Summary
Background ozone concentration displayed distinct seasonal variations at all stations, with a maximum of about 40 ppb in spring and a minimum of about 23 ppb in autumn. The monthly and yearly mean diurnal variations of ozone at urban stations exhibited bi-modal characteristics not evident at rural sites. Two daily maxima and minima were recorded at urban stations, but only one maximum and one minimum at rural sites. The second minimu min the evening at urban stations was shown to be related to high levels of nitric oxide, while nighttime heat island was considered the main cause of the second maximum. One-day resultant trajectories show that the most frequent and highest ozone concentrations were associated with a relatively light to moderate south to southwesterly flow but not with light or with strong winds. Although there were several recognizable meteorological regimes associated with elevated ozone concentrations over Ontario, those with weak pressure gradients favourable for the onset of lake breeze or with southwesterly flows on the west side of slow-moving high pressure centres crossing the eastern United States were the most frequent and resulted in the highest ozone concentrations. Ozone maxima over Ontario may mostly be attributed to long distance transport of emissions from major industrial areas south of the border. However, tropospheric and stratospheric contributions cannot be ruled out and may even be a major source at times. A case study of possible stratospheric ozone influx into the troposphere is presented.
Zusammenfassung
Der natiirliche Pegel der Ozonkonzentrationen zeigt an allen Stationen einen ausgeprägten jahreszeitlichen Gang mit einem FrüWingsmaximum von 40 ppb and einem Herbstminimum von ungefär 23 ppb. Stationen in städtischer Umgebung weisen einen bimodalen, monatlichen und jährlichen, mittleren Tagesgang auf, der an ländlichen Stationen fehlt. Zwei tägliche Maxima und Minima warden an Stadtstationen registriert, jedoch nur je ein Maximum und Minimum an Landstationen. Das abendliche Sekundärminimum an Stadtstationen ist mit hohen Stickoxydpegeln verbunden, während der nächtliche Wärme-inseleffekt für das sekundäre Ozonmaximum verantwortlich ist. Vie rundzwanzigstündige Trajektorien das resultierenden Windes zeigen, daß die haufigsten and höchsten Ozon-konzentratione nbei relativ leichten bis mäßigen südlichen bis südwestlichen Luftströmunge nauftreten, jedoch nicht bei ganz leichten and bei starken Winden. Mehrere meteorologische Regime sind erkennbar mit erhöhten Ozonkonzentrationen über Ontario verbunden. Am häufigsten führen allerdings jene mit schwachem Druckgradienten, die das Auftrete neines Seewindes begünstigen und jene, die mit einer südwestlichen Strömung an der Westseite eines sich langsam verlagemden Hochdruckgebietes, das die östlichen Vereinigten Staaten iiberquert, verbunden sind, zu hohen Ozonpegeln. Ozonmaxima über Ontario können hauptsäcluich dem Ferntransport der Emissionen aus den größeren Industriegebieten südlich der Grenze zugeschrieben werden. Troposphärische and stratosphärische Quellen dürfen jedoch nicht ausgeschlossen werden und konnen manchmal sogar den Hauptbeitrag liefern. Ein Beispiel für stratosphärisches Einströmen von Ozon in die Troposphere wird geboten.
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Mukammal, E.I., Neuman, H.H. & Nichols, T.R. Some features of the ozone climatology of Ontario, Canada and possible contributions of stratospheric ozone to surface concentrations. Arch. Met. Geoph. Biocl. A. 34, 179–211 (1985). https://doi.org/10.1007/BF02277447
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DOI: https://doi.org/10.1007/BF02277447