Mass budget studies of photochemical ozone production over the U.K.

Abstract

During late April 1987 an ozone episode occurred during which mid-afternoon hourly O₃concentrations reached between 80 and 100 ppbV at many sites in the U.K. during a 5-day period. Such an occurrence is exceptional for April. Data from the recently-established U.K. network of O₃-monitoring stations is used to estimate O₃concentrations in the mixed layer of the lower troposphere as air moves across the U.K. Detailed analyses of individual episode days have enabled estimates to be made of the contribution of ‘local’ photochemical O₃production to the observed concentrations at various U.K. sites.

Ozone budgets are derived by following air columns between well-mixed phases on successive days along trajectories, which take into account their variation with height. Surface deposition is estimated using a diurnally-varying deposition velocity and observed O₃concentrations along the trajectory. Net photochemical production is deduced as a residual, knowing O₃concentrations at the end points of the trajectory.

The results from 10 budget calculations show a clear relationship between the apparent generation of O₃in the mixed layer and the sunshine experienced along the respective trajectory. A net O₃generation of about 4 ppbV h ⁻¹ of bright sunshine is indicated, implying maximum daily generations of about 45 ppbV in April and about 60 ppbV in mid-summer under similar conditions.