| 16 Oct 2025
Hygroscopic growth characteristics of anthropogenic aerosols over central China revealed by lidar observations
Abstract. Lidar-derived particle backscatter coefficient is commonly used to assess air pollution levels; however, hygroscopic growth can amplify particle backscatter and hinder accurate assessment of particle concentration. This study investigated the hygroscopic growth characteristics of urban anthropogenic aerosols in Wuhan (30.5° N, 114.4° E), central China, using ground-based 532-nm polarization lidar observations during 2010–2024. A total of 192 cases were identified based on the following criteria: (1) the presence of a layer thicker than 300 m; (2) a lidar-derived backscatter coefficient that increases monotonically with simultaneously-measured relative humidity (RH) from radiosonde, and (3) limited variations in key meteorological parameters, including water vapor mixing ratio, potential temperature, and wind speed and direction. Using the Hänel parameterization method, the hygroscopic growth parameter γ was estimated as 0.62 (±0.24), corresponding to a backscatter coefficient enhancement factor of 2.36 at 85 % RH. No evident differences in γ were observed between the boundary layer (0.63±0.25) and free troposphere (0.60±0.24). The annual mean γ increased from 0.49 in 2014 to 0.63 in 2017 and stabilized within 0.6–0.7 after 2018, closely following the evolution of the annual mean NO2-to-SO2 concentration ratio. The minimum seasonal average γ occurred in winter (0.56), while the maximum was observed in autumn (0.64). These results provide a comprehensive characterization of the long-term and seasonal hygroscopicity of pollutants over central China, enhancing our understanding of the influence of hygroscopic growth on lidar-observed particle backscatter coefficients and offering valuable insights for urban air pollution control strategies.
This manuscript presents a valuable long-term study of aerosol hygroscopicity in Wuhan, Central China, utilizing a 15-year dataset of ground-based polarization lidar observations. The authors successfully identify 192 cases of hygroscopic growth and provide a detailed statistical analysis of the hygroscopic growth parameter, exploring its inter-annual trends, seasonal variations, and vertical distribution.
The contribution of this work is significant. Long-term, vertically resolved datasets of aerosol hygroscopicity are rare, and this study offers critical insights into how emission control policies, specifically the shifting ratio of NO2 to SO2, may be altering the optical properties of urban aerosols over time. The approach of combining lidar with radiosonde and reanalysis data is sound, and the manuscript is generally well-structured and clear.
However, to ensure the robustness of the retrieved optical properties and the subsequent hygroscopic parameters, I have a few specific concerns regarding the retrieval assumptions and aerosol classification methods. Addressing the following points would strengthen the physical interpretation of the results.
1. The study utilizes the POLIPHON method to separate dust and non-dust (anthropogenic) components. However, the manuscript does not explicitly state the specific particle depolarization ratios for pure dust and non-dust used for this separation. Since the derived non-dust backscatter coefficient is highly sensitive to these threshold values, please explicitly list them in the methodology section.
2. The methodology assumes that dust and non-dust aerosols are externally mixed. In a humid environment like Wuhan, it is common for dust particles to become internally mixed (e.g., coated) with soluble pollutants during transport. This coating process typically lowers the particle depolarization ratio of the dust. Could the POLIPHON method be misclassifying these "coated dust" particles as "anthropogenic" due to their reduced depolarization? If misclassification occurs, the "non-dust" component would be contaminated by less hygroscopic dust cores. Could this artificially lower the reported hygroscopicity of the anthropogenic component? A brief discussion on this potential contamination and its impact on the final statistics would be very helpful.
3. The particle backscatter coefficient is retrieved using a fixed lidar ratio of 50 sr. This assumption requires further elaboration. As aerosols absorb water and grow, their microphysical properties (size distribution, refractive index, shape) change, which typically causes the lidar ratio to vary rather than remain constant. Please discuss or quantify the potential error introduced by holding this value fixed during humidification events. A simple sensitivity analysis showing how a varying lidar ratio affects the calculated hygroscopic growth parameter would improve the robustness of the conclusions.
I hope my comments can be helpful in refining this interesting study.