vg =is the gravitational settling velocity (m/s),
Ur =is the approach wind speed at 10m (m/s),
α =is the proportionality constant in the relationship between flux from the pit and the product of Ur and concentration in the pit (Thompson, 1994).
The gravitational settling velocity, vg, is computed as described in Section 1.3.2 for each particle size category. Thompson (1994) used laboratory measurements of pollutant residence times in a variety of pit shapes typical of actual mines and determined that a single value of α = 0.029 worked well for all pits studied.
The adjusted emission rate (Qi) for each particle size category is then computed as:
where Q is the total emission rate (for all particles) within the pit, φi is the original mass fraction for the given size category, and ε is the escape fraction calculated from Equation (1-68). The adjusted total emission rate (for all particles escaping the pit), Qa, is the sum of the Qi for all particle categories calculated from Equation 1-69. The mass fractions (of particles escaping the pit), φai, for each category is:
Because of particle settling within the pit, the distribution of mass escaping the pit is different than that emitted within the pit. The adjusted total particulate emission rate, Qa, and the adjusted mass fractions, φai, reflect this change, and it is these adjusted values that are used for modeling the open pit emissions.
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