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Secondary organic aerosols (SOAs) account for a large fraction of atmospheric aerosol mass and play significant roles in visibility impairment by scattering solar radiation. However, comprehensive evaluations of SOA scattering abilities under ambient relative humidity (RH) conditions on the basis of field measurements are still lacking due to the difficulty of simultaneously direct quantifications of SOA scattering efficiency in dry state and SOA water uptake abilities. In this study, field measurements of aerosol chemical and physical properties were conducted in winter in Guangzhou (lasting about 3 months) using a humidified nephelometer system and aerosol chemical speciation monitor. A modified multilinear regression model was proposed to retrieve dry-state mass scattering efficiencies (MSEs, defined as scattering coefficient per unit aerosol mass) of aerosol components. The more oxidized oxygenated organic aerosol (MOOA) with an O/C ratio of 1.17 was identified as the most efficient light scattering aerosol component. On average, 34 % mass contribution of MOOA to total submicron organic aerosol mass contributed 51 % of dry-state organic aerosol scattering. The overall organic aerosol hygroscopicity parameter κOA was quantified directly through hygroscopicity closure, and hygroscopicity parameters of SOA components were further retrieved using a multilinear regression model by assuming hydrophobic properties of primary organic aerosols. The highest water uptake ability of MOOA among organic aerosol factors was revealed with κMOOA reaching 0.23, thus further enhancing the fractional contribution of MOOA in ambient organic aerosol scattering. In particular, the scattering abilities of MOOA were found to be even higher than those of ammonium nitrate under RH of <70% , which was identified as the most efficient inorganic scattering aerosol component, demonstrating that MOOA had the strongest scattering abilities in ambient air (average RH of 57 %) during winter in Guangzhou. During the observation period, secondary aerosols contributed dominantly to visibility degradation (∼70%), with substantial contributions from MOOA (16 % on average), demonstrating significant impacts of MOOA on visibility degradation. The findings of this study demonstrate that more attention needs to be paid to SOA property changes in future visibility improvement investigations. Also, more comprehensive studies on MOOA physical properties and chemical formation are needed to better parameterize its radiative effects in models and implement targeted control strategies on MOOA precursors for visibility improvement. [ABSTRACT FROM AUTHOR]

The hygroscopicity parameter (κ) has been widely used in studies of aerosol hygroscopic growth and cloud condensation nucleus activation, but the effect of relative humidity (RH) variations on ambient aerosol hygroscopicity is rarely discussed and is often ignored in the application of κ‐Köhler theory. In this study, an improved humidified nephelometer system was used to measure the scattering enhancement (f(RH)) of bulk aerosols and the RH‐dependent κ was derived. It was found that the κ of both pure inorganic water‐soluble electrolytes and ambient aerosols is significantly RH‐dependent. The particle size had a great influence on f(RH), but κ had no obvious size dependence. Compared with using the RH‐dependent κ, the scattering coefficient of ambient aerosols with the fixed κ value would produce a relative error of −10%–25%. Parameterization schemes of RH‐dependent κ of ambient aerosols were established, which could serve as a reference for the future research. Plain Language Summary: A large proportion of environmental aerosols is hygroscopic substances. In the case of high RH, aerosol particles will absorb liquid water and will be in a state of deliquescence. The water uptake ability of aerosol is its hygroscopicity which is a very important parameter in the studies of climate effect, environmental impact, formation, and evolution process of aerosols. The κ‐Köhler theory was established and a hygroscopicity parameter (κ) was proposed in 2007 to simplify the calculation of hygroscopic growth or liquid water content for aerosol particles. The κ is commonly treated as a constant value for a certain particle or a specific mass of bulk ambient aerosols. In this study, it is found that the κ of ambient aerosol is significantly RH‐dependent, and this feature cannot be ignored in the calculation of aerosol hygroscopic growth. Key Points: A humidified nephelometer system was improved and used to measure the aerosol hygroscopicity under different relative humidityThe hygroscopicity of ambient aerosols showed significant relative humidity dependenceThe parameterization schemes of relative humidity‐dependent κ of ambient aerosols were established [ABSTRACT FROM AUTHOR]

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Water condensed on ambient aerosol particles plays significant roles in atmospheric environment, atmospheric chemistry and climate. Before now, no instruments were available for real-time monitoring of ambient aerosol liquid water contents (ALWCs). In this paper, a novel method is proposed to calculate ambient ALWC based on measurements of a three-wavelength humidified nephelometer system, which measures aerosol light scattering coefficients and backscattering coefficients at three wavelengths under dry state and different relative humidity (RH) conditions, providing measurements of light scattering enhancement factor f (RH). The proposed ALWC calculation method includes two steps: the first step is the estimation of the dry state total volume concentration of ambient aerosol particles, Va(dry), with a machine learning method called random forest model based on measurements of the "dry" nephelometer. The estimated Va(dry) agrees well with the measured one. The second step is the estimation of the volume growth factor Vg(RH) of ambient aerosol particles due to water uptake, using f (RH) and the Ångström exponent. The ALWC is calculated from the estimated Va(dry) and Vg(RH). To validate the new method, the ambient ALWC calculated from measurements of the humidified nephelometer system during the Gucheng campaign was compared with ambient ALWC calculated from ISORROPIA thermodynamic model using aerosol chemistry data. A good agreement was achieved, with a slope and intercept of 1.14 and -8.6 μm3 cm-3 (r2 =0.92), respectively. The advantage of this new method is that the ambient ALWC can be obtained solely based on measurements of a three-wavelength humidified nephelometer system, facilitating the real-time monitoring of the ambient ALWC and promoting the study of aerosol liquid water and its role in atmospheric chemistry, secondary aerosol formation and climate change. [ABSTRACT FROM AUTHOR]

Aerosol surface area concentration ( S A ) is crucial for studying atmospheric chemical reactions happened in aerosol water or on aerosol surface. However, there is no commercial instrument that can provide direct measurements of ambient S A . In this paper, we propose a method to calculate ambient S A based only on measurements of a three-wavelength humidified nephelometer system, which measures aerosol optical properties at three wavelengths under dry state and different relative humidity (RH) conditions. Two critical steps are required in this method: (1) Calculating surface area concentration of ambient aerosols in dry state, S A (dry) , using a trained random forest machine learning model based only on optical properties measured by the "dry" nephelometer. The proposed machine learning method is evaluated with particle number size distributions (PNSD) datasets from eight field campaigns conducted on the North China Plain during different seasons. The square of correlation coefficients between predicted and calculated S A (dry) for PNSD is about 0.99, the average ratio between predicted and calculated S A (dry) is 1.01 and 70% of data points has a relative difference less than 10%. (2) Calculating the surface area growth factor f S (RH) of ambient aerosol particles due to water uptake using the proposed f S (RH) parameterization scheme f S (RH) = (1 + κ S R H 100 − R H) 2 3 , and the hygroscopicity parameter κ S can be calculated using measured aerosol light scattering enhancement factor and Å ngstr o ¨ m exponent. The ambient S A values during two field campaigns which are conducted on the North China Plain (Wangdu campaign and Gucheng campaign, in summer and winter respectively) are calculated using the proposed method. The ambient S A ranges from 42 to 1871 μ m 2 / c m 3 with an average of 319 μ m 2 / c m 3 during Wangdu campaign, and ranges from 19 to 4156 μ m 2 / c m 3 with an average of 788 μ m 2 / c m 3 during Gucheng campaign. Drastic daily variations of ambient S A are observed during these two campaigns. The results demonstrate that aerosol hygroscopic growth impacts significantly on variations in ambient S A especially under high RH conditions. The f S (RH) ranges from near 1 to 4 with an average of 1.4 during Wangdu campaign, and ranges from near 1 to 2.7 with an average of 1.3 during Gucheng campaign. The results indicate that the larger RH, the more sensitive f S (RH) becomes to variations in κ S which highlights that real-time measurements of aerosol hygroscopicity are required for accurate calculations of ambient S A . The advantage of the proposed method is that the ambient S A can be obtained solely based on measurements of a three-wavelength humidified nephelometer system, facilitating real-time measurements of ambient S A and promoting studies in aerosol heterogeneous reactions. • Humidified nephelometer system performs great in predicting aerosol surface area. • Proposed method can be used in different regions. • Aerosol hygroscopic growth impacts significantly on aerosol surface area. [ABSTRACT FROM AUTHOR]

Abstract The light scattering enhancement factor f (RH), defined as the ratio of the light scattering coefficient (σ S P ) observed under an elevated relative humidity (RH) conditions to that under dry conditions, is a crucial parameter for estimating aerosol direct radiative effects and atmospheric visibility. In this paper, a new f (RH) parameterization scheme considering both the influence of aerosol chemical compositions and that of particle number size distribution (PNSD) is proposed based on in situ measurements in the North China Plain. The development of this parameterization scheme includes three steps. First, aerosol hygroscopicity parameter κ is parameterized with aerosol chemical compositions. Then, the ratio between f (RH) fitted parameter κ s c a (f (R H) = 1 + κ s c a R H 100 − R H ) and κ is introduced to correlate f (RH) with κ. The ratio, influenced mostly by PNSD, is described as a function of scattering Ångström exponent (SAE) because the SAE can represent the predominant size of aerosol particles to some extent. Finally, f (RH) cycle is parameterized with κ s c a . Validation results show that a good consistency between the parameterized and measured f (RH = 80%) is achieved and the correlation coefficient is 0.80. This parameterization scheme between aerosol chemistry and f (RH) can be used in chemical transport models for reducing uncertainties in estimations of direct aerosol radiative forcing and air quality assessments. Highlights • A relationship between the f (RH) fitted parameter (κ sca) and the aerosol hygroscopicity parameter (κ f(RH)) is found. • The ratio of κ sca and κ f(RH) increases linearly with the increase of the scattering Ångström exponent. • A new parameterization of f (RH) considering the aerosol chemical compositions is developed. [ABSTRACT FROM AUTHOR]