Notes

Ranges of years in the chart represent 3-year averages. For example, 1989-1991 is the average concentration of 1989,1990 and 1991, as calculated from gridded output for each of the years. The three annual grids from the 3-year period were averaged to derive the mean for the 3-year period. Only sites meeting completeness criteria for at least two of the three years of the averaging period were included.

Color gradient maps were generated in ArcInfo GIS software using an inverse distance weighting (IDW) interpolation technique. Using IDW, the surface is most influenced by the nearest point values and less so by more distant points. Only sites within 400 km of each grid point were used in the computation. White areas on the maps are areas where monitoring sites are absent and no information is available.