In situ ozone production under free tropospheric conditions during FREETEX '98 in the Swiss Alps

The second Free Tropospheric Experiment (FREETEX '98) took place at the Jungfraujoch Observatory (3580 m above sea level), Switzerland, in March/April 1998. A number of trace gases and photolysis parameters were measured, including peroxy radicals (HO2+RO2), the photolysis rate coefficients j(NO2) and j(O1D), O3, NOx, NOy, HCHO, peroxyacetylnitrate (PAN), CO, CN, and hydrocarbons. The average midday concentration of HO2+RO2 observed during the campaign is 13±3 parts per trillion by volume (pptv). The peroxy radical concentrations measured during FREETEX '98 are discussed in terms of the respective NO measurements and box model calculations. By screening out polluted and cloudy conditions during FREETEX '98, 8 relatively clean days are selected. These 8 days represent near free tropospheric conditions. The average midday concentration of HO2+RO2 observed during these 8 days is 17±5 pptv and the modeled diurnal cycle of [HO2]+[CH3O2] simulated by a photochemical box model shows very good agreement with the mean diurnal cycle of the peroxy radicals over these 8 days. The in situ net ozone production rate is calculated for each day during FREETEX '98 using the peroxy radical, j(O1D), NO, H2O, and O3 measurements. The mean net ozone production rate during the 8 relatively clean selected days is 0.09 ppbv h-1. If more stringent criteria are applied, 3 free tropospheric days can be selected with a mean net ozone production rate of 0.05±0.01 ppbv h-1, indicating the positive role of in situ photochemistry during springtime in the free troposphere over the Alps and supporting the previous results of the first FREETEX campaign in April/May 1996. Using the experimental data, it is estimated that the ozone compensation point, that is, the point where photochemical ozone production equals destruction, occurs at 24±9 pptv of NO in good agreement with theoretical calculations.