Plume dilution

Point sources are a relatively small contributor of NO emissions compared to S02, but still substantial. Both NO and N02 have low solubility in water. Virtually no NO is removed from fresh plumes. HN03 formed by gas-phase oxidation of N02 is very soluble in water and the principal source of nitrate in precipitation. Since the secondary products are much more easily scavenged than NO, its scavenging increases with plume dilution and oxidation. Mesoscale studies show much variation in the efficiency of wet scavenging of SO and NO, depending on the storm type and plume history. About one-third of the anthropogenic NO emissions in the United States are estimated to be removed by wet... 

Figure 26-53 shows the affect of initial momentum and buoyancy of the release. If the material is released as a jet, then the effective height of the release is increased. Furthermore, if the material released is heavier than air (which is the usual case for the release of most hydrocarbons), the plume initially slumps toward the ground until subsequent dilution by air results in a neutrally buoyant cloud. 

One of the effects of wind speed is to dilute continuously released pollutants at the point of emission. Whether a source is at the surface or elevated, this dilution takes place in the direction of plume transport. Figure 19-2 shows this effect of wind speed for an elevated source with an emission of 6 mass units per second. For a wind speed of 6 m s", there is 1 unit between the vertical parallel planes 1 m apart. When the wind is slowed to 2 m s there are 3 units between those same vertical parallel planes 1 m apart. Note that this dilution by the wind takes place at the point of emission. Because of this, wind speeds used in estimating plume dispersion are generally estimated at stack top. 

In Gaussian plume computations the change in wind velocity with height is a function both of the terrain and of the time of day. We model the air flow as turbulent flow, with turbulence represented by eddy motion. The effect of eddy motion is important in diluting concentrations of pollutants. If a parcel of air is displaced from one level to another, it can carry momentum and thermal energy with it. It also carries whatever has been placed in it from pollution sources. Eddies exist in different sizes in the atmosphere, and these turbulent eddies are most effective in dispersing the plume. 

An alternative simple model for contaminant dilution of rooftop exhaust stacks is presented in Halitsky. This model combines a jet region specification for the upward exhaust movement with a more traditional Gaussian plume region controlled by atmospheric and building-generated turbulent dilution. 

Capture efficiency measurements may be used to evaluate the function of a canopy hood (see Section 10.5). Capture velocity is not a feasible evaluation tool, since a canopy hood does not generate an air velocity close to the source. It is also possible to use exposure measurements for workers outside the plume area. Since most hot processes generate visible contaminants, visual inspection of the flow, especially around hood edges, might provide a qualitative evaluation. Many contaminants could however be invisible when diluted and smoke generators (Section 10.5) may be necessary to find leakages (temporary or permanent) around the hood edges. 

As the wind speed increases, the plume in Figure 5-1 becomes longer and narrower the substance is carried downwind faster but is diluted faster by a larger quantity of air. 

For a given release scenario, estimate the state of the released contaminant after it has depressurized and become airborne (including any initial dilution). The initial mole fraction of hazardous components will be applied to the final reported concentrations and hazardous endpoint concentrations throughout the process. If source momentum is important (as in a jet release or for plume rise), other models are available that can address these considerations. Disregarding the dilution due to source momentum will likely result in higher concentrations downwind, but not always. 

The concentrations of both parent surfactant and lipophilic metabolites decreased sharply with increasing distance from the sewage outlet. At a distance of 100 m, the concentrations had all dropped to below 0.5 and 0.1 pg L-1 for A9PEOn and metabolites, respectively. The decrease was mainly explained by an efficient dilution. The concentrations were significantly higher near the water surface, as a consequence of the wastewater plume spreading primarily into the upper freshwater layer, while leaving the saline layer less affected. 

The log-linear solubility enhancement by cosolutes may be important in characterizing concentrated leachate plumes or chemical spills, but will be of little importance in characterizations of the dilute aqueous systems that predominate in nature [19,55,143,145,158,184,226,241-247,249-263]. 

Hydrothermal venting injects fluids into seawater as buoyant, jetlike pliunes. These turbulent flows mix rapidly with seawater becoming diluted by factors of lO" to 10. This mixing eventually makes the plumes neutrally buoyant, after which they are transported laterally through the ocean basins as part of the intermediate and deepwater currents. Hydrothermal plumes have the potential to greatly affect seawater chemistry. From global estimates of hydrothermal fluid emissions and dilution ratios, a volume of seawater equivalent to the entire ocean can be entrained in the hydrothermal plumes every few thousand years. 

In addition to observations in Los Angeles, Blumenthal and White have reported measurements of a power-plant plume and an urban plume 35 and 46 km downwind from St. Louis, Nfissouri. Bgute 4-25 shows the evidence of extensive ozone buildup in the urban plume. Simultaneous measurements of scattering coefficient, 6>cat, trace the spread and dilution of suspended particulate material. It is interesting that in the urban plume, which spreads to 20 km in width, the ozone increases while the particulate matter decreases this suggests considerable photochemical production at an altitude of 750 m. Contrary to the statements of Davis and co-workers reported above, the power-plant plume causes a decrease, rather than an increase, in ozone. Nitric oxide in the plume reacts with the ozone as it mixes. This is clearly indicated by the distribution of particulate matter, which acts as a tracer. 

Estimates of visibility in smoke plumes have been made and are on the order of less than 1 meter near the source (i.e., high smoke concentration). Generic smoke dilution factors for large plumes have also been estimated and are presented in the CMPT Handbook (Spouge, 1999). Outdoor smoke plume models can be used to estimate the specific areas of smoke involvement. 

As described above, the plume becomes wider and more dilute as it evolves in the streamwise direction, thus ccenteriine and a are changing with x. The decrease of the time-averaged concentration along the centerline of the plume follows a v 1 profile for x/H > 2 (Fig. 5.8). This power law decrease agrees well with the time-averaged concentration field predicted by modeling efforts that assume... 

The sample data presented in this chapter were collected for fairly simple flow conditions. The flow was a unidirectional open-channel flow without large-scale flow meander, and the release condition was isokinetic in the direction of the bulk flow. Thus, chemical filaments were advected by the bulk flow in the stream-wise direction, while turbulent mixing acted to expand the plume size and dilute the chemical concentration. Changes in the flow and release conditions lead to significant variation in the plume characteristics and structure. 

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