大气气溶胶的光学特性受吸湿性影响较大。并且由于气溶胶颗粒的理化性质不尽相同，它们的吸湿性也存在着差异，这将影响不同相对湿度环境下的气溶胶后向散射系数。在云附近气溶胶通常更容易发生吸湿增长，而云和晴空的过渡带（Twilight Zone）占据了大部分地球表面积，气溶胶的吸湿性在云-气溶胶相互作用中有着重要贡献。而激光雷达作为具有高时空分辨率的观测仪器，可以连续地测量真实环境下大气中的气溶胶和水汽，为研究云下过渡带的气溶胶提供技术基础。因此本研究利用美国能源部大气辐射测量（ARM）计划在美国南部大平原（SGP）站点的超级站，2021年4月28日至2022年4月23日为期一年的地面气溶胶综合观测及拉曼激光雷达和无线电探空仪的廓线观测，深入探讨了季节性变化对地面及云下过渡带气溶胶吸湿性和光学性质的影响，并分析了云下过渡带中云碎片对气溶胶后向散射系数（β）的贡献，主要结论包括：

（1）利用拉曼激光雷达探测的大气温度（T）和水汽混合比（w）廓线，结合Tetens公式计算得到高时空分辨率的相对湿度（RH）廓线。与1.5km以下无线电探空仪的RH廓线对比验证表明，全天数据的相关系数达到0.69，显示出良好的一致性。特别地，夜间数据的相关系数增加至0.72，说明夜间观测数据减少了太阳光的干扰而更为精确。

（2）由于气溶胶理化特性季节变化导致吸湿性明显的季节变化，秋冬季节气溶胶的吸湿性强于夏季，这与秋冬季无机物占比增加以及老化程度高有关。此外，发现新粒子的生成现象全年皆有，但在秋冬季节尤为显著。这可能与夏季有机物参与的内混程度较高且无机物含量占比较低，进一步造成了日变化谱分布的相对稳定有关。

（3）云下过渡带气溶胶后向散射系数（β）随着离云底距离的减小和RH的增加而增强。夏季云底附近的增强效应尤为显著，这可能与夏季对流云底频繁的云破碎过程有关。通过比较云下和非云下廓线的个例研究，排除了气溶胶数浓度谱分布以及化学组分变化的影响，发现至少约33.3%的β增强可以归因于云碎片的贡献，而剩余的66.7%由气溶胶的吸湿增长贡献。此外，近云区域观察到的类似气溶胶活化成云的现象，也可能是气溶胶β增强的一个重要因素。

本研究对了解当地气溶胶吸湿性强弱成因，理解季节性变化如何影响气溶胶的吸湿性及光学性质提供了新的见解，并揭示了云碎片在云下过渡带中对气溶胶后向散射特性的显著贡献。这些发现对深入认识云过渡带的气溶胶特性、云-气溶胶相互作用机制以及改进相关的云下云凝结核估算、大气辐射模型和气候模拟具有重要意义。

The hygroscopicity of atmospheric aerosols significantly alters their optical properties. However, due to the variable physicochemical properties of different aerosol particles, their hygroscopic behaviors also differ, affecting the backscatter coefficient of aerosols in varying Relative Humidity (RH) environments. Near clouds, aerosols are more prone to hygroscopic growth, and the twilight zone between clouds and clear skies, which covers a vast area of the Earth's surface, plays a crucial role in cloud-aerosol interactions. Raman lidar, with its high spatiotemporal resolution, offers uninterrupted and non-intrusive measurements of real atmospheric aerosols and water vapor, laying a technical foundation for studying aerosols in the cloud transition zone. Thus, this study utilized the Atmospheric Radiation Measurement (ARM) data acquired at the super site in the Southern Great Plains (SGP), USA, of one year worth of comprehensive ground-based aerosol observations combined with lidar and radiosonde profile measurements from April 28, 2021, to April 23, 2022, to delve into the effects of seasonal changes on the hygroscopicity and optical properties of ground-level and twilight zone aerosols. It also analyzed the contribution of cloud fragments to the aerosol backscatter coefficient (β) within the cloud transition zone. The main finding are:

(1) The atmospheric profiles of temperature (T) and water vapor mixing ratio (w) profiles can be derived from the Raman lidar data, combined with the Tetens formula.  High-resolution RH profiles were calculated and validated against coinciding radiosonde RH profiles below 1.5km. The correlation coefficient for the entire dataset reached 0.69, indicating good consistency. At night, the correlation coefficient increased to 0.72, due to reduced solar noise interference.

(2) Due to the seasonal variations in the physical and chemical properties of aerosols, their hygroscopicity exhibits significant seasonal changes, being higher in autumn and winter than in summer. This is related to the increased proportion of inorganic materials and the higher degree of aging during the colder seasons. Additionally, new particle formation events were observed throughout the year but were particularly pronounced during autumn and winter. This phenomenon could be related to the higher degree of internal mixing involving organic matter and the lower proportion of inorganic content in summer, which further leads to relatively stable diurnal spectrum distributions.

(3) The study indicated that the backscattering coefficient (β) of aerosols in the twilight zone significantly increased as the distance from the cloud base decreased and RH increased. The enhancement effect near cloud bases in summer was especially notable, likely related to frequent cloud fragmentation processes at the bases of convective clouds. Through case studies comparing cloud and non-cloud conditions and excluding the effects of aerosol number concentration spectrum and chemical composition changes, it was found that at least approximately 33.3% of the β enhancement could be attributed to cloud fragments, while the remaining 66.7% was contributed by aerosol hygroscopic growth. Additionally, phenomena resembling aerosol activation into clouds observed near clouds could also be a significant factor in enhancing aerosol β.

This study gains new insights into the causes of local aerosol hygroscopicity strengths and weaknesses, how seasonal changes affect aerosol hygroscopicity and optical properties, and reveals the significant contribution of cloud fragments to the backscatter characteristics of aerosols in the cloud transition zone. These findings are crucial for enhancing our understanding of aerosol properties in the cloud transition zone, the mechanisms of cloud-aerosol interactions, and for improving estimates of cloud condensation nuclei below clouds, atmospheric radiation models, and climate simulations.