ESTIMATION OF SPATIALLY-VARIABLE ATMOSPHERIC CONCENTRATIONS DEDUCED FROM REGIONAL MASS-BALANCE MODELS

Multimedia box models of contaminant fate in a regional environment ge nerate estimates of average atmospheric concentrations, ie. the total mass of substance in a defined volume. In this study the issue of esti mating the distribution of concentrations, ie. spatial variability, co nsistent with the estimated average, is addressed. Simulations show th at the distribution depends on the number and location of emission poi nts, the local atmospheric dispersion or diffusion characteristics and the rate of chemical removal by reaction and irreversible deposition. The form of a correlating equation is suggested to relate these quant ities to the relative standard deviation and the spread factor in the Weibull distribution, the latter being preferred for this analysis. An alysis of monitoring data for volatile organic chemicals (VOCs) in Ont ario suggests that the value of the Weibull spread factor is predictab le (with error limits), thus enabling average concentrations to be exp ressed also as approximate spatial distributions. It is also shown tha t spatial distributions differ depending on whether they are expressed on a sampling site, area, or population basis. Implications for analy sis of data generated by multimedia box models to yield distributions and hence estimates of fractions of the environment exceeding specifie d exposure concentrations are discussed. (C) 1998 Elsevier Science Ltd . All rights reserved.