Relationships of the Hygroscopicity of HULIS With Their Degrees of Oxygenation and Sources in the Urban Atmosphere

Abstract Humic‐like substances (HULIS) are unresolved compounds isolated from organic matter in atmospheric aerosols. Understanding the chemical structures and sources that control the hygroscopicity of HULIS is important for predicting its environmental impacts and possibly useful for parameterization of the hygroscopicity of organic aerosol (OA). Here, the hygroscopic growth of HULIS from total suspended particles samples in Beijing was analyzed using a hygroscopicity tandem differential mobility analyzer. The hygroscopicity parameter of HULIS ( κ HULIS ) was determined to be 0.06 ± 0.03, and was found to correlate positively with O:C and f 44 from offline aerosol mass spectrometry. Additionally, κ HULIS was correlated positively with oxygenated organic aerosol (OOA) and correlated negatively with cooking OA and biomass burning OA, for which the source‐related OA types were from mass‐spectral‐based positive matrix factorization analyses. Further analysis resolved substantial κ values of 0.09 and 0.22, respectively, for less‐oxidized and more‐oxidized OOA, in contrast to the low κ values for cooking OA and biomass burning OA. Extensions of O:C‐ and source‐based parameterizations reasonably estimated the hygroscopicity of total OA with a moderate contribution from HULIS. This result and the fact that the obtained OOA contributions to κ of OA were similar to those from different environments suggest the applicability of parameterizing the hygroscopicity for a wide range of OA in particular based on source‐related structural characteristics. Plain Language Summary Humic‐like substances (HULIS) are ubiquitous components of atmospheric organic aerosol (OA). Understanding and parameterizing the hygroscopicity of HULIS is expected to facilitate predictions of aerosol budgets and related environmental impacts. In this study, the hygroscopicity parameters of HULIS were determined and found to be positively correlated with the oxygen‐to‐carbon elemental ratio (O:C). Furthermore, contributions from the sources of HULIS to their hygroscopicity were resolved, and a significant influence of oxygenated OA mass on the hygroscopicity of HULIS was revealed. The parameterization of hygroscopicity based on O:C or source information reconstructed the hygroscopicity of HULIS well, particularly for source‐based approach. These two parameterization approaches reasonably estimated the hygroscopicity of total OA in the studied aerosol, and the contributions from oxygenated OA masses were consistent with those in other environments, both of which suggest wide applicability of this parameterization method for OA fractions in addition to total OA. Key Points The hygroscopicity parameter ( κ ) of Humic‐like substances (HULIS) was 0.06 ± 0.03, and it correlated positively with O:C and f 44 Organic mass assigned to the oxygenated organic aerosol (OA) controls the hygroscopicity of HULIS Source‐based parameterization of the κ of OAs is applicable for different ranges of OA components