The impacts of air quality and secondary organic aerosols formation on traffic accidents in heavy fog–haze weather

This study estimated changes in the levels of three components of regional haze, namely fine particulate matter (PM 2.5 ), relative humidity (RH), and secondary organic aerosols (SOAs), at the time of two severe traffic accidents on a coastal expressway and a freeway in the Jianan Plain in southwestern Taiwan to understand the impact of weather and air quality factors on the low visibility. Monitoring data and surveillance images from four nearby air quality monitoring stations were collected to determine the precise causes of the poor visibility–related accidents. The study applied a haze extraction method to the images to achieve demisting, and the processed data were used to assess the relationship between the haze components and visibility during the accidents. The correlation between visibility and the haze components was assessed. The results revealed that the RH levels dropped significantly at the time of the accidents, signifying that moisture was not the main haze–fog component. The haze components can be ordered as follows in terms of their correlation with (and thus effect on) local visibility: PM 2.5 > SOAs > RH. The spatial distributions and evolutions of the three components indicated that the PM 2.5 concentrations remained high from midnight until early morning but decreased slightly at the time of both accidents. By contrast, the concentration of ultrafine SOA particles, which can scatter and absorb light to reduce road visibility, increased rapidly before both accidents. Therefore, PM 2.5 and SOAs were two non-negligible factors of low visibility during the accidents, especially SOAs.