Aerosol Hygroscopicity and Surface-Active Coverage for the Droplet Growth of Aerosol Mixtures

Nahin Ferdousi-Rokib, Kotiba A. Malek, Ian Mitchell, Laura M. Fierce and Akua A. Asa-Awuku*, 
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Abstract

The partitioning between inorganic salts and organic compounds within individual particles is a key factor that influences the uptake of water by particles. In this study, we investigated the aerosol hygroscopicity of ammonium sulfate (AS) and 2-methylglutaric acid (2-MGA) mixtures. 2-MGA is a moderately surface-active compound. Dilute surface tension measurements of 2-MGA/AS mixtures were taken by using a pendant drop goniometer. Hygroscopicity at subsaturated conditions was determined using a hygroscopicity tandem differential mobility analyzer (H-TDMA) and relative humidity was kept constant at 89 ± 0.9% RH. The droplet activation was also measured at supersaturated conditions using a cloud condensation nuclei counter (CCNC) from 0.4 to 1% supersaturation (SS). The single-hygroscopicity parameter κ was derived from measurements. Mixtures predominantly composed of AS, up to a 60 wt% 2-MGA, exhibit κ-values close to pure AS. However, κ decreases significantly as the organic fraction increases (>60 wt% 2-MGA). Previous predictions of κ-hygroscopicity assume full dissolution of both the organic and inorganic compounds. However, organic partitioning can influence the κ-hygroscopicity. A coverage-based parametrization, ϕ, assumes the probability of surface-active organics at the droplet surface. By estimation of the bulk and surface organic contribution, overall κ-hygroscopicity can be calculated. The model is computationally efficient, and the results indicate that organic solute depletion should be considered for fully soluble surface-active organics. Hygroscopicity predictions that account for the role of organic surface-active partitioning agree best with experimental results (R2 > 0.95). Therefore, this study helps to enhance our understanding of cloud-forming properties of complex chemical mixtures containing surface-active organic and inorganic compounds.

Abstract Image

气溶胶吸湿性和气溶胶混合物液滴生长的表面活性覆盖
单个颗粒内无机盐和有机化合物之间的分配是影响颗粒吸收水分的关键因素。在这项研究中,我们研究了硫酸铵(AS)和2-甲基戊二酸(2-MGA)混合物的气溶胶吸湿性。2-MGA是一种中等表面活性化合物。用垂滴角计测量了2-MGA/AS混合物的稀释表面张力。采用吸湿性串联差分迁移率分析仪(H-TDMA)测定亚饱和条件下的吸湿性,相对湿度保持在89±0.9% RH。使用云凝结核计数器(CCNC)在过饱和(SS)为0.4%至1%的过饱和条件下测量液滴活化。单吸湿参数κ由测量得到。主要由AS组成的混合物,高达60 wt% 2-MGA,表现出接近纯AS的κ-值。然而,κ随着有机组分的增加而显著降低(>60 wt% 2-MGA)。先前对κ-吸湿性的预测假设有机和无机化合物都能完全溶解。然而,有机分配会影响κ-吸湿性。基于覆盖的参数化,φ,假设表面活性有机物在液滴表面的概率。通过估算体积和表面有机物的贡献,可以计算出总体吸湿性。该模型计算效率高,结果表明,对于完全可溶的表面活性有机物,应考虑有机溶质耗竭。考虑有机表面活性分配作用的吸湿性预测与实验结果最吻合(R2 >;0.95)。因此,这项研究有助于提高我们对含有表面活性有机和无机化合物的复杂化学混合物的云形成特性的理解。
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