Marine Volatile Organic Compounds Promote the Chloride Depletion in Sea Salt Aerosols

Despite chloride depletion of sea salt aerosols (SSA) is one of the most important phenomena in the coastal atmosphere with significant impacts on air quality and human health, the factors influencing this depletion remain unclear. We analyzed the mole ratio of Cl⁻/Na⁺ to evaluate the effects of coastal atmospheric enriched SO₂ and some marine volatile organic compounds (namely isoprene and dimethyl sulfide (DMS)) on chloride depletion by laboratory experiments. Using ultrahigh performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometer (UPLC/ESI-HR-Q-TOF-MS), the formation of organic chlorinated compounds during chloride depletion of SSA was identified and analyzed to verify marine sources of Cl radicals. Our results show that chloride depletion increased (1.1%–33.7%) with increasing SO₂ concentration (0–400 ppb) due to the production of inorganic acids. This chloride depletion was also enhanced with the addition of isoprene and DMS, with DMS having the greatest effect. In the presence of DMS, the mole ratio of Cl⁻/Na⁺ was 0.721 at an SO₂ concentration of 200 ppb, causing greater chloride depletion than isoprene (0.820). Many organic chlorinated compounds were identified, with CHClSO compounds being identified in experiments in the presence of SO₂. The mass spectrometry results confirm that the activation of chloride ions, another important factor affecting chloride depletion, is essential for the formation of organic chlorinated compounds. These results have implications for model predictions of chloride depletion in polluted coastal areas.