Estimation of Long-Term Concentrations Using a “Universal” Wind Speed Distribution

Abstract

Estimation of long-term average concentrations from fixed sources is completely straightforward using standard dispersion formulas and occurrence statistics of wind speed classes and wind directions. However, in many cases of practical interest, only the fraction of time within a direction interval and its associated mean wind speed are available from a standard wind rose. While this information is sufficient to make an estimate of the mean concentration, estimates that better reflect the distributed nature of wind speeds can be made using some observed properties of the wind speed distribution. In particular, it is shown that speed distributions from many diverse sites possess a quasi-universal shape which, when approximated analytically, can be adjusted to yield a distribution of wind speeds which have some specified mean value. The distributions are shown to be satisfactorily described with a log-normal function having a typical geometric standard deviation of 1.9 which, in turn, yields a value of 1.5 for the ratio of long-term concentration determined by distributed winds to that determined from the average wind.