Abstract: The radiative forcing from natural background aerosol particles in the atmosphere represents a key uncertainty in evaluating historic climate change. Oceanic emissions of dimethyl sulfide (DMS) to the atmosphere and subsequent oxidation chemistry are the controlling processes for natural aerosol abundance over many oceanic regions. However, despite longstanding research on DMS emissions and chemistry, there remain large uncertainties as to the chemical fate of DMS and its oxidation products and how efficiently they contribute to aerosol formation and growth. In particular, the recently discovered formation of hydroperoxyl methylthioformate (HPMTF) from DMS oxidation, and the controlling role of heterogeneous uptake of HPMTF onto cloud droplets has prompted major revisions to our understanding of DMS reaction pathways and atmospheric fate.

Here I will present results from the recent Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) airborne field campaign which sampled over the mid-latitude eastern Pacific Ocean in the summer of 2023. This campaign included the most comprehensive measurements to date of marine sulfur species (including DMS and HPMTF) in both clear sky and cloudy conditions. Through targeted sampling under variable cloud regimes, we derive direct constraints on HPMTF cloud loss rates and uptake kinetics, including the reactive uptake coefficient. Finally, I will briefly discuss how these in-situ constraints can be used to inform chemical transport models and the remaining critical uncertainties in current model implementations of DMS chemistry.

Bio: Gordon Novak received his PhD in analytical chemistry from the University of Wisconsin in 2020, advised by Prof. Timothy Bertram. There he conducted targeted field studies to probe sources and sinks of reactive trace gases and heterogeneous chemistry in the marine atmosphere. He then joined the National Oceanic and Atmospheric Administration Chemical Sciences Laboratory (NOAA CSL) first as a postdoc and then as a permanent research scientist. At CSL he continued to use field measurements to constrain atmospheric heterogeneous reactions through numerous airborne research campaigns ranging from the marine boundary layer to the stratospheric polar vortex. In 2025 he joined the Wolfson Atmospheric Chemistry Laboratory at the University of York as a research fellow, where he is working on improving the representation of heterogeneous reaction processes in the GEOS-Chem chemical transport model.