2. EmpiricalMethods for Estimation of y and z
It becomes necessary, under certain conditions, to estimate pollutant concentrations
on the basis of standard data, such as sunshine, cloud amount, and wind speed.
Pasquill and Gifford generated P–G curves for estimating y and z based on these
data (Gifford, 1961). P–G curves are presented in terms of stability classes A–F
(Turner, 1969) which are based on wind speed, insolation and cloud cover. Here
class A is the most unstable and class F is the most stable, with class B moderately
unstable and class E slightly stable. Class D is the neutral class assumed mostly
for overcast conditions. The P–G curves provide graphs of y and z as a function
of downwind distance for the different stability classes. It is important to mention
here that these curves are prepared from observations over smooth terrain, and
represent the average values over a few minutes.
It is difficult to use the graphical forms of y and z in mathematical modelling.
Therefore, regression formulae for these curves have been given in the following
form (EPA, 1977):