Atmospheric visibility trends in an urban area in Taiwan 1961–2003

Climatological observations made in Tainan urban area, southern Taiwan, between 1961 and June 2003 were analyzed along with critical air pollutants monitored from 1994 to June 2003 in order to establish the relationship between atmospheric visibility and major air pollutants and meteorological parameters in the urban area. The visibility discrepancy between weekend and week days was also examined. Average annual visibility in the complete period 1961–2003 was 12.4±4.2 km. However, during the early 1960s it was >20 km, against only 6–7 km between 2002 and 2003. This study revealed a correlation between PM10 and NOx levels, which were higher on weekdays than on weekends, and low weekday visibility. Furthermore, decreased visibility was related mostly to increases in PM10. A weekend effect, in which weekend ozone concentrations exceed weekday concentrations, was also revealed. Mixing layer height is an most important meteorological parameter involved in visibility reduction. Principal component analysis demonstrated that increased vehicular emissions, road traffic dust and industrial activity markedly impacted visibility. Pollutant standard index (PSI) values >100 were characterized by concentrations of PM10 and NOx and atmospheric pressure higher than normal, but with wind speed lower than normal. Regression results for various empirical models of visibility demonstrated that higher PM10 concentrations implied lower visibility ranges, and the parameter of ln[PM10] represented the most significant impact on visibility. Because PM2.5 has a significant impact on visibility, a targeted reduction of PM10 would not completely improve the visual range. However, there is a strong association between presence of PM10 and presence of PM2.5, such that a targeted reduction in PM10 is likely to lead to an increase in visibility.