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

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. 10 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 15 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 20 the size-resolved chemical composition data. That is, the relationship between the number fraction of each hygroscopicity mode and volume fraction of different chemical composition was investigated. For example, the HOA volume fraction explained about 60 % of the variation of less hygroscopic (LH) mode number fraction for externally mixed aerosols. https://doi.org/10.5194/acp-2020-450 Preprint. Discussion started: 4 June 2020 c © Author(s) 2020. CC BY 4.0 License.


Temporal variation of aerosol chemical composition
shows the temporal variations of aerosol chemical compositions, including sulfate, nitrate, ammonium and, organics, at Olympic Park during the campaign period. The bulk mass concentration of PM1 (=NR-PM1+BC) ranged from 4.4 to 57.1 μg m -3 with a mean value of 19.1 μg m -3 and there was substantial variation of chemical composition (Fig.1a). Among non-refractory aerosols, organics occupied 180 about 42.5 % of total mass concentration of PM1 aerosols during the whole period followed by sulfate (28.4%), nitrate (16.3%), ammonium (12.2%) and chloride (0.6%). Campaign averaged BC mass concentration was about 2.5 μg m -3 . In this study, 1300 kg m-3 and 1700 kg m-3 were assumed for densities of organic (Cross et al., 2007;Florou et al., 2017) and BC (Wu et al., 2013), respectively, to calculate the volume for each species. For BC, PM 2.5 mass concentration is used for calculation, 185 assuming that BC mass is mainly determined by submicron particles (e.g., Clarke et al., 2004;Wu et al., 2013). It can be said from the good agreement between predicted and measured 4 + that observed anions ( 4 2− , 3 − and − ) are fully neutralized by 4 + (Fig. S2) and ion species mainly existed in the form of ( 4 ) 2 4 and 4 3 (Reilly and Wood (1969); Gysel et al. (2007)). Predominant volume fractions of ( 4 ) 2 4 and 4 3 among inorganic compounds can also be found in Fig   190 1b. For organics, HOA, SV-OOA, and LV-OOA accounted for 32.0%, 8.8%, and 59.2%, respectively, of the total OA mass concentration during the campaign.
Chemical composition of PM1 aerosol showed substantial variation, especially for periods A and B. Organics were dominant in period A when stagnant conditions prevailed due to persistent high atmospheric pressure and weak synoptic flow (Kim et al. 2018a). The average ratio of organic to 195 (inorganic + BC) was 1.60 ±0.82, ranging from 0.48 to 3.60. The average mass concentrations of each chemical species during period A were 7.9 μg m-3 (organic), 3.7 μg m-3 (sulfate), 2.9 μg m-3 (nitrate), 2.2 μg m-3 (ammonium) and 2.4 μg m-3 (BC). At the beginning of period A, mass concentrations of both HOA and LV-OOA increased sharply, and that of LV-OOA remained high until 23 May (Fig.1c). For period B, was much lower than the normal period that excludes periods A and B, although particle sizes are larger 205 than those in the normal period.

Size-resolved hygroscopicity of urban aerosols
As mentioned above, size-resolved hygroscopicity for four dry diameters (30, 50, 100, and 150 nm) was measured during the campaign. The average value of κ, a representative single hygroscopicity 210 parameter (Petters and Kreidenweis 2007), ranged from 0.11 to 0.24 with distinct diurnal variation (Kim et al., 2018a). Figure 2 shows the size-resolved κ values measured in SMA from the two campaigns (MAPS-Seoul and KORUS-AQ) as well as the results from some other urban measurements including Shanghai (Ye et al., 2013), Beijing (Wang et al., 2018), the Pearl River Delta (PRD) region (Jiang et al., 2016) and Paris (Juŕ nyi et al., 2013). The κ values in the figure were derived from HTDMA GF 215 measurements except for Paris that derived κ from CCN measurement. The ĸ values of SMA were lower than those in Shanghai and similar to Beijing but the lowest κ values were observed from Paris for most diameters. According to Fig.2, most κ values increase with particle size. It is closely related to the fact that the mass fraction of inorganic species increases with increasing particle size (Fig. S3). Inorganic components measured by AMS are considered as the major water-soluble chemical components, 220 influencing the hygroscopic behavior of atmospheric aerosols. Wu et al. (2016) showed increase of the particle number fraction of hydrophilic mode with increasing particle size, and this trend was more

2013)
. Although the Kelvin effect may cause some decrease of κ with decreasing particle size, this effect is small, less than 5%, for particles in the diameter ranged from 50 to 200 nm (Swietlicki et al., 2008;Wang et al., 2018). The average κ values of urban aerosols shown in Fig. 2 are smaller than 0.3 for diameters smaller than 300 nm, implying that the suggested typical continental κ value of 0.3 by Andreae and Rosenfeld (2008) is an overestimation for these urban aerosols. Consequently, it can cause the over-

ĸ closure
Closure on hygroscopicity has been studied to understand the relationship between chemical composition and aerosol hygroscopicity (Chang et al., 2010;Gunthe et al., 2009;Gysel et al., 2007;Kim et al., 2017;Wu et al., 2013). The ZSR mixing rule (Eq. 2) with a volume fraction of aerosol composition 240 is generally applied for the hygroscopicity closure.
where ℎ is the κ value of the mixed particle, is the hygroscopicity value of the chemical component, , in pure form and is the volume fraction of this chemical component. Unlike inorganic species, the hygroscopicity of organic aerosols is relatively unknown, and many estimation methods have  (Gysel et al., 2007;Topping et al., 2005). BC is assumed to be hydrophobic. Figure 3 presents the scatterplot of κ vs. κ ℎ , which incorporates the κ values derived from the two estimation methods above. Only 150 nm results are used for ĸ . The agreement between κ and κ ℎ looks good regardless of the κ estimation method and 255 therefore it can be said that such oxidation parameters are suitable to use for estimating hygroscopicity of organic aerosols. Perhaps the similar results of the two methods was in part due to the fact that inorganic species having high κ values compared to organics occupied a major portion of the total mass. In this study, we adopted the method using f44 for further analysis because it produced better results than the https://doi.org/10.5194/acp-2020-450 Preprint. Discussion started: 4 June 2020 c Author(s) 2020. CC BY 4.0 License. determination) and the average ratio between κ and κ ℎ values (Table S1). According to Fig.   4, however, a good agreement between κ and κ ℎ is shown only for 150 nm. As particle size becomes smaller, widely dispersed scatterplots between κ and κ ℎ are shown. Furthermore, the overestimation of κ ℎ is clearly shown for small particles. It is because large particles mainly determine the volume fraction in bulk chemical composition data. This result implies that size-resolved 265 chemical composition data should be accompanied when we analyze the relationship between hygroscopicity and chemical composition, especially for small particles.  In other words, organic mass concentration in this study can be explained substantially by the two organic 315 factors. Also, m/z 57 and 44 can be considered as first-order tracers of the two major organic components.
The correlation coefficient between measured and reconstructed HOA is slightly lower than that of OOA ( Fig. S5) because the contribution of m/z 57 on HOA varies depending on time and/or sources, whereas m/z 44 contains a broader range of OOA. Figure 5 shows the campaign averaged size distribution of reconstructed HOA and OOA (From now on, 'reconstructed' HOA and OOA are just called as HOA and 320 OOA in short.). The mode diameter of OOA is somewhat larger than that of HOA. Mass fraction of HOA is larger than that of OOA for small particles (< 120 nm) but the opposite is true for larger particles (> 120 nm).
where is the particle density and 0 is the standard density (1000 kg m-3). In this study, 1300 335 kg m-3 is used as the particle density since organics are the most dominant chemical composition in the particle size range of hygroscopicity measurement. As mentioned above, the κ value of 30 nm particle is excluded due to high uncertainties. Densities of chemical species are assumed for calculation of volume fraction: 930 kg m-3 (HOA), 1500 kg m-3 (OOA), and 1769 kg m-3 (inorganics). For small particles, volume fraction is dominated by organics (= HOA+OOA) and HOA, widely known to be hydrophobic, 340 explains more than 50%. However, the volume fraction of inorganics, which is hygroscopic, increases as particle size increases. Among organics, a sharp decrease of HOA volume fraction and an increase of OOA with size are clearly shown. These results support the size-dependent hygroscopicity. Moreover, the dominant organic volume fraction for small particles ( < 100 nm) manifests the importance of sizeresolved organic factors to explain the variation of hygroscopicity. Figure 7 illustrates the diurnal variation Therefore, it can be said that the effect of chemical composition on diurnal variation of κ is more sensitive for small particles than for large particles. Such results demonstrate that, without proper specification of organic factors, it is difficult to explain the diurnal variation of κ. Also noted is that κ variation for small 355 particles is mostly affected by the volume fraction of organics rather than that of inorganics.   photochemical oxidation. The size-dependent κ is also reflected in the degree of oxidation, as can be seen 370 from the increase in size-resolved 44 with increasing particle diameter (Fig.8a). The positive relationship between size-resolved 44 and κ values for 50 nm particles (Fig.8b) also explains that the oxidation of organics affects the hygroscopic properties of particles. It is noted that data that the volume fraction of organics is larger than 0.7 were only used to exclude the effect of inorganics. Figure 9 presents scatterplots between (30, 50, and 150 nm) and volume fraction of HOA and OOA among organics.

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is calculated by subtracting inorganic part from . As expected, the volume fraction of HOA was negatively correlated with κ values, whereas that of OOA was positively correlated with κ values for all sizes of particles. These results demonstrate that the specification of size-resolved organic factor is an indispensable part of describing the relationship between size-resolved hygroscopicity and chemical composition of aerosols.

Relevance to mixing state
HTDMA measurement data can provide information on the mixing state of atmospheric particles, i.e., external or internal mixing. We can also infer the extent of chemical mixing of particles from this information (Swietlicki et al., 2008). External mixing was prevalently observed in Seoul during the MAPS-Seoul (2015), and the KORUS-AQ (2016)  is safe to assume that externally mixed particles are bimodal. Then the first peak (denoted as Peak 1) in the GF distribution is defined as less hygroscopic (LH) mode that usually had GF value lower than 1.1, and the second peak (denoted as Peak 2) is defined as more hygroscopic (MH) mode that has GF value 405 larger than 1.1.   Table 1 presents the area ratio, GF, and κ value of LH and MH modes for the four different dry diameters. The area ratio of each mode is directly related to the number fraction of each mode as the area of each mode is calculated by integrating the GF distribution, / ( ), for each mode. The results

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in Table 1 contain all three types of aerosols. LH mode includes Peak 1 of Type 1 aerosols and all Type 3 aerosols. MH mode includes Peak 2 of Type 1 aerosols and all Type 2 aerosols. The area ratio of LH mode is substantially high for small particles compared to MH mode, and the area ratio of MH mode becomes larger as particle size increases. It is directly connected to the size-dependency of κ. The GF value of MH mode increases as particle size increases.  , Bhattu et al., 2015;Ervens et al., 2010;Ren et al., 2018;Wang et al., 2010). For 435 externally mixed aerosols, chemical species can be divided into two modes, LH and MH mode, based on their hygroscopic properties. In general, BC and organics (or only HOA) are classified into LH mode, whereas inorganics and/or OOA are classified into MH mode in externally mixed aerosols. In this study, we identify and quantify chemical species of each mode for externally mixed aerosols with GF distribution data and size-resolved chemical data. Figure 10 shows the scatterplot of the Peak 1 (LH mode) 440 area ratio vs. the volume fraction of each chemical species for different diameters. As mentioned above, the area ratio of each mode in the GF distribution represents the number fraction of particles in each mode and thereby can be compared directly with the volume fraction of each chemical species for a diameter.
Note that only the observed externally mixed aerosols (Type 1) are used for comparison. The volume fraction of HOA is positively correlated with Peak 1 area ratio (Fig.10a) when all sizes are combined but 445 not for each diameter. The slope between them and the coefficient of determination ( 2 ) were 0.73 and 0.58, respectively. In other words, the HOA volume fraction can explain about 58% of the variation of number fraction for LH mode in externally mixed aerosols. We can infer that the unexplained part can be complemented by BC, which is known to be hydrophobic. Unfortunately, size-resolved BC is not available in this study. The results in Fig. 10b and 10c also support this speculation. The volume fraction 450 of all organics, including both HOA and OOA (Fig. 10b), is much higher than the number fraction of LH mode. Furthermore, negative and even weak correlation was shown between the volume fraction of the OOA and Peak 1 area ratio (Fig. 10c).
https://doi.org/10.5194/acp-2020-450 Preprint. Discussion started: 4 June 2020 c Author(s) 2020. CC BY 4.0 License. but it was not strong enough to explain a significant portion of MH mode. The sum of OOA and inorganic volume fraction (Fig. 11c) does explain a significant portion of MH mode variation in externally mixed aerosols, whereas a negative correlation is clearly shown between the volume fraction of HOA and Peak 2 area ratio (Fig. 11d). For individual diameters, correlations tended to be stronger for larger (100 nm and 150 nm) than smaller (50 nm) diameters. It is related to the fact that there are high uncertainties of size-  size-resolved organic factor can give a detailed explanation of the diurnal variation of κ for small particles.
Low κ in the morning is associated with the large volume fraction of HOA, whereas high κ in the afternoon is related to the large volume fraction of OOA. Scatterplots of volume fraction of organic factors vs. κ values clearly illustrate that chemical composition is closely associated with hygroscopic properties of 510 aerosols, not only for large particles but also for small particles.
Lastly, the characteristics of the mixing state of aerosols were investigated in association with size-resolved chemical composition data. Externally mixed aerosols were observed about 50% of the time during the campaign period, especially for large particles. Importantly, the number fraction and GF value of MH mode increased as particle size increased. The relationship between the number fraction of each 515 hygroscopicity mode and volume fraction of different chemical composition is analyzed. For example, the HOA volume fraction explained about 60% of the variation of LH mode number fraction for externally mixed aerosols. It can be inferred that the volume fraction of BC can explain the rest. On the other hand, the chemical composition of MH mode can be explained by the sum of inorganics and OOA. Such relationship between chemical composition and mixing state of atmospheric particles can be of crucial 520 use in accurate prediction.
It can be concluded that size-resolved chemical composition data did provide more detailed and essential information than bulk data, which are highly needed when examining the relationship between chemical composition and hygroscopic properties of aerosols as well as the mixing state. Specified organic factors were found to be critically important mainly in estimating the hygroscopicity of small 525 particles as organics occupied a significant portion of these particles. Although the two OA factors, HOA and OOA, can represent the total organic mass concentration and can also explain the variability of κ reasonably well, more detailed analysis can be made when more spectral tracers are added to derive subdivided organic factors. The results presented here were obtained during spring/summer season. It would be very informative to make the observation during other seasons to find seasonal variability, especially 530 https://doi.org/10.5194/acp-2020-450 Preprint. Discussion started: 4 June 2020 c Author(s) 2020. CC BY 4.0 License.