Hygroscopicity of urban aerosols and its link to size-resolved chemical composition during spring/summertime in Seoul, Korea

Abstract. Chemical effects on the size-resolved hygroscopicity of urban aerosols were examined based on the KORUS-AQ field campaign data. The information on size-resolved hygroscopicity and chemical composition of aerosols were obtained by a hygroscopic tandem differential mobility analyzer (HTDMA) and a high-resolution time of flight aerosol mass spectrometer (HR-ToF-AMS), respectively. Good correspondences were shown between the measured and estimated κ values calculated from the combination of bulk chemical composition data and oxidation parameters of organic aerosols (f44 and O/C). These results infer that chemical composition is closely associated with aerosol hygroscopicity. However, the correlation between measured and estimated κ values degraded as particle size decreased, implying that the size-resolved chemical composition data is required for more detailed hygroscopicity analysis. In addition to size-resolved chemical data, the m/z tracer method was applied for size-resolved organic factors. Specifically, m/z 57 and 44 were used as AMS spectral markers for HOA and OOA, respectively. These size-resolved chemical composition data were found to be critical in explaining the size-dependent hygroscopicity as well as the diurnal variation of κ for small particles, i.e., low κ in the morning and high κ in the afternoon. Additionally, aerosol mixing state information was associated with the size-resolved chemical composition data. That is, the relationship between the number fraction of each hygroscopicity mode and the volume fraction of different chemical composition was investigated. It was found that the HOA volume fraction explained about 60 % of the variation of less hygroscopic (LH) mode number fraction for externally mixed aerosols.