Modeling the formation of secondary organic aerosol (SOA). 2. The predicted effects of relative humidity on aerosol formation in the alpha-pinene-, beta-pinene-, sabinene-, Delta(3)-Carene-, and cyclohexene-ozone systems

Atmospheric oxidation of volatile organic compounds can lead to the formati on of secondary organic aerosol (SOA) through the gas/particle (G/P) partit ioning of the oxidation products. Since water is ubiquitous in the atmosphe re, the extent of the partitioning for any individual organic product depen ds not only on the amounts and properties of the partitioning organic compo unds, but also on the amount of water present. Predicting the effects of wa ter on the atmospheric G/P distributions of organic compounds is, therefore , central to understanding SOA formation. The goals of the current work are to gain understanding of how increases in RH affect (1) overall SOA yields , (2) water uptake by SOA, (3) the behaviors of individual oxidation produc ts, and (4) the fundamental physical properties of the SOA phase that gover n the G/P distribution of each of the oxidation products. Part 1 of this se ries considered SOA formation from five parent hydrocarbons in the absence of water. This paper predicts how adding RH to those systems uniformly incr eases both the amount of condensed organic mass and the amount of liquid wa ter in the SOA phase. The presence of inorganic components is not considere d. The effect of increasing RH is predicted to be stronger for SOA produced from cyclohexene as compared to SOA produced from tour monoterpenes. This is likely a result of the greater general degree of oxidation (and hydrophi licity) of the cyclohexene products. Good agreement was obtained between pr edicted SOA yields and laboratory SOA yield data actually obtained in the p resence of water. As RH increases, the compounds that play the largest role s in changing both the organic and water masses in the SOA phase are those with va por pressures that a re intermediate between those of essentially n onvolatile and highly volatile species. RH-driven changes in the compound-d ependent G/P partitioning coefficient K-p result from changes in both the a verage molecular weight MWom of the absorbing organic/water phase, and the compound-dependent activity coefficient zeta values. Adding water to the SO A phase by increasing the RH drives down MWom and thereby uniformly favors SOA condensation. The effect of RH on zeta values is compound specific and depends on the hydrophilicity of the specific compound of interest; the mor e hydrophilic a compound, the more increasing RH will favor its condensatio n into the SOA phase. The results also indicate that it may be a useful fir st approximation to assume that zeta = 1 for many compounds making up SOA m ixtures.