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Abstract: Particulate carbon is routinely measured in the IMPROVE (interagency monitoring of protected visual environments) program by analysis of samples collected on quartz filters. The analysis is performed at Desert Research Institute using the thermal optical reflectance method. Measurements of Si, Al, Ca, Ti, and Fe by X-ray fluorescence at Crocker Nuclear Laboratory are used by IMPROVE to calculate a SOIL parameter by weighting these elements to account for the oxygen and minor elements that are not measured. It is desirable to have alternative methods to measure both carbon and oxygen for data validation purposes. We have tested a method to measure carbon and oxygen concentrations from air samples deposited on PTFE membrane filters using the backscattered proton spectrum. The measurements were performed with a 4.5MeV proton beam from the cyclotron of the Crocker Nuclear Laboratory during routine IMPROVE measurements of hydrogen by proton elastic scattering analysis. A surface barrier detector at 155° below the plane of the beam was employed in a Cornell geometry setup to measure the proton spectrum. We will discuss a consistent method to estimate the carbon from the PTFE (CF2) n membrane substrate that must be subtracted from the measured carbon (filter plus deposit). This method is independent of the number and arrangement of the fibers and the unknown stretching of the substrate. The measured carbon at multiple IMPROVE sites using this new method is generally slightly higher than carbon measured using thermal optical reflectance. The sum of all elements, including the oxygen and carbon determined by proton backscattering, compares somewhat better to gravimetric mass than the same sum using carbon by TOR instead of backscattering. [Copyright &y& Elsevier]