DIURNAL AND SEASONAL MODELING OF THE TROPOSPHERIC HALF-LIVES OF POLYCYCLIC AROMATIC-HYDROCARBONS

The half-lives of polycyclic aromatic hydrocarbons (PAH) were calculated under different conditions of location, season, and time of day, based on the reaction of PAH with the hydroxyl radical OH as the major sink. The calculations required a method of estimating the diurnal variation in the concentration of OH, which is not normally available experimentally. For naphthalene as a prototype PAH, the half-lives were of the order of several hours, but varied by more than an order of magnitude with season and geographical location. The previously developed model applicable to rural atmospheres was modified to estimate the half-lives of PAH in the urban troposphere by including of the reaction of OH with NO2 as a major urban sink for OH. The half-lives of several PAH were estimated for North York, Ontario, under different conditions of NO2 and ozone concentrations during a period of high tropospheric air pollution in July 1988.