Derivation of a series of annual values for N2O

The series is derived from the IPCC values for 1970,1980,1990 and 2000 
and from the series of measurements in ice from Machida et al (1995). The 
observed values range from 1735 to 1964 (their table 1)

Machida, T., T. Nakazawa, Y. Fujii, S. Aoki and O. Watanabe, 1995: Increase in
the atmospheric nitrous oxide concentration during the last 250 years.
Geophysical Research Letters, 22, 2921-2924.

The values are joined together in a dataset N2O_for_fit1.dat. Artificial 
values for 2010, 2020, 2030 are generated so that the value for year 2000 
will be preserved in the moving average operations

The computations are done within the xmgrace package. The series is 
imported in xmgrace using Data | Import | ASCII. First a series of annual 
values is generated using the function Data| Transformations | 
Interpolation/splines | Method: Cubic spline, Sampling: Linear mesh, Start 
at: 1735 Stop at: 2030 Length: 296

The series of annual values 1735-2030 is then subjected to a smoothing to 
remove short period oscillation by using Data| Transformations | Running 
averages | Running: average, Length of average: 21, Restrictions: None

This operation is done three times thus resulting in a smoothed series over 
the period 1765-2000. It has been checked that the second and third passes 
give very similar results

The resulting series for 1850-2000 is then written to file N20_fit.dat 
using Data| Export | ASCII | Format: %12.2f 

To make sure that it is clear that the series represent annual mean (that is 
central values) for the year in question 0.49 is added to the year before 
writing the series. This has been chosen so that a rounding of the year 
when writing it in integer form will not round the year to the upper 
integer.