Anthropogenic-Biogenic Interaction Drives Chloride Depletion in Coastal Atmosphere

Chloride depletion from sea salt aerosols (SSA) represents a key process in a coastal atmosphere, yet persistent discrepancies between field observations and modeling indicate unresolved mechanisms. Given the role of anthropogenic and biogenic emissions in altering atmospheric processes, we systematically investigated chloride depletion driven by NO_x and its interaction with isoprene. Chamber experiments showed a nonlinear dependence of chloride depletion on NO_x levels, with depletion increasing from 1.2% to 24.4% as NO_x increased from 0 to 141 ppb and then decreasing to 11.3% at 275 ppb. Experimental and modeling results demonstrate that oxidant-initiated reactions (OH, NO₃, etc.) are the primary factors driving chloride depletion. Isoprene further modulates NO_x-induced chloride depletion by shifting oxidant regimes, suppressing it by up to 3.5% at low NO_x concentrations but enhancing it by up to 12.1% at high NO_x concentrations. Additionally, several organic chlorinated compounds (e.g., C₅H₇ClO₄, C₈H₁₃ClO₆, etc.) that have been previously observed in ambient particles were identified, indicating that reactive chlorine produced from chloride depletion serves as a critical oxidant for the formation of organic chlorinated compounds. This study advances the understanding of chloride depletion influenced by anthropogenic-biogenic interactions in a coastal atmosphere and provides evidence for the formation of organic chlorinated compounds in coastal ambient particles.