Exploring DMS oxidation and implications for global aerosol radiative forcing

Abstract

Abstract. Aerosol indirect radiative forcing (IRF), which characterizes how aerosols alter cloud formation and properties, is very sensitive to the preindustrial (PI) aerosol burden. Dimethyl sulfide (DMS), emitted from the ocean, is a dominant natural precursor of non-sea-salt sulfate in the PI and pristine present-day (PD) atmospheres. Here we revisit the atmospheric oxidation chemistry of DMS, particularly under pristine conditions, and its impact on aerosol IRF. Based on previous laboratory studies, we expand the simplified DMS oxidation scheme used in the Community Atmospheric Model version 6 with chemistry (CAM6-chem) to capture the OH-addition pathway as well as the H-abstraction pathway and the associated isomerization branch. These additional oxidation channels of DMS produce several stable intermediate compounds, e.g., methanesulfonic acid (MSA) and hydroperoxymethyl thioformate (HPMTF), delay the formation of sulfate, and hence, alter the spatial distribution of sulfate aerosol and radiative impacts. The expanded scheme improves the agreement between modeled and observed concentrations of DMS, MSA, HPMTF, and sulfate over most marine regions based on the NASA Atmospheric Tomography (ATom), the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA), and the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) measurements. We find that the global HPMTF burden, as well as the burden of sulfate produced from DMS oxidation are relatively insensitive to the assumed isomerization rate, but the burden of HPMTF is very sensitive to a potential additional cloud loss. We find that global sulfate burden under PI and PD emissions increase to 412 Gg-S (+29 %) and 582 Gg-S (+8.8 %), respectively, compared to the standard simplified DMS oxidation scheme. The resulting annual mean global PD direct radiative effect of DMS-derived sulfate alone is −0.11 W m−2. The enhanced PI sulfate produced via the gas-phase chemistry updates alone dampens the aerosol IRF as anticipated (−2.2 W m−2 in standard versus −1.7 W m−2 with updated gas-phase chemistry). However, high clouds in the tropics and low clouds in the Southern Ocean appear particularly sensitive to the additional aqueous-phase pathways, counteracting this change (−2.3 W m−2). This study confirms the sensitivity of aerosol IRF to the PI aerosol loading, as well as the need to better understand the processes controlling aerosol formation in the PI atmosphere and the cloud response to these changes.

Arctic observations of Hydroperoxymethyl Thioformate (HPMTF) – seasonal behavior and relationship to other oxidation products from Dimethyl Sulfide at the Zeppelin Observatory, Svalbard

“…low occurrence of OH radicals and/or meteorological factors. This hypothesis is supported by the higher importance of OHinitiated DMS oxidation at lower latitudes in the study by Fung et al (2022), and the DMS conversion yields of MSA and HPMTF in the different model runs by Wollesen de Jonge et al ( 2021), where the HPMTF/MSA ratio was considerably lower in the AtmMain case compared to cases with higher temperature and lower RH. This could be both due to differing sources and sinks, such as dry/wet deposition of MSA (Bergin et al, 1995) and HPMTF (Khan et al, 2021), heterogeneous oxidation of MSA (g) (Mungall et al, 2018) and evaporation of MSA from the particle-to the gas phase (Baccarini et al, 2021).…”

“…1) during these months (Berndt et al, 2019;R. Wu et al, 2015), despite the low occurrence of this mechanism compared to the addition pathway in the Nordic Seas according to Fung et al (2022). However, the discrepancies that exist in the relative temporal evolution between measured MSA (g) and HPMTF (g) in summer could also be a sign that HPMTF is not produced as efficiently as MSA close to Svalbard due to e.g.…”

“…Since the discovery of HPMTF, it has inspired several new studies (e.g., Khan et al, 2021;Novak et al, 2021;Vermeuel et al, 2020;Wollesen de Jonge et al, 2021). Fung et al, (2022) developed an understanding of where certain DMS oxidation products can dominate in the world due to variation in dominating DMS oxidation mechanisms. This clarified why the highest HPMTF concentrations were found over tropical oceans with a high degree of OH oxidation through the abstraction pathway, and lower over equally microbiologically productive waters with a relatively higher degree of oxidation through the addition pathway, such as the North Atlantic and Canadian Arctic (Fung et al, 2022;Veres et al, 2020).…”

Influence of Air Mass Source Regions on Signatures of Surface-Active Organic Molecules in Size Resolved Atmospheric Aerosol Particles

“…17,18 Non-sea salt sulfate can also be formed through the oxidation of dimethyl sulfide (DMS), emitted from the ocean, and contribute to a larger fraction of sulfate in marine aerosol. 19,20 Coastal and estuarine plants can emit VOCs that form organic acids, including formate and acetate. 21−23 Another major natural, terrestrial source of particles is mineral and soil dust, which can be identified through the presence of calcium and carbonate.…”