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.