Abstract
Abstract. New techniques have recently been developed and applied to capture reactive nitrogen species, including nitrogen oxides (NOx=NO+NO2), nitrous acid (HONO), nitric acid (HNO3), and particulate nitrate (pNO3-), for accurate measurement of their isotopic composition. Here, we report – for the first time – the isotopic composition of HONO from biomass burning (BB) emissions collected during the Fire Influence on Regional to Global Environments Experiment (FIREX, later evolved into FIREX-AQ) at the Missoula Fire Science Laboratory in the fall of 2016. We used our newly developed annular denuder system (ADS), which was verified to completely capture HONO associated with BB in comparison with four other high-time-resolution concentration measurement techniques, including mist chamber–ion chromatography (MC–IC), open-path Fourier transform infrared spectroscopy (OP-FTIR), cavity-enhanced spectroscopy (CES), and proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF). In 20 “stack” fires (direct emission within ∼5 s of production by the fire) that burned various biomass materials from the western US, 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 ‰ to +5.8 ‰ and +5.2 ‰ to +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<0.001), suggesting HONO is directly formed via subsequent chain reactions of NOx emitted from biomass combustion. Only 5 of 20 pNO3- samples had a sufficient amount for isotopic analysis and showed δ15N and δ18O of pNO3- ranging from −10.6 ‰ to −7.4 ‰ and +11.5 ‰ to +14.8 ‰, respectively. Our δ15N of NOx, HONO, and pNO3- ranges can serve as important biomass burning source signatures, useful for constraining emissions of these species in environmental applications. The δ18O of HONO and NO3- obtained here verify that our method is capable of determining the oxygen isotopic composition in BB plumes. The δ18O values for both of these species reflect laboratory conditions (i.e., a lack of photochemistry) and would be expected to track with the influence of different oxidation pathways in real environments. The methods used in this study will be further applied in future field studies to quantitatively track reactive nitrogen cycling in fresh and aged western US wildfire plumes.
Highlights
Biomass burning (BB), which occurs in both anthropogenic processes and natural wildfire, is a significant source of atmospheric reactive nitrogen species, including nitrogen oxides (NOx = NO + NO2), nitrous acid (HONO), nitric acid (HNO3), particulate nitrate, organic nitrates, peroxyacyl nitrate (PAN), and ammonia (NH3), that have major impacts on air quality and climate from regional to global scales (Crutzen and Andreae, 1990)
The time series of HONO and HNO3 concentrations measured by mist chamber–ion chromatography (MC–IC) at 5 min resolution for the majority of the stack burns are shown in Fig. 1, and original data can be found in the NOAA data archive (FIREX, 2016)
Fire no. 15 and no. 17 have relatively low MCE (∼ 0.89), the pulse of HONO in first 5–10 min suggests an active flaming phase followed by a longer smoldering phase. This indicates that both fires had combustion conditions that consisted of a mixture of flaming and smoldering, and significant HONO was still produced
Summary
Fibiger et al (2014) developed a method to quantitatively collect NOx in solution as NO−3 for isotopic analysis, which has been verified to avoid any isotopic fractionation during collection in both lab and field studies This allows for high-resolution measurement of δ15N–NOx in minutes to hours depending on ambient NOx concentrations (δ15N = [(15N/14N)sample/(15N/14N)air−N2 − 1] × 1000 ‰, and δ18O = [(18O/16O)sample/(18O/16O)VSMOW − 1] × 1000 ‰ where VSMOW is Vienna Standard Mean Ocean Water). Δ15N has been used to track gaseous NOx from a variety of major sources including emissions from biomass burning (Fibiger and Hastings, 2016), vehicles (Miller et al, 2017), and agricultural soils (Miller et al, 2018) Using this method, Fibiger and Hastings (2016) systematically investigated BB δ15N–NOx from different types of biomass from around the world in a controlled environment during the fourth Fire Lab at Missoula Experiment (FLAME-4). This work offers a characterization and quantification of the BB source signatures of these species, which can be applied in the interpretation of observations in future field studies
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