Concentration variations of tropospheric nitrous oxide over Japan

Aircraft measurements of the tropospheric N 2 O concentration were made over Japan during the period from October 1991 to June 1999. The observed values of the N 2 O concentration showed clear evidence of the secular increase, with mean rates of 0.50 to 0.64 ppbv/yr for selected height intervals of 0–2, 2–4, 4–6, 6–8 km and 8 km‐tropopause. Although the seasonality of the N 2 O concentration was hardly observable throughout the troposphere, interannual variations with periods of about 2 years were clearly found. The average N 2 O concentrations over the observation period for the above 5 height intervals were almost identical with each other, showing no appreciable vertical gradient of the concentration. This suggests that N 2 O emissions from the ground surface around Japan are very weak and the emitted N 2 O is mixed well in the troposphere.