The diurnal variation of photochemical oxidants (O
x) over central Japan during clear summer days was investigated to clarify how it is intimately associated with the thermally-induced local circulation using hourly O
x, NO
2, NO
x concentrations and surface meteorological variables for the period of 1992-2009. In the Kanto plain, the peak time of the O
x concentration is gradually delayed from urban areas to inland and mountain areas. A similar situation is also evident in the Nobi plain, suggesting that air pollutants emitted in coastal areas are transported into the southeastern part of the Gifu prefecture by large-scale sea breezes. The high O
x concentrations in the inland area of the Kanto plain, which are observed before the occurrence of the large-scale sea breeze, tend to occur during successive clear summer days, and the high concentration area expands along the primary route of the large-scale sea breeze. These features might result from the nighttime accumulation of NO
2 produced by the decomposition of O
x that had already been transported into the inland areas on the preceding day. In the basins of the mountain areas, the O
x concentration increases until around noon, but it remains constant or slightly decreases after that time. The second peak of the O
x concentration appears at 17JST in the Kofu basin and at 19JST in the Iida basin. Since such a decrease in the afternoon is also evident for the diurnal variation in the potential ozone (PO) concentration, it is postulated that the consumption of O
3 by NO is not a major factor in the inhibition of the O
x concentration within the basin. Other factors, such as the transport of air pollutants due to the development of local circulations over the basin, should also be considered.
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