A Near-Explicit Mechanistic Evaluation of Isoprene
Photochemical Secondary Organic Aerosol Formation and Evolution: Simulations
of Multiple Chamber Experiments with and without Added NOx
Posted on 2020-06-22 - 17:04
Experimentally
determined yields of secondary organic aerosol (SOA)
from the photochemical oxidation of isoprene in the absence of aqueous
acidic aerosol vary substantially, both within a given experiment
and across different environmental chamber conditions. The underlying
mechanisms driving this variation remain poorly evaluated, leading
to significant uncertainty in how to extrapolate laboratory chamber
results to the atmosphere. Herein, we compare SOA predictions from
a near-explicit gas-phase chemical mechanism of isoprene oxidation
by the hydroxyl radical (OH) in the presence and absence of nitrogen
oxide radicals (NOx), to multiple chamber
experiments on non-aqueous isoprene photochemical SOA (ipSOA) conducted
by different groups in different chambers. SOA is predicted by volatility-driven
gas-particle partitioning of hundreds of individual reaction products.
The mechanism includes simplified descriptions of particle-phase organic
chemistry, including organic hydroperoxide photolysis, and organic
nitrate hydrolysis and accretion reactions. The model has good skill
(mean normalized bias typically within 25%) at predicting the observed
formation and evolution of ipSOA across a range of chambers and conditions
at low NOx. The model has much less skill
at describing the observed non-linear response of ipSOA to elevated
NOx. Organic nitrate hydrolysis is unable
to explain significant ipSOA at high NOx, whereas particle-phase accretion reactions of tertiary nitrates
may play a role. Uncertainties in the chamber radical environment
and fate of key organic peroxy radicals (RO2) remain as
or even more important than vapor losses to chamber walls in determining
how best to extrapolate chamber-based yields to the atmosphere. Implications
for likely atmospheric yields of ipSOA and recommendations for future
chamber experiments are discussed.
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Thornton, Joel A.; Shilling, John E.; Shrivastava, Manish; D’Ambro, Emma L.; Zawadowicz, Maria A.; Liu, Jiumeng (2020). A Near-Explicit Mechanistic Evaluation of Isoprene
Photochemical Secondary Organic Aerosol Formation and Evolution: Simulations
of Multiple Chamber Experiments with and without Added NOx. ACS Publications. Collection. https://doi.org/10.1021/acsearthspacechem.0c00118
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AUTHORS (6)
JT
Joel A. Thornton
JS
John E. Shilling
MS
Manish Shrivastava
ED
Emma L. D’Ambro
MZ
Maria A. Zawadowicz
JL
Jiumeng Liu
KEYWORDS
Multiple Chamber ExperimentsSOA predictionsperoxy radicalsnear-explicit gas-phase chemical mechanismchamber conditionsIsoprene Photochemicalchamber experimentsOrganic Aerosol FormationOHparticle-phase accretion reactionschamber wallsnitrate hydrolysisOrganic nitrate hydrolysisNear-Explicit Mechanistic Evaluationisoprene oxidationipSOARO 2chamber-based yieldsnon-aqueous isopreneaccretion reactionsvolatility-driven gas-particle partitioninglaboratory chamber resultsnitrogen oxide radicalsacidic aerosolfuture chamber experimentsvapor lossesreaction productshydroperoxide photolysis