Figure 2.5. Hemispheric monthly mean N_2O mole fractions (ppb) (crosses for the NH and triangles for the SH). Observations (in situ) of N_2O from the Atmospheric Lifetime Experiment (ALE) and GAGE (through the mid-1990s) and AGAGE (since the mid-1990s) networks (Prinn et al., 2000, 2005b) are shown with monthly standard deviations. Data from NOAA/GMD are shown without these standard deviations (Thompson et al., 2004). The general decrease in the variability of the measurements over time is due mainly to improved instrumental precision. The real signal emerges only in the last decade.