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.
Highlights
The IPCC AR5 (Fifth Assessment Report of the United Nations Intergovernmental Panel on Climate Change; Myhre et al, 2013) and the recent preliminary release of AR6 (Sixth Assessment Report; https://www.ipcc.ch/report/ sixth-assessment-report-cycle/, last access: 30 November 2021) indicate that atmospheric aerosol particles are a dominant source of uncertainty in global climate forcing
Both theoretical and laboratory studies have proposed that a pristine environment favors the H-abstraction reaction when Dimethyl sulfide (DMS) is oxidized by OH, generating methylthiomethylperoxy radicals (MSP; CH3SCH2OO), which further undergo a series of rapid intramolecular H-shift isomerization reactions, yielding a stable intermediate hydroperoxymethyl thioformate (HPMTF; HOOCH2SCHO; Wu et al, 2015; Berndt et al, 2019)
Contrasting [MOD_RE] and [GAS_RE] reveals that the introduction of the aqueous-phase pathway contributes to large changes in the PD–PI sulfate co-located with high clouds over the tropics and low clouds over the Southern Ocean
Summary
The IPCC AR5 (Fifth Assessment Report of the United Nations Intergovernmental Panel on Climate Change; Myhre et al, 2013) and the recent preliminary release of AR6 (Sixth Assessment Report; https://www.ipcc.ch/report/ sixth-assessment-report-cycle/, last access: 30 November 2021) indicate that atmospheric aerosol particles are a dominant source of uncertainty in global climate forcing. The Community Atmosphere Model with chemistry (CAM-chem) includes only the oxidation of DMS by OH and NO3 radicals, directly producing SO2, which further oxidizes to produce sulfate (Emmons et al, 2020; Lamarque et al, 2012) This simplification ignores some potentially important reaction intermediates and pathways. MSA has been found to form efficiently via the multiphase OH-addition DMS oxidation pathway followed by a reaction with OH(aq) to form sulfate aerosol in the MBL (von Glasow and Crutzen, 2004; Milne et al, 1989; Zhu et al, 2006) Both theoretical and laboratory studies have proposed that a pristine environment favors the H-abstraction reaction when DMS is oxidized by OH, generating methylthiomethylperoxy radicals (MSP; CH3SCH2OO), which further undergo a series of rapid intramolecular H-shift isomerization reactions, yielding a stable intermediate hydroperoxymethyl thioformate (HPMTF; HOOCH2SCHO; Wu et al, 2015; Berndt et al, 2019). We examine how the natural aerosol background from DMS oxidation simulated with the modified model impacts estimates of aerosol radiative forcing
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