The use of cavity-enhanced methods in environmental measurements has exploded in the last ten years. This talk will introduce our use of the cavity ring-down method to measure aerosol optical extinction (the sum of scattering and absorption by airborne particulate matter) in conjunction with a commercial measurement of scattering. Results from measurements of aerosol and chemical properties in Portland, OR neighborhoods that are very near a major interstate highway (I5) will be used to highlight the utility of the coordinated and time-resolved optical measurements. For example, particulate absorption (derived by subtracting the two extensive properties extinction and scattering) coefficients are found to correlate well with other common tracers of the strong nearby mobile source. Single scattering albedo (the ratio of scattering to extinction) is an intensive property that we believe can be used to follow the mixing and/or transformation of the diesel and other black carbon containing PM as it merges with the urban background aerosol. A case will be made for the use of this instrument in urban settings as a cost effective “diesel-o-meter” by regulatory agencies empowered to examine air toxics and their control/abatement.

The same methods and treatment of the data will also be applied in a new airborne instrument. Although plume tracking from urban sources and settings would be an interesting application of the new instrument, its primary objective is the measurement of aerosol optical properties that influence the Earth's radiation balance. A short, highly simplified explanation of the extant science questions will be presented and the expected data products from our new instrument will be highlighted. In this case, the transformation of the PM, mapped by the aerosol single scattering albedo, will include the influence of clouds, an area of great current interest and scientific inquiry.