Daytime meteorological structures causing elevated photochemical oxidants concentrations in north Kanto, Japan

The north Kanto area is about 150 km north of the heavily populated and industrialized Tokyo metropolis, and local emissions of photochemical oxidants (Ox) precursors are limited because of little anthropogenic emission. Nevertheless, high Ox concentrations exceeding the warning level for Ox (>120 ppbv, 1-h average) are often observed there. We recently reported that high Ox concentrations in the north Kanto area occur under the coupled conditions of a southerly extended sea breeze transporting Ox from the southern urban area and a subsidence inversion layer restricting vertical dispersion. However, these meteorological conditions do not always lead to high Ox concentrations. To identify other conditions affecting the occurrence of high Ox concentrations in north Kanto, we examined meteorological conditions. Key meteorological structures were two thermal lows occurring on sunny days in districts neighboring the Kanto district on the west and north. When both thermal lows persisted in their morning positions until evening, a southerly sea breeze toward the thermal lows was maintained throughout the afternoon, transporting high levels of Ox formed in south Kanto, including Tokyo, to north Kanto. When a thermal low present only in the western district moved eastward to cover the central Kanto area during the afternoon, isobars in north Kanto acquired an east–west orientation and observed wind direction became consistently northerly, thus restricting transport of high Ox from south Kanto.