A Chemical Ionization Mass Spectrometer for ambient measurements of Ammonia

Abstract. 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 ~60 pptv at a 1 min integration time (3 sigma). The CIMS time response was <30 s. 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.