Large‐scale variability of ozone and aerosols in the summertime Arctic and sub‐Arctic troposphere

Measurements of ozone (O 3 ) and aerosol distributions were made with an airborne lidar system in the Arctic and sub‐Arctic during July–August 1988 as part of the NASA Global Tropospheric Experiment/Arctic Boundary Layer Expedition (ABLE 3A). Aerosol and O 3 profiles were measured simultaneously above and below the Electra aircraft from near the surface to above the tropopause. In situ measurements of O 3 mixing ratios and aerosol size distributions and number densities were also made on the aircraft. Many different atmospheric conditions were investigated on long‐range survey flights in the Arctic and on intensive flights over the tundra, ice, and marine regions near Barrow and Bethel, Alaska. The tropospheric composition at high latitudes was found to be strongly influenced by stratospheric intrusions. Regions of low‐aerosol scattering and enhanced O 3 mixing ratios were correlated with descending air from the lower stratosphere. Over 37% of the troposphere along our flight track at latitudes >57°N had significantly enhanced O 3 levels due to stratospheric intrusions, and in the 4‐ to 6‐km altitude range the tropospheric extent of the enhanced O 3 exceeded 56%. Ozone mixing ratios of 80 ppbv at 6 km were common, with vertical O 3 gradients of over 11 ppbv km −1 observed across the base of strong intrusions. In the mixed layer over the tundra, O 3 was in the 25–35 ppbv range with a gradient of 5.5 ppbv km −1 , while in continental polar air masses, the average gradient in the lower troposphere was 7.4 ppbv km −1 , indicating more downward transport of O 3 at higher latitudes. Due to the many forest fires that year, plumes from biomass burning sources were observed on several flights over Alaska. Plumes influenced about 10% of the air below 4 km, and in some photochemically active plumes, O 3 was enhanced by 10–20 ppbv over ambient levels. Pollution plumes from industrial sources were infrequently observed; however, a few large plumes were found over the North Pacific with greatly enhanced aerosol scattering and with O 3 levels exceeding 75 ppbv.