Direct Spectroscopic Quantification of the Absorption and Scattering Properties for Single Aerosol Particles

Abstract

Understanding the optical properties of micrometer-scale light-absorbing aerosol particles is of paramount importance in addressing key challenges in atmospheric and physical chemistry. For example, the absorption of solar radiation by atmospheric aerosols represents one of the largest uncertainties in climate models. Moreover, reaction acceleration within the unique environments of aerosol droplets cannot be replicated in bulk solutions. The causes of these reaction rate enhancements remain controversial, but ultrasensitive spectroscopic measurements of evolving aerosol optical properties should provide new insights. We demonstrate a new approach using cavity ring-down spectroscopy that allows the first direct spectroscopic quantification of the continuously evolving absorption and scattering cross sections for single, levitated, micrometer-scale particles as their size and chromophore concentration change. For two-component droplets composed of nigrosin and 1,2,6-hexanetriol, the unprecedented sensitivity of our measurements reveals the evolving real and imaginary components of the refractive index caused by changes in concentration as 1,2,6-hexanetriol slowly evaporates.

Figure 1

Schematic diagram of the single-particle CRDS approach for spectroscopic interrogation at a wavelength of 405 nm for single aerosol particles levitated in an LEQ trap, with angularly resolved elastic light scattering from the 532 nm laser to retrieve the particle size. AOM is an acousto-optic modulator, PZT is a piezo-electric actuator, PD is a photodiode, and I is the elastic light scattering intensity.

Figure 2

Measured (black points) variation in σ_ext with the particle radius averaged to a 1 Hz sampling rate for (a) a nonabsorbing HT droplet and for absorbing HT-nigrosin droplets with (b) w_N = 0.002 and (c) w_N = 0.004. The best-fit Mie theory distributions are overlaid (red lines). Insets show expanded regions of the plots to highlight changes in the whispering gallery mode structure with increasing nigrosin absorption.

Figure 3

Retrieved (solid lines) and predicted (dashed lines) variations of the n and k components of the complex refractive indices with particle radius for pure HT droplets (w_N = 0.000; top) and for HT-nigrosin droplets with initial w_N values (corresponding to the solution composition loaded into our droplet dispensers) of 0.002 (middle) and 0.004 (bottom). The shaded envelopes represent the uncertainties in the predicted n and k values as a function of particle size. The three solid lines of different colors in each panel correspond to the retrieved refractive indices for three separate repeat droplets that were studied for each initial droplet w_N value. The data series extends to different lower values for droplet radii because droplets fell out of the trap stochastically upon evaporation to radii of <900 nm.