scholarly journals Low hygroscopicity of organic material in anthropogenic aerosols under pollution episode in China

2018 ◽  
Author(s):  
Juan Hong ◽  
Hanbing Xu ◽  
Haobo Tan ◽  
Changqing Yin ◽  
Liqing Hao ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Growth Factor ◽  
Organic Material ◽  
Large Scale ◽  
Hygroscopic Growth ◽  
Pearl River Delta Region ◽  
Anthropogenic Aerosols ◽  
Gaseous Species ◽  
Oxidation Level ◽  
Suburban Site

Abstract. Simultaneous measurements of aerosol hygroscopicity and particle phase chemical composition were performed at a suburban site over the Pearl River Delta Region in the late summer of 2016 using a self-assembled Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) and an Aerodyne Quadruple Aerosol Chemical Speciation Monitor (ACSM), respectively. Hygroscopic growth factor (HGF) of Aitken (30 nm, 60 nm) and accumulation mode (100 nm, 145 nm) particles was obtained under 90 % relative humidity (RH). An external mixture was observed for all-sized particles during this study, with a dominant mode of more hygroscopic (MH) particles as aged aerosols dominated due to the anthropogenic influence. The HGF of less hygroscopic (LH) mode particles increased, while their number fractions decreased, during the daytime due to a reduced degree of external mixing probably from the condensation of gaseous species. These LH mode particles in the early morning or late afternoon could be possibly dominated by carbonaceous material emitted from local automobile exhaust during the rush hours. During polluted days with air masses mainly from the coastal areas, the chemical composition of aerosols had a clear diurnal variation and a strong correlation with the mean HGF. Closure analysis was carried out between the HTDMA-measured HGF and the ACSM-derived hygroscopicity using various approximations for hygroscopic growth factor of organic compounds (HGForg). The result with HGForg of below 1.1 suggests that the organic material in the aerosol from this suburban site, which represents a more anthropogenically influenced aerosol, is close to being hydrophobic even under 90 % RH. According to the closure analysis, a simple linear relationship between HGForg and the oxidation level inferred from the O : C ratio of the organic material was suggested for the current type of aerosols during this study. Compared with the results from other environments, HGForg obtained from our measurements appeared to be less sensitive to the variation of its oxidation level. This finding suggests it is important to consider the differences in the relationship between HGForg and the O : C atomic ratio observed in different laboratory and field environments. Moreover, this result provides an improved hygroscopicity parameterization of organic material in anthropogenically influenced aerosols that can be incorporated into large-scale modeling frameworks, especially for the suburban context in China.

scholarly journals Mixing state and particle hygroscopicity of organic-dominated aerosols over the Pearl River Delta region in China

2018 ◽  
Vol 18 (19) ◽  
pp. 14079-14094 ◽  
Author(s):  
Juan Hong ◽  
Hanbing Xu ◽  
Haobo Tan ◽  
Changqing Yin ◽  
Liqing Hao ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Growth Factor ◽  
Organic Material ◽  
Pearl River Delta ◽  
Urban Atmosphere ◽  
Pearl River ◽  
Hygroscopic Growth ◽  
Pearl River Delta Region ◽  
The Pearl River Delta ◽  
The Pearl River

Abstract. Simultaneous measurements of aerosol hygroscopicity and particle-phase chemical composition were performed at a suburban site over the Pearl River Delta region in the late summer of 2016 using a self-assembled hygroscopic tandem differential mobility analyzer (HTDMA) and an Aerodyne quadruple aerosol chemical speciation monitor (ACSM), respectively. The hygroscopic growth factor (HGF) of the Aitken mode (30 nm, 60 nm) and accumulation mode (100 nm, 145 nm) particles were obtained under 90 % relative humidity (RH). An external mixture was observed for particles of every size during this study, with a dominant mode of more-hygroscopic (MH) particles, as aged aerosols dominated due to the anthropogenic influence. The HGF of less-hygroscopic (LH) mode particles increased, while their number fractions decreased during the daytime due to a reduced degree of external mixing that probably resulted from the condensation of gaseous species. These LH mode particles in the early morning or late afternoon could be possibly dominated by carbonaceous material emitted from local automobile exhaust during rush hours. During polluted days with air masses flowing mainly from the coastal areas, the chemical composition of aerosols had a clear diurnal variation and a strong correlation with the mean HGF. Closure analysis was carried out between the HTDMA-measured HGF and the ACSM-derived hygroscopicity using various approximations for the hygroscopic growth factor of organic compounds (HGForg). Considering the assumptions regarding the differences in the mass fraction of each component between PM1 and 145 nm particles, the hygroscopicity-composition closure was achieved using an HGForg of 1.26 for the organic material in the 145 nm particles and a simple linear relationship between the HGForg and the oxidation level inferred from the O:C ratio of the organic material was suggested. Compared with the results from other environments, HGForg obtained from our measurements appeared to be less sensitive to the variation of its oxidation level, which is, however, similar to the observations in the urban atmosphere of other megacities in China. This finding suggests that the anthropogenic precursors or the photooxidation mechanisms might differ significantly between the suburban and urban atmosphere in China and those in other background environments. This may lead to different characteristics of the oxidation products in secondary organic aerosols (SOA) and therefore to a different relationship between the HGForg and its O:C ratio.

scholarly journals Relating the hygroscopic properties of submicron aerosol to both gas- and particle-phase chemical composition in a boreal forest environment

2015 ◽  
Vol 15 (11) ◽  
pp. 15511-15541
Author(s):  
J. Hong ◽  
J. Kim ◽  
T. Nieminen ◽  
J. Duplissy ◽  
M. Ehn ◽  
...  
Keyword(s):  
Organic Matter ◽  
Chemical Composition ◽  
Growth Factor ◽  
Boreal Forest ◽  
Gas Phase ◽  
Mixing Rule ◽  
Particle Phase ◽  
Hygroscopic Growth ◽  
Forest Environment ◽  
Composition Data

Abstract. Measurements of the hygroscopicity of 15–145 nm particles in a boreal forest environment were conducted using two Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) systems during the Pan-European Gas-AeroSOIs-climate interaction Study (PEGASOS) campaign in spring 2013. Measurements of the chemical composition of non-size segregated particles were also performed using a High-Resolution Aerosol Mass Spectrometer (HR-AMS) in parallel with hygroscopicity measurements. On average, the hygroscopic growth factor (HGF) of particles was observed to increase from the morning until afternoon. In case of accumulation mode particles, the main reasons for this behavior were increases in the ratio of sulfate to organic matter and oxidation level (O : C ratio) of the organic matter in the particle phase. Using an O : C dependent hygroscopic growth factor of organic matter (HGForg), fitted using the inverse Zdanovskii–Stokes–Robinson (ZSR) mixing rule, clearly improved the agreement between measured HGF and that predicted based on HR-AMS composition data. Besides organic oxidation level, the influence of inorganic species was tested when using the ZSR mixing rule to estimate the hygroscopic growth factor of organics in the aerosols. While accumulation and Aitken mode particles were predicted fairly well by the bulk aerosol composition data, the hygroscopicity of nucleation mode particles showed little correlation. However, we observed them to be more sensitive to the gas phase concentration of condensable vapors: the more there was sulfuric acid in the gas phase, the more hygroscopic the nucleation mode particles were. No clear dependence was found between the extremely low-volatility organics (ELVOCs) concentration and the HGF of particles of any size.

scholarly journals An alternative method for estimating hygroscopic growth factor of aerosol light-scattering coefficient: a case study in an urban area of Guangzhou, South China

2014 ◽  
Vol 14 (14) ◽  
pp. 7631-7644 ◽  
Author(s):  
Z. J. Lin ◽  
Z. S. Zhang ◽  
L. Zhang ◽  
J. Tao ◽  
R. J. Zhang ◽  
...  
Keyword(s):  
Light Scattering ◽  
Chemical Composition ◽  
Growth Factor ◽  
Scattering Coefficient ◽  
Hygroscopic Growth ◽  
Alternative Approach ◽  
Potential Applications ◽  
Dry Conditions ◽  
Empirical Curve

Abstract. A method was developed to estimate hygroscopic growth factor (f(RH)) of aerosol light-scattering coefficient (bsp), making use of the measured size- and chemically resolved aerosol samples. In this method, chemical composition of the measured aerosol samples were first reconstructed using the equilibrium model ISORROPIA II. The reconstructed chemical composition, which varies with relative humidity (RH), was then employed to calculate bsp and hygroscopic growth factor of bsp (fsp(RH)) using the Mie model. Furthermore, the calculated fsp(RH) was fitted with an empirical curve. To evaluate the applicability of fsp(RH), the curve of fsp(RH) was used to correct the long-term records of the measured bsp from the values under comparative dry conditions to the ones under ambient RH conditions. Compared with the original bsp data, the fsp(RH)-corrected bsp had a higher linear correlation with, and a smaller discrepancy from, the bsp derived directly from visibility and absorption measurements. The fsp(RH) determined here was further compared with that reported in previous studies. The method described in this manuscript provides an alternative approach to derive credible fsp(RH) with high accuracy and has many potential applications in aerosol-related research.

scholarly journals Impacts of organic aerosols and its oxidation level on CCN activity from measurement at a suburban site in China

2016 ◽  
Vol 16 (8) ◽  
pp. 5413-5425 ◽  
Author(s):  
Fang Zhang ◽  
Zhanqing Li ◽  
Yanan Li ◽  
Yele Sun ◽  
Zhenzhu Wang ◽  
...  
Keyword(s):  
Organic Material ◽  
Volume Fraction ◽  
Aerosol Particles ◽  
Strong Dependence ◽  
Organic Aerosols ◽  
Number Concentration ◽  
Accurate Estimation ◽  
Oxidation Level ◽  
Suburban Site ◽  
Ccn Activity

Abstract. This study is concerned with the impacts of organic aerosols on cloud condensation nuclei (CCN) activity based on field measurements made at a suburban site in Northern China. The sensitivity of the estimated CCN number concentration (NCCN) to both volume fraction of organic material (xorg) and aerosol oxidation level (using f44, the fraction of m∕z 44 in aerosol organic material) are examined. A strong dependence of CCN number concentration (NCCN) on the xorg and f44 was noted. The sensitivity of NCCN to volume fraction of organics increased with increasing xorg. The impacts of the aerosol particles oxidization or aging level on estimating NCCN were also very significant. When the particles were mostly composed of organics (xorg > 60 %), the NCCN at the supersaturation of 0.075 and 0.13 % was underestimated by 46 and 44 %, respectively, if aerosol particles were freshly emitted with primary organics (f44 < 11 %); the underestimation decreased to 32 and 23 % at the corresponding supersaturations, however, if the particles were with more hygroscopic secondary organics (f44 > 15 %). The NCCN at the supersaturation of 0.76 % was underestimated by 11 and 4 %, respectively, at f44 < 11 and f44 > 15 %. However, for the particles composed of low organics (e.g., xorg < 40 %), the effect caused by the f44 was quite insignificant both at high and low supersaturations. This is because the overall hygroscopicity of the particles is dominated by inorganics such as sulfate and nitrate, which are more hygroscopic than organic compounds. Our results indicated that it would decrease the uncertainties in estimating NCCN and lead to a more accurate estimation of NCCN to increase the proportion of secondary organics, especially when the composition of the aerosols is dominated by organics. The applicability of the CCN activation spectrum obtained at Xinzhou to the Xianghe site, about 400 km to the northeast of Xinzhou, was investigated, with the aim of further examining the sensitivity of NCCN to aerosol type. Overall, the mean CCN efficiency spectrum derived from Xinzhou performs well at Xianghe when the supersaturation levels are > 0.2 % (overestimation of 2–4 %). However, NCCN was overestimated by  ∼  20 % at supersaturation levels of < 0.1 %. This suggests that the overestimation is mainly due to the smaller proportion of aged and oxidized organic aerosols present at Xianghe compared to Xinzhou.

scholarly journals Particle hygroscopicity and its link to chemical composition in the urban atmosphere of Beijing, China, during summertime

2016 ◽  
Vol 16 (2) ◽  
pp. 1123-1138 ◽  
Author(s):  
Z. J. Wu ◽  
J. Zheng ◽  
D. J. Shang ◽  
Z. F. Du ◽  
Y. S. Wu ◽  
...  
Keyword(s):  
Particle Size ◽  
Chemical Composition ◽  
Growth Factor ◽  
Mass Concentration ◽  
Particle Formation ◽  
High Time Resolution ◽  
New Particle Formation ◽  
Hygroscopic Growth ◽  
Hygroscopic Properties ◽  
Beijing China

Abstract. Simultaneous measurements of particle number size distribution, particle hygroscopic properties, and size-resolved chemical composition were made during the summer of 2014 in Beijing, China. During the measurement period, the mean hygroscopicity parameters (κs) of 50, 100, 150, 200, and 250 nm particles were respectively 0.16  &amp;pm;  0.07, 0.19  &amp;pm;  0.06, 0.22  &amp;pm;  0.06, 0.26  &amp;pm;  0.07, and 0.28  &amp;pm;  0.10, showing an increasing trend with increasing particle size. Such size dependency of particle hygroscopicity was similar to that of the inorganic mass fraction in PM1. The hydrophilic mode (hygroscopic growth factor, HGF  >  1.2) was more prominent in growth factor probability density distributions and its dominance of hydrophilic mode became more pronounced with increasing particle size. When PM2.5 mass concentration was greater than 50 μg m−3, the fractions of the hydrophilic mode for 150, 250, and 350 nm particles increased towards 1 as PM2.5 mass concentration increased. This indicates that aged particles dominated during severe pollution periods in the atmosphere of Beijing. Particle hygroscopic growth can be well predicted using high-time-resolution size-resolved chemical composition derived from aerosol mass spectrometer (AMS) measurements using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule. The organic hygroscopicity parameter (κorg) showed a positive correlation with the oxygen to carbon ratio. During the new particle formation event associated with strongly active photochemistry, the hygroscopic growth factor or κ of newly formed particles is greater than for particles with the same sizes not during new particle formation (NPF) periods. A quick transformation from external mixture to internal mixture for pre-existing particles (for example, 250 nm particles) was observed. Such transformations may modify the state of the mixture of pre-existing particles and thus modify properties such as the light absorption coefficient and cloud condensation nuclei activation.

scholarly journals Hygroscopic and chemical characterisation of Po Valley aerosol

2013 ◽  
Vol 13 (2) ◽  
pp. 3247-3278 ◽  
Author(s):  
J. Bialek ◽  
M. Dall'Osto ◽  
P. Vaattovaara ◽  
J. Ovadnevaite ◽  
S. Decesari ◽  
...  
Keyword(s):  
Growth Factors ◽  
Chemical Composition ◽  
Growth Factor ◽  
Organic Aerosol ◽  
Diurnal Changes ◽  
Hygroscopic Growth ◽  
Organic Mass ◽  
Po Valley ◽  
Aerosol Mass ◽  
Mass Concentrations

Abstract. Continental summer-time aerosol in the Italian Po Valley was characterized in terms of hygroscopic properties and the influence of chemical composition therein. The campaign-average minima in hygroscopic growth factors (HGFs) occurred just before and during sunrise from 03:00–06:00, but more generally, the whole night shows very low hygroscopicity, particularly in the smaller particle sizes. The average HGFs increased from 1.18 for the smallest sized particles (35 nm) to 1.38 for the largest sizes (165 nm) for the lowest HGF period while during the day, the HGF gradually increased to achieve maximum values in the early afternoon hours from 12:00–15:00, reaching 1.32 for 35 nm particles and 1.46 for 165 nm particles. Two contrasting case scenarios were encountered during the measurement period: Case 1 was associated with westerly air flow moving at a moderate pace and Case 2 was associated with more stagnant, slower moving air from the north-easterly sector. Case 1 exhibited low diurnal temporal patterns and was associated with moderate non-refractory aerosol mass concentrations (for 50% size cut at 1 μm) of the order of 4.5 μg m−3. For Case 1, organics contributed typically to 50% of the mass. Case 2 was characterized by > 9.5 μg m−3 total mass (< 1 μm) in the early morning hours (04:00), decreasing to ∼ 3 μg m−3 by late morning (10:00) and exhibited strong diurnal changes in chemical composition, particularly in nitrate mass but also in total organic mass concentrations. Organic growth factors (OGFs) exhibited a minimum around 15:00, 1–2 h after the peak in HGF. Particles sized 165 nm exhibited moderate diurnal variability in HGF, ranging from 80% at night to 95% of "more hygroscopic" growth factors (i.e. GF = 1.35–1.9) around noon. The diurnal changes in HGF progressively became enhanced with decreasing particle size, decreasing from 95% "more hygroscopic" growth factor fraction at noon to 10% fraction at midnight, while the "less hygroscopic" growth factor fraction (1.13–1.34) increased from 5% at noon to > 60% and the "barely hygroscopic" growth factor fraction (1.1–1.2) increased from less than 2% at noon to 30% at midnight. OGFs were generally anti-correlated to HGF and also total organic mass as measured by the aerosol mass spectrometer due to a high sulphate/organics ratio. Surprisingly, the lowest HGFs occurred for periods when nitrate mass reached peak concentrations. This may suggest formation of organonitrates and organosulphates, which significantly decreased the OGF. Coincident with the peak in nitrate was a peak in Hydrocarbon-like Organic Aerosol (HOA) and Semi-Volatile Oxygenated Organic Aerosol (SV-OOA) and analysis of the HGF probability distribution function (PDF) reveals a transformation of a predominant "More Hygroscopic" (MH) mode with HGF of 1.5 around noon, into two modes, one with a "less hygroscopic" (LH) HGF of 1.26, and another with a "barely hygroscopic" (BH) mode of 1.05. The analysis points to an internal mixture of larger size inorganic species, mainly nitrates, coated with a hydrophobic organic layer which suppresses water uptake. In addition, a new, externally mixed BH ultrafine mode appears and persists through the night.

scholarly journals Hygroscopic and chemical characterisation of Po Valley aerosol

2014 ◽  
Vol 14 (3) ◽  
pp. 1557-1570 ◽  
Author(s):  
J. Bialek ◽  
M. Dall Osto ◽  
P. Vaattovaara ◽  
S. Decesari ◽  
J. Ovadnevaite ◽  
...  
Keyword(s):  
Growth Factors ◽  
Chemical Composition ◽  
Growth Factor ◽  
Organic Aerosol ◽  
Early Morning ◽  
Organic Nitrates ◽  
Diurnal Changes ◽  
Hygroscopic Growth ◽  
Po Valley ◽  
Aerosol Mass

Abstract. Continental summer-time aerosol in the Italian Po Valley was characterised in terms of hygroscopic properties and the influence of chemical composition therein. Additionally, the ethanol affinity of particles was analysed. The campaign-average minima in hygroscopic growth factors (HGFs, at 90% relative humidity) occurred just before and during sunrise from 03:00 to 06:00 LT (all data are reported in the local time), but, more generally, the hygroscopicity during the whole night is very low, particularly in the smaller particle sizes. The average HGFs recorded during the low HGF period were in a range from 1.18 (for the smallest, 35nm particles) to 1.38 (for the largest, 165 nm particles). During the day, the HGF gradually increased to achieve maximum values in the early afternoon hours 12:00–15:00, reaching 1.32 for 35 nm particles and 1.46 for 165 nm particles. Two contrasting case scenarios were encountered during the measurement period: Case 1 was associated with westerly air flow moving at a moderate pace and Case 2 was associated with more stagnant, slower moving air from the north-easterly sector. Case 1 exhibited weak diurnal temporal patterns, with no distinct maximum or minimum in HGF or chemical composition, and was associated with moderate non-refractory aerosol mass concentrations (for 50% size cut at 1 μ) of the order of 4.5 μg m−3. For Case 1, organics contributed typically 50% of the mass. Case 2 was characterised by >9.5 μg m−3 total non-refractory mass (<1 μ) in the early morning hours (04:00), decreasing to ~3 μg m−3 by late morning (10:00) and exhibited strong diurnal changes in chemical composition, particularly in nitrate mass but also in total organic mass concentrations. Specifically, the concentrations of nitrate peaked at night-time, along with the concentrations of hydrocarbon-like organic aerosol (HOA) and of semi-volatile oxygenated organic aerosol (SV-OOA). In general, organic growth factors (OGFs) followed a trend which was opposed to HGF and also to the total organic mass as measured by the aerosol mass spectrometer. The analysis of the HGF probability distribution function (PDF) reveals an existence of a predominant "more hygroscopic" (MH) mode with HGF of 1.5 around noon, and two additional modes: one with a "less hygroscopic" (LH) HGF of 1.26, and another with a "barely hygroscopic" (BH) mode of 1.05. Particles sized 165 nm exhibited moderate diurnal variability in HGF, ranging from 80% at night to 95% of "more hygroscopic" growth factors (i.e. HGFs 1.35–1.9) around noon. The diurnal changes in HGF progressively became enhanced with decreasing particle size, decreasing from 95% "more hygroscopic" growth factor fraction at noon to 10% fraction at midnight, while the "less hygroscopic" growth factor fraction (1.13–1.34) increased from 5% at noon to > 60% and the "barely hygroscopic" growth factor fraction (1.1–1.2) increased from less than 2% at noon to 30% at midnight. Surprisingly, the lowest HGFs occurred for the period when nitrate mass reached peak concentrations (Case 2). We hypothesised that the low HGFs of nitrate-containing particles can be explained by a) an organic coating suppressing the water-uptake, and/or by b) the existence of nitrates in a less hygroscopic state, e.g. as organic nitrates. The latter hypothesis allows us to explain also the reduced OGFs observed during the early morning hours (before dawn) when nitrate concentrations peaked, based on the evidence that organic nitrates have significant lower ethanol affinity than other SV-OOA compounds.

scholarly journals An alternative method estimating hygroscopic growth factor of aerosol light scattering coefficient: a case study in an urban area of Guangzhou, South China

2014 ◽  
Vol 14 (1) ◽  
pp. 435-469 ◽  
Author(s):  
Z. J. Lin ◽  
Z. S. Zhang ◽  
L. Zhang ◽  
J. Tao ◽  
R. J. Zhang ◽  
...  
Keyword(s):  
Light Scattering ◽  
Chemical Composition ◽  
Growth Factor ◽  
Scattering Coefficient ◽  
Hygroscopic Growth ◽  
Practical Applications ◽  
Formula One ◽  
Potential Applications ◽  
Dry Conditions ◽  
Two Parameters

Abstract. A method was developed to estimate hygroscopic growth factor (f(RH)) of aerosol light scattering coefficient (bsp), making use of the measured size- and chemically-resolved aerosol samples. Regarding this method, chemical composition of the measured aerosol samples were first reconstructed using the equilibrium model ISOPPORIA II. The model reconstructed chemical composition varies with a varying relative humidity (RH) input, which was then employed to calculate bsp and f(RH) of bsp using Mie Model. Further, the RH dependence of f(RH) of bsp (denoted as f(RH) derived from model calculation was empirically fitted with a two-parameter formula. One of the two parameters was set to be a constant for practical applications. For validation, the developed formula of fsp(RH) was applied to correct the long-term records of measured bsp from the values under comparative dry conditions to the ones under ambient RH conditions. Compared with the original bsp data, the f(RH)-corrected bsp had a higher linear correlation with and a smaller discrepancy from the bsp data derived directly from visibility and absorption measurements. The method described in this paper provides an alternative approach to estimate fsp(RH) and has many potential applications.

scholarly journals Relating the hygroscopic properties of submicron aerosol to both gas- and particle-phase chemical composition in a boreal forest environment

2015 ◽  
Vol 15 (20) ◽  
pp. 11999-12009 ◽  
Author(s):  
J. Hong ◽  
J. Kim ◽  
T. Nieminen ◽  
J. Duplissy ◽  
M. Ehn ◽  
...  
Keyword(s):  
Organic Matter ◽  
Chemical Composition ◽  
Growth Factor ◽  
Boreal Forest ◽  
Gas Phase ◽  
Mixing Rule ◽  
Particle Phase ◽  
Hygroscopic Growth ◽  
Forest Environment ◽  
Composition Data

Abstract. Measurements of the hygroscopicity of 15–145 nm particles in a boreal forest environment were conducted using two Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) systems during the Pan-European Gas-Aerosols-climate interaction Study (PEGASOS) campaign in spring 2013. Measurements of the chemical composition of non-size segregated particles were also performed using a high-resolution aerosol mass spectrometer (HR-AMS) in parallel with hygroscopicity measurements. On average, the hygroscopic growth factor (HGF) of particles was observed to increase from the morning until afternoon. In case of accumulation mode particles, the main reasons for this behavior were increases in the ratio of sulfate to organic matter and oxidation level (O : C ratio) of the organic matter in the particle phase. Using an O : C dependent hygroscopic growth factor of organic matter (HGForg), fitted using the inverse Zdanovskii–Stokes–Robinson (ZSR) mixing rule, clearly improved the agreement between measured HGF and that predicted based on HR-AMS composition data. Besides organic oxidation level, the influence of inorganic species was tested when using the ZSR mixing rule to estimate the hygroscopic growth factor of organics in the aerosols. While accumulation and Aitken mode particles were predicted fairly well by the bulk aerosol composition data, the hygroscopicity of nucleation mode particles showed little correlation. However, we observed them to be more sensitive to the gas phase concentration of condensable vapors: the more sulfuric acid in the gas phase, the more hygroscopic the nucleation mode particles were. No clear dependence was found between the extremely low-volatility organics concentration (ELVOC) and the HGF of particles of any size.

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