Lower tropospheric ozone trend observed in 1989-1997 at Okinawa, Japan

In order to elucidate recent tropospheric ozone trends in Northeast Asia, 8 year-long ozone sounding data obtained between 1989 and 1997 at Naha (Okinawa Island), Japan, were analyzed incorporating with backward trajectory categorization. Focusing on the regionally polluted continental outflow, only data associated with air masses that reached Naha from northern, northwestern, and western directions during autumn/ winter/early spring seasons (from October to March) were selected for analysis. The concentration of ozone shows an increase of about 2.5 +/- 0.6 % (one standard deviation) per year for a 0 - 2 km layer from the ground representing the planetary boundary. The surface ozone concentrations obtained at Cape Hedo (northern tip of Okinawa Island) at the same timing as the selected ozone sounding also showed an increasing trend of 2.6% per year but with a large statistical uncertainty of + 2.0 %. This result is in accordance with the 22-year period (1969-1990) of tropospheric ozone trends, 1.5 - 2.5 % per year, obtained previously at three other Japanese ozone sounding stations, Kagoshima, Tsukuba, and Sapporo [Akimoto et al. 1994], demonstrating the continuous increase of tropospheric ozone in Northeast Asia in 1990s. It is suggested that this tropospheric ozone trend would be related to the increasing emission of NOx from Northeast Asian region (China, Japan, South Korea, Taiwan) during this period with a rate of 3.9% per year.