The Pacific Exploratory Missions (PEM) were designed to study the chemistry of tropospheric air within the Pacific Rim region extending from the equator to about 50 ¿N. Missions emphasized the importance of Asian outflow to the chemistry of tropospheric air. PEM-West A was conducted in September and October 1991, and PEM-West B was conducted in February and March 1994. The PEM-West B winter mission coincides with the time of maximum impact of Asian outflow on the Pacific Rim region. This paper examines the chemical composition of air measured during PEM-West B aircraft ascents/descents. Chemical composition of tropospheric air is related to its history as determined from 5- to 10-day back trajectory calculations at multiple altitudes of the vertical profiles. Locations and the altitudes for trajectory calculations are selected to elucidate relationships between Asian source regions, transport within the region, and the chemical characteristics of tropospheric air. Data are over-ocean measurements at locations ranging from hundreds of kilometers from the Asian coast to remote ocean sites thousands of kilometers east of Asia. Seasonal differences are illustrated by comparing PEM-West A and B results. In general, the chemical composition of tropospheric air throughout the Pacific Rim region is influenced by Asian outflow, and transported continental emissions are an important source of pollution to the region during both seasons. ¿ 1997 American Geophysical Union