Improved method for estimating chlorine depletion in sea salt aerosols using single particle aerosol mass spectrometer

Chloride-containing aerosols significantly influence tropospheric chemistry and cloud microphysics. Sea salt aerosols (SSAs) are a major source of chlorine-containing aerosols in coastal regions. However, existing methods for estimating chlorine depletion using Single Particle Aerosol Mass Spectrometry (SPAMS) suffer from significant limitations. This study demonstrates that relying solely on [Cl]^- signal at m/z = −35 and −37 to quantify chlorine content leads to substantial overestimation of chlorine depletion, particularly in fresh SSAs, where additional chlorine is contributed by [Na₂Cl]⁺ and [NaCl₂]^- ions. Using SPAMS data collected at a coastal site in Hong Kong during winter, spring and summer, we propose an improved methodology that accounts for all potential chlorine-depleting components. This improved method shows a better correlation with bulk observations, reducing the average estimated chlorine depletion from 98 ± 1 % to 57 ± 3 %, aligning closely with bulk results (49 ± 22 %). The new method significantly improves the refined analysis of chemical processes affecting chlorine depletion by precisely resolving size-dependent biases, organic acid, and NH₃ concentration-dependent dual roles. Seasonal analyses highlight the strong influence of dominant air masses, with winter exhibiting the highest depletion due to air masses originating from industrial inland/coastal regions, while summer shows minimal depletion under clean marine conditions. The improved method establishes a more reliable framework for quantifying chlorine depletion in individual SSAs, advancing mechanistic understanding of coastal chlorine cycling.