Radiative Properties of Soot Superaggregates in the Infrared

Soot is one of the largest man-made contributors to global warming and its influence on climate has been greatly underestimated. This underestimation is due to poor understanding of the microphysical properties of soot and its parameterization in climate models and satellite retrieval algorithms. Very recently, researchers observed a new kind of soot particle “super aggregates”, emitted from large-scale forest fires globally. The morphology of these particles is described by a mass fractal dimension ≈ 2.6, mobility diameter > 1 µm, and aerodynamic diameter in the range of 0.5 - 20 µm. Given their large size, it is expected that these soot superaggregates would impact radiative forcing in the longer wavelengths (i.e. infrared (IR)) of the incoming solar spectrum. Here, we quantitatively investigate the microphysical and optical properties of soot superaggregates using controlled laboratory-scale experiments. Soot superaggregates were produced in our laboratory using a novel diffusion flame aerosol reactor operated in a negative gravity. Next, an Integrated Photoacoustic-Nephelometer (IPN) System using a 1047nm laser source was built to simultaneously measure absorption and scattering coefficient of these particles in real-time. Along with the optical properties, the total number concentration and aerodynamic size distribution were measured using a TSI Aerodynamic Particle Sizer. The mobility size distribution was also inferred from electron microscopy analysis of a statistically significant number of particles. Our results show that soot superaggregates have distinct physical and optical properties compared to conventional sub-micron soot aggregates, and could significantly impact direct radiative forcing in the atmosphere.