Observations of ozone and carbon monoxide at Mei-Feng mountain site (2269 m a.s.l.) in Central Taiwan: Seasonal variations and influence of Asian continental outflow

Abstract Continuous measurements of ozone (O 3 ) and carbon monoxide (CO) were carried out at Mei-Feng (24.05°N, 120.10°E, 2269 m above sea level), a remote mountain site in central Taiwan, to investigate the influence of long-range transported air pollution on O 3 and CO variations in the subtropical Pacific region. Data collected from March 2009 to September 2010 revealed average mixing ratios of 37 ± 14 ppb for O 3 and 188 ± 82 ppb for CO at this remote site. Diurnal variations for both O 3 and CO were observed as well in all seasons. The higher levels for O 3 and CO in the afternoon were attributed to transport of boundary layer pollution to the site during daytime upslope flow. Monthly means of both O 3 and CO showed maxima in spring and in the continental air masses from Southeast Asia, coastal China, and Korea/Japan. On the contrary, the lower O 3 and CO levels found in summer were due to the marine air masses originating from the Philippine Sea and Pacific Ocean. The relationship between O 3 and CO was analyzed, using nighttime data to minimize any local influence. The results showed a fairly good correlation between O 3 and CO from March to September. The contribution of CO from the Asian outflow reached a maximum in spring (88 ppb) and had a minimum in summer (27 ppb). The photochemical buildup of O 3 resulting from anthropogenic emissions in continental Asia was estimated to be 15 ppb in spring, while its production was insignificant, with an average of 4 ppb, in summer. A positive correlation between O 3 and CO plus high ozone levels in springtime suggested that the enhancements of O 3 were likely due to O 3 which was photochemically produced over this region.