F.3.2 Off-site exposure concentration

c. The contaminant mass emission should be computed for one residence

time for the three wind speeds. The emission should be computed by

first estimating the contaminant flux rate in mg/m2/s for the given locale

and wind speed.

d. The flux rate is then multiplied by the area of the locale in m2 to obtain

the contaminant emission rate in mg/s.

e. The emission rate is then multiplied by the residence time in seconds to

obtain the contaminant emission in milligrams for one control volume of

air. The contaminant site exposure concentration in mg/m3 or ug/L is

then computed by dividing the contaminant emission for the three wind

conditions by the control volume. The highest of the three site exposure

concentrations is used for evaluations of air quality at the site.

The contaminant exposure concentration is compared with the air quality

standard to determine the acceptability of the volatile emission. If an air quality

standard is not available, a health and safety standard in terms of an inhalation

reference dose may be available. The reference dose in mg/kg/day can be con-

verted to an air quality standard in ug/L by multiplying the dose by the weight of

the receptor (person being protected) and dividing the result by the volume of air

breathed by the receptor at the exposure point in a day considering the receptor's

activity level. If the receptor were a worker, exposure might be limited to 9 hr

per day while a nearby resident might be exposed 24 hr per day.

F.3.2 Off-site exposure concentration

To evaluate off-site air quality, the off-site exposure concentration is

predicted using a Gaussian dispersion model for the same three wind conditions.

The Gaussian dispersion equation given below describes a ground level source

with no thermal or momentum flux.

Cx, 0, 0 =

(F-2)

π σyσz u

where

Cx, 0,0 = concentration of pollutants at coordinate x above background,

mg/m3

Q = emission rate of pollutants, mg/s

s y = horizontal standard deviation of pollutant concentration along the

centerline of plume at X distance, m

s z = vertical standard deviation of pollutant concentration along the

centerline of plume at X distance, m

u = mean wind velocity, m/s

Appendix F Laboratory Evaluation of Volatile Emissions and Volatile Dispersion Modeling