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

Aircraft measurements of total reactive nitrogen (NOy) and ozone (O3) were made over the sea, south and north of Japan in February and November 1993, respectively. NOy 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 O3 mixing ratios were also high, about 70 ppbv at these low altitudes. These high levels of NOy and O3 extended horizontally for more than 200 km. Back trajectory calculations indicate that the air with these high levels of NOy and O3 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 Ny and O3 should have a significant impact on the chemical environment over the western Pacific. The relationship between NOy and O3 shows that the gradient of O3 mixing ratios with respect to the NOy values (ΔO3/ΔNOy) is systematically lower in polluted air at altitudes below 2 km, than those in the relatively clean free troposphere.