Ozone distribution in coastal central Taiwan under sea-breeze conditions

Sea-breeze circulation influences ozone concentration differently for two types of synoptic winds across the westcentral coastal plain of Taiwan. During summer months, when the synoptic flow is southerly, a strong westerly sea-breeze confined to the lower 700–800 m produces a northeastward flow. Ozone concentration is low, with a core centered at about 100 m, leading to moderate ozone concentrations in the low population density northeastern foothills. In autumn (and spring), northerly synoptic winds of about the same strength combine with less energetic westerly sea breeze to produce a southeastward flow that carries higher ozone levels into the heavily populated Taichung basin. Ozone levels are high from 80 to 400 m, with a core up to 120 ppb from about 150 to 300 m, contributing to serious ozone episodes at the southern (downwind) end of the basin. Analyzing the backward trajectories and ozone concentration showed that the weak southeastward breeze is the dominant factor affecting the occurrence of high ozone events in the region. The horizontal distribution is based on 3 yr data obtained from a network of air-pollution monitoring sites in the study area, while the vertical data comes from two 2-day tethersonde experiments, measuring wind-speed, direction, temperature and humidity, NO, NO2, NMHC and O3, conducted during August and November 1999.