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Exploring dimethyl sulfide (DMS) oxidation and implications for global aerosol radiative forcing
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2022
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Source: Atmospheric Chemistry and Physics, 22(2), 1549-1573
[PDF-6.61 MB]
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Journal Title:Atmospheric Chemistry and Physics
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Description: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 and 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 radiativeimpacts. The expanded scheme improves the agreement between modeled andobserved concentrations of DMS, MSA, HPMTF, and sulfate over most marineregions, based on the NASA Atmospheric Tomography (ATom), the Aerosol andCloud Experiments in the Eastern North Atlantic (ACE-ENA), and theVariability of the American Monsoon Systems (VAMOS)Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx)measurements. We find that the global HPMTF burden and the burden of sulfate produced from DMS oxidation are relatively insensitive to theassumed isomerization rate, but the burden of HPMTF is very sensitive to apotential 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 confirmsthe sensitivity of aerosol IRF to the PI aerosol loading and theneed to better understand the processes controlling aerosol formation in the PI atmosphere and the cloud response to these changes.
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Source:Atmospheric Chemistry and Physics, 22(2), 1549-1573
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ISSN:1680-7324
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Rights Information:CC BY
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Compliance:Submitted
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