# Isotopic characterization of nitrogen oxides (NO x), nitrous acid (HONO), and nitrate ( pNO 3−) from laboratory biomass burning during FIREX

New techniques have recently been developed and applied to capture reactive nitrogen species, including nitrogen oxides (inline-formula $M6inlinescrollmathmlchem{\mathrm{normal NO}}_{x}=chem\mathrm{normal NO}+chem{\mathrm{normal NO}}_{normal 2}$ 85pt13ptsvg-formulamathimgf0add4bbe2151ecfa7cd944e28fa7e9e amt-12-6303-2019-ie00004.svg85pt13ptamt-12-6303-2019-ie00004.png ), nitrous acid (HONO), nitric acid (inline-formulaHNO3), and particulate nitrate (inline-formulapinline-formula $M9inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimgb3512ed4eb493ff037a5c39221523c47 amt-12-6303-2019-ie00005.svg25pt16ptamt-12-6303-2019-ie00005.png ), 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 inline-formula∼5 s of production by the fire) that burned various biomass materials from the western US, inline-formulaδ15Ninline-formulaNOx ranges from inline-formula−4.3 ‰ to inline-formula+7.0 ‰, falling near the middle of the range reported in previous work. The first measurements of inline-formulaδ15N–HONO and inline-formulaδ18O–HONO in biomass burning smoke reveal a range of inline-formula−5.3 ‰ to inline-formula+5.8 ‰ and inline-formula+5.2 ‰ to inline-formula+15.2 ‰, respectively. Both HONO and inline-formulaNOx are sourced from N in the biomass fuel, and inline-formulaδ15N–HONO and inline-formulaδ15Ninline-formulaNOx are strongly correlated (inline-formulaR2=0.89, inline-formulap<0.001), suggesting HONO is directly formed via subsequent chain reactions of inline-formulaNOx emitted from biomass combustion. Only 5 of 20 inline-formulapinline-formula $M29inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimgf406d9210c9988b6f1f99fbfd13290fc amt-12-6303-2019-ie00006.svg25pt16ptamt-12-6303-2019-ie00006.png samples had a sufficient amount for isotopic analysis and showed inline-formulaδ15N and inline-formulaδ18O of inline-formulapinline-formula $M33inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimg2fee02b4e30ea20d69bb3f8f98615992 amt-12-6303-2019-ie00007.svg25pt16ptamt-12-6303-2019-ie00007.png ranging from inline-formula−10.6 ‰ to inline-formula−7.4 ‰ and inline-formula+11.5 ‰ to inline-formula+14.8 ‰, respectively.

Our inline-formulaδ15N of inline-formulaNOx, HONO, and inline-formulapinline-formula $M41inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimg12193b9fdcecd5060489ac774923195d amt-12-6303-2019-ie00008.svg25pt16ptamt-12-6303-2019-ie00008.png ranges can serve as important biomass burning source signatures, useful for constraining emissions of these species in environmental applications. The inline-formulaδ18O of HONO and inline-formula $M43inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimg2bdfa65311a7f5f5135cb65d40e792b4 amt-12-6303-2019-ie00009.svg25pt16ptamt-12-6303-2019-ie00009.png obtained here verify that our method is capable of determining the oxygen isotopic composition in BB plumes. The inline-formulaδ18O 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 inpage6304 future field studies to quantitatively track reactive nitrogen cycling in fresh and aged western US wildfire plumes.

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Chai, Jiajue / Miller, David J. / Scheuer, Eric / et al: Isotopic characterization of nitrogen oxides (NOx), nitrous acid (HONO), and nitrate (pNO3−) from laboratory biomass burning during FIREX. 2019. Copernicus Publications.

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