The hygroscopicity of atmospheric aerosol particles plays an important role in their effects on the environment and the public health. Oxalic acid (OA) has been recognized as the dominant dicarboxylic acid in the urban and the remote aerosol particles, so a number of studies have investigated the hygroscopicity of aerosols consisted of pure OA and its mixture with inorganic salt. However, few experimental studies have focused on the hygroscopicity of nanoscale particles at high relative humidity (RH) below saturation of water vapor (i.e., RH = 90–100%) due to the limitation of traditional measuring instruments. In this work, the hygroscopic growth factor (GF) of the aerosol particles composed of pure OA and internally mixed OA-ammonium sulfate (AS) at RH = 80–99.5% were studied using a high humidity tandem differential mobility analyzer (HHTDMA). The experimental results were used to verify the applicability of models including the ideal solution model, the extended aerosol inorganics model (E-AIM) combining with the revised universal quasi-chemical functional group activity coefficients (UNIFAC-Peng) model, the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model and the Zdanovskii–Stokes–Robinson (ZSR) model. According to the experimental results, the effect of RH, the composition and the initial dry particle diameter (D₀) on the hygroscopic growth factor (GF) and the effective hygroscopicity parameter (κ) of particles at RH = 80–99.5% were analyzed in details. It has been found that for pure OA particles with D₀ = 80/100 nm, the prediction of the UNIFAC-Peng model and the AIOMFAC model becomes more accurate than the ideal solution model with the increasing of RH, while all models cannot describe the GF of smaller particles at RH = 80–99.5%. For mixed OA-AS aerosol particles, the chemical reaction is an important reason for the discrepancies between the HHTDMA results and the model prediction. For the pure OA aerosol particles at RH ≤ 96% and particles containing AS at RH ≤ 99.5%, the sensitivity of GF to D₀ is positively and negatively correlated with RH and D₀, respectively. However, for all aerosol types in this study, the sensitivity of GF to RH is positively correlated with both D₀ and RH. In addition, the κ based on the measurement is clearly dependent on D₀ (at RH = 95–99.5%), RH and the particle composition (at RH = 80–99.5%). The measured hygroscopicity of aerosol particles at high RH might be useful in addressing challenges in solving the discrepancies between the κ measured under subsaturated and supersaturated conditions.

大气气溶胶颗粒的吸湿性在其对环境和公众健康的影响中起着重要作用。草酸（OA）被公认为是城市和偏远气溶胶颗粒中的主要二元羧酸，因此许多研究研究了由纯OA及其与无机盐混合而成的气溶胶的吸湿性。但是，由于传统测量仪器的局限性，很少有实验研究集中在低于水蒸气饱和度（即RH = 90–100％）的高相对湿度（RH）下的纳米级颗粒的吸湿性。在这项工作中，使用高湿度串联差动迁移率分析仪（HHTDMA）研究了由纯OA和内部混合的OA-硫酸铵（AS）在RH = 80-99.5％组成的气溶胶颗粒的吸湿生长因子（GF）。实验结果用于验证模型的适用性，包括理想溶液模型，扩展的气溶胶无机物模型（E-AIM）以及经修订的通用准化学官能团活性系数（UNIFAC-Peng）模型，气溶胶无机-有机混合物功能组活性系数（AIOMFAC）模型和Zdanovskii–Stokes–Robinson（ZSR）模型。根据实验结果，RH的影响，组成和初始干粒径（气溶胶无机-有​​机混合物功能团活度系数（AIOMFAC）模型和Zdanovskii-Stokes-Robinson（ZSR）模型。根据实验结果，RH的影响，组成和初始干粒径（气溶胶无机-有​​机混合物功能团活度系数（AIOMFAC）模型和Zdanovskii-Stokes-Robinson（ZSR）模型。根据实验结果，RH的影响，组成和初始干粒径（详细分析了RH = 80–99.5％时，颗粒的吸湿性生长因子（GF）和有效吸湿性参数（κ）的D ₀。已经发现，对于D ₀  = 80/100 nm的纯OA粒子，随着RH的增加，UNIFAC-Peng模型和AIOMFAC模型的预测比理想溶液模型更准确，而所有模型都无法描述相对较小的颗粒在RH时的GF = 80-99.5％。对于混合的OA-AS气溶胶颗粒，化学反应是HHTDMA结果与模型预测之间差异的重要原因。对于RH≤96％的纯OA气溶胶颗粒和RH≤99.5％的含AS的颗粒，GF对D ₀的敏感性分别与RH和D ₀正相关和负相关。但是，对于本研究中的所有气溶胶类型，GF对RH的敏感性与D ₀和RH均呈正相关。此外，基于测量值的κ明显取决于D ₀（在RH = 95–99.5％时），RH和颗粒组成（在RH = 80–99.5％时）。在高RH下测得的气溶胶颗粒的吸湿性可能有助于解决挑战，以解决在过饱和和过饱和条件下测得的κ之间的差异。