| 10 Mar 2026
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.
This paper describes the detection and quantification of sea salt aerosol with a TOF-ACSM equipped with a capture vaporizer. The authors performed calibration experiments with size-selected NaCl particles and used the results to analyze ambient data from two field campaigns as well as chamber data with NaCl seeds. This paper is a nice contribution to the literature and should be accepted after consideration of the following comments:
Major comments:
Minor comments:
Lines 56-58: You need another sentence or two explaining what the vaporizer is.
Line 58: Be more specific about the impact of the CV on thermal decomposition. Instead of “can alter” say “increases.”
Line 60: The difference in temperature between the SV at 600 C and the CV and 550 C has very little impact on fragmentation. It is really about the structure and residence time. I would delete the end of this sentence.
Figure 1 caption: I don’t see a purple shaded area. The org signal does not go to zero at the end. Is that 58 or something else?
Lines 197-198. I don’t see how 58 can be the most dominant ion in Org after the a-pinene injection.
Figure 2. You need some kind of legend for the different concentrations. The y-axes should be labeled with mass and surface area because it is easy to miss the change in units. Scale both y-axes from 0. I don’t understand what the /sc and /mc in the legend are referring to.
Line 221 and line 234. I don’t think these slopes are useful to report since you are not using them to calculate anything and you would need the intercepts, too. You say there’s a factor of two change for mass concentration. State what the relative change is for surface area.
Line 228. “it is impossible” seems a bit strong. The size dependence does introduce some uncertainty but one could still report concentration.
Line 235 “confirming the absence” is also a bit strong. There is a size dependence when normalized by surface area. It is just less pronounced.
Line 261 and Figure 3 legends. Do you really think you have three significant digits in R2?
Lines 296-299. This is an interesting observation. Do you see enough SO4 or NO3 to confirm? Do you see different slopes at Cabauw?
Line 343. The background signal also changes with time, even for a given instrument.
Line 347 and line 364. I don’t understand this comment about NaCl before aging. Do you think aging changes the surface area? How is what you report related to the actual aerosol?
Line 355. This claim of minimal interference is contradicted later in the paragraph. Do you mean for marine air masses?
SI line 3. “inclusion in the analysis” rather than “data acquisition”
Figure S1. Can you use different symbols for the different sizes? Those colors are very hard to tell apart. Also, “analyzed” rather than “collected” in the caption.
Figure S2. I don’t see a purple shaded area. Do you know why the organic is rising before you inject a-pinene?
Typographical/grammatical errors:
Line 74: “in” rather than “at” the samples.
Line 98: Define AIDA.
Line 122: Define CAINA.
Lines 139-141: Repeats information in lines 141-143. Delete this sentence.
Line 158: Maybe “instrument” or “sampling” rather than “sample container.”
Line 163: Insert “at” before coordinates.
Line 226: Remove the “±”
Lines 247-251. This is a long, somewhat confusing sentence. I would put a period after “Germany.” Then start the next sentence “This allowed investigation of truly marine air, although…” and get rid of the parentheses.
Line 258. List the m/z’s as y vs x and in the order shown in Fig. 3, meaning 23, 58,23, 60, 60, 58. Same thing on line286.
Line 274. I think you mean offshore (continental) rather than onshore.
Line 361. Say “lack of” rather than “missing” correlation.
SI Section 2. “held” rather than “remained”