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

Abstract

Abstract. Chemical effects on the size-resolved hygroscopicity of urban aerosols were examined based on the Korea–US Air Quality Study (KORUS-AQ, 2020) field campaign data. The information on size-resolved hygroscopicity and the 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 correspondence was shown between 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 imply 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 size-resolved chemical composition data are 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 hydrocarbon-like organic aerosol (HOA) and oxygenated organic aerosol (OOA), respectively. These size-resolved chemical composition data were found to be critical in explaining size-dependent hygroscopicity, as well as the diurnal variation in 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 to say that the relationship between the number fraction of each hygroscopicity mode and the volume fraction of different chemical composition was investigated. For example, the HOA volume fraction comprised about 60 % of the variation in less hygroscopic (LH) mode number fractions for externally mixed aerosols.

Measurement Report: Distinct size dependence and diurnal variation of OA hygroscopicity, volatility, and CCN activity at a rural site in the Pearl River Delta (PRD) region, China

“…3) ranged from 0.058 to 0.09, within the range (0.05-0.15 at 100 nm) previously reported in the PRD region (Hong et al, 2018) and slightly higher than that (0.03-0.06 at 250 nm) at a mountain site in Germany (Wu et al, 2013). In general, the κOA values increased with particle sizes from 0.058 at 30 nm to 0.09 at 150 and 200 nm, similar to the feature observed in urban and forest environments (Kim et al, 2020;Deng et al, 2019). The increases of the κOA values with particle sizes could be explained by the oxidation level of organic aerosols (Massoli et al, 2010;Lambe et al, 2011;Xu et al, 2021).…”

“…6). As reported in literature, the hygroscopicity of organics was partly dependent on the aging degree (Liu et al, 2021;Zhao et al, 2016;Kim et al, 2020). The diurnal characteristics of the size-resolved κOA indicate that the OA in small particles (30-100 nm) was fresh and became aged in large particles.…”

“…S8). Previous field studies also indicated that 𝑓 44 increased with particle diameters (Kim et al, 2020;Cai et al, 2018), leading to a higher κOA value.…”

“…A positive correlation between the hygroscopicity values and the oxidation degree of OA, including the ratio of atomic oxygen to atomic carbon (O:C), the oxidation state (OS C ̅̅̅̅̅ ), or the mass fraction of m/z 44 (for CO2 + ) ion fragments in the organic spectra (𝑓 44 ) from chemical composition, were widely reported in the literature (Wu et al, 2013;Pajunoja et al, 2015;Chang et al, 2010). Kim et al (2020) found that the κOA was positively and negatively correlated with OOA and HOA at different size ranges, respectively. Deng et al (2019) reported a decreasing trend of κOA at a size range of 100-360 nm during daytime in a forest environment, suggesting the formation of biogenic SOA (BSOA) through photochemical oxidation of biogenic volatile organic compounds (BVOCs).…”

Measurement report: Distinct size dependence and diurnal variation in organic aerosol hygroscopicity, volatility, and cloud condensation nuclei activity at a rural site in the Pearl River Delta (PRD) region, China

“…2 ) ion fragments in the organic spectra (f 44 ) from chemical composition, was widely reported in the literature (Wu et al, 2013;Pajunoja et al, 2015;Chang et al, 2010). Kim et al (2020) found that the κ OA was positively and negatively correlated with OOA and HOA at different size ranges, respectively. Deng et al (2019) reported a decreasing trend of κ OA at a size range of 100-360 nm during daytime in a forest environment, suggesting the formation of biogenic SOA (BSOA) through photochemical oxidation of biogenic volatile organic compounds (BVOCs).…”