In many areas of the U.S. and particularly in Oregon, residential wood burning has been identified as a major source of fine particle pollution in the wintertime. In this paper, evidence is presented that open burning of twigs, branches, leaves, orchard prunings, and other domestic yard debris, which is called backyard burning, may on the average contribute as much to the daily fine particle pollution during the spring as wood burning does during the winter. Whether backyard burning causes significant air pollution has been a controversial issue in the Portland, Oregon, area, especially over the past few years. Efforts were made in 1980 to legislate a total ban on backyard burning. In 1981 when the ban was scheduled to take effect, a new state statute was legislated which required that before a ban could be placed on residential backyard burning, the Environmental Quality Commission would have to establish the necessity of the ban in meeting air quality standards and show the availability of alternate methods for the disposal of debris at acceptable costs. The feasibility of using a gaseous tracer model will be demonstrated here to estimate the contribution of backyard burning to air pollution in the Portland area. In the past receptor modeling relying on chemical mass balances has been inadequate for accurately determining the contribution of backyard burning or wood burning to urban air pollution primarily because of the great variability in the patterns of elemental tracers emitted from these sources. Two of the important species characteristic of woodburning, organic and elemental carbon, have many sources other than vegetative burning, and show variabilities in emissions from wood combustion of 16% and 81% respectively. The other important elemental species in the emission profile, S, Cl, and K, all show variabilities of over 100% in source emissions from woodburning. Recently a gaseous tracer has been identified which can be used to make almost real time estimates of fine particle pollution from sources of biomass burning such as backyard burning, residential woodburning, and field burning. This tracer is methylchloride (CH3Cl), and it can be used to determine the pollution from biomass burning more accurately than the elemental tracers used in the past. The uniqueness of this tracer for pollution from woodburning arises from three important characteristics. First, CH3Cl can be measured over short times, providing estimates of particulate pollution from combustion sources over time scales of several minutes compared to 8-24 hours for elemental tracer methods. Second, there are no significant local sources of CH3Cl other than biomass combustion, thus making CH3Cl a unique tracer of wood burning. Finally, our recent studies show that the source emissions of this tracer are far less variable than elemental tracers. The major source of CH3Cl in the background atmosphere is oceanic, and its global concentration and seasonal cycle have been well characterized. Our study using CH3Cl as a tracer of residential wood burning in the suburban Portland area showed source contributions to fine particles in agreement with estimates from other receptor modeling studies when its short-term contributions were extrapolated to 24-h averages. Because of the relatively high background concentration of CH3Cl, the propagation of errors in the calculation of the contribution of this source to the ambient aerosol shows that the uncertainties may be as high as 40-50%. However, these uncertainties in estimating the woodburning contribution are no greater than those in the chemical mass balance. Hence, at present it is expected that the gaseous tracer model using CH3Cl as a tracer for woodburning is at least as accurate as the chemical mass balance methods, and has the advantage of resolving the contribution of wood burning to ambient pollution over short time periods. The gaseous tracer model developed previously and CH3Cl data collected over the past year are utilized to estimate the fine particle pollution from the three categories of biomass burning mentioned above. It is shown how backyard burning might significantly contribute to fine particle pollution in the Portland, Oregon, area during the springtime.