A Review on Laboratory Studies and Field Measurements of Atmospheric Organic Aerosol Hygroscopicity and Its Parameterization Based on Oxidation Levels

Purpose of Review The study of organic aerosol hygroscopic growth and cloud droplet activation is crucial for accurately quantifying their climate and environmental impacts. However, the physical mechanisms behind organic aerosol hygroscopicity variations are not well understood. In this review, we summarized laboratory and field measurements of the organic aerosol hygroscopicity parameter kappa(OA), discussed the physical understanding of why kappa(OA)was generally positively correlated with organic aerosol oxidation level, summarized proposed kappa(OA)parameterization schemes, and examined possible explanations for the marked differences among these parameterization schemes. Recent Findings Recent findings challenged the general cognition that cloud condensation nuclei (CCN) activity of secondary organic aerosol depended largely on solubility, showing it to be mainly controlled by molecular weight, yet the universality of this finding needs to be further examined. It was found that carbon chain length and functional groups had significant impacts on kappa(OA)and additional parameters other than O/C ratio need to be included when parameterizing kappa(OA)of multifunctional compounds, which is typically the case for ambient atmospheric aerosols. Additionally, laboratory results of secondary organic aerosol suggest that kappa(OA)might be highly RH-dependent under sub-saturated conditions, especially for biogenic secondary organic aerosols. This review summarized laboratory and field measurements of atmospheric organic aerosol hygroscopicity parameter kappa(OA)and its parameterization schemes. The results demonstrate that representing kappa(OA)with a single oxidation level parameter still bears large uncertainty, and physical mechanisms associated with hygroscopic growth and cloud activation processes of organic aerosol still remain unresolved and need further investigation.