Showing papers on "Particulates published in 1998"

Formation of atmospheric particles from organic acids produced by forests

Abstract: Aerosol formation in the atmosphere is an important process to understand, in that such particles may act as the cloud condensation nuclei responsible for the ‘cloud–climate’ effect1, and could locally be hazardous to health. The number-concentration of total atmospheric aerosols and cloud condensation nuclei is largely contributed by organic aerosols1. Much of the organic aerosol is formed from atmospheric gas-to-particle conversion, and the common and widespread non-methane hydrocarbons emitted by vegetation have been investigated as possible precursors2. But strong evidence for a quantitative link between biogenic hydrocarbon emission and organic aerosol formation has so far been lacking. Here we present measurements of gaseous and particulate atmospheric species from a forested area to show that some hydrocarbons (for example, terpenes) emitted by vegetation are photo-oxidized to organic acids (for example, pinonic acids), which condense to form organic aerosols. Thus the forests, through their production of large quantities of organic aerosols, could be of considerable significance both for climate (through cloud-condensation-nuclei formation) and for heterogeneous atmospheric chemical processes.

Emission factors of hydrocarbons, halocarbons, trace gases and particles from biomass burning in Brazil

Abstract: Airborne measurements of the emissions of gases and particles from 19 individual forest, cerrado, and pasture fires in Brazil were obtained during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) study in August-September 1995. Emission factors were determined for a number of major and minor gaseous and particulate species, including carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, methane, nonmethane hydrocarbons, halocarbons, particulate (black and organic) carbon, and particulate ionic species. The magnitude of the emission factors for gaseous species were determined primarily by the relative amounts of flaming and smoldering combustion, rather than differences in vegetation type. Hydrocarbons and halocarbons were well correlated with CO, which is indicative of emissions primarily associated with smoldering combustion. Although there was large variability between fires, higher emission factors for SO2 and NOχ were associated with an increased ratio of flaming to smoldering combustion; this could be due to variations in the amounts of sulfur and nitrogen in the fuels. Emission factors for particles were not so clearly associated with smoldering combustion as those for hydrocarbons. The emission factors measured in this study are similar to those measured previously in Brazil and Africa. However, particle emission factors from fires in Brazil appear to be roughly 20 to 40% lower than those from North American boreal forest fires.

Gas-Phase and Particle-Phase Organic Compounds Emitted from Motor Vehicle Traffic in a Los Angeles Roadway Tunnel

Abstract: The emission rates for 221 vapor-phase, semivolatile, and particle-phase organic compounds from motor vehicles plus fine particulate matter mass and some inorganic particle-phase species are calculated based on measurements made inside and outside a Los Angeles roadway tunnel in 1993. These emission rates are calculated based on fuel consumption to remove any uncertainties based on tunnel dilution rates or air circulation. The results show carbon monoxide emissions rates of 130 g L^(-1) of gasoline-equivalent fuel burned and volatile organic compound (VOC) emissions of 9.1 g L^(-1). These values are higher than predicted by the baseline version of California's EMFAC 7G emissions inventory program but are within the coemission rate range of 108 ± 25 g L^(-1) reported by roadside remote sensing studies in Los Angeles [Singer, B. C.; Harley, R. A. J. Air Waste Manage. Assoc. 1996, 46, 581−593]. When the VOC emissions composition in the tunnel is compared to that of tailpipe emissions source test data and to the composition of additional unburned whole gasoline, the tunnel atmosphere is found to be consistent with a linear combination of these major contributors over a fairly broad range of about 74−97% vehicle exhaust depending on the tailpipe profiles used. Fine particulate emissions within the tunnel consist largely of carbonaceous material accompanied by a significant amount of ammonium nitrate apparently formed by gas-to-particle conversion processes within the tunnel atmosphere. Certain gas-phase and particulate organic compounds traditionally thought to be the secondary products of atmospheric chemical reactions are enriched inside the tunnel, and from this enrichment, the primary emission rates of aromatic alcohols, aliphatic dicarboxylic acids, and aromatic polycarboxylic acids are calculated. Data on petroleum biomarkers emissions rates in the tunnel can be used in the future to estimate primary vehicle exhaust fine particulate matter concentrations in the urban atmosphere.

Spectroscopic characterization of humic-like substances in airborne particulate matter

Abstract: In a standard reference air dust (NIST 1648 – urban particulate matter) as well as in airborne particulate matter collected in German urban and rural areas (Dortmund, Sauerland) 10% and more of the organic carbon can be attributed to macromolecular substances like humic and fulvic acids (HA, FA). Indirect evidence for the presence of humin was also obtained. HA and FA extracted from NIST 1648 and other urban and rural atmospheric particles using 0.1 m NaOH and isolated by ion-exchangers were characterized by their molecular-size distribution applying multistage ultrafiltration, their carbon content and their UV/VIS, Fourier-transformed infrared (FTIR) and nuclear magnetic resonance (1H-NMR) spectra. Summarizing, the structural studies exhibit that these humic-like substances (HULIS) contained in air dust samples are small in their molecular size and rich in aliphatic and carbohydrate substructures compared to HA and FA from soils and aquatic systems.

Airborne studies of aerosol emissions from savanna fires in southern Africa: 2. Aerosol chemical composition

Abstract: We investigated smoke emissions from fires in savanna, forest, and agricultural ecosystems by airborne sampling of plumes close to prescribed burns and incidental fires in southern Africa. Aerosol samples were collected on glass fiber filters and on stacked filter units, consisting of a Nuclepore prefilter for particles larger than ∼1–2 μm and a Teflon second filter stage for the submicron fraction. The samples were analyzed for soluble ionic components, organic carbon, and black carbon. Onboard the research aircraft, particle number and volume distributions as a function of size were determined with a laser-optical particle counter and the black carbon content of the aerosol with an aethalometer. We determined the emission ratios (relative to CO2 and CO) and emission factors (relative to the amount of biomass burnt) for the various aerosol constituents. The smoke aerosols were rich in organic and black carbon, the latter representing 10–30% of the aerosol mass. K+ and NH4+ were the dominant cationic species in the smoke of most fires, while Cl− and SO42− were the most important anions. The aerosols were unusually rich in Cl−, probably due to the high Cl content of the semiarid vegetation. Comparison of the element budget of the fuel before and after the fires shows that the fraction of the elements released during combustion is highly variable between elements. In the case of the halogen elements, almost the entire amount released during the fire is present in the aerosol phase, while in the case of C, N, and S, only a small proportion ends up as particulate matter. This suggests that the latter elements are present predominantly as gaseous species in the fresh fire plumes studied here.

Organic molecular tracers for particulate air pollution sources

Abstract: The contribution of the major urban air pollution sources to regional particulate air quality problems can be diagnosed by using organic molecular marker techniques. Distinctive organic compounds or compound groups that are present in the emissions from some source types but not others are used to quantify the presence of the effluent from different air pollution sources within atmospheric fine particle samples. The tracer compounds are measured in source samples and in atmospheric samples by gas chromatography–mass spectrometry. Some of the most valuable tracers include hopanes and steranes from petroleum use in motor vehicles; phytosterols, lignans, phenolic products from lignins, and diterpenoids from resins in wood smoke; cholesterol in meat charbroiling smoke; high-molecular-weight odd carbon number n-alkanes in the particles shed as leaves rub together in the wind, and iso- and anteiso-alkanes in cigarette smoke. To date it has been demonstrated that the particles from up to nine different source types can be distinguished from each other simultaneously in the southern California atmosphere by this method. In the future, new analytical methods will increase the family of tracer compounds, assist in the identification of markers for additional sources, and speed the chemical analysis process in the laboratory.

Plasma regenerated particulate trap and NOx reduction system

Abstract: A non-catalytic two-stage process for removal of NOx and particulates from engine exhaust comprises a first stage that plasma converts NO to NO2 in the presence of O2 and hydrocarbons, and a second stage, which preferably occurs simultaneously with the first stage, that converts NO2 and carbon soot particles to respective environmentally benign gases that include N2 and CO2. By preconverting NO to NO2 in the first stage, the efficiency of the second stage for NOx reduction is enhanced while carbon soot from trapped particulates is simultaneously converted to CO2 when reacting with the NO2 (that converts to N2). For example, an internal combustion engine exhaust is connected by a pipe to a chamber where carbon-containing particulates are electrostatically trapped or filtered and a non-thermal plasma converts NO to NO2 in the presence of O2 and hydrocarbons. Volatile hydrocarbons (Cx Hy) from the trapped particulates are oxidized in the plasma and the remaining soot from the particulates reacts with the NO2 to convert NO2 to N2, and the soot to CO2. The nitrogen exhaust components remain in the gas phase throughout the process, with no accompanying adsorption.

Chlorine loss from marine aerosol in a coastal atmosphere

Abstract: The horizontal gradient concentrations of particulate and gaseous pollutants were measured on the western coast of the Iberian Peninsula. In summer, a large fraction of particulate sea-salt chlorine (67% for fine particles and 24% for coarse particles) is already lost during the sea breezes by the time marine air masses arrive to the coast. The volatilization values increase to 86% and 68%, 30 km inland. Acidic sulphur compounds are responsible for 64–71% of the degassing of Cl from fine particles, while nitric acid is the main cause of the coarse particles, contributing with 60-68% for the particulate chlorine loss. There is more or less an 11% chlorine degassing per hour during the air mass transport inland. Thermodynamic equilibrium is not attained during the first 3 hours of aerosol transport inland for both fine and coarse sea-salt particles.

Transport of Carbon and Nitrogen to the Northern Adriatic Sea by the Po River

Abstract: Concentrations of dissolved and particulate organic carbon (DOC, POC) and particulated nitrogen (PN) were measured in samples taken monthly for a year at two stations in the lower Po River and on four separate occasions in the mixing zone. From the results, the influence of suspended matter concentrations on POC and PN contents in the solid phase, and of flow variations on POC and PN contents in the water phase were evaluated. The behaviour of organic matter in the mixing zone was conservative in low to medium solid transport conditions, but had a strong negative deviation with respect to the dilution line during high solid transport, presumably due to both settling of particulate matter in the prodelta region and bacterial utilization of organic matter. Organic carbon was preferentially transported in the solid phase, in the lower Po, compared to world rivers in temperate zones. The loads of total organic carbon and nitrogen were estimated as 25·5×104and 15·5×104tonnes year−1respectively, making this river the most important contributor of organic matter and nutrients to the Mediterranean. These high loads together with lesser inputs from other Italian rivers flowing in to the northern Adriatic exert a significant impact on the productivity and trophic dynamics of this basin.

Where do particulate toxins reside ? An improved paradigm for the structure and dynamics of the urban mid-Atlantic aerosol

Abstract: Discussions of excess mortality and morbidity from exposure to urban aerosol particles typically invoke the now 20-year-old trimodal aerosol paradigm proposed by Whitby to explain the structure and behavior of ambient aerosol volume and its major constituent, sulfate. However, this paradigm largely ignores the primary high-temperature combustion (HTC) components of the urban aerosol, which contribute minor amounts of the aerosol mass, but carry the bulk of the particulate toxins and numbers of aerosol particles. Studies encompassing the analyses of >100 size distributions of important intrinsic tracers of primary particles from HTC sources collected over the past decade in various environments show that urban aerosol contains a complex mixture of physically-discrete fresh and aged, primary particle populations from a variety of sources. Furthermore, whereas the behavior of fine-particulate aerosol mass and sulfate was described in terms of coagulation and accumulation aerosol scavenging of new secondary sulfate nuclei, studies reviewed herein suggest that the behavior of primary aerosol is mediated more by hygroscopic growth and cloud processing, accompanied by oxidation of SO{sub 2} on wet particles and droplets. The authors conclude that the distribution of airborne particulate toxins and their atmospheric behavior is far more complex than commonly conceptualized on the basismore » of the classical trimodal model, and they develop an extended paradigm in which the focus is on the primary accumulation aerosol.« less

Control of oxides of nitrogen from diesel engines using diluents while minimising the impact on particulate pollutants

Abstract: The investigation was conducted on a high-speed direct-injection diesel engine and was concerned with the effects of exhaust gas recirculation (EGR) on diesel engine combustion and emissions. In particular, the effects on combustion and emissions of carbon dioxide (CO2) and water vapour (H2O), principal constituents of EGR, were analysed and quantified experimentally. The effects of increased inlet temperature and thermal throttling of the inlet charge, both arising from the use of hot EGR, were also investigated. Finally, tests were carried out during which the CO2 added to the engine air flow increased the charge mass flow rate to the engine, rather than displacing some of the oxygen (O2) in the inlet air. It was found that when CO2 or H2O displaced O2 in the inlet charge, both the chemical and thermal effects on exhaust emissions were small. However, the dilution effect was substantial, and resulted in very large reductions in exhaust NOx at the expense of higher particulate and unburnt hydrocarbon emissions. Higher inlet charge temperature increased exhaust NOx and particulate emissions, but reduced unburnt hydrocarbon emissions. Reduction in the inlet charge due to thermal throttling reduces NOx emissions but raises all the other pollutants. Finally, when CO2 was additional to the inlet air charge (rather than displacing O2), large reductions in NOx were recorded with little increase in particulate emissions.

Characterization of Polycyclic Aromatic Hydrocarbon Particulate and Gaseous Emissions from Polystyrene Combustion

Abstract: The partitioning of polycyclic aromatic hydrocarbons (PAHs) between the particulate and gaseous phases resulting from the combustion of polystyrene was studied. A vertical tubular flow furnace was used to incinerate polystyrene spheres (100−300 μm) at different combustion temper atures (800−1200 °C) to determine the effect of temperature and polystyrene feed size on the particulate and gaseous emissions and their chemical composition. The furnace reactor exhaust was sampled using real-time instruments (differential mobility particle sizer and/or optical particle counter) to determine the particle size distribution. For chemical composition analyses, the particles were either collected on Teflon filters or split into eight size fractions using a cascade impactor with filter media substrates, while the gaseous products were collected on XAD-2 adsorbent. Gas chromatography/mass spectroscopy (GC/MS) was used to identify and quantify the specific PAH species, their partitioning between the gas and particulate ...

Total and hydrolizable particulate organic matter (carbohydrates, proteins and lipids) at a coastal station in Terra Nova Bay (Ross Sea, Antarctica)

Abstract: We analysed quantity and quality of particulate organic matter during the austral summer 1994/1995 at a coastal station in Terra Nova Bay (Ross Sea, Antarctica). Our main aims were to investigate the origin and biochemical composition of particulate organic matter (POM), to measure its availability for consumers through the study of its digestible fraction (measured by using different enzymes separately) and to highlight the role of hydrolizable compounds in the organic matter diagenesis in the coastal waters at Terra Nova Bay. Temporal and spatial patterns of chlorophyll-a concentrations were reflected by the particulate organic carbon, nitrogen and total biopolymeric carbon concentrations, suggesting that most POM originated directly from phytoplankton. The most evident feature of POM in the coastal waters at Terra Nova Bay was the dominance of proteins (on average 57% of total biopolymeric particulate carbon), followed by carbohydrates (25%) and lipids (18%). We found that about 30% of the refractory particulate organic carbon (assumed to be present only after the complete exploitation of particulate organic nitrogen) did not originate from biopolymeric carbon (as sum of carbohydrate, protein and lipid carbon). This allows us to suggest the use of the digestible fraction of particulate biopolymeric carbon as a more accurate measure of the food availability of POM for consumers. In Terra Nova Bay coastal waters, most of the particulate protein pool was associated with large phytoplankton cells or phytodetritus. As a result, the protein pool appeared less available (i.e. less digestible) than the one present in oligotrophic waters where, conversely, most particulate organic nitrogen is sequestered into bacteria. The relative low availability of the protein pool, together with the rapid sinking of POM and the low remineralization rates of benthic heterotrophic microbes, are suggested as possible factors in determining the “inefficiency” in organic matter recycling of coastal waters at Terra Nova Bay, which behaves as a “loss type” system.

A Thermodynamic Approach for Modeling Partitioning of Semivolatile Organic Compounds on Atmospheric Particulate Matter: Humidity Effects

Abstract: Humidity effects on the gas/particle partitioning of the different types of semivolatile organic compounds (SOCs) in the organic layer of wood soot, diesel soot, and secondary aerosols were studied in outdoor environmental chambers Experimental partitioning coefficients, Kp, of different SOCs were measured using outdoor Teflon film chambers and compared to theoretical Kp values calculated with the aid of activity coefficients and vapor pressures A thermodynamic model based on group contribution methods was used to estimate SOC activity coefficients in the liquid organic layer of different atmospheric particles The equilibrated water content in the organic phase of chemi cally different particles was estimated from the activity coefficient of water in the particle's organic liquid and the ambient relative humidity (RH) It was found that predicted SOC activity coefficients (i ) for diesel soot particles were not a strong function of RH There was, however, a dramatic change in i on wood soot particles f

Determination of particulate and unburned hydrocarbon emissions from diesel engines fueled with biodiesel

Abstract: Methyl esters of soybean oil, known as biodiesel, are receiving increasing attention as an alternative fiiel for diesel engines. Biodiesel is a nontoxic, biodegradable, and renewable fiiel with the potential to reduce engine exhaust emissions. However, previous results have shown that biodiesel produces a higher fraction of soluble organic material (SOF) in the exhaust particulate matter than petroleum-based diesel fiiel although its total particulate emissions were lowered. Also, because biodiesel has a high boiling point and low volatility compared with diesel fuel, its unbumed hydrocarbon vapor could be lost by deposition in the hydrocarbon sampling line. This loss could be the source of reported reductions in hydrocarbon emissions from biodiesel-fiaeled engines. No information about whether the typical measurement temperature for unbumed hydrocarbons is suitable for biodiesel hydrocarbon measurement is available. In this project, the formation of SOF in diesel exhaust particulates and the deposition of hydrocarbons in heated sampling lines were studied experimentally and theoretically. The experimental results showed that biodiesel produced a higher SOF fraction in its total particulates than diesel fuel under virtually all engine operating conditions. The SOF fraction decreased with increasing filter temperature at constant dilution ratio and with increasing dilution ratio at constant filter temperature. Biodiesel injection tests, where a small quantity of neat biodiesel was injected into the exhaust gas showed that most of the unbumed biodiesel showed up in the soluble organic fraction of the collected particulates. The response of the heated flame ionization detector did not match the concentration change of unbumed biodiesel in the exhaust gas. Based on the predicted results from condensation and multicomponent adsorption models, adsorption of vapor phase hydrocarbons on the carbon particle surface is the primary source of the SOF in the total particulate matter. The condensation process, by itself, can not explain the formation of SOF during the dilution process. Adsorption of hydrocarbons is also shown to take place during transport of the hydrocarbon sample, even at high temperature. At the standard hydrocarbon measurement temperature, adsorption onto the sampling line walls could afifect the concentration of

Particulate matter fluxes into the benthic boundary layer at a long time-series station in the abyssal NE Pacific: composition and fluxes

Abstract: Sediment traps were deployed to collect contiguous 10-day samples of sedimenting particulate matter at 600 and 50 meters above bottom (mab) at an abyssal station off of the central California coast (Sta. M, 4100-m depth) over a 6-yr time period. Interannual variations were manifested in fluxes at both depths with high fluxes in 1991, 1993, and 1994. Low particulate matter fluxes observed in 1992 may have resulted from a well-documented El Nino-Southern Oscillation (ENSO) event, but the mechanisms responsible for the low fluxes observed in 1995 and 1996 were not resolved. Particulate fluxes were impacted by upwelling with a lag time of ∼50 days between the Bakun upwelling index and the arrival of particulate matter to the 600-mab trap. Allowing time between the upwelling of nutrients and the flux of material from the photic zone, we estimate a sinking rate of ∼100 m day-1. There was generally a peak in particulate fluxes in the summer with a secondary peak in the fall. Decreased concentrations (mg gdwt-1 of total particulates) of organic carbon, CaCO3, and particulate nitrogen in the summer were attributed to dilution effects from siliceous plankton. Particulate fluxes were higher at 50 mab (4050 m depth) than at 600 mab (3500-m depth), suggesting an input of laterally advected material at 50 mab, possibly from the Monterey Canyon northeast of Sta. M. The variability of particulate and component fluxes during this 6-yr period illustrates the importance of long time-series studies for interpreting organic matter input and its importance to deep-sea communities.

Methods and apparatus for continuous ambient particulate mass monitoring

Abstract: Methods and apparatus for the continuous monitoring of ambient particulate mass encompassing the monitoring of PM10 and coarse particles (25 to 10 νm), the determination of the size distribution of particles, the determination of particle density and the determination of particle-bound water

Influence of Different Indoor Activities on the Indoor Particulate Levels in Residential Buildings

Abstract: This study reports 24-hour measurements of indoor particulate levels in 8 residential premises in Hong Kong. The 24-hour respirable suspended particulate (RSP) levels varied from 44.9 to 119.4 µg·m–3 and the corresponding total suspended particulate (TSP) levels varied from 45.8 to 122.2 µg·m–3. These levels are higher than those measured in other countries. This was found to be related to the poor quality of outdoor air found in Hong Kong and also to the different indoor activities practised by these families. During the measurements indoor activities were recorded and ventilation rates at some of the sites were also measured using the carbon dioxide decay technique. A significant rise of particulate level was detected during cooking, smoking and burning of incense. In the study, particulate levels over 5,000 µg·m–3 were observed during some cooking activities. Particulate levels when people were smoking were 2–3 times higher than the relative background level. Incense burning in some families produced a peak concentration of particulates around 2,000 µg·m–3 if the ventilation was poor. The effect of rain was to wash the outdoor particulates from the air and at the same time the indoor particulate level fell if the ventilation rate was high enough. It was observed that about 20% of the indoor dust level could be reduced in less than 1 h when there was heavy rain. It was also found in the study that a very high indoor RSP to TSP ratio of 82–98% existed indoors.

Correlations of Exhaust Emissions from a Diesel Engine with Diesel Fuel Properties

Abstract: Mathematical expressions are presented in this paper that correlate the exhaust emissions from a single-cylinder diesel engine with some of the most important properties of the fuels used. Exhaust emissions measured were carbon monoxide, unburned hydrocarbons, nitrogen oxides, and particulate matter. The experiments were performed using a matrix of 68 fuels. The cetane number of the fuels covered the range 42−58, the density varied between 0.84 and 0.860 g/mL, and the sulfur content varied from 0.05 to 0.20 wt %. All predictions were based on specific points of the distillation curve, the cetane number, and the density of the fuels. In the case of particulate matter emissions, sulfur content was also employed. Very good predictions were obtained for all the emissions considered. The aromatic content was not used as a predictor variable because it was found to have a strong intercorrelation with the cetane number, density, and the 90% distillation point.

The effect of included participates on the spectral albedo of sea ice

Abstract: Sediments and other participates are often entrained into sea ice formed over shallow shelves in the Arctic, causing significant changes in the albedo of the ice and in the amount of shortwave radiation absorbed and transmitted by the ice. A structural-optical model was used in conjunction with a four-stream radiative transfer model to examine the effects of such particulates on the optical properties of sea ice. Albedo data from well-characterized ice with moderate particulate loading were combined with model calculations to infer a spectral absorption coefficient and effective size for the particulates. Results indicate that sediment particles contained in the ice have an effective radius (R) of ∼9 μm, assuming absorption coefficients similar to those of Saharan dust. With these values, model predictions are in close agreement with spectral albedo observations over a broad range of particulate loading. For particle size distributions commonly observed in sea ice, the calculations indicate that particles with R>30 μm have little effect on the bulk optical properties of the ice. The albedo data also suggest that even apparently “clean” ice contains trace amounts (5–10 g m−3) of particulates that reduce albedos by as much as 5–10% in the visible part of the spectrum. The calculations show that particulates in sea ice primarily affect radiative transfer at visible wavelengths, whereas apparent optical properties in the near-infrared tend to be governed by ice structure rather than by the presence of particulates. Particle-bearing layers occurring below ∼20–30 cm are found to have little effect on albedo, although they can still have a substantial effect on transmission. Estimates of total particle loading cannot be obtained from reflectance data without some additional information on particle size, vertical distribution, and ice structure.

Transport and retention of suspended particulate matter and bacteria in the Humber-Ouse Estuary, United Kingdom, and their relationship to hypoxia and anoxia

Abstract: Data are presented on dissolved oxygen (DO) concentrations and their relationship to salinity, suspended particulate matter (SPM), concentrations, and the turbidity maximum in the Humber-Ouse Estuary, United Kingdom, during summer 1995. Measurements in the upper Humber during March 1995 showed DO in the range 82% to 87% of saturation. Suspended particulate matter concentrations were 60,000 mg l−1) occurred within 1 m of the bed in the turbidity maximum region. A spring-neap record showed a dramatic and tidally controlled decrease in DO at very low salinities as the tides progressed from neaps to springs. An anchor station located down-channel of the turbidity maximum showed that about 95% of the variance in DO, which varied from 28% at low-water slack to 67% at high-water slack, could be explained in terms of salinity variation. At the up-channel margins of the turbidity maximum, DO increased from zero (anoxia) near high water to 60% near low water slack, in contrast to the behavior down-channel of the turbidity maximum. About 82% of the variance in DO could be explained in terms of salinity variations alone. Only 43% of the DO variance could be explained in terms of SPM alone. Up-channel of the turbidity maximum, SPM concentrations were relatively low (<3000 mg l−1) and DO levels varied from 48% of saturation near high water to 83% near low water slack. About 76% of the variance in DO could be explained in terms of salinity variations alone. Within the turbidity maximum region, DO varied from <2% saturation on the early flood and late ebb and maximized around 7% at high water slack. About 63% of the variance in DO could be explained in terms of salinity variation alone. This increased to 70% when suspended particulate matter was taken into account. Only 29% of the DO variance could be explained in terms of suspended particulate matter alone. Because bacteria were likely to have been the cause of the observed reduction in DO, the numbers of bacteria, both free-living and attached to particles, were measured in the turbidity maximum region. Numbers of free-living bacteria were low and most of the bacteria were attached to sediment particles. There was a linear correlation between total bacterial number and suspended particulate matter concentration, suggesting that the strong DO demand was exerted locally as a result of bacterial activity associated with increased suspended particulate matter concentrations. An order of magnitude analysis of DO consumption within the Ouse’s turbidity maximum, based on the premise that DO depletion was directly related to suspended particulate matter concentrations and that DO addition was due to reaeration, indicates that complete deoxygenation could have occurred with an oxygen depletion rate of ∼0.01 mg DO h−1/g suspended particulate matter during the residence time of waters within the turbidity maximum (∼7 d). This rate was sufficiently fast that anoxic to aerobic conditions were able to develop a spring-neap periodicity within the turbidity maximum, but too slow to generate substantial intratidal fluctuations in DO. This is in accordance with the observations, which show that relatively little of the intratidal variance in DO could be explained in terms of suspended particulate matter fluctuations, whereas most of the variance could be explained in terms of salinity, which behaved as a surrogate measure for the proximity of the turbidity maximum.