PHOTOCHEMICAL OZONE PRODUCTION IN THE CONVECTIVE MIXED-LAYER, STUDIEDWITH A TETHERED BALLOON SOUNDING SYSTEM

In 1987, 1988, and 1990, four measuring campaigns with a tethered ball oon sounding system were performed in a semirural area in Belgium to s tudy the ozone distribution in the boundary layer in relation to meteo rological parameters, during episodes of photochemical ozone productio n in the convective mixed layer. The detailed profiles of the ozone co ncentration between sunrise and sunset were used for a calculation of the change rate of the ozone column density in the boundary layer. The se values allowed us to determine for the first time the integrated oz one production rates in the convective mixed layer, making use of a si mplified form of the continuity equation of ozone mass. From all the m easuring campaigns a range of integrated ozone production rates from 0 to 9 mu g 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 noo n. The upper envelope of a scatterplot of all ozone production rates a s a function of the direct solar radiation shows a linear course up to about 650 W m. Beyond this value the intensity of direct solar radiat ion is no longer a rate-limiting factor in the photochemical ozone pro duction process in the boundary layer; under these conditions the ozon e production is limited by the supply rate of hydrocarbons. In general , the measured ozone production rates also show an increase with incre asing ambient temperatures. ht temperatures larger than 24 degrees C t he integrated net ozone production rate during the campaigns was alway s at least 3 mu g m(-2) s(-1) when the direct solar radiation values w ere larger than 350 W m(-2). The surface ozone deposition velocity at the site of the campaigns was determined from the ozone data through a n indirect method that does not require any parameterization of the ve rtical ozone flux; at daylight conditions an upper limit of 1.4 cm s(- 1) was found.