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

Reported here are results from an airborne photochemical/sulfur field study in the equatorial Pacific. This study was part of NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A program. The 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‐type sampling configuration, this sortie was initiated under pre‐sunrise conditions and terminated in early afternoon with both boundary layer (BL) as well as buffer layer (BuL) sampling being completed. Chemical species sampled included the gas phase sulfur species dimethyl sulfide (DMS), sulfur dioxide (SO 2 ), methane sulfonic acid (MSA) g , and sulfuric acid (H 2 SO 4 ) g . Bulk aerosol samples were collected and analyzed for methane sulfonate (MS), non‐sea‐salt sulfate (NSS), Na + ,Cl − , and NH 4 + . 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 ratios for OH, SO 2 , and H 2 SO 4 and post‐sunrise maxima in the levels of DMS, OH, and H 2 SO 4 . Thus, unlike several previous studies involving coincidence DMS and SO 2 measurements, the Christmas Island data revealed that DMS and SO 2 were strongly anticorrelated. Our “best estimate” of the overall efficiency for the conversion of DMS to SO 2 is 72±22%. These results clearly demonstrate that DMS was the dominant source of SO 2 in the marine BL. Using as model input measured values for SO 2 and OH, the level of agreement between observed and simulated BL H 2 SO 4 (g) profiles was shown to be excellent. This finding, together with supporting correlation analyses, suggests that the dominant sulfur precursor for formation of H 2 SO 4 is SO 2 rather than the more speculative sulfur species, SO 3 . Optimization of the fit between the calculated and observed H 2 SO 4 values was achieved using a H 2 SO 4 first‐order loss rate of 1.3 × 10 −3 s −1 . On the basis of an estimated total “wet” aerosol surface area of 75 µm 2 /cm 3 , a H 2 SO 4 sticking coefficient of 0.6 was evaluated at a relative humidity of ≃95%, in excellent agreement with recent laboratory measurements. The Christmas Island data suggest that over half of the photochemically generated SO 2 forms NSS, but that both BL NSS and MS levels are predominantly controlled by heterogeneous processes involving aerosols. In the case of MS, the precursors species most likely responsible are the unmeasured oxidation products dimethyl sulfoxide (DMSO) and methane sulfinic acid (MSIA). Gas phase production of MSA was shown to account for only 1% of the observed MS; whereas gas phase produced H 2 SO 4 accounted for ∼20% of the NSS. These results are of particular significance in that BL‐measured values of the ratio MS/NSS have often 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 complex and still poorly characterized heterogeneous processes, it would suggest that for some environmental settings a simple interpretation of this ratio might be subject to considerable error.