PEROXYNITRATE FORMATION DURING THE NIGHTTIME OXIDATION OF DIMETHYLSULFIDE - ITS ROLE AS A RESERVOIR SPECIES FOR AEROSOL FORMATION

The reaction between dimethyl sulphide (DMS) and the nitrate radical NO3 in dark air was studied in a Teflon bag, monitoring products formed in the gas phase together with aerosol composition and size distributions in the course.of the experiment. The formation of the condensable products methane sulphonic acid (MSA) and sulphuric acid (H22SO4) was found to happen via a gaseous, relatively stable PAN-like peroxynitrate intermediate (CH3S(O)O2NO2 or CH3S(O)2O2NO2, called MSPN) which can build up to concentrations in the gas phase that are a multiple of MSA and H2SO4. A coupled gas chemistry-aerosol dynamics model was fitted to the experimental data and led to a consistent description of the partitioning of the S-containing products (SO2, MSPN, MSA and H2SO4) over gas and aerosol phase. The optimized chemical model reproduces adequately the observed strong NO dependence of the MSPN-to-(MSA + H2SO4) conversion rate by gas-phase reactions, The fitted loss rate for MSPN via pathways not included in the gas-phase mechanisms (e.g. reaction on aerosol particles or on the wall) is 100-500 times smaller than for N2O5. The model predicts further that about 50% of the initial DMS is transformed to SO2. Fitting the aerosol dynamics model to the observed aerosol growth rate, led to an estimate for the MSA condensation accommodation coefficient (alpha(MSA) > 0.1) and for the MSA/H2SO4 formation ratio (1.2-3). The chemical model predicts that MSPN might be an important reservoir species for nitrogen, sulphur, and for aerosol formation in marine regions that are impacted by NO(x)-rich air masses.