Abstract
In order to investigate the increasing photochemical oxidants(Ox) in Japan, the occurrence of high Ox events were analyzed, with a focus on its relation with the local climate in the Kanto Plain for the summer months in 1989-1991 and 1999-2001.
During this ten year period, the occurrence frequency of higher ranks of daily maximum Ox concentrations significantly increased, whereas similar frequency shifts were also found in daytime and nighttime averages.
As a result of general grouping of daily weather patterns for the two examined periods, recent increase of days during which the Kanto region experienced high pressure systems was recognized. High pressure systems are associated with clear skies and development of sea breezes, and an increase of the latter was also confirmed by screening simple criteria. An increase in clear sky days could be connected with a rise in air temperature as well as increased ultraviolet radiation, and could be one factor causing increase in high Ox events.
Two groups of days, STL and LTL, were extracted, in which the sea breeze from Tokyo Bay penetrated into a suburban site through the urban areas of Tokyo with a few hours or more, respectively. Recently STL occurred more frequently. On STL days, Ox levels only slightly changed in the urban and suburban areas of Tokyo during the ten years, while they obviously increased in far inland areas. On LTL days, afternoon Ox peaks enhanced sharply in most parts of the examined region.
