Abstract
High summertime concentrations of ozone in the central Kanto plain, extending northward from the Tokyo metropolitan area, are prominent phenomena. The behavior of ozone precursors is an important factor dominating them, and has been studied in relation to the meteorological conditions. Ambient concentrations of NMHC, NOx, and photochemical oxidants (Ox), and wind data, routinely measured at the air quality monitoring stations during July and August of 2000–2011, have been employed. For the screening of sea breeze days, the sunshine duration and wind data from the AMeDAS were referred to.
High Ox events, defined as a daily peak concentration over 120 ppb in the studied areas, occur with a very high probability on the sea breeze days, and are preceded by higher concentrations of the precursors, NMHC and NOx, in a 3 h average between 06 and 09 h LT than on general days in the central Tokyo (the 23-wards area). In particular, convergence flow under calm conditions before the sea breeze develops in this area raises these precursor concentrations, and tends to induce high Ox over a wide region.
In the later six years of the studied term, however, the NMHC concentrations have been conspicuously reduced as a result of the VOC emission control, and wide high- Ox events scarcely occurred. Instead, high Ox mainly in the northern downwind areas, which tends to occur in a relatively low NMHC regime, has remained even in the later six years.
