On the Discrepancy between Modeled and Measured Mass Absorption Cross Sections of Light Absorbing Carbon Aerosols

Abstract

Recent modeling studies based on the Rayleigh-Debye-Gans (RDG) approximation have revealed a discrepancy between modeled and measured mass absorption cross sections (MAC) for atmospheric light absorbing carbon (LAC) aerosols. One plausible explanation is that this discrepancy is due to errors introduced by neglecting electromagnetic interactions among monomers in LAC aggregates within the RDG approximation. Here we compute MAC by use of numerically exact solutions to Maxwell's equations and investigate the sensitivity of the results to a variation in the aggregates’ physical properties and refractive index. The results do confirm that approximate methods can introduce large errors in the results for the optical properties. However, these errors alone cannot explain the discrepancy between measured and modeled values of MAC. An agreement between observations and theoretical results can only be attained when assuming a fairly high value of the real and imaginary parts of the refractive index along the void-fraction curve and a mass density not exceeding 1.5–1.7 g/cm³.

The author thanks D. Mackowski for providing his program for aggregated particle generation. D. Mackowski, M. Mishchenko, Y. Okada, Y. Xu, and B. Gustafson are acknowledged for making their electromagnetic scattering programs publicly available. Two anonymous reviewers are gratefully acknowledged for helpful suggestions. This work was supported by the Swedish Research Council under contract number 80438701.