n this chapter we describe the results of airborne studies of smokes from 17 biomass fuel fires, including 14 prescribed fires and 3 wildfires, burned primarily in the temperate zone of North America between 34° and 49°N latitude. The prescribed fires were in forested lands and logging debris and varied in areas burned from 10 to 700 hectares (ha) (over a few hours). One of the wildfires ultimately consumed 20,000 ha and burned over a period of weeks. The larger fires produced towering columns of smoke and capping water clouds. As an indication of scale, the prescribed fires were visible only as small features in meteorological satellite imagery, but one of the wildfires studied produced a persistent, visible plume more than 1000 kilometers (km) long. Details of these fires and their fuels are given in Table 28.1. The measurements were made aboard the University of Washington's C-131A research aircraft. This twin-engined, 20,000 kilogram (kg), propeller-driven airplane carries instrumentation for measuring the size and nature of aerosol particles, trace gas concentrations, and meteorological parameters. Major portions of the aerosol system have been described by Radke (1983) and the trace gas instrumentation has been described by Hegg et al. (1987). Details concerning the analysis of data, in addition to those presented in subsequent sections, can be found in Radke et al. (1988) and Hegg et al. (1990). Our studies have focused on factors that could impact global climate through alteration of the earth's radiation balance. These include emissions of trace gases and smoke particles from biomass burning, the optical properties of the smoke, and the interaction of the smoke particles with water clouds.