Seinfeld J H, Erdakos G B, Asher W E, Pankow J F
Department of Chemical Engineering, Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, USA.
Environ Sci Technol. 2001 May 1;35(9):1806-17. doi: 10.1021/es001765+.
Atmospheric oxidation of volatile organic compounds can lead to the formation of secondary organic aerosol (SOA) through the gas/particle (G/P) partitioning of the oxidation products. Since water is ubiquitous in the atmosphere, the extent of the partitioning for any individual organic product depends not only on the amounts and properties of the partitioning organic compounds, but also on the amount of water present. Predicting the effects of water 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 products, and (4) the fundamental physical properties of the SOA phase that govern the G/P distribution of each of the oxidation products. Part 1 of this series considered SOA formation from five parent hydrocarbons in the absence of water. This paper predicts how adding RH to those systems uniformly increases both the amount of condensed organic mass and the amount of liquid water in the SOA phase. The presence of inorganic components is not considered. The effect of increasing RH is predicted to be stronger for SOA produced from cyclohexene as compared to SOA produced from four monoterpenes. This is likely a result of the greater general degree of oxidation (and hydrophilicity) of the cyclohexene products. Good agreement was obtained between predicted SOA yields and laboratory SOA yield data actually obtained in the presence of water. As RH increases, the compounds that play the largest roles in changing both the organic and water masses in the SOA phase are those with vapor pressures that are intermediate between those of essentially nonvolatile and highly volatile species. RH-driven changes in the compound-dependent G/P partitioning coefficient Kp result from changes in both the average molecular weight MWom of the absorbing organic/water phase, and the compound-dependent activity coefficient zeta values. Adding water to the SOA 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 more hydrophilic a compound, the more increasing RH will favor its condensation into the SOA phase. The results also indicate that it may be a useful first approximation to assume that zeta = 1 for many compounds making up SOA mixtures.
挥发性有机化合物的大气氧化可通过氧化产物的气/粒（G/P）分配导致二次有机气溶胶（SOA）的形成。由于水在大气中无处不在，任何单一有机产物的分配程度不仅取决于参与分配的有机化合物的数量和性质，还取决于水的含量。因此，预测水对有机化合物大气G/P分布的影响是理解SOA形成的核心。当前工作的目标是了解相对湿度（RH）的增加如何影响：（1）SOA的总体产率；（2）SOA对水的摄取；（3）单个氧化产物的行为；（4）控制各氧化产物G/P分布的SOA相的基本物理性质。本系列的第1部分考虑了在无水情况下由五种母体烃形成SOA。本文预测了向这些体系中添加RH如何一致地增加SOA相中冷凝有机物质的量和液态水的量。未考虑无机成分的存在。预计与由四种单萜类化合物生成的SOA相比，由环己烯生成的SOA中，增加RH的影响更强。这可能是环己烯产物氧化程度（和亲水性）总体较高的结果。预测的SOA产率与在有水情况下实际获得的实验室SOA产率数据之间取得了良好的一致性。随着RH增加，在改变SOA相中有机物质和水的量方面起最大作用的化合物是那些蒸气压介于基本不挥发和高挥发性物质之间的化合物。RH驱动的与化合物相关的G/P分配系数Kp的变化是由吸收性有机/水相的平均分子量MWom以及与化合物相关的活度系数ζ值的变化引起的。通过增加RH向SOA相中加水会降低MWom，从而一致地有利于SOA的冷凝。RH对ζ值的影响因化合物而异，取决于所关注的特定化合物的亲水性；化合物的亲水性越强，增加RH就越有利于其冷凝进入SOA相。结果还表明，对于构成SOA混合物的许多化合物，假设ζ = 1可能是一个有用的一阶近似。
