Reply on RC2

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 US. 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 six wildfires in the Western US, two prescribed grassland fires in the Central US, one prescribed forest fire in the Southern US, and 66 small agricultural fires in the Southeastern US. 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 NHx, with NHx = NH3 + NH4+. Average NH3 and NHx emission factors for wildfires in the Western US were 1.86 ± 0.75 g kg-1 of fuel burned and 2.47 ± 0.80 g kg-1, respectively. Average NH3 and NHx emission factors for agricultural fires in the Southeastern US were 0.89 ± 0.58 g kg-1 and 1.74 ± 0.92 g kg-1, respectively. Our data show no clear inverse correlation between modified combustion efficiency (MCE) and NH3 emissions. Importantly, we found that NH3 emissions in ambient sampling were significantly higher than observed in previous laboratory experiments with similar fuel types.