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| - Modeling the Formation ofSecondary Organic Aerosol (SOA). 2.The Predicted Effects of RelativeHumidity on Aerosol Formation inthe α-Pinene-, β-Pinene-,Sabinene-, Δ3-Carene-, andCyclohexene-Ozone Systems
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| - Atmospheric oxidation of volatile organic compounds canlead to the formation of secondary organic aerosol(SOA) through the gas/particle (G/P) partitioning of theoxidation products. Since water is ubiquitous in theatmosphere, the extent of the partitioning for any individualorganic product depends not only on the amounts andproperties of the partitioning organic compounds, but alsoon the amount of water present. Predicting the effectsof water on the atmospheric G/P distributions of organiccompounds is, therefore, central to understanding SOAformation. The goals of the current work are to gainunderstanding of how increases in RH affect (1) overallSOA yields, (2) water uptake by SOA, (3) the behaviors ofindividual oxidation products, and (4) the fundamentalphysical properties of the SOA phase that govern the G/Pdistribution of each of the oxidation products. Part 1 ofthis series considered SOA formation from five parenthydrocarbons in the absence of water. This paper predictshow adding RH to those systems uniformly increasesboth the amount of condensed organic mass and the amountof liquid water in the SOA phase. The presence ofinorganic components is not considered. The effect ofincreasing RH is predicted to be stronger for SOA producedfrom cyclohexene as compared to SOA produced fromfour monoterpenes. This is likely a result of the greatergeneral degree of oxidation (and hydrophilicity) of thecyclohexene products. Good agreement was obtainedbetween predicted SOA yields and laboratory SOA yielddata actually obtained in the presence of water. AsRH increases, the compounds that play the largest rolesin changing both the organic and water masses in the SOAphase are those with vapor pressures that are intermediatebetween those of essentially nonvolatile and highlyvolatile species. RH-driven changes in the compound-dependent G/P partitioning coefficient Kp result from changesin both the average molecular weight MWom of theabsorbing organic/water phase, and the compound-dependent activity coefficient ζ values. Adding water tothe SOA phase by increasing the RH drives down MWom andthereby uniformly favors SOA condensation. The effectof RH on ζ values is compound specific and depends onthe hydrophilicity of the specific compound of interest; themore hydrophilic a compound, the more increasing RHwill favor its condensation into the SOA phase. The resultsalso indicate that it may be a useful first approximationto assume that ζ = 1 for many compounds making up SOAmixtures.
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