Abstract
Visibility is important because it influences transportation safety. This study examined the relationships among sea–land breezes, relative humidity (RH), and the urban heat island (UHI) effect. The study also sought to understand how the synergistic effects of fine particulate matter (PM2.5) and RH influence visibility. Hourly meteorological, PM2.5 concentration, and visibility data from 2016 to 2019 were obtained from government-owned stations. This study used quadratic equations, exponential functions, and multi-regression models, along with a comparison test, to analyse the relationships between these variables. While sea breezes alone cannot explain the presence of PM2.5, UHI circulation coupled with sea breezes during winter can promote the accumulation of PM2.5. The synergistic effects of RH, PM2,5, and aerosol hygroscopicity exist in synoptic patterns type I and type III. PM2.5 was negatively correlated with visibility in the winter, when the RH was 67–95% and the continental cold high-pressure (CCHP) system was over the Asian continent (type I), or when the RH was 49–89% and the CCHP had moved eastward, with its centre located beyond 125° E (type III). The synergistic predictor variable PM2.5×RH was more important than PM2.5 and RH individually in explaining the variation in visibility.
Highlights
Academic Editor: Antonio DonateoVisibility is closely related to air pollution [1,2,3,4], and its decrease has mainly been attributed to fine particulate matter (PM2.5) [5]
‘Longitude < 121° E’ indicates that the continental cold high-pressure (CCHP) was over the Asian continent
In group 1 (G1), the CCHP was over the Asian continent, and the mean hourly air temperature of the Taipei metropolis was less than 20 °C, meaning that the north-easterly monsoon influenced the weather conditions in the Taipei metropolis
Summary
Visibility is closely related to air pollution [1,2,3,4], and its decrease has mainly been attributed to fine particulate matter (PM2.5) [5]. In some areas, owing to low air quality monitoring budgets, air quality is assessed by observing regional visibility. Visibility is affected by many factors such as rainfall, fog, and haze. To avoid misjudging the air quality, it is important to identify the factors that influence visibility. When rainfall and fog are absent, visibility can be reduced owing to absorption or scattering of light by particulate matter or gases. The daily mean concentration of NO2 was low in the Taipei metropolis; light absorption by gases was too low to be considered as a factor influencing visibility. This study focused on the relationship between visibility and PM2.5
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