Measurements of trace gases and photolysis frequencies during SLOPE96 and a coarse estimate of the local OH concentration from HNO3 formation

Abstract

The Schauinsland Ozone Precursor Experiment (SLOPE96) campaign was conducted in June 1996 to study the physicochemical transformation of pollutants and the production of photooxidants during transport from the city of Freiburg to the Schauinsland mountain. For this purpose, chemical surface measurements were made at the entrance of the valley Großes Tal, and close to the Schauinsland summit, at 1200 m altitude on a saddle at the end of the valley. In addition, measurements of ozone, NO2, and meteorological parameters were made on two tethered balloons and aboard a small aircraft. This paper describes the experimental setup and the measurements of ozone, odd‐nitrogen compounds, carbonyl compounds, CO, and photolysis frequencies made during SLOPE96. The various instruments used on the different platforms were harmonized on the basis of intercomparison experiments in order to achieve a consistent picture. Large precursor concentrations from the nearby city of Freiburg are transported to Schauinsland in a valley wind system during stagnant high‐pressure conditions. These conditions occurred only on 2 days of the campaign, and only 1 day (June 5) was predictable enough to allow for deployment of the aircraft and the balloons. An OH concentration of 8 ‐ 10× 06 cm−3 is derived from the rate of change of HNO3 and NO2 on June 5 at Schauinsland (1.5–3 ppb NO2; JO1D ≈ 2 × 10−5 s−1), assuming quasi steady state and a homogeneous air mass. The ozone production rate as determined from the photostationary state of NOx at Schauinsland reached maximum daily values between 15 and 60 ppb h 1 around noontime. On average, P(O3) comprised about 30% of the photolysis rate of NO2. Similar to earlier observations at Schauinsland, a good correlation between P(O3) and the product of UV radiation and precursor concentrations was found.