The atmospheric mixing ratios of CCl2F2 (F-12- and CCl3F (F-11) increase abruptly up to 14 ppt (F-12) and 8 ppt (F-11) after strong cold fronts pass and decrease abruptly after warm fronts pass at a semi-continuous monitoring station near Pullman, Washington (46¿42'N, 117¿10'W). We conclude that air masses from higher latitudes (>60¿N) can have F-12 and/or F-11 concentrations as much as 5% higher than mid-latitude air masses. Conversely, nitrous oxide show no significant changes during frontal passages and is uniformly distributed between air masses.