Dimethyl sulfide oxidation in the equatorial Pacific: Comparison of model simulations with field observations for DMS, SO2, H2SO4(g), MSA(g), MS, andNSS

Reported here are results from an airborne photochemical/sulfur field study in the equatorial Pacific. This study was part of NASA's Global Tropospher ic Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A program. Th e focus of this paper is on data gathered during an airborne mission (P-3B flight 7) near the Pacific site of Christmas Island. Using a Lagrangian-typ e sampling configuration, this sortie was initiated under pre-sunrise condi tions and terminated in early afternoon with both boundary layer (BL) as we ll as buffer layer (BuL) sampling being completed. Chemical species sampled included the gas phase sulfur species dimethyl sulfide (DMS), sulfur dioxi de (SO2), methane sulfonic acid (MSA)(g), and sulfuric acid(H2SO4)(g) Bulk aerosol samples were collected and analyzed for methane sulfonate (MS), non -sea-salt sulfate (NSS), Na+, Cl-, and NH4+. Critical non-sulfur parameters included real-time sampling of the hydroxyl radical (OH) and particle size /number distributions. These data showed pre-sunrise minima in the mixing r atios for OH, SO2, and H2SO4 and post-sunrise maxima in the levels of DMS, OH, and H2SO4 ThuS, unlike several previous studies involving coincidence D MS and SO2 measurements, the Christmas Island data revealed that DMS and SO 2 were strongly anticorrelated. Our "best estimate" of the overall efficien cy for the conversion of DMS to SO2 is 72+/-22%. These results clearly demo nstrate that DMS was the dominant source of SO2 in the marine BL. Using as model input measured values for SO2 and OH, the level of agreement between observed and simulated BL H2SO4(g) profiles was shown to be excellent. This finding, together with supporting correlation analyses, suggests that the dominant sulfur precursor for formation of H2SO4 is SO2 rather than the mor e speculative sulfur species, SO3. Optimization of the fit between the calc ulated and observed H2SO4 Values was achieved using a H2SO4 first-order los s rate of 1.3 x 10(-3) s(-1). On the basis of an estimated total "wet" aero sol surface area of 75 mu m(2)/cm(3), a H2SO4 sticking coefficient of 0.6 w as evaluated at a relative humidity of similar or equal to 95%, in excellen t agreement with recent laboratory measurements. The Christmas Island data suggest that over half of the photochemically generated SO2 forms NSS, but that both BL NSS and MS levels are predominantly controlled by heterogeneou s processes involving aerosols. In the case of MS, the precursors species m ost likely responsible are the unmeasured oxidation products dimethyl sulfo xide (DMSO) and methane sulfinic acid (MSIA). Gas phase production of MSA w as shown to account for only 1% of the observed MS; whereas gas phase produ ced H2SO4 accounted for similar to 20% of the NSS. These results are of par ticular significance in that BL-measured values of the ratio MS/NSS have of ten been used to estimate the fraction of NSS derived from biogenic DMS and to infer the temperature environment where DMS oxidation occurred.. If our conclusions are correct and both products are predominantly formed from co mplex and still poorly characterized heterogeneous processes, it would sugg est that for some environmental settings a simple interpretation of this ra tio might be subject to considerable error.