Photochemical ozone production in the convective mixed layer, studied with a tethered balloon sounding system

Abstract

In 1987, 1988, and 1990, four measuring campaigns with a tethered balloon sounding system were performed in a semirural area in Belgium to study the ozone distribution in the boundary layer in relation to meteorological parameters, during episodes of photochemical ozone production in the convective mixed layer. The detailed profiles of the ozone concentration between sunrise and sunset were used for a calculation of the change rate of the ozone column density in the boundary layer. These values allowed us to determine for the first time the integrated ozone production rates in the convective mixed layer, making use of a simplified form of the continuity equation of ozone mass. From all the measuring campaigns a range of integrated ozone production rates from O to 9 μg m-2 s-1 was found. Peak values of the ozone production rate were observed when the direct solar radiation intensity rises for the first time in the course of a day to about 400 W m-2 before noon. The upper envelope of a scatterplot of all ozone production rates as a function of the direct solar radiation shows a linear course up to about 650 W m-2. Beyond this value the intensity of direct solar radiation is no longer a rate-limiting factor in the photochemical ozone production process in the boundary layer; under these conditions the ozone production is limited by the supply rate of hydrocarbons. In general, the measured ozone production rates also show an increase with increasing ambient temperatures. At temperatures larger than 24°C the integrated net ozone production rate during the campaigns was always at least 3 μg m-2 s-1 when the direct solar radiation values were larger than 350 W m-2. The surface ozone deposition velocity at the site of the campaigns was determined from the ozone data through an indirect method that does not require any parameterization of the vertical ozone flux; at daylight conditions an upper limit of 1.4 cm s-1 was found.