A chemical ionization mass spectrometer (CIMS) utilizing protonated acetone dimer ion chemistry to measure gas-phase ammonia (NH3) from the NOAA WP-3D aircraft is described. The average sensitivity determined from in-flight standard addition calibrations ranged from 2.6 to 5 ion counts s−1 pptv−1, depending on flow conditions, for 1 MHz of reagent ion signal. The instrument time response was determined to be 5 s from the 2 e-folding signal decay time after removal of a standard addition calibration. The instrumental background varied from flight to flight ranging from 0.5 to 1.3 ppbv. The variability between successive background measurements ranged from 50 pptv to 100 pptv. Total uncertainty for the 5 s data was conservatively estimated to be ±(30% + 125 pptv). Two NH3 sources were sampled during the New England Air Quality Study–Intercontinental Transport and Chemical Transformation (NEAQS-ITCT) 2004 campaign, one urban and one agricultural. During the 25 July flight, enhancements in NH3 mixing ratios were coincident with enhancements in CO, NOx, and SO2 mixing ratios downwind of New York City. The NH3 mixing ratios in the urban outflow plume ranged from 0.4 to 1 ppbv, or enhancements of 0.2 to 0.8 ppbv above local background. During the 15 August flight, NH3 mixing ratios were enhanced 0.3 to 0.45 ppbv above local background directly downwind of an agricultural area northeast of Atlanta, Georgia. The NH3 CIMS instrument has shown the ability to measure sub-ppbv NH3 levels at high time resolution from an aircraft.