Measurements of NaCl in ambient air with a Capture Vaporizer-ToF-ACSM

Abstract. Sea spray aerosol is important for climate and atmospheric chemistry by influencing 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 (Na³⁵Cl) and m/z 60 (Na³⁷Cl) were identified through a controlled laboratory calibration. The calibration experiments show that when the ACSM response (ions/s) is normalized by the available particle surface area, the response is independent of particle concentration and only weakly dependent on particle size for monodisperse NaCl aerosol. When considering aerosol mass concentrations without normalization for the available particle surface area, it is only possible to derive ambient sea salt mass concentrations from the ACSM signal with prior information on particle size due to the observed size dependence. 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 amounts 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 (R² = 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 lower 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 ageing, 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.