The NASA Pacific Exploratory Mission in the Western Pacific Ocean (PEM-West) is a major component of the East Asia/North Pacific Regional Study (APARE), a project within the International Global Atmospheric Chemistry (IGAC) Program. The broad objectives of the PEM-West/APARE initiative are to study chemical processes and long-range transport of atmospheric trace species over the northwest Pacific Ocean and to estimate the magnitude of the human impact on these species over this region. The first phase of PEM-West (PEM-West A) was conducted in September--October 1991, a period characterized by minimum outflow from the Asian continent. The second phase of this mission, PEM-West B, was conducted during February--March 1994, a period characterized by enhanced outflow from the Asian continent. Both field campaigns of PEM-West included intensive airborne measurements of trace gases and aerosols from the NASA DC-8 aircraft coordinated with measurements at surface sites. This paper reports the experimental design for PEM-West B and provides a brief summary of the salient results of the PEM-West B campaign with particular emphases on the difference/similarities between phases A and B. Results from the two campaigns clearly quantify, from a trace gas perspective, the seasonal differences in the continental outflow that were qualitatively anticipated based upon meteorological considerations, and show the impact of major meteorological features within the region on the quality of tropospheric air over the North Pacific Ocean regions. The PEM-West database provides a baseline tool by which future assessments of a continuing impact of Asian emissions on remote Pacific regions can be judged. . ¿ 1997 American Geophysical Union