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| Titel |
Chemical aging of m-xylene secondary organic aerosol: laboratory chamber study |
| VerfasserIn |
C. L. Loza, P. S. Chhabra, L. D. Yee, J. S. Craven, R. C. Flagan, J. H. Seinfeld |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 1 ; Nr. 12, no. 1 (2012-01-03), S.151-167 |
| Datensatznummer |
250010427
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| Publikation (Nr.) |
 copernicus.org/acp-12-151-2012.pdf |
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| Zusammenfassung |
| Secondary organic aerosol (SOA) can reside in the atmosphere for a week or
more. While its initial formation from the gas-phase oxidation of volatile
organic compounds tends to take place in the first few hours after emission,
SOA can continue to evolve chemically over its atmospheric lifetime.
Simulating this chemical aging over an extended time in the laboratory has
proven to be challenging. We present here a procedure for studying SOA aging
in laboratory chambers that is applied to achieve 36 h of oxidation. The
formation and evolution of SOA from the photooxidation of m-xylene
under low-NOx conditions and in the presence of either neutral or
acidic seed particles is studied. In SOA aging, increasing molecular
functionalization leads to less volatile products and an increase in SOA
mass, whereas gas- or particle-phase fragmentation chemistry results in more
volatile products and a loss of SOA. The challenge is to discern from
measured chamber variables the extent to which these processes are important
for a given SOA system. In the experiments conducted, m-xylene SOA
mass, calculated under the assumption of size-invariant particle composition,
increased over the initial 12–13 h of photooxidation and decreased beyond
that time, suggesting the existence of fragmentation chemistry. The oxidation
of the SOA, as manifested in the O:C elemental ratio and fraction of organic
ion detected at m/z 44 measured by the Aerodyne aerosol mass
spectrometer, increased continuously starting after 5 h of irradiation until
the 36 h termination. This behavior is consistent with an initial period in
which, as the mass of SOA increases, products of higher volatility partition
to the aerosol phase, followed by an aging period in which gas- and
particle-phase reaction products become increasingly more oxidized. When
irradiation is stopped 12.4 h into one experiment, and OH generation ceases,
minimal loss of SOA is observed, indicating that the loss of SOA is either
light- or OH-induced. Chemical ionization mass spectrometry measurements of
low-volatility m-xylene oxidation products exhibit behavior
indicative of continuous photooxidation chemistry. A condensed chemical
mechanism of m-xylene oxidation under low-NOx conditions is
capable of reproducing the general behavior of gas-phase evolution observed
here. Moreover, order of magnitude analysis of the mechanism suggests that
gas-phase OH reaction of low volatility SOA precursors is the dominant
pathway of aging in the m-xylene system although OH reaction with
particle surfaces cannot be ruled out. Finally, the effect of size-dependent
particle composition and size-dependent particle wall loss rates on different
particle wall loss correction methods is discussed. |
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