Surface carbon monoxide (CO) data were acquired continuously at Heimaey, Iceland (63¿24'N, 20¿18'W), Mace Head, Ireland (53¿19'N, 9¿54'W), and Ragged Point, Barbados (13¿15'N, 59¿30'W), during April and October 1994, in support of Measurement of Air Pollution From Satellite (MAPS) Space Radar Laboratory (SRL) missions SRL-1 and SRL-2, respectively, measuring middle tropospheric CO from space. Observed median CO levels from the three surface sites during these two MAPS missions approximate the monthly median for 1994 and are mostly typical of data from prior years. For two of the sites, computed mission isentropic back-trajectory ensemble probability fields are compared to seasonal (March--May and September--November) probability fields for 1994 and 1986--1995. Such comparisons help gauge the representativeness of (1) observed surface air quality at, and (2) isentropic flow to, these sites during the mission periods, in terms of intraseasonal and interannual variability. Results appear consistent with longer-term flow climatological data and confirm the SRL-1 and SRL-2 mission periods are generally representative of the climatology applicable to these sites for the time of year. Lower free troposphere in situ CO data were acquired from an aircraft over the Maryland Eastern Shore on April 14 and October 3, 4, and 6. During the April flight a nearly linear gradient in CO with pressure from 1000--650 mbar of 225--150 parts per billion by volume (ppbv) was observed. At 650 mbar, CO was quite steady around 150 ppbv; this value compares favorably with the MAPS CO data for the closest 5¿¿5¿ grid box averaged April 13--15 of 105--120 ppbv. During SRL-2 a three flight CO average of 125 ppbv observed at ~725 mbar is in good agreement with the closest MAPS 5¿¿5¿ grid box averaged October 3--7 of 90--105 ppbv. A layer of elevated CO at 845--740 mbar, most likely the result of synoptic-scale transport, was observed during the October flights and seen to dissipate with time. The MAPS cloud-filtered second-by-second CO data during concurrent shuttle overflights show temporal structure consistent with the in situ observations, indicating the MAPS weighting function may be capable of discerning features at lower altitudes than thought previously. ¿ 1998 American Geophysical Union