<p>Particulate chloride (Cl⁻) is an essential substance in regulating chlorine chemistry, which has significant impacts on the atmospheric oxidation capacity and secondary aerosol formation. However, the level of chloride loss in particles depends on the availability of acidic gases, and the loss rate varies in different environments. Here, we measured chloride concentrations in the fine aerosols at an island site (Da Wan Shan, DWS) in the Pearl River Estuary (PRE) during the winter of 2023 to be in the range of 0.10–0.93&#xa0;µg m^− 3. The Cl⁻/Na⁺ ratios varied from 0.48 to 3.22, and about 83% of them exhibited ratios below 1.81, indicating that the particulate samples underwent significant Cl⁻ depletion. When air masses originated from coastal and marine areas, higher particulate chloride loss was observed, with mean Cl⁻/Na⁺ ratio of 1.14 ± 0.50 and 1.11 ± 0.35, respectively, whereas inland-originated air masses had a larger mean Cl⁻/Na⁺ ratio of 1.50 ± 0.60. The higher ratio in inland-originated air masses may indicate that anthropogenic chlorine sources from the continent can replenish the chloride content in aerosol, thereby increasing the Cl⁻/Na⁺ ratio. The variation of Cl⁻ depletion observed at DWS is very likely controlled by the aging process of the arriving air mass, the level of nitric acid, and changes in ambient temperature. Our study reveals the significant Cl⁻ loss in the fine particles in PRE together with the factors affecting their loss, which will be important for a better understanding of chlorine chemistry in the marine-estuary atmosphere.</p>