The Regional Ozone Problem

Ozone is an important secondary pollutant in the boundary layer and comes from three sources, the stratosphere, the free troposphere and from production in the boundary layer itself. Although the contributions from the first two sources are not well defined, there is no question that ozone concentrations in excess of 80 to over 200 ppb observed during photochemical episodes on a regional scale require a predominantly photochemical source in the boundary layer. The regional scale is defined here by the spatial scale of precursor source regions and by typical transport distances for ozone, 1000–2000 km.