Isotopic characterization of nitrogen oxides (NOx), nitrous acid (HONO), and nitrate (NO3−(p)) from laboratory biomass burning during FIREX

New techniques have recently been developed to capture reactive nitrogen species for accurate measurement of their isotopic composition. Reactive nitrogen species play important roles in atmospheric oxidation capacity (hydroxyl radical and ozone formation) and may have impacts on air quality and climate. Tracking reactive nitrogen species and their chemistry in the atmosphere based upon concentration alone is challenging. Isotopic analysis provides a potential tool for tracking the sources and chemistry of species such as nitrogen oxides (NOx = NO + NO2), nitrous acid (HONO), nitric acid (HNO3) and particulate nitrate (NO3−(p)). Here we study direct biomass burning (BB) emissions during the Fire Influence on Regional to Global Environments Experiment (FIREX, later evolved into FIREX-AQ) laboratory experiments at the Missoula Fire Laboratory in the fall of 2016. An annular denuder system (ADS) developed to efficiently collect HONO for isotopic composition analysis was deployed to the Fire Lab study. Concentrations of HONO recovered from the ADS collection agree well with mean concentrations averaged over each fire measured by 4 other high time resolution techniques, including mist chamber/ion chromatography (MC/IC), open-path Fourier transform infrared spectroscopy (OP-FTIR), cavity enhanced spectroscopy (CES), proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF). The concentration validation ensures complete collection of BB emitted HONO, of which the isotopic composition is preserved during the collection process. In addition, the isotopic composition of NOx and NO3−(p) from direct BB emissions were also characterized. In 20 stack fires (direct emission within ~ 5 seconds of production by the fire) that burned various biomass materials, δ15N-NOx ranges from −4.3 ‰ to +7.0 ‰, falling near the middle of the range reported in previous work. The first measurements of δ15N-HONO and δ18O-HONO in biomass burning smoke reveal a range of −5.3 – +5.8 ‰ and +5.2 – +15.2 ‰ respectively. Both HONO and NOx are sourced from N in the biomass fuel and δ15N-HONO and δ15N-NOx are strongly correlated (R2 = 0.89, p