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| Titel |
Organic photolysis reactions in tropospheric aerosols: effect on secondary organic aerosol formation and lifetime |
| VerfasserIn |
A. Hodzic, S. Madronich, P. S. Kasibhatla, G. Tyndall, B. Aumont, J. L. Jimenez, J. Lee-Taylor, J. Orlando |
| 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 ; 15, no. 16 ; Nr. 15, no. 16 (2015-08-20), S.9253-9269 |
| Datensatznummer |
250119979
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| Publikation (Nr.) |
 copernicus.org/acp-15-9253-2015.pdf |
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| Zusammenfassung |
| This study presents the first modeling estimates of the potential effect of
gas- and particle-phase organic photolysis reactions on the formation and
lifetime of secondary organic aerosols (SOAs). Typically only photolysis of
smaller organic molecules (e.g., formaldehyde) for which explicit data exist
is included in chemistry–climate models. Here, we specifically examine the
photolysis of larger molecules that actively partition between the gas and
particle phases. The chemical mechanism generator GECKO-A is used to
explicitly model SOA formation from α-pinene, toluene, and C12
and C16 n-alkane reactions with OH at low and high NOx.
Simulations are conducted for typical mid-latitude conditions and a solar
zenith angle of 45° (permanent daylight). The results show that
after 4 days of chemical aging under those conditions (equivalent to
8 days in the summer mid-latitudes), gas-phase photolysis leads to a
moderate decrease in SOA yields, i.e., ~15 % (low NOx) to
~45 % (high NOx) for α-pinene, ~15 % for toluene, ~25 % for C12 n-alkane, and
~10 % for C16 n-alkane. The small effect of gas-phase
photolysis on low-volatility n-alkanes such as C16 n-alkane is due to the
rapid partitioning of early-generation products to the particle phase, where
they are protected from gas-phase photolysis. Minor changes are found in the
volatility distribution of organic products and in oxygen to carbon ratios.
The decrease in SOA mass is increasingly more important after a day of
chemical processing, suggesting that most laboratory experiments are likely
too short to quantify the effect of gas-phase photolysis on SOA yields. Our
results also suggest that many molecules containing chromophores are
preferentially partitioned into the particle phase before they can be
photolyzed in the gas phase. Given the growing experimental evidence that
these molecules can undergo in-particle photolysis, we performed sensitivity
simulations using an empirically estimated SOA photolysis rate of
JSOA = 4 × 10−4 JNO2. Modeling results indicate that
this photolytic loss rate would decrease SOA mass by 40–60 % for most
species after 10 days of equivalent atmospheric aging at mid-latitudes in
the summer. It should be noted that in our simulations we do not consider
in-particle or aqueous-phase reactions which could modify the chemical
composition of the particle and thus the quantity of photolabile species. The
atmospheric implications of our results are significant for both the SOA
global distribution and lifetime. GEOS-Chem global model results suggest
that particle-phase photolytic reactions could be an important loss process
for SOA in the atmosphere, removing aerosols from the troposphere on
timescales of less than 7 days that are comparable to wet deposition. |
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