Measurement report: Emission factors of NH3 and NHx for wildfires and agricultural fires in the United States

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 con-tained 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 emis-sion factors of NH3 and NHX, with NHX = NH3 +NH4+. Average NH3 and NHX 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 NHX 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 ef-ficiency (MCE) and NH3 emissions. The observed NH3 emissions were significantly higher than measured in previous laboratory experiments in the FIREX FireLab 2016 study.