| 05 Mar 2026
Uncovering precursors for VOC production from ozonolysis of seawater
Abstract. Large uncertainty exists in the role that the ocean plays as a net source for atmospheric volatile organic carbon (VOC) compounds, in part because of poorly quantified processes near the air–sea interface. Laboratory studies imply that heterogeneous reactions of ozone at the ocean surface may be a key source of VOCs to the marine atmosphere, but the representativeness of such experiments to the chemically complex surface ocean waters is unclear due to limited field evidence. Here, we determined the production ratios of select VOCs formed during ozonolysis of fresh, natural seawater from laboratory experiments under turbulent conditions using a proton-transfer-reaction quadrupole mass spectrometer (PTR-MS). To quantify seasonal variability, near surface water samples were collected from a temperate marine station during different seasons and measured for ozone-driven VOC production ratios. In all experiments, out of the VOCs monitored the dominant product of ozonolysis were m/z 69 (C5H9+, assumed to be isoprene), followed by m/z 59 (acetone/propanal) and m/z 45 (acetaldehyde). Clear seasonal differences in VOC production imply that marine biogeochemistry likely drives the availability and composition of VOC precursors. Further ozonolysis experiments using diluted VOC precursors (senescent algal culture, fatty acids, certified natural organic matter (NOM)) resulted in different VOC production compared to those using natural seawater. Our results show that ocean biogeochemical cycling is a key driver of variability in ozone driven VOC production at the sea surface.
Hopkins et al. present the production of VOCs from the ozonolysis of natural seawater via controlled laboratory experiments. The study found that VOC production ratios exhibit strong seasonal patterns linked to phytoplankton dynamics, identified fatty acids and algal-derived dissolved organic matter (DOM) as potential key precursors, and provided a preliminary scaling of the process to estimate its global significance. The manuscript is generally well-structured, the methodology is clearly described, and the research addresses an important gap in understanding marine VOC sources. However, I have several concerns on the manuscript that need to be addressed before considering for publication.
Line 14: In the phrase “representativeness of such experiments to the chemically complex”, it is better to revise the word “to” to “for”.
Line 66: Typo for “quiestcent”.
Line 74: Please delete or revise the word “roughly” as it induces high uncertainties or a weak result.
Line 82-83: It is not clear how the percentage in “(59 % within 3 h, 94 % within 24 h)” represent the samples used in the experiments.
Line 113: In the O3 generation methodology, it was mentioned that the O3 levels were set at 7.7 ± 0.7, 1.7 ± 0.7, and 2.1 ± 1.1 ppmv for spring, summer and winter experiments. These levels are substantially higher than ambient marine O3 levels. In other words, do the experiments have environmental implications and what are the basis for choosing high O3 levels for the ozonolysis experiments.
Line 134, Section 2.3: Using the bubble-column system with natural sea water, a significant number of sea-salt particles can be produced. These particles may provide a large surface area for heterogeneous reactions, and may produce VOCs in certain cases. How can the authors assure that the particles will not affect the results of the study? The authors need to clarify and exclude these influences.
Line 169, Section 3.1: In this work, high concentration of O3 were used in the experiments. The high concentration of O3 can react with these VOCs, generating SOA and its intermediate precursors, may also affect the results that the VOCs are from the emission of sea water. How can the authors make sure that the measured VOCs are not secondary produced in the experimental system?
Line 285, Figure 5: Error bars should be added to the graph.
Line 290, Section 3.3: The laboratory used high concentrations of O3 (1.7-9 ppmv), while the real environmental concentration was three orders of magnitude lower. This means that if the rate of O3 dry deposition on the ocean and VOCs emission rate remain unchanged, the amount of VOCs may be significantly reduced at the ambient O3 levels. I think extrapolating the experimental results to the global ocean is not accurate enough. Furthermore, if the DOM is the key precursor, the uneven distribution of DOM in the global ocean may also affect the VOCs emissions. Therefore, I think this section is unnecessary in the text. Instead, the authors can discuss more on the potential implications of the ozonolysis.
SI, Table S4: What is the limit of detection (LOD) for these VOCs measurements?
SI, Line 17: Please explain why the VOC peak height was used here is a more appropriate method than the peak area technique?