The chemical composition and size of individual particles derived from combustion products of several species found in Southern California were obtained using aerosol time-of-flight mass spectrometry. The major inorganic species observed in >90% of all biomass burning particles is potassium, indicated by the atomic ion, as well as clusters containing chloride, nitrate, and sulfate ions in the mass spectra. By obtaining positive and negative ion mass spectra it is possible to identify distinct chemical marker combinations in particles resulting from the burning of plant species, which in turn allows for differentiation from particles produced from other combustion sources such as vehicle emissions. Using these markers, particles derived from biomass burning were identified in ambient aerosol samples. © 1999 American Chemical Society.