Aircraft measurements of total reactive nitrogen and ozone over the Western Pacific in late autumn and winter

Aircraft measurements of total reactive nitrogen (NO y ) and ozone (O 3 ) were made over the sea, south and north of Japan in February and November 1993, respectively. NO y mixing ratios between 1000 and 2000 pptv were observed at altitudes below 1.5 km and 2 km on both days. These values were much higher than those of about 300 pptv in the free troposphere. In the November measurements, the O 3 mixing ratios were also high, about 70 ppbv at these low altitudes. These high levels of NO y and O 3 extended horizontally for more than 200 km. Back trajectory calculations indicate that the air with these high levels of NO y and O 3 in November and February was transported from North Korea and the northeast part of China within about 1 or 1.5 d. Since westerly winds dominate from late autumn to early spring, the large-scale transport of air with high levels of NO y and O 3 should have a significant impact on the chemical environment over the western Pacific. The relationship between NO and O 3 shows that the gradient of O 3 mixing ratios with respect to the NO values (ΔO 3 /ΔNO y ) is systematically lower in polluted air at altitudes below 2 km, than those in the relatively clean free troposphere.