Measurements of NaCl in ambient air with a capture vaporizer-ToF-ACSM

Sea spray aerosol is important for climate and atmospheric chemistry as it influences radiative forcing and heterogeneous reactions, but few online methods exist for quantifying sub-micron sea spray concentrations. Common chemical speciation instruments, such as aerosol mass spectrometers (AMS) and aerosol chemical speciation monitors (ACSM), are usually not used for sea salt quantification due to the incomplete evaporation of refractory sodium chloride (NaCl). This study evaluates the capability of the time-of-flight-ACSM (ToF-ACSM) equipped with a capture vaporizer (CV) to detect and quantify sea salt aerosol for the first time. Key NaCl marker ions (m/z 23 (Na), m/z 58 (Na 35 Cl), and m/z 60 (Na 37 Cl)) were identified through controlled laboratory calibration. The calibration experiments show that the ACSM response (ions s −1 ) at characteristic NaCl fragments scales with the available particle surface area, yielding a surface-normalized signal that is only weakly dependent on particle size for monodisperse NaCl aerosol. This indicates that the CV-ToF-ACSM provides a direct and robust measure of sea salt aerosol surface area. If a prior estimate of the NaCl size distribution is available, this signal can be converted to aerosol mass concentrations. Furthermore, controlled chamber experiments indicated that secondary organic aerosol (SOA) formed from α -pinene as a precursor and condensed on the NaCl particles does not produce significant quantities of fragments at the m/z values characteristic for NaCl. Field experiments at a coastal site showed that conditions with onshore winds resulted in high correlations ( R 2 = 0.949–0.977) between the three key NaCl marker ions. By applying the laboratory-derived calibration formula to the raw CV-ToF-ACSM m/z 23 signal, sea salt aerosol surface concentrations could be quantitatively determined in real time. However, the slope between the fragments at m/z 23 and m/z 58 is higher in the ambient data than in the laboratory calibration, suggesting reduced Cl relative to Na due to aging reactions of the sea salt particles in the coastal environment. Overall, these results demonstrate that the CV-ToF-ACSM can provide quantitative real-time information on submicron sea salt aerosol before aging, particularly in terms of surface area, while accurate mass concentration retrieval requires additional NaCl size distribution information. These findings highlight the potential to improve the characterization of marine aerosol sources and their role in atmospheric processes.