Sources,transport, and sinks of SO<Subscript>2</Subscript> over the equatorial Pacific during the Pacific Atmospheric Sulfur Experiment

The Pacific Atmospheric Sulfur Experiment (PASE) is the first sulfur-budget field experiment to feature simultaneous flux measurements of DMS marine emissions and SO₂ deposition to the ocean surface. We make use of these data to constrain a 1-D chemical transport model to study the production and loss pathways for DMS and SO₂ over the equatorial Pacific. Model results suggest that OH is the main sink for DMS in the boundary layer (BL), and the average DMS-to-SO₂ conversion efficiency is ~73%. In an exploratory run involving the addition of 1 pptv of BrO as a second oxidant, a 14% increase in the DMS flux is needed beyond that based on OH oxidation alone. This BrO addition also reduces the DMS-to-SO₂ conversion efficiency from 73% to 60%. The possibility of non-DMS sources of marine sulfur influencing the estimated conversion efficiency was explored and found to be unconvincing. For BL conditions, SO₂ losses consist of 48% dry deposition, while transport loss to the BuL and aerosol scavenging each account for another 19%. The conversion of SO₂ to H₂SO₄ consumes the final 14%. In the BuL, cloud scavenging removes 85% of the SO₂, thus resulting in a decreasing vertical profile for SO₂. The average SO₂ dry deposition velocity from direct measurements (i.e., 0.36 cm sec⁻¹) is approximately 50% of what is calculated from the 1-D model and the global GEOS-Chem model. This suggests that the current generation of global models may be significantly overestimating SO₂ deposition rates over some tropical marine areas. Although the specific mechanism cannot be determined, speculation here is that the dry deposition anomalous results may point to the presence of a micro-surface chemical phenomenon involving partial saturation with either S(IV) and/or S(VI) DMS oxidation products. This could also appear as a pH drop in the ocean’s surface microfilm layer in this region. Finally, we propose that the enhanced SO₂ level observed in the lower free troposphere versus that in the upper BuL during PASE is most likely the result of transported DMS/SO₂-rich free-tropospheric air parcels from the east of the PASE sampling area, rather than an inadequate representation in the model of local convection.     

A complete dynamical ozone budget measured in the tropical marine boundary layer during PASE

The Pacific Atmospheric Sulfur Experiment (PASE) was a field mission that took place aboard the NCAR C-130 airborne laboratory over the equatorial Pacific Ocean near Christmas Island (Kirimati, Republic of Kiribati) during August?CSeptember, 2007. Eddy covariance measurements of the ozone fluxes at various altitudes above the ocean surface, along with simultaneous mapping of the horizontal gradients provided a unique opportunity to observe all of the dynamical components of the ozone budget in this remote marine environment. The results of six daytime and two sunrise flights indicate that vertical transport into the marine boundary layer from above and horizontal advection by the tradewinds are both important source terms, while photochemical destruction consisting of 82% photolysis (leading to OH production), 11% reaction with HO₂, and 7% reaction with OH provides the main sink. The overall photochemical lifetime of ozone in the marine boundary layer was found to be 6.5 days. Ocean uptake of ozone was observed to be fairly slow (mean deposition velocity of 0.024?±?0.014 cm s^?1) accounting for a diurnally averaged loss rate that was ??30% as large as the net photochemical destruction. From the measurement of deposition velocity an ozone reactivity of ??50 s^?1 in seawater is inferred. Due to the unprecedented measurement accuracy of the dynamical budget terms, unobserved photochemistry was able to be deduced, leading to the conclusion that 3.9?±?3.0 ppt (parts per trillion by volume) of NO is present on average in the daytime tropical marine boundary layer, broadly consistent with several previous studies in similar environments. It is estimated, however, that each ppt of BrO hypothetically present would counter each ppt of NO above the requisite 3.9 ppt needed for budget closure. The long-term budget of ozone is further analyzed in the buffer layer, between the boundary layer and free troposphere, and used to derive an entrainment velocity across the trade wind inversion of 0.51 ± 0.38 cm s^?1.     

Sulfur dioxide in the tropical marine boundary layer: dry deposition and heterogeneous oxidation observed during the Pacific Atmospheric Sulfur Experiment

Research flights with the National Center for Atmospheric Research (NCAR) C-130 airborne laboratory were conducted over the equatorial ocean during the Pacific Atmospheric Sulfur Experiment (PASE). The focused, repetitive flight plans provided a unique opportunity to explore the principal pathways of sulfur processing in remote marine environments in close detail. Fast airborne measurements of SO₂ using the Drexel University APIMS (Atmospheric Pressure Ionization Mass Spectrometer) instrument further provided unprecedented insight into the complete budget of this important sulfur gas. In general, turbulent mixing in the marine boundary layer (MBL) continuously depletes SO₂ due to the shallow convection of the tropical trade wind regime by venting the gas into the buffer layer (BuL) above. However, on nearly one-third of the flights a net import of SO₂ into the MBL from the BuL was observed. Concurrent measurements of the DMS budget allowed for a heterogeneous S(IV) oxidation rate to be inferred from the SO₂ budget residual. The average heterogeneous loss rate was found to be 0.05 h⁻¹, which taken in conjunction with the observed aerosol surface area distributions and O₃ levels indicates that the supermicron aerosols maintain a near neutral pH. The average dry deposition velocity of SO₂ was found to be 0.4 cm s⁻¹, about 30% lower than predicted by standard parameterizations. The yield of SO₂ from DMS oxidation was found to be near unity. The mission averages indicate that approximately 57% of the SO₂ in the MBL is lost to aerosols, 27% is subject to dry deposition, 7% is mixed into the BuL, and 10% is oxidized by OH.     

西太平洋区域二甲硫酸及其他硫化物的模拟研究

本文利用一维光化学模式，以二甲硫醚（ＤｉｍｅｔｈｙｌＳｕｌｆｉｄｅ，简称ＤＭＳ）为源模拟了西太平洋对流层的硫化物循环。ＤＭＳ海－气通量由“ｓｔａｇｎａｎｔ－ｆｉｌｍ”模式进行计算。尽管海洋大气中ＯＣＳ浓度比ＤＭＳ大一个量级，但它对ＳＯ２的贡献很小，ＤＭＳ仍是海洋大气中ＳＯ２的主要源。在大气垂直湍流输送过程中，ＤＭＳ白天与ＯＨ反应，夜间与ＮＯ３反应被氧化成ＳＯ２，ＳＯ２大部分通过非均相转化形成Ｈ２ＳＯ４。模拟结果与ＰＥＭ－ＷＥＳＴ－Ａ观测资料对比，取得了较好的一致性。     

Modelling the multiphase near-surface chemistry related to ozone depletions in polar spring

Near-total depletions of ozone have been observed in the Arctic spring since the mid 1980s. The autocatalytic reaction cycles involving reactive halogens are now recognized to be of main importance for ozone depletion events in the polar boundary layer. We present sensitivity studies using the model MISTRA in the box-model mode on the influence of chemical species on these ozone depletion processes. In order to test the sensitivity of the chemistry under polar conditions, we compared base runs undergoing fluxes of either Br₂, BrCl, or Cl₂ to induce ozone depletions, with similar runs including a modification of the chemical conditions. The role of HCHO, H₂O₂, DMS, Cl₂, C₂H₆, HONO, NO₂, and RONO₂ was investigated. Cases with elevated mixing ratios of HCHO, H₂O₂, DMS, Cl₂, and HONO induced a shift in bromine speciation from Br / BrO to HOBr/HBr, while high mixing ratios of C₂H₆ induced a shift from HOBr/HBr to Br/BrO. The shifts from Br/BrO to HOBr/HBr accelerated the aerosol debromination, but also increased the total amount of deposited bromine at the surface (mainly via increased deposition of HOBr). For all NO_y species studied (HONO, NO₂, RONO₂) the chemistry is characterized by an increased bromine deposition on snow reducing the amount of reactive bromine in the air. Ozone is less depleted under conditions of high mixing ratios of NO_x. The production of HNO₃ led to the acid displacement of HCl, and the release of chlorine out of salt aerosol (Cl₂ or BrCl) increased.     

FT-IR Study of the Kinetics and Products of the Reactions of Dimethylsulphide, Dimethylsulphoxide and Dimethylsulphone with Br and BrO

Kinetics and products of the gas-phase reactions of dimethylsulphide (DMS), dimethylsulphoxide (DMSO) and dimethylsulphone (DMSO₂) with Br atoms and BrO radicals in air have beeninvestigated using on-line Fourier Transform Infrared Spectroscopy (FT-IR) as analytical technique at 740 ± 5 Torr total pressure and at 296 ± 3 K in a480 L reaction chamber. Using a relative rate method for determining the rate constants; the following values (expressed in cm³molecule^–1 s^–1) were found: k_DMS+Br = (4.9 ±1.0) ×10^–14, k_DMSO + Br < 6 × 10^–14,k_{DMSO 2} + Br 1 × 10^–15,k_DMSO + BrO = (1.0 ± 0.3) × 10^–14 andk_{DMSO 2} + BrO 3 × 10^–15 (allvalues are given with one on the experimental data). DMSO, SO₂, COS, CH₃SBr andCH₃SO₂Br were identified as the main sulphur containing products of the oxidation of DMS by Br atoms. From the reaction between DMSO and Br atoms, DMSO₂and CH₃SO₂Br were the only sulphur containing products thatwere identified. DMSO, DMSO₂ and SO₂ were identified as themain sulphur containing products of the reaction between DMS and BrO.DMSO₂ was found to be the only product of the reaction between DMSO and BrO. For the reactions of DMSO₂ with Br and BrO no products were identified because the reactions were too slow.The implications of these results for atmospheric chemistry are discussed.     

Seasonal characteristics of tropical marine boundary layer air measured at the Cape Verde Atmospheric Observatory

Observations of the tropical atmosphere are fundamental to the understanding of global changes in air quality, atmospheric oxidation capacity and climate, yet the tropics are under-populated with long-term measurements. The first three years (October 2006–September 2009) of meteorological, trace gas and particulate data from the global WMO/Global Atmospheric Watch (GAW) Cape Verde Atmospheric Observatory Humberto Duarte Fonseca (CVAO; 16° 51′ N, 24° 52′ W) are presented, along with a characterisation of the origin and pathways of air masses arriving at the station using the NAME dispersion model and simulations of dust deposition using the COSMO-MUSCAT dust model. The observations show a strong influence from Saharan dust in winter with a maximum in super-micron aerosol and particulate iron and aluminium. The dust model results match the magnitude and daily variations of dust events, but in the region of the CVAO underestimate the measured aerosol optical thickness (AOT) because of contributions from other aerosol. The NAME model also captured the dust events, giving confidence in its ability to correctly identify air mass origins and pathways in this region. Dissolution experiments on collected dust samples showed a strong correlation between soluble Fe and Al and measured solubilities were lower at high atmospheric dust concentrations. Fine mode aerosol at the CVAO contains a significant fraction of non-sea salt components including dicarboxylic acids, methanesulfonic acid and aliphatic amines, all believed to be of oceanic origin. A marine influence is also apparent in the year-round presence of iodine and bromine monoxide (IO and BrO), with IO suggested to be confined mainly to the surface few hundred metres but BrO well mixed in the boundary layer. Enhanced CO₂ and CH₄ and depleted oxygen concentrations are markers for air-sea exchange over the nearby northwest African coastal upwelling area. Long-range transport results in generally higher levels of O₃ and anthropogenic non-methane hydrocarbons (NMHC) in air originating from North America. Ozone/CO ratios were highest (up to 0.42) in relatively fresh European air masses. In air heavily influenced by Saharan dust the O₃/CO ratio was as low as 0.13, possibly indicating O₃ uptake to dust. Nitrogen oxides (NO_x and NO_y) show generally higher concentrations in winter when air mass origins are predominantly from Africa. High photochemical activity at the site is shown by maximum spring/summer concentrations of OH and HO₂ of 9?×?10⁶ molecule cm^?3 and 6?×?10⁸ molecule cm^?3, respectively. After the primary photolysis source, the most important controls on the HO_x budget in this region are IO and BrO chemistry, the abundance of HCHO, and uptake of HO_x to aerosol.     

A Study of DMS Oxidation in the Tropics: Comparison of Christmas Island Field Observations of DMS, SO2, and DMSO with Model Simulations

This study reports comparisonsbetween model simulations, based on current sulfurmechanisms, with the DMS, SO₂ and DMSOobservational data reported by Bandy et al.(1996) in their 1994 Christmas Island field study. For both DMS and SO₂, the model results werefound to be in excellent agreement with theobservations when the observations were filtered so asto establish a common meteorological environment. Thisfiltered DMS and SO₂ data encompassedapproximately half of the total sampled days. Basedon these composite profiles, it was shown thatoxidation of DMS via OH was the dominant pathway withno more than 5 to 15% proceeding through Cl atoms andless than 3% through NO₃. This analysis wasbased on an estimated DMS sea-to-air flux of 3.4 ×10⁹ molecs cm^-2 s^-1. The dominant sourceof BL SO₂ was oxidation of DMS, the overallconversion efficiency being evaluated at 0.65 ± 0.15. The major loss of SO₂ was deposition to theocean's surface and scavenging by aerosol. Theresulting combined first order k value was estimated at 1.6 × 10^-5 s^-1. In contrast to the DMSand SO₂ simulations, the model under-predictedthe observed DMSO levels by nearly a factor of 50. Although DMSO instrument measurement problems can notbe totally ruled out, the possibility of DMSO sourcesother than gas phase oxidation of DMS must beseriously considered and should be explored in futurestudies.     

DMS and SO2 at Baring Head, New Zealand: Implications for the Yield of SO2 from DMS

Atmospheric dimethyl sulfide (DMS) and sulfur dioxide (SO₂) concentrations were measured at Baring Head, New Zealandduring February and March 2000. Anti-correlated DMS and SO₂ diurnalcycles, consistent with the photochemical production of SO₂ from DMS, were observed in clean southerly air off the ocean. The data is used to infer a yield of SO₂ from DMS oxidation. The estimated yields are highly dependent on assumptions about the DMS oxidation rate. Fitting the measured data in a photochemical box model using model-generated OH levels and the Hynes et al. (1986) DMS + OH rate constant suggests that theSO₂ yield is 50–100%, similar to current estimates for the tropical Pacific.However, the observed amplitude of the DMS diurnal cycle suggests that the oxidation rate is higher than that used by the model, and therefore, that theSO₂ yield is lower in the range of 20–40%.     

Covariations in oceanic dimethyl sulfide,its oxidation products and rain acidity at Amsterdam Island in the Southern Indian Ocean

Simultaneous measurements of rain acidity and dimethyl sulfide (DMS) at the ocean surface and in the atmosphere were performed at Amsterdam Island over a 4 year period. During the last 2 years, measurements of sulfur dioxide (SO₂) in the atmosphere and of methane sulfonic acid (MSA) and non-sea-salt-sulfate (nss-SO₄ ^2-) in rainwater were also performed. Covariations are observed between the oceanic and atmospheric DMS concentrations, atmospheric SO₂ concentrations, wet deposition of MSA, nss-SO₄ ^2-, and rain acidity. A comparable summer to winter ratio of DMS and SO₂ in the atmosphere and MSA in precipitation were also observed. From the chemical composition of precipitation we estimate that DMS oxidation products contribute approximately 40% of the rain acidity. If we consider the acidity in excess, then DMS oxidation products contribute about 55%.     

A global three-dimensional model of the tropospheric sulfur cycle

The tropospheric part of the atmospheric sulfur cycle has been simulated in a global three-dimensional model. The model treats the emission, transport, chemistry, and removal processes for three sulfur components; DMS (dimethyl sulfide), SO₂ and SO₄ ^2– (sulfate). These processes are resolved using an Eulerian transport model, the MOGUNTIA model, with a horizontal resolution of 10° longitude by 10° latitude and with 10 layers in the vertical between the surface and 100 hPa. Advection takes place by climatological monthly mean winds. Transport processes occurring on smaller space and time scales are parameterized as eddy diffusion except for transport in deep convective clouds which is treated separately. The simulations are broadly consistent with observations of concentrations in air and precipitation in and over polluted regions in Europe and North America. Oxidation of DMS by OH radicals together with a global emission of 16 Tg DMS-S yr^–1 from the oceans result in DMS concentrations consistent with observations in the marine boundary layer. The average turn-over times were estimated to be 3, 1.2–1.8, and 3.2–6.1 days for DMS, SO₂, and SO₄ ^2– respectively.     

The biogeochemical sulfur cycle in the marine boundary layer over the Northeast Pacific Ocean

The major components of the marine boundary layer biogeochemical sulfur cycle were measured simultaneously onshore and off the coast of Washington State, U.S.A. during May 1987. Seawater dimethylsulfide (DMS) concentrations on the continental shelf were strongly influenced by coastal upwelling. Concentration further offshore were typical of summer values (2.2 nmol/L) at this latitude. Although seawater DMS concentrations were high on the biologically productive continental shelf (2–12 nmol/L), this region had no measurable effect on atmospheric DMS concentrations. Atmospheric DMS concentrations (0.1–12 nmol/m³), however, were extremely dependent upon wind speed and boundary layer height. Although there appeared to be an appreciable input of non-sea-salt sulfate to the marine boundary layer from the free troposphere, the local flux of DMS from the ocean to the atmosphere was sufficient to balance the remainder of the sulfur budget.     

Examination of sulfate chemistry sensitivity in a mid-latitude and tropical storm using a cloud resolving model

We examine the sensitivities of heterogeneous sulfate chemistry in a mid-latitude and tropical storm using a cloud resolving model. Both thermodynamic environments show unstable conditions favorable for development of intensive convection, with more CAPE in tropical compared to mid-latitude storm. Compared with the observed severe storms, modeled results show a relatively good agreement with the radar and surface chemical observations. Microphysical evaluation indicates that the accretion and autoconversion appear to be most important processes in such considered clouds. This sensitivity simulation is an upper bound for conversion of S (IV) to sulfate. The tropical convective storm produces for about 2.5 times more sulfate compared to mid-latitude storm and converts more SO₂ to sulfate, increasing wet deposition of sulfur. The results for a midlatitude run indicate that aerosol nucleation and impact scavenging account for between 18.9% and 28.9% of the in-cloud sulfate ultimately deposited. As a result of greater rainfall efficiency, tropical storm shows about two times higher sub-cloud scavenging rate than mid-latitude storm. The oxidation of S (IV) to SO₄ ^?2 in cloud droplets and in precipitation is found to be dominant in both convective storms accounting almost with the same percentage contribution of 45.4% and 46.3% to sulfur deposition, respectively. In-cloud oxidation contribute a larger fraction of the total amount of sulfur deposited in tropical case (29.2%) when compared to the mid-latitude case (11.8), respectively. Neglecting aqueous-phase chemistry in ice-phase hydrometeors in both convective clouds led to overpredict deposition of about 40% to 33% relative to the base runs.     

Sulfur dioxide and dimethyl sulfide in the central Pacific troposphere

Boundary-layer and free-troposphere measurements of sulfur dioxide, dimethyl sulfide, and carbon disulfide were made during transits of the central and southern Pacific Ocean between Hawaii and Australia. Sulfur dioxide was generally less than 100 pptv and highly variable with no correlation with respect to geographic location or altitude. Dimethyl sulfide in the boundary layer had a concentration range of <10 to 200 pptv. Highest concentrations of DMS were in the equatorial region of the southern hemisphere although the concentrations were dependent on location and meteorological regime. In the region of the Fiji Islands several boundary layer samples had SO₂, DMS, and CS₂. In 1989, additional SO₂ measurements were made between Hawaii and the equator and to the west of Hawaii downwind of the Kilauea volcano plumes.Paper submitted to the 7th International Symposium of the Commission for Atmospheric Chemistry and Global Pollution on the Chemistry of the Global Atmosphere held in Chamrousse, France, from 5 to 11 September 1990.     

Model Simulations on the Variability of Particulate MSA to Non-Sea-Salt Sulfate Ratio in the Marine Environment

A box model was constructed to investigate connections between the particulate MSA to non-sea-salt sulfate ratio, R, and DMS chemistry in a clean marine boundary layer. The simulations demonstrated that R varies widely with particle size, which must be taken into account when interpreting field measurements or comparing them with each other. In addition to DMS gas-phase chemistry, R in the submicron size range was shown to be sensitive to the factors dictating sulfate production via cloud processing, to the removal of SO₂ from the boundary layer by dry deposition and sea-salt oxidation, to the entrainment of SO₂ from the free troposphere, to the relative concentration of sub- and supermicron particles, and to meteorology. Three potential explanations for the increase of R toward high-latitudes during the summer were found: larger MSA yields from DMS oxidation at high latitudes, larger DMSO yields from DMS oxidation followed by the conversion of DMSO to MSA at high latitudes, or lower ambient H₂O₂ concentrations at high latitudes leading to less efficient sulfate production in clouds. Possible reasons for the large seasonal amplitude of R at mid and high latitudes include seasonal changes in the partitioning of DMS oxidation to the OH and NO₃ initiated pathways, seasonal changes in the concentration of species participating the DMS-OH reaction pathway, or the existence of a SO₂ source other than DMS oxidation in the marine boundary layer. Even small anthropogenic perturbations were shown to have a potential to alter the MSA to non-sea-salt sulfate ratio.     

The impact of tropical western Pacific convection on the North Pacific atmospheric circulation during the boreal winter

In this study, we investigate the impact of atmospheric convection over the western tropical Pacific (100–145°E, 0–20°N) on the boreal winter North Pacific atmosphere flow by analyzing National Center for Environmental Prediction Reanalysis 1, Extended Reconstructed Sea Surface Temperature and Global Precipitation Climatology Project data. The western tropical Pacific convection is not only the main energy source driving the local Hadley and Walker circulations, but it also significantly influences North Pacific circulation, by modifying a mid-latitude Jet stream through the connection with the local Hadley circulation. On the one hand, this strong convection leads to a northward expansion of local Hadley cells simultaneous with a northward movement of the western North Pacific jet because of the close correlation between the Jet and Hadley circulation boundaries. On the other hand, this strong convection also intensifies tropical Pacific Walker circulation, which reduces the eastern Pacific sea surface temperature, resembling a La Nina state through the enhanced equatorial upwelling. The cooling of the eastern tropical Pacific has an inter-tropical convergence zone located further north; thus, the local Hadley circulation moves northward. As a result, the jet axis over the eastern North Pacific, which also corresponds to the boundary of the local Hadley circulation, moves to higher latitude. Consequently, this northward movement of the Jet axis over the North Pacific is reflected as a northwest–southeast dipole sea level pressure (SLP) pattern. The composite analysis of SLP over the North Pacific against the omega (Ω) (Pa/s) at 500 hPa over the western tropical Pacific actually reveals that this northwest-southeast dipole structure is attributed to the intensified tropical western Pacific convection, which pushes the Pacific Jet to the north. Finally we also analyzed south Pacific for the austral winter as did previously to North Pacific, and found that the results were consistent.     

Seasonal variations of atmospheric sulfur dioxide and dimethylsulfide concentrations at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric sulfur dioxide (SO₂) concentrations were performed from March 1989 to January 1991 at Amsterdam Island (37°50 S–77°30 E), a remote site located in the southern Indian Ocean. Long-range transport of continental air masses was studied using Radon (²²²Rn) as continental tracer. Average monthly SO₂ concentrations range from less than 0.2 to 3.9 nmol m^-3 (annual average = 0.7 nmol m^-3) and present a seasonal cycle with a minimum in winter and a maximum in summer, similar to that described for atmospheric DMS concentrations measured during the same period. Clear diel correlation between atmospheric DMS and SO₂ concentrations is also observed during summer. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces the seasonal variations in the measured atmospheric SO₂ concentrations within ±30%. Comparing between computed and measured SO₂ concentrations allowed us to estimate a yield of SO₂ from DMS oxidation of about 70%.     

The relative importance of tropical variability forced from the North Pacific through ocean pathways

To what extent is tropical variability forced from the North Pacific through ocean pathways relative to locally generated variability and variability forced through the atmosphere? To address this question, in this study we use an anomaly-coupled model, consisting of a global, atmospheric general circulation model and a 4½-layer, reduced-gravity, Pacific-Ocean model. Three solutions are obtained; with coupling over the entire basin (CNT), with coupling confined to the tropics and wind stress and heat fluxes in the North and South Pacific specified by climatology (TP), and with coupling confined to the Tropics and wind stress and heat fluxes in the North Pacific specified by output from CNT (NPF). It is found that there are two distinct signals forced in the North Pacific that can impact the tropics through ocean pathways. These two signals are forced by wind stress and surface heat flux anomalies in the subtropical North Pacific. The first signal is relatively fast, impacts tropical variability less than a year after forcing, is triggered from November to March, and propagates as a first-mode baroclinic Rossby wave. The second signal is only triggered during springtime when buoyancy forcing can effectively generate higher-order baroclinic modes through subduction anomalies into the permanent thermocline, and it reaches the equator 4–5 years after forcing. The slow signal is found to initiate tropical variability more efficiently than the fast signal with one standard deviation in subtropical zonal wind stress forcing tropical SST anomalies centered on the equator at 135°W of approximately 0.5°C. Allowing extratropically forced tropical variability is found to shift primarily 2-year ENSO variability in a tropics-alone simulation to a more realistic range of 2–6 years.     

Observation of DMSO and CH3S(O)OH from the gas phase reaction between DMS and OH

Products and mechanisms have been investigated for the reactions between dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) and the hydroxyl radical (OH) in the presence of NO_x. All of the experiments were performed in a 480 L reaction chamber, applying Fourier transform infrared spectroscopy (FT-IR) and ion chromatography as the analytical techniques.In addition to the sulfur containing products that are known to be produced from the gas phase reaction between DMS and OH (SO₂, dimethylsulfone, methylsulfonyl peroxynitrate, methanesulfonic acid, H₂SO₄), DMSO and methanesulfinic acid (CH₃S(O)OH) were also observed as products. Only SO₂, DMSO₂ and methylsulfonyl peroxynitrate were found as sulfur containing products in the reaction between DMSO and OH. Based on these new results we propose a mechanism for the atmospheric oxidation of DMS and DMSO by OH radical.     

Simulations of seasonal variations of sulfur compounds in the remote marine atmosphere

A photochemical box model is used to simulate seasonal variations in concentrations of sulfur compounds at latitude 40° S. It is assumed that the hydroxyl radical (OH) addition reaction to sulfur in the dimethyl sulfide (DMS) molecule is the predominant pathway for methanesulfonic acid (MSA) production, and that the rate constant increases as the air temperature decreases. Concentration of the nitrate radical (NO₃) is a function of the DMS flux, because the reaction of DMS with NO₃ is the most important loss mechanism of NO₃. While the diurnally averaged concentration of OH in winter is a factor of about 8 smaller than in summer, due to the weak photolysis process, the diurnally averaged concentration of NO₃ in winter is a factor of about 4–5 larger than in summer, due to the decrease of DMS flux. Therefore, at middle and high latitudes in winter, atmospheric DMS is mainly oxidized by the reaction with NO₃. The calculated ratio of the MSA to SO₂ production rates is smaller in winter than in summer, and the MSA to non-sea-salt sulfate (nssSO₄ ^2-) molar ratio varies seasonally. This result agrees with data on the seasonal variation of the MSA/nssSO₄ ^2- molar ratio obtained at middle and high latitudes. The calculations indicate that during winter the reaction of DMS with NO₃ is likely to be a more important sink of NO_x (NO+NO₂) than the reaction of NO₂ with OH, and to serve as a significant pathway of the HNO₃ production. If dimethyl sulfoxide (DMSO) is produced through the OH addition reaction and is heterogeneously oxidized in aqueous solutions, half of the nssSO₄ ^2- produced in summer may be through the oxidation process of DMSO. It is necessary to further investigate the oxidation products by the reaction of DMS with OH, and the possibility of the reaction of DMS with NO₃ during winter.