Isotopic characterization of nitrogen oxides (NO<sub>x</sub>), nitrous acid (HONO), and nitrate (NO<sub>3</sub><sup>−</sup>(p)) from laboratory biomass burning during FIREX

Abstract

1 2 New techniques have recently been developed to capture reactive nitrogen species for 3 accurate measurement of their isotopic composition. Reactive nitrogen species play 4 important roles in atmospheric oxidation capacity (hydroxyl radical and ozone formation) 5 and may have impacts on air quality and climate. Tracking reactive nitrogen species and 6 their chemistry in the atmosphere based upon concentration alone is challenging. Isotopic 7 analysis provides a potential tool for tracking the sources and chemistry of species such 8 as nitrogen oxides (NO x = NO + NO 2 ), nitrous acid (HONO), nitric acid (HNO 3 ) and 9 particulate nitrate (NO 3 -(p)). Here we study direct biomass burning (BB) emissions 10 during the Fire Influence on Regional to Global Environments Experiment (FIREX, later 11 evolved into FIREX-AQ) laboratory experiments at the Missoula Fire Laboratory in the 12 fall of 2016. 13 14 An annular denuder system (ADS) developed to efficiently collect HONO for isotopic 15 composition analysis was deployed to the Fire Lab study. Concentrations of HONO 16 recovered from the ADS collection agree well with mean concentrations averaged over 17 each fire measured by 4 other high time resolution techniques, including mist 18 chamber/ion chromatography (MC/IC), open-path Fourier transform infrared 19 spectroscopy (OP-FTIR), cavity enhanced spectroscopy (CES), proton-transfer-reaction 20 time-of-flight mass spectrometer (PTR-ToF). The concentration validation ensures 21 complete collection of BB emitted HONO, of which the isotopic composition is 22preserved during the collection process. In addition, the isotopic composition of NO x and 23 NO 3 -(p) from direct BB emissions were also characterized. 24In 20 "stack" fires (direct emission within ~5 seconds of production by the fire) that 25 burned various biomass materials, δ 15 N-NO x ranges from -4.3 ‰ to +7.0 ‰, falling near 26 the middle of the range reported in previous work. The first measurements of δ 15 N-27HONO and δ 18 O-HONO in biomass burning smoke reveal a range of -5.3 -+5.8 ‰ and 28 +5.2 -+15.2 ‰ respectively. Both HONO and NO x are sourced from N in the biomass 29 fuel and δ 15 N-HONO and δ 15 N-NO x are strongly correlated (R 2 = 0.89, p<0.001), 30suggesting NO x and HONO are connected via formation pathways. 31 32Our δ 15 N of NO x , HONO and NO 3 -(p) ranges can serve as important biomass burning 33 source signatures, useful for constraining direct emissions of these species in 34 environmental applications. The δ 18 O of HONO and NO 3 obtained here verify our 35 method is capable of determining oxygen isotopic composition in BB plumes. The δ 18 O 36for both species in this study reflect the laboratory conditions (i.e. a lack of 37 photochemistry), and would be expected to track with the influence of ozone (O 3 ), 38 photochemistry and nighttime chemistry in real environments. The methods used in this 39 study will be further applied in future field studies to quantitatively track reactive 40 nitrogen cycling in fresh and aged Western US wild...

Collection of Nitrogen Dioxide for Nitrogen and Oxygen Isotope Determination─Laboratory and Environmental Chamber Evaluation

“…Interference trials indicate that NO 2 • was not detectable on the NaCl denuder extracts; however, significant amounts of NO 2 – were collected on Na 2 CO 3 coated denuder extracts, indicating a slight interference associated with NO 2 • collection (I = 2.2 ± 0.1%) (Figure b). This value is consistent with previous reports of NO 2 • collection on Na 2 CO 3 denuders that have indicated a collection amount ranging between 1 and 7% of total NO 2 • . − While the NaCl denuder extracts indicated no significant NO 2 – , there was a detectable and consistent collection of NO 3 – (1 ± 0.1 ppb v ). Neither NO 2 – nor NO 3 – were detected on NaCl denuders when only flowing N 2 .…”

HONO Emissions from Western U.S. Wildfires Provide Dominant Radical Source in Fresh Wildfire Smoke

“…Akagi et al observed Δ H ONO/ΔNO x molar ratios spanning from 0.16 to 0.33 pptv pptv –1 in South Carolina biomass burning plumes. Chai et al determined an overall range of HONO/NO x ratio from 0.13 to 0.53 pptv pptv –1 for fresh emissions of 20 laboratory fires of different fuels. The laboratory studies by Burling et al found the fire-integrated molar emission ratios of HONO relative to NO x ranged from approximately 0.03 to 0.20 pptv pptv –1 , with higher values observed for southeastern fuels, while later airborne measurements from prescribed burning in southwestern U.S. by Burling et al revealed systematically higher Δ H ONO/ΔNO x molar ratios spanning from 0.077 to 0.22 pptv pptv –1 .…”

“…At combustion temperatures over 1000 K, increased fuel rank tends to produce more HCN, while decreased fuel rank favors NH 3 formation. These pyrolysis products then undergo further in-flame processing in the flaming stage. , It is hypothesized that HONO emission results from conversion of NO or NO 2 in series depending on temperature in the vicinity of the flame, where fast oxidations of HCN and NH 3 also occur. , Thus, HONO emissions are expected to anticorrelate with HCN and NH 3 , as they are intermediate products that are being oxidized in the flame chemistry, while HONO emissions are expected to correlate with MCE and NO x , corresponding to higher combustion temperatures which favor more oxidized nitrogen. MCE ranged from 0.86 to >0.94 in WE-CAN, suggesting some of the fire to fire variation in our reported ERs results from burning conditions alone.…”

Formation Mechanisms and Source Apportionments of Airborne Nitrate Aerosols at a Himalayan-Tibetan Plateau Site: Insights from Nitrogen and Oxygen Isotopic Compositions

“…To quantify the potential sources of nitrate aerosols, the δ 15 N-NO 3 – values in ambient samples and δ 15 N values in NO x (δ 15 N-NO x ) from various emission sources must serve as input in the SIAR model. Here, the δ 15 N-NO x values of emission sources were obtained from those reported in the literature studies. − Vehicle emission and coal combustion are well-known important sources of atmospheric NO x . , In addition, BB (e.g., open burning for agricultural waste and weeds to obtain land for sowing) and biogenic soil emission also contributed a part of atmospheric NO x . , Consequently, four emission sources mentioned above were considered in apportioning the emission sources of nitrate aerosols at QOMS. Table S1 lists the δ 15 N-NO x values in the four emission sources used in the current study.…”