Model study of multiphase DMS oxidation with a focus on halogens

[摘要] We studied the oxidation of dimethylsulfide (DMS) in the marineboundary layer (MBL) with a one-dimensional numerical model andfocused on the influence of halogens.Our model runs show that thereis still significant uncertainty about the end products of theDMS addition pathway, which is especially caused by uncertaintyin the product yield of the reaction of the intermediate product methyl sulfinic acid(MSIA) with OH.BrO strongly increases the importanceof the addition branch in the oxidation of DMS even whenpresent at mixing ratios smaller than 0.5pmol mol^-1.Theinclusion of halogen chemistry leads to higher DMS oxidationrates and smaller DMS to SO₂ conversionefficiencies. The DMS to SO₂ conversion efficiency isalso drastically reduced under cloudy conditions.In cloud-free modelruns between 5 and 15% of the oxidized DMS reacts further toparticulate sulfur, in cloudy runs this fraction is almost 100%.Sulfate production by HOCl_aq and HOBr_aq isimportant in cloud droplets even for small Br^- deficits andrelated small gas phase halogen concentrations.In general, moreparticulate sulfur is formed when halogen chemistry is included.Apossible enrichment of HCO₃^- in fresh sea salt aerosol wouldincrease pH values enough to make the reaction of S(IV)^*(=SO_2,aq+HSO₃^-+SO₃^2-)with O₃ dominant for sulfate production. It leads to a shiftfrom methyl sulfonic acid (MSA) to non-seasalt sulfate (nss-SO₄^2-) production but increases thetotal nss-SO₄^2- only somewhat because almost all availablesulfur is already oxidized to particulate sulfur in the basescenario. We discuss how realistic this is for the MBL. We found thereaction MSA_aq+OH to contribute about 10% to theproduction of nss-SO₄^2- in clouds. It is unimportant forcloud-free model runs.Overall we find that the presence of halogensleads to processes that decrease the albedo of stratiform clouds in theMBL.

[学科分类] 大气科学