A Study of the Winter Surface Ozone in Taipei

Abstract

In the Taipei basin in winter, surface ozone concentration usually displays a diurnal pattern with double peaks, one in the early morning, the other in early afternoon. The occurrence probability of double-peak days in a particular winter can be as high as 45%. In this study, a field experiment is carried out to test a hypothesis proposed by the authors in a previous paper to explain the formation of the double peaks.
The field experiment shows that the double peak days tend to appear after the passage of a coldfront over northern Taiwan. At that time the Taipei basin is characterized by a strong northeasterly monsoon, low temperature, and overcasting sky with occasional light rain. An upstream hilltop ozone station shows that a background ozone of about 30ppbv is carried by the monsoon into the Taipei basin. Effective transport processes replenish the near surface ozone that is otherwise titrated out by vehicle-emitted nitrogen oxide (NO). The diurnal ozone double peaks occur at times when the NO concentration is at its minimum which are the result of the combined effect of diurnal traffic pattern and transport processes.