Ozon-Stickoxid-Korrelationen in der Troposphäre

During 13 flights aboard the UK Met. Office Research Aircraft Hercules (C130) around the British Isles in the summers of 1990 and 1991 trace gases in the altitude range 0-10 km were measured in a coilabaration between the ICG-2 of KFA-Jülich and the MRF-Atmospheric Chemistry Group of the UK Met. Office. The measurements include O$^{3}$, NO$_{y}$, NO, H$_{2}$O and hydrocarbons (< C6). In addition, JNO$_{2}$ was determined by a novel photoelectric detectordescribed here. This allowed an estimate of NO$_{x}$ from the simultaneous measurements of NO and O$_{3}$ using the photostationary state assumption. The prevailing meteorological conditions resulted in the southward transport of polar air masses to midlatitudes. In these air masses maximum O$_{3}$ and NO$_{y}$ mixing ratios of 345 ppb and 2600 ppt, respectively, were measured 2 km above the meteorologically defined tropopause (typically 8 km). During different flights the stratospheric NO$_{y}$/O$_{3}$ ratio was found to be constant at (7.6±1)$\cdot$10$^{-3}$ (fig.7.2 p. 96). This ratio also was found as the slope of a linearcorrelation between NO$_{y}$ and O$_{3}$ observed over the whole range of O$_{3}$ mixing ratios between 40 and 345 ppb (fig.7.1 p.94). As it is shown this result has to be explained by mixing between tropospheric air, relatively poor in ozone and NO$_{y}$, and stratospheric air, containing a constant NO$_{y}$/O$_{3}$-ratio. Thus a significant stratospheric contribution to NO$_{y}$ and ozone in the free troposphere can be identified, although the stratospheric source of NO$_{y}$ is relatively small. The correlation between O$_{3}$ and NO$_{y}$ for O$_{3}$ < 100 ppb is similar to that found from otherexperiments as GTE-CITE-2, MLOPEX and AASB (fig. 7.4 p.105). Conversely, measured NO$_{y}$/O$_{3}$ ratios of (3-4.5) $\cdot$ 10$^{-3}$ in the lower stratosphere of midlatitudes (Murphy et al., 1993) are significantly lower than our observed value of (7.6$\pm$1). 10$^{-3}$ in polar air masses. This resultis unlikely to be due to the influence of tropospheric air since we observed slightly smaller NO$_{y}$/O$_{3}$-ratios below the tropopause than above. There are two possible explanations: (1) Photochemical ozone reduction, (2) inhanced influence of aircraft emissions. However, the negative NO$_{y}$-correlation with long lived tracers, such as CF$_{2}$CL$_{2}$ (F12) indicates the absence of extra sources of NO$_{y}$. Model calculations initialized with the NO$_{x}$-data yielded an average ozone production of 5.6 $\cdot$ 10$^{10}$ cm$^{-2}$s$^{-1}$ between 3 and 10 km. In order to quantify the influence of NO$_{x}$ for each model run a second model run was made with identical initialization except for NO$_{x}$ which was set to zero. The difference between the resulting netto ozone production of both runs provides a quantification of the photochemical NO$_{x}$ contribution. The average NO$_{x}$ contribution between 3 and 10 km was 24.4 $\cdot$ 10$^{10}$ cm$^{-2}$s$^{-1}$ neglecting possible effects from non- urethan-hydrocarbons. Measurements in the PBL showed a nonlinear correlation between O$_{3}$ and NO$_{y}$ in photochemically aged airmasses (fig.9.1 p.141) which was due to photochemical ozone production. The observed curvature indicates a nonlinearity of the ozone-ozone precursorrelationship which is in line with results from current photochemical models. As a consequence of the nonlinearity a general reduction of precursor emissions changing neither the spatial and temporal distribution nor the relative contributions from different precursor species probably would cause only a minor change of peak ozone concentrations during smog episodes