Aircraft measurements of tropospheric ozone over the Western Pacific Ocean

The latitudinal distribution of the tropospheric ozone over the western Pacific Ocean has been observed during two aircraft experiments, namely the 1993 Pre-PACE and the 1994 PACE-I experiments. The ozone concentration in the northern middle latitudes ( > 20 degrees N) was high (40-50 ppbv) and stable in both the experiments, however, the air originating from the Pacific Ocean showed lower concentration of ozone compared to the one that came from the Eurasian Continent. A sharp gradient of the ozone concentration was observed around 10-25 degrees N in these experiments. To the north of this gradient, the air mass with high ozone and low water vapor concentration came from the Eurasian Continent. South of the gradient, the air mass with low ozone and high water vapor concentration came from the tropical Pacific. Extremely low ozone concentrations (as low as 20 ppbv) were observed around the equator during the field experiments. The reason is probably that the ozone was photochemically destroyed by the high water vapor concentration in the boundary layer near the equator and this air mass was lifted up by the Walker circulation. The air mass which came from the middle of Australia showed high concentration of ozone (similar to 50 ppbv) around 20 degrees S on the flight course; on the other hand, the air mass which came from the ocean showed low concentration of ozone around 24 degrees S. The air mass with high concentration of ozone may be affected by mixing with the boundary layer in which photochemical ozone production occurs.