Abstract.  During the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) study, the NASA DC-8 carried out in situ chemical measurements in smoke plumes emitted from wildfires and agricultural fires in the contiguous United States. The DC-8 payload included a modified proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) for the fast measurement of gaseous ammonia (NH₃) and a high-resolution time-of-flight aerosol mass spectrometer (AMS) for the fast measurement of submicron particulate ammonium (NH₄⁺). We herein report data collected in smoke plumes emitted from 6 wildfires in the Western United States, 2 prescribed grassland fires in the Central United States, 1 prescribed forest fire in the Southern United States, and 66 small agricultural fires in the Southeastern United States. Smoke plumes contained double to triple digit ppb levels of NH₃. In the wildfire plumes, a significant fraction of NH₃ had already been converted to NH₄⁺ at the time of sampling (≥2 h after emission). Substantial amounts of NH₄⁺ were also detected in freshly emitted smoke from corn and rice field fires. We herein present a comprehensive set of emission factors of NH₃ and NH_x, with NH_x = NH₃ + NH₄⁺. Average NH₃ and NH_x emission factors for wildfires in the Western United States were 1.86±0.75 g kg⁻¹ and 2.47±0.80 g kg⁻¹ of fuel burned, respectively. Average NH₃ and NH_x emission factors for agricultural fires in the Southeastern United States were 0.89±0.58 and 1.74±0.92 g kg⁻¹, respectively. Our data show no clear inverse correlation between modified combustion efficiency (MCE) and NH₃ emissions. The observed NH₃ emissions were significantly higher than measured in previous laboratory experiments in the FIREX FireLab 2016 study.