Microwave Scattering Properties of Laboratory‐Produced Pyrometeors and Carbonaceous Aerosols

Abstract

Weather radar has become a vital tool for monitoring wildfire plume evolution and estimating emission source strengths. However, its quantitative application remains constrained by a limited understanding of the microwave scattering properties of wildfire smoke particles. This study characterizes the scattering behavior of three key particulate types released during wildfires: black carbon (BC), brown carbon (BrC), and coarse‐mode pyrometeors. By integrating experimentally derived refractive indices into discrete dipole approximation and Mie theory, we quantitatively analyzed the microwave scattering characteristics of these particles. The calculation accounts for the horizontal orientation of pyrometeors, random orientation of BC, and orientation‐independent scattering of spherical BrC. Results indicate that at microwave frequencies, BC exhibits a higher real part of the refractive index, while BrC displays a relatively larger imaginary part, indicating strong absorption. In contrast, although pyrometeors possess lower refractive indices than carbonaceous aerosols, they exhibit the highest single scattering albedo due to their significantly larger size parameters. Carbonaceous aerosols outnumber pyrometeors by ∼4 orders of magnitude. Simulations confirm that this numerical advantage is offset by the massive ∼11‐order‐of‐magnitude advantage in the pyrometeor backscattering cross‐section. Consequently, pyrometeors are the dominant source of radar reflectivity. While BC and BrC exhibit near‐zero differential reflectivity (ZDR), pyrometeors display significantly elevated ZDR. Furthermore, the low‐ZDR signature observed in upper plumes is consistent with the abundance of the lofted carbonaceous aerosols. These results provide essential dielectric and scattering parameters for improving wildfire plume microphysical retrievals.