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.
