A METEOROLOGICAL INTERPRETATION OF THE ARCTIC BOUNDARY-LAYER EXPEDITION (ABLE) 3B FLIGHT SERIES

The Arctic Boundary Layer Expedition (ABLE) 3B was conducted to determ ine the summertime tropospheric distribution, sources, and sinks of im portant trace gas and aerosol species over the wetlands and boreal for ests of central and eastern Canada. Isentropic trajectories and analyz ed midtropospheric circulation patterns were used to group flights acc ording to the transport histories of polar, midlatitude, or tropical a ir mas ses which were sampled. These data were then divided into bands of potential temperature levels representing the low, middle, and max imum aircraft altitudes to assess the effects of both local and long d istance transport and natural and man-made pollutants to the measured chemical species. Detailed case studies are provided to depict the com plex three-dimensional airflow regimes that transported air with diffe ring chemical signatures to the study area. Mission 6 details the larg e-scale movement of smoke in the generally prevailing west to northwes terly airflow that was observed on the majority of flights. Mission I analyzes the horizontal and vertical motions of maritime Pacific air i n the upper troposphere that was routinely mixed downward to the aircr aft altitude. Finally, mission 14 tracks the far northward excursion o f tropical air that had been associated with a Pacific typhoon. The fo llowing three factors all had important influences on the collected ch emical data sets: (1) local and distant stratospheric in puts into the upper and middle troposphere; (2) biomass-burning plumes from active fires in Alaska and Canada; (3) a band of ''low ozone'' upper troposph eric air that was observed by airborne differential absorption lidar ( DIAL) above the aircraft maximum altitude. Other modification factors observed on some flights included urban pollution from U.S. and Canadi an cities, tropical air that had been associated with a Pacific typhoo n, and precipitation scavenging by clouds and rain. Many flights were affected by several of the above factors which led to complex chemical signatures that will be discussed in other companion papers.