Bedeutung von Stickoxiden für die Ozonbilanz in Reinluftgebieten : Untersuchung der Photochemie in Reinluft anhand von Spurengasmessungen auf Teneriffa

In August 1993 trace gas measurements were performed at the Global Atmospheric Watch station Izana an Tenerife. The measurements were part of the European research program OCTA (Oxidising Capacity of the Tropospheric Atmosphere). Peroxy radicals were measured using two different methods: a chemical amplifier and matrix isolation ESR spectroscopy. Both data sets show reasonable agreement and exhibit a pronounced diumal variation with noontime maximum concentrations reaching 40 to 100 pptv as is expected from the range of 80-1500 pptv for the NO$_{x}$ concentrations observed. The site Izana is influenced by local upslope during daytime. Using the concentrations of water vapour, NO$_{x}$, and NMHCs, it could be demonstrated that trace gases which are emitted below the trade wind inversion can wach the station and significantly enhance the measured concentrations of these species and their photochemical reaction products. The ratio of NO$_{y}$/NO$_{x}$ was consistent with the assumption that (for the time period under investigation) most of the locally emitted trace gases have originated in the town of Puerto de la Cruz which is located at the Northern coast of Tenerife. A photochemical box-model was used for simulating the Jong-range transportfrom the North American continent to the canary Islands. The model was able to reproduce most of the trace gas concentrations that were measured at Izana atnight, except for the concentrations of ethene and propene. A possible explanation for the enhanced ethene and propene concentrations at Izana is that these species are emitted from the ocean around the canaiy islands and that a mixing of marine boundary layer air with free tropospheric air occurs close to the Island of Tenerife. Since most of the trace gas concentrations were well reproduced by the model, the model results were then used to complete the measured data set for the analysis of the local budgets of OH, HO$_{x}$, and ozone. To simulate the daytime free troposphere at Izana, further model calculations were performed, and the resulting trace gas budgets were compared to those derived from the measurements as well as to results obtained at Mauna Loa/Hawaii and Scotia/Pennsylvania. The data, together with the model calculations, led to the following conclusions: • In some cases the budgets which were derived from the measurements show greater similarity to expected budgets of continental sites with high concentrations of biogenic hydrocarbons and moderately NO$_{x}$concentrations, as in Scotia, whereas the simulated free tropospheric budget is very simular to the budget of Mauna Loa. • Noontime OH concentrations between 5 $\cdot$10$^{6}$ and 1.5 $\cdot$ 10$^{7}$ molecules/cm$^{3}$ were derived from steady state assumptions. The value of 5 $\cdot$ 10$^{6}$ molecules/cm$^{3}$ agrees well with the result of the model calculation simulating the daytime free troposphere at Izana. The range of concentrations is comparable to values reported in the literature for similar conditions. • Formaldehyde photolysis plays a major role in the budget of HO$_{x}$ at Izana. The observed high concentrations of formaldehyde are likely due to the oxidation of reactive biogenic hydrocarbons which are emitted from a nearby canarian pine forest or from scattered juniper bushes in the vicinity of the station. • A major source of uncertainty in the HO$_{x}$-budget are the rate coefficients for the reactions of organic peroxy radicals with NO. Compared to the usual approach of assuming equal rate coefficients of 8 $\cdot$ 10$^{-12}$ cm$^{3}$/molecules$\cdot$s for all peroxy radicals, much better agreement between HO$_{x}$ production and loss was found when recent laboratory data showing slower rates for branched radicals was taken into account. At Izana, the average reactivity of organic peroxy radicals towards NO seems to be substantially smaller than the rate coefficient for HO$_{2}$ or CH$_{3}$O$_{2}$. This has important consequences for the rate of photochemical ozone production: for the conditions of Izana that were analyzed in this work, the net ozone production is up to 30% smaller than what would be calculated by using the average rate coefficient of 8 $\cdot$ 10$^{-12}$ cm$^{3}$/molecules$\cdot$s. • From the model simulations for free tropospheric conditions, the ozone budget was found to be approximately balanced in the area around Izana. This is consistent with the ozone budget of the free troposphere at Mauna Loa and with airborne measurements over the Atlantic ocean. The longrange transport simulation of this work also shows only small variations of the ozone concentrations over most of the simulated period. This leads to the hypothesis that a balanced ozone budget may be representative for wide areas of the marine troposphere of the northem hemisphere