Applying a Phase-Separation Parameterization in Modeling Secondary Organic Aerosol Formation from Acid-Driven Reactive Uptake of Isoprene Epoxydiols under Humid Conditions.

Secondary organic aerosol (SOA) from acid-driven reactive uptake of isoprene epoxydiols (IEPOX) contributes up to 40% of organic aerosol (OA) mass in fine particulate matter. Previous work showed that IEPOX substantially converts particulate inorganic sulfates to surface-active organosulfates (OSs). This decreases aerosol acidity and creates a viscous organic-rich shell that poses as a diffusion barrier, inhibiting additional reactive uptake of IEPOX. To account for this "self-limiting" effect, we developed a phase-separation box model to evaluate parameterizations of IEPOX reactive uptake against time-resolved chamber measurements of IEPOX-SOA tracers, including 2-methyltetrols (2-MT) and methyltetrol sulfates (MTS), at ~ 50% relative humidity. The phase-separation model was most sensitive to the mass accommodation coefficient, IEPOX diffusivity in the organic shell, and ratio of the third-order reaction rate constants forming 2-MT and MTS ( k M T / k M T S ). In particular, k M T / k M T S had to be lower than 0.1 to bring model predictions of 2-MT and MTS in closer agreement with chamber measurements; prior studies reported values larger than 0.71. The model-derived rate constants favor more particulate MTS formation due to 2-MT likely off-gassing at ambient-relevant OA loadings. Incorporating this parametrization into chemical transport models is expected to predict lower IEPOX-SOA mass and volatility due to the predominance of OSs.

异戊二烯环氧二醇（IEPOX）在酸驱动下的反应性吸收所形成的二次有机气溶胶（SOA）在细颗粒物中的有机气溶胶（OA）质量中占比高达40%。先前的研究表明，IEPOX会使颗粒状无机硫酸盐大量转化为表面活性有机硫酸盐（OSs）。这会降低气溶胶的酸性，并形成一个富含有机物的粘性外壳，作为扩散屏障，抑制IEPOX的额外反应性吸收。为了解释这种“自限”效应，我们开发了一个相分离箱式模型，以在约50%相对湿度下，根据对IEPOX - SOA示踪物（包括2 - 甲基赤藓醇（2 - MT）和甲基赤藓醇硫酸盐（MTS））的时间分辨箱式测量结果，评估IEPOX反应性吸收的参数化。相分离模型对质量容纳系数、IEPOX在有机外壳中的扩散率以及形成2 - MT和MTS的三级反应速率常数之比（kMT/kMTS）最为敏感。特别是，kMT/kMTS必须小于0.1，才能使2 - MT和MTS的模型预测与箱式测量结果更吻合；先前的研究报告的值大于0.71。由于在与环境相关的OA负荷下2 - MT可能会逸出，模型得出的速率常数更有利于颗粒状MTS的形成。将这种参数化纳入化学传输模型中，预计由于OSs占主导地位，会预测出更低的IEPOX - SOA质量和挥发性。