This study presents the first modeling estimates of the potential effect ofgas- and particle-phase organic photolysis reactions on the formation andlifetime of secondary organic aerosols (SOAs). Typically only photolysis ofsmaller organic molecules (e.g., formaldehyde) for which explicit data existis included in chemistry–climate models. Here, we specifically examine thephotolysis of larger molecules that actively partition between the gas andparticle phases. The chemical mechanism generator GECKO-A is used toexplicitly model SOA formation from α-pinene, toluene, and Cand C n-alkane reactions with OH at low and high NO.Simulations are conducted for typical mid-latitude conditions and a solarzenith angle of 45° (permanent daylight). The results show thatafter 4 days of chemical aging under those conditions (equivalent to8 days in the summer mid-latitudes), gas-phase photolysis leads to amoderate decrease in SOA yields, i.e., ~15 % (low NO) to~45 % (high NO) for α-pinene, ~15 % for toluene, ~25 % for C n-alkane, and~10 % for C n-alkane. The small effect of gas-phasephotolysis on low-volatility n-alkanes such as C n-alkane is due to therapid partitioning of early-generation products to the particle phase, wherethey are protected from gas-phase photolysis. Minor changes are found in thevolatility distribution of organic products and in oxygen to carbon ratios.The decrease in SOA mass is increasingly more important after a day ofchemical processing, suggesting that most laboratory experiments are likelytoo short to quantify the effect of gas-phase photolysis on SOA yields. Ourresults also suggest that many molecules containing chromophores arepreferentially partitioned into the particle phase before they can bephotolyzed in the gas phase. Given the growing experimental evidence thatthese molecules can undergo in-particle photolysis, we performed sensitivitysimulations using an empirically estimated SOA photolysis rate of = 4 × 10 . Modeling results indicate thatthis photolytic loss rate would decrease SOA mass by 40–60 % for mostspecies after 10 days of equivalent atmospheric aging at mid-latitudes inthe summer. It should be noted that in our simulations we do not considerin-particle or aqueous-phase reactions which could modify the chemicalcomposition of the particle and thus the quantity of photolabile species. Theatmospheric implications of our results are significant for both the SOAglobal distribution and lifetime. GEOS-Chem global model results suggestthat particle-phase photolytic reactions could be an important loss processfor SOA in the atmosphere, removing aerosols from the troposphere ontimescales of less than 7 days that are comparable to wet deposition.
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