Measurements of the imaginary component of the refractive index of weakly absorbing single aerosol particles

The interaction of atmospheric aerosols with radiation remains a significant source of uncertainty in modelling radiative forcing. Laboratory measurements of the microphysical properties of atmospherically relevant particles is one approach to reduce this uncertainty. We report a new method to investigate light absorption by a single aerosol particle, inferring changes in the imaginary part of the refractive index with change in environmental conditions (e.g. relative humidity) and inferring the size dependence of the optical extinction cross-section. More specifically, we present measurements of the response of single aerosol particles to near infrared (NIR) laser induced heating at a wavelength 1520 nm. Particles were composed of aqueous NaCl or (NH4)2SO4 and were studied over ranges in relative humidity (40 - 85%), particle radius (1 – 2.2 µm) and NIR laser power. The ensuing size change and real component of the refractive index were extracted from measurements of the angular variation in elastically scattered light. From the heating-induced size change at varying NIR beam intensities, we retrieved the change in the imaginary component of the refractive index. In addition, cavity ring-down spectroscopy measurements monitored the change in extinction cross-section with modulation of the heating
laser power.