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 $\mathrm{O}/\mathrm{C}$ 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 κ_OA 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 κ_MOOA 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.

中文翻译：

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高氧有机气溶胶的强光散射对能见度下降有显着影响

二次有机气溶胶 (SOA) 占大气气溶胶质量的很大一部分，并通过散射太阳辐射在能见度损害中发挥重要作用。然而，由于难以同时直接量化干燥状态下的 SOA 散射效率和 SOA 吸水能力，因此基于现场测量的环境相对湿度 (RH) 条件下 SOA 散射能力的综合评估仍然缺乏。在这项研究中，使用加湿浊度计系统和气溶胶化学形态监测仪在广州冬季（持续约 3 个月）对气​​溶胶化学和物理特性进行了现场测量。提出了一种改进的多线性回归模型来检索干态质量散射效率（MSE，定义为气溶胶成分的每单位气溶胶质量的散射系数。氧化程度更高的氧化有机气溶胶 (MOOA)$\mathrm{○}/\mathrm{C}$ 1.17 的比率被确定为最有效的光散射气溶胶组分。平均而言，MOOA 对总亚微米有机气溶胶质量的 34% 质量贡献占干态有机气溶胶散射的 51%。整体有机气溶胶吸湿性参数κ _OA 通过吸湿闭合直接量化，并通过假设初级有机气溶胶的疏水特性，使用多线性回归模型进一步检索 SOA 组分的吸湿性参数。κ _MOOA揭示了有机气溶胶因子中 MOOA 的最高吸水能力达到 0.23，从而进一步增强 MOOA 在环境有机气溶胶散射中的分数贡献。特别是在相对湿度<70  %的情况下，MOOA的散射能力甚至高于硝酸铵，被认为是最有效的无机散射气溶胶组分，表明MOOA在环境空气中具有最强的散射能力（广州冬季平均相对湿度为 57%）。在观测期间，二次气溶胶对能见度下降的贡献最大（~ 70  %)，MOOA 的贡献很大（平均为 16%），表明 MOOA 对能见度下降的显着影响。这项研究的结果表明，在未来的可见性改进调查中，需要更多地关注 SOA 属性的变化。此外，还需要对 MOOA 物理性质和化学形成进行更全面的研究，以更好地在模型中参数化其辐射效应，并对 MOOA 前体实施有针对性的控制策略以提高能见度。