Aircraft Measurements of Dimethyl Sulfide (DMS) Using a Whole Air Sampling Technique

We present a technique for the measurement of dimethyl sulfide (DMS) from airborne and ground-based platforms, using whole air sampling followed by gas chromatography with mass spectrometer and flame ionization detection. DMS measurements that were obtained during the 1999 NASA Pacific Exploratory Mission-Tropics B showed excellent agreement with independent in-flight DMS measurements, over a wide range of concentrations. The intercomparison supports two key results from this study, first that DMS can be accurately quantified based on ethane and propane per-carbon-response-factors (PCRFs), and second that DMS is stable in water-doped electropolished stainless steel canisters for at least several weeks. In addition, our sampling frequency and duration are flexible and allow detail in the vertical structure of DMS to be well captured. Sampling times as fast as 8 s were achieved and these data are suitable for DMS flux calculations using the mixed-layer gradient technique. Correlations between DMS and other marine tracers can also be readily investigated by this whole air sampling technique, because DMS is analyzed together with more than 50 simultaneously sampled hydrocarbons, halocarbons, and alkyl nitrates. The detection limit of the DMS measurements is 1 part per trillion by volume (pptv), and we conservatively estimate the accuracy to be ±20% or 3 pptv, whichever is larger. The measurement precision (1 ) is 2–4% at high mixing ratios (> 25 pptv), and 1 pptv or 15%, whichever is larger, at low mixing ratios (<10 pptv).     

A time series for carbonyl sulfide in the northern hemisphere

The variation of OCS in the northern hemisphere for the period 1977–1991 was investigated by grouping all measurements made by our research group for that period. The data set contained 1066 measurements made in the northern hemisphere over a longitude range of 52 E to 155 W and a latitude range of 10 N to 85 N. About 50% of the measurements were made from aircraft. The overall data set had a mean of 512 parts per trillion by volume (pptv) and a standard deviation of 119 pptv. The data obtained from aircraft had a mean of 514 pptv and a standard deviation of 64 pptv. A study of the time series constructed from the data set and several subsets indicate that the change in global OCS with time is between –1.5 and 1.5 parts per trillion per year at the 95% confidence level. The data had no seasonal dependence within the precision of the data set.     

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.     

Salinization: the ultimate threat to temperate lakes,with particular reference to Southeastern Wisconsin (USA)

Many lakes in Southeastern Wisconsin(the metropolitan-Milwaukee area) are gradually becoming increasingly "salty".While these waterbodies would not be considered presently to be saline lakes,there has been a rapid increase in the chloride concentrations in most of these lakes over the last 30 years,with the lakes increasing from a mean chloride concentration of about 19 mg/L to over 100 mg/L in some cases.While ecological impacts can be expected when chloride values exceed 250 mg/L,the rate of increase presents a basis for concern,especially since the underlying geology of the region is based on limestone/dolomite which is deficient in chlorides.Thus,the origin of the chlorides is anthropogenic:human and industrial wastewaters(treatment of which has effected improvements in trophic status but has not affected other water-borne contaminants) and winter de-icing practices based upon large quantities of sodium chloride are major contributors to the increasing concentrations of chloride in the region's waterways.Without taking remedial measures,the rate of salinization is expected to continue to increase,resulting,ultimately,in the alteration of the freshwater systems in the region.     

Trends in eutrophication research and control

Eutrophication is the natural ageing process of lakes. It is characterized by a geologically slow shift from in-lake biological production driven by allochthonous (external to the water body) loading of nutrients, to production driven by autochthonous (in-lake) processes. This shift typically is accompanied by changes in species and biotic community composition, as an aquatic ecosystem is ultimately transformed into a terrestrial biome. However, this typically slow process can be greatly accelerated by human intervention in the natural biogeochemical cycling of nutrients within a watershed; the resulting cultural eutrophication can create conditions inimical to the continued use of the water body for human-driven economic purposes. Excessive algal and rooted plant growth, degraded water quality, extensive deoxygenation of the bottom water layers and increased fish biomass accompanied by decreased harvest quality, are some features of this process. Following the Second World War, concern with cultural eutrophication achieved an intensity that spurred a significant research effort, culminating in the identification of phosphorus as the single most significant, and controllable, element involved in driving the eutrophication process. During the late 1960s and throughout the 1970s, much effort was devoted to reducing phosphorus in wastewater effluents, primarily in the developed countries of the temperate zone. These efforts generally resulted in the control of eutrophication in these countries, albeit with varying degrees of success. The present effort in the temperature zone, comprising mostly developed nations, has now shifted to the control of diffuse sources of a broader spectrum of contaminants that impact human water use. In the developing countries of the inter-tropical zone, however, rapidly expanding populations, a growing industrial economy and extensive urbanization have only recently reached an intensity at which cultural eutrophication can no longer be ignored. Further, initial attempts at applying temperate zone control measures in this region have been largely unsuccessful. Modification of the temperate zone eutrophication paradigm will be needed, especially to address the differing climatic and hydrological conditions, if cultural eutrophication is to be contained in this region, where eutrophication-related diseases continue to be a primary cause of human distress.     

Influence of soil minerals on chromium(VI) reduction by sulfide under anoxic conditions

The effects of soil minerals on chromate (Cr^VIO₄ ^2-, noted as Cr(VI)) reduction by sulfide were investigated in the pH range of 7.67 to 9.07 under the anoxic condition. The examined minerals included montmorillonite (Swy-2), illite (IMt-2), kaolinite (KGa-2), aluminum oxide (γ-Al₂O₃), titanium oxide (TiO₂, P-25, primarily anatase), and silica (SiO₂). Based on their effects on Cr(VI) reduction, these minerals were categorized into three groups: (i) minerals catalyzing Cr(VI) reduction – illite; (ii) minerals with no effect – Al₂O₃; and (iii) minerals inhibiting Cr(VI) reduction- kaolinite, montmorillonite, SiO₂ and TiO₂ . The catalysis of illite was attributed primarily to the low concentration of iron solubilized from the mineral, which could accelerate Cr(VI) reduction by shuttling electrons from sulfide to Cr(VI). Additionally, elemental sulfur produced as the primary product of sulfide oxidation could further catalyze Cr(VI) reduction in the heterogeneous system. Previous studies have shown that adsorption of sulfide onto elemental sulfur nanoparticles could greatly increase sulfide reactivity towards Cr(VI) reduction. Consequently, the observed rate constant, k _obs, increased with increasing amounts of both iron solubilized from illite and elemental sulfur produced during the reaction. The catalysis of iron, however, was found to be blocked by phenanthroline, a strong complexing agent for ferrous iron. In this case, the overall reaction rate at the initial stage of reaction was pseudo first order with respect to Cr(VI), i.e., the reaction kinetics was similar to that in the homogeneous system, because elemental sulfur exerted no effect at the initial stage prior to accumulation of elemental sulfur nanoparticles. In the suspension of kaolinite, which belonged to group (iii), an inhibitive effect to Cr(VI) reduction was observed and subsequently examined in more details. The inhibition was due to the sorption of elemental sulfur onto kaolinite, which reduced or completely eliminated the catalytic effect of elemental sulfur, depending on kaolinite concentration. This was consistent with the observation that the catalysis of externally added elemental sulfur (50 μM) on Cr(VI) reduction would disappear with a kaolinite concentration of more than 5.0 g/L. In kaolinite suspension, the overall reaction rate law was:

Assessment of MTBE biodegradation in contaminated groundwater using C and C analysis: Field and laboratory microcosm studies

Radiolabelled assays and compound-specific stable isotope analysis (CSIA) were used to assess methyl tert-butyl ether (MTBE) biodegradation in an unleaded fuel plume in a UK chalk aquifer, both in the field and in laboratory microcosm experiments. The ¹⁴C-MTBE radiorespirometry studies demonstrated widespread potential for aerobic and anaerobic MTBE biodegradation in the aquifer. However, δ¹³C compositions of MTBE in groundwater samples from the plume showed no significant ¹³C enrichment that would indicate MTBE biodegradation at the field scale. Carbon isotope enrichment during MTBE biodegradation was assessed in the microcosms when dissolved O₂ was not limiting, compared with low in situ concentrations (2 mg/L) in the aquifer, and in the absence of O₂. The microcosm experiments showed ubiquitous potential for aerobic MTBE biodegradation in the aquifer within hundreds of days. Aerobic MTBE biodegradation in the microcosms produced an enrichment of 7‰ in the MTBE δ¹³C composition and an isotope enrichment factor (ε) of −1.53‰ when dissolved O₂ was not limiting. However, for the low dissolved O₂ concentration of up to 2 mg/L that characterizes most of the MTBE plume fringe, aerobic MTBE biodegradation produced an enrichment of 0.5-0.7‰, corresponding to an ε value of −0.22‰ to −0.24‰. No anaerobic MTBE biodegradation occurred under these experimental conditions. These results suggest the existence of a complex MTBE-biodegrading community in the aquifer, which may consist of different aerobic species competing for MTBE and dissolved O₂. Under low O₂ conditions, the lower fractionating species have been shown to govern overall MTBE C-isotope fractionation during biodegradation, confirming the results of previous laboratory experiments mixing pure cultures. This implies that significant aerobic MTBE biodegradation could occur under the low dissolved O₂ concentration that typifies the reactive fringe zone of MTBE plumes, without producing detectable changes in the MTBE δ¹³C composition. This observed insensitivity of C isotope enrichment to MTBE biodegradation could lead to significant underestimation of aerobic MTBE biodegradation at field scale, with an unnecessarily pessimistic performance assessment for natural attenuation. Site-specific C isotope enrichment factors are, therefore, required to reliably quantify MTBE biodegradation, which may limit CSIA as a tool for the in situ assessment of MTBE biodegradation in groundwater using only C isotopes.