Aircraft measurements of the tropospheric N2O concentration were made over
Japan during the period from October 1991 to June 1999. The observed values
of the N2O concentration showed clear evidence of the secular increase, wi
th 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 N2O
concentration was hardly observable throughout the troposphere, interannual
variations with periods of about 2 years were clearly found. The average N
2O concentrations over the observation period for the above 5 height interv
als were almost identical with each other, showing no appreciable vertical
gradient of the concentration. This suggests that N2O emissions from the gr
ound surface around Japan are very weak and the emitted N2O is mixed well i
n the troposphere.