Measurements of CH4, N2O, and SF6 from a globally-distributed network of air sampling sites are first smoothed in time. Values are extracted from the smooth curves at equally-spaced intervals (48 time steps per year), then curves are fitted to these values as a function of latitude. Values extracted from these curves at spacing of sine(latitude)=0.05 define a matrix of Greenhouse Gas values in units of dry air mole fraction as functions of latitude and time. Values in this matrix, already weighted for their relative surface area, are averaged to calculate global and other zonal means.
See Marine Boundary Layer Reference for more details.
Uncertainties are based on two terms, one that varies our network with a bootstrap method (also called a resampling method) and one that accounts for analytical uncertainty using a Monte Carlo method. For each term, 100 iterations are prepared, parameters of interested are calculated, standard deviations are determined from the 100 iterations, and the network and analytical terms are then added in quadrature to get the total uncertainty.
Please cite these globally averaged products as: