Response to ?Comment on Measurement of Aerosol Absorption Coefficient from Teflon Filters Using Integrating Plate and Integrating Sphere Techniques by D. Campbell, S. Copeland, and T. Cahill,? by Antony Clarke, John Ogren ? Robert Charlson

We strongly disagree with Clarke et al. on their conclusion that using a fibrous filter substrate, rather than Nuclepore, will necessarily result in gross overestimation of the aerosol absorption coefficient, ba/ We agree on many other points of their comment, however, including the inadequate level of validation provided for the LIPM technique in the recent paper. We welcome this opportunity to provide some additional information that supports the use of stretched Teflon membrane filters (Gelman Teflo™) in a variant of the IPM. As pointed out in both the original paper and the comment, measuring the absorption of particles deposited on a filter as the logarithm of the transmittance relative to the transmittance of the blank filter, while acting as the foundation for an IPM measurement of bap ʹ does not by itself necessarily allow accurate calculation of the aerosol absorption coefficient, bapʹ This is true regardless of whether the measurement is performed with the IPM apparatus or a more absolute method such as the integrating sphere. The integrating sphere does, however, provide the absorbance of the sample, filter plus particles, through reflectance and transmission values, without the need to be normalized. The device is calibrated using NIST traceable standards provided by the manufacturer. Using the calibration procedure we found that the sum of reflectance, I Rʹ and transmittance, ITʹ for a clean Teflo filter is equal to 0.994 ± 0.009. It is in the conversion of the absorption measurement to bap that problems can arise, including but not limited to the altered scattering properties of a collection of particles when they are condensed onto/into a surface/matrix. The conversion of filtrate absorbance to ba is not simple and requires experimentaf validation to be accepted because no quantitative theoretical treatments of scattering in such complex systems are currently possible. Simplified cases utilizing carefully controlled concentrations of laboratory-generated aerosols with "known optical properties," such as employed by Lin, et al. (1973),