Secondary organic aerosol formation from camphene oxidation: measurements and modeling
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Published:2022-03-09
Issue:5
Volume:22
Page:3131-3147
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Li Qi, Jiang JiaORCID, Afreh Isaac K., Barsanti Kelley C.ORCID, Cocker III David R.
Abstract
Abstract. While camphene is one of the dominant monoterpenes
measured in biogenic and pyrogenic emission samples, oxidation of camphene
has not been well-studied in environmental chambers and very little is known
about its potential to form secondary organic aerosol (SOA). The lack of
chamber-derived SOA data for camphene may lead to significant uncertainties
in predictions of SOA from oxidation of monoterpenes using existing
parameterizations when camphene is a significant contributor to total
monoterpenes. Therefore, to advance the understanding of camphene oxidation
and SOA formation and to improve representation of camphene in air quality
models, a series of experiments was performed in the University of
California Riverside environmental chamber to explore camphene SOA mass
yields and properties across a range of chemical conditions at
atmospherically relevant OH concentrations. The experimental results were
compared with modeling simulations obtained using two chemically detailed
box models: Statewide Air Pollution Research Center (SAPRC) and Generator
for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A).
SOA parameterizations were derived from the chamber data using both the
two-product and volatility basis set (VBS) approaches. Experiments performed
with added nitrogen oxides (NOx) resulted in higher SOA mass yields (up
to 64 %) than experiments performed without added NOx (up to 28 %).
In addition, camphene SOA mass yields increased with SOA mass (Mo) at
lower mass loadings, but a threshold was reached at higher mass loadings in
which the SOA mass yields no longer increased with Mo. SAPRC modeling
of the chamber studies suggested that the higher SOA mass yields at higher
initial NOx levels were primarily due to higher production of peroxy
radicals (RO2) and the generation of highly oxygenated organic
molecules (HOMs) formed through unimolecular RO2 reactions. SAPRC
predicted that in the presence of NOx, camphene RO2 reacts with NO
and the resultant RO2 undergoes hydrogen (H)-shift isomerization
reactions; as has been documented previously, such reactions rapidly add
oxygen and lead to products with very low volatility (i.e., HOMs). The end
products formed in the presence of NOx have significantly lower
volatilities, and higher O : C ratios, than those formed by initial camphene
RO2 reacting with hydroperoxyl radicals (HO2) or other RO2.
Further analysis reveals the existence of an extreme NOx regime, wherein
the SOA mass yield can be suppressed again due to high NO / HO2 ratios.
Moreover, particle densities were found to decrease from 1.47 to 1.30 g cm−3 as [HC]0 / [NOx]0 increased and O : C decreased. The
observed differences in SOA mass yields were largely explained by the
gas-phase RO2 chemistry and the competition between RO2+
HO2, RO2+ NO, RO2+ RO2, and RO2 autoxidation
reactions.
Funder
Division of Atmospheric and Geospace Sciences
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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