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Info box: "Road Traffic induced Ozone"

Keywords:
  • Road traffic
  • Ozone
  • PBL
  • ITCZ
  • medium lifetime
  • Seasonal cicle
  • Steady state
Input parameter:
  • Lifetime: 100 days / ~ 3 months
  • Emission rate: 100-200e9 molecules cm**-2 s**-1
  • Emission source:
    • Process: Catalytic photochemical reaction chain
      initiated by NOx emissions from road traffic
    • Geographical distribution:
      Mainly Europe, North America, and some minor sources in South Africa, Asia (Russia, India, Japan), Australia, and South America.
    • Altitude: 1000hPa ~ Surface level
  • Start distribution: Zero
  • Simulation time: 3 years + 1 month
Vocabulary: Model results:
  • Concentrations:
    ~50-140 ppbv in the major source regions
    ~ 5 ppbv throughout the SH (except for some higher spots at local sources there).
  • Spatial variability:
    Gradients around the emission source locations smoother than those of the corresponding NOx.
    Vertical exchange stronger than in the aircraft case (convective updraft).
  • Steady state:
    Approximately reached after two years - see station diagram.
  • Seasonality:
    Well pronounced, especially inside the source regions:
    Seasonal maxima during NH winter (oops) and scattered minima during summer e.g. Mainz which is a place inside the PBL containing the emission source "traffic".
    Here the lack of a proper formulation of the ozone photocamistry drastically turns out: Remember the ban of car driving in summer in the early 90s which was motivated by "ozone alert"! The photochemistry as parameterised here in form of a prescribed chemical lifetime of ozone without any spatial or seasonal variation cannot reproduce the feature of summer smog!
    The seasonality at the Mainz receptor site is just the result of the "breathing" PBL whichs vertical extension is much higher in summer than in winter. Photochemistry in nature is driven by solar radiation and a proper numerically formulation would turn this behaviour upside down. Simulations like that have been performed and the observed seasonality could be reproduced (Crutzen and Zimmermann, 1992).
    Very small amplitude at Zugspitze (3000m) but better seasonality: The station is situated aloft the model-PBL. Photochemistry would enhance the amplitude here in the right direction. Tasmania (SH) is influenced by its minor local source - that is why it also reveals an odd seasonality due to lack of photochemistry.
  • Conclusion:
    The important role of photochemistry is clearly pointed out by this simulation experiment. The inclusion of a chemical reaction module however would go beyond the scope of thise simple exercises.
    Features of the global circulation and the "breathing" of the PBL and of the hemispheres with season are demonstrated: The PBL almost threefolds in summer especially over the continents while the volume of the hemispheres grow in winter because of the migration of the ITCZ which moves south in NH-winter. A hemisphere in the meteorological / thermodynamical sence is not the space between the Equator and Pole but the volume between the rising air belt in the tropics and the Pole. Thus the volume between the ITCZ position and the North Pole increases when the sun moves south. The "Circulation pattern" plates clearly illustrate this: Switch between the Jan. and July patterns and view the shift of the strong upwinds in the tropics.
    As a result of that, concentrations outside the PBL go down while those inside increase - up to some compensation effect. These and even more features make the atmospheric circulation system very complex and interactive and numerical models serve here as interpretation aids.
  • Other remarks:
    The calculated "aircraft ozone" concentrations might be a bit high. They are a result of first guess production rates per NOx molecule. However, validation is hard because there is no color or flag which identifies ozone molecules by their origin.