Mixing height

Trajectory models require spatiaUy and temporaUy resolved wind fields, mixing-height fields, deposition parameters, and data on the spatial distribution of emissions. Lagrangian trajectory models assume that vertical wind shear and horizontal diffusion are negligible. Other limitations of trajectory and Eulerian models have been discussed (30). 

In addition to short-term emission estimates, normally for hourly periods, the meteorological data include hourly wind direction, wind speed, and Pasquill stability class. Although of secondary importance, the hourly data also include temperature (only important if buoyant plume rise needs to be calculated from any sources) and mixing height. 

What factors contribute to a high mixing height ... 

For these large tr values, eddy reflection has occurred repeatedly both at the ground and at the mixing height, so that the vertical expanse of the plume has been uniformly mixed through the mixing height, i.e., 1/L. 

When there is an upwind background concentration Xb and the mixing height is rising with time into a layer aloft having an average concentration of Xa/ equation of continuity is... 

In the application of the model to eastern North America, the mixing height is varied seasonally, and hourly precipitation data are used. 

What is the steady-state concentrahon derived from the box model for a 10-km city with average emissions of 2 x 10" g m s when the mixing height is 500 m and the wind speed is 4 m s ... 

Fig. 21-2. Mean annual afternoon mixing height, in hundreds of meters. Source Adapted from Holzworth (2).

Note that for stable conditions and/or mixing heights greater than or equal to 10,000 m, unlimited mixing is assumed and the summation term is assumed to be zero, as noted by expressions presented earlier in this chapter. Equation (9) is used to model the plume impacts from point sources, flare releases, and volume releases in SCREEN. The SCREEN volume source option uses a virtual point source approach. The model uses a numerical integration algorithm for modeling impacts from area sources. 

The mixing height used in SCREEN for neutral and unstable conditions (classes A-D) is based on an estimate of the mechanically driven mixing height. The mechanical mixing height, z (m), is calculated (Randerson, 1984) as ... 

Mixing height The height above an internal or external pollutant source within which emitted pollutants are dispersed and mixed with the surrounding atmosphere. In meteorological terms, this is the area below the inversion layer. 

A Rule 57 day is one on which the inversion bese et 4 AM (PST) is lower then 1500 feet, the mexintunt mixing height is not above 3500 feet and the average surface wind speed between 600 AM end 12 noon (PST) does not exceed 5 MPH. 

Gradient diffusion was assumed in the species-mass-conservation model of Shir and Shieh. Integration was carried out in the space between the ground and the mixing height with zero fluxes assumed at each boundary. A first-order decay of sulfur dioxide was the only chemical reaction, and it was suggested that this reaction is important only under low wind speed. Finite-difference numerical solutions for sulfur dioxide in the St. Louis, Missouri, area were obtained with a second-order central finite-difference scheme for horizontal terms and the Crank-Nicolson technique for the vertical-diffusion terms. The three-dimensional grid had 16,800 points on a 30 x 40 x 14 mesh. 

Bruntz et al. applied multiple regression analysis and found that the method of least squares yielded a set of coefficients that produced a 0.84 correlation of ozone concentration with the data. Adding mixing height to the correlation yielded no statistically significant improvement in agreement with the assertions of Hanna. ... 

Meteorological information which has been useful to receptor model studies includes temperature, relative humidity, mixing height, windspeed and wind direction. The wind direction is particularly important for the verification of certain receptor model source contribution predictions definite differences should exist between samples on which the source is upwind and downwind of the receptor. 

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