(click map to enlarge)
1) no aircraft sites or shipboard measurements;
2) continuous and flask stations included, if both, shown as continuous;
3) must have been producing good quality data for at least the past 12 months (presently Nov07-Nov08);
4) must receive "clean, background air" at least some of the time;
5) must be in *routine* operation today, and planned to continue for forseeable future.
Alex Etchells 2008