. The CIMS detection limit for NH3 was similar to 60 pptv at a 1 min integration time (3 sigma). The CIMS time response was < 30s. This new NH3-CIMS has been used for ambient measurements in Kent, Ohio, for several weeks throughout three seasons. The measured NH3 mixing ratios were usually at the sub-ppbv level and higher in spring (200 +/- 120 pptv) than in winter (60 +/- 75 pptv) and fall (150 +/- 80 pptv). High emissions of SO2 from power plants in this region, and thus possible high acidity of aerosol particles, may explain these low NH3 mixing ratios in general.