New approaches for the sensitive and accurate quantification of aerosol optical properties are needed to improve the current understanding of the unique physical chemistry of airborne particles and to explore their roles in fields as diverse as chemical manufacturing, healthcare, and atmospheric science. We have pioneered the use of cavity ring-down spectroscopy (CRDS), with concurrent angularly resolved elastic light scattering measurements, to interrogate the optical properties of single aerosol particles levitated in optical and electrodynamic traps. This approach enables the robust quantification of optical properties such as extinction cross sections for individual particles of known size. Our measurements can now distinguish the scattering and absorption contributions to the overall light extinction, from which the real and imaginary components of the complex refractive indices can be retrieved and linked to chemical composition. In this Feature Article, we show that this innovative measurement platform enables accurate and precise optical measurements for spherical and nonspherical particles, whether nonabsorbing or absorbing at the CRDS probe wavelength. We discuss the current limitations of our approach and the key challenges in physical and atmospheric chemistry that can now be addressed by CRDS measurements for single aerosol particles levitated in controlled environments. 13 , 14 Over the remaining 7–10 days of the particle’s lifetime, the coatings undergo chemical and physical transformations, caused by photobleaching and oxidative bleaching of molecular chromophores and gas-to-particle partitioning of further semivolatile or low-volatility organic species. These processes change the intrinsic optical properties of the coating material as well as the magnitude of the lensing effect. 8 For different types of aerosol particles undergoing such transformations, accurate characterizations of the changing optical properties and the associated kinetics are crucial for an improved understanding of how aerosols impact atmospheric composition and climate. Recent demonstrations of accelerated reactions in micron-scale droplets offer new avenues for sustainable chemical synthesis, but the reasons for significantly enhanced reaction rates remain the subject of intensive investigation. 16 − 18 A newfound ability to monitoring changes in chemical composition of micron-scale single aerosol particles using absorption spectroscopy may help to unravel these causes. In healthcare applications, the optical properties of pathogen-containing droplets may influence pathogen survival rates upon droplet exposure to UV germicidal radiation. Whether in the natural environment, an aerosol reaction chamber that simulates atmospheric processing, 21 or droplets in exhaled breath, the aerosol particles present are typically heterogeneous in size, shape, chemical composition, and constituent phases. Although aqueous aerosol microdroplets are spherical, they may contain inclusions or transform to nonspherical particles upon drying and efflorescence. Moreover, solid particles adopt a wide range of morphologies from crystalline to nanostructured or amorphous structures with shapes that depend on chemical composition and the rate of particle drying. 35 − 42 CRDS measures absolute particle scattering and absorption losses directly, and a sequence of such measurements for a trapped particle evolving in size can be analyzed to extract the corresponding real and imaginary components of the refractive index, both of which are indicative of chemical composition and are parameters required for atmospheric radiative transfer calculations. Such an analysis involves comparing the sequence of CRDS-measured extinctions to predictions from an optical model that best represents the interaction of the particles with light (such as Lorenz–Mie theory for particles known to be homogeneous spheres). 2 Optical Properties of Aerosol Particles Aerosol particles in the atmosphere can reflect, refract, diffract, and absorb radiation, thereby influencing the radiative forcing of the atmosphere by light scattering and absorption. The degree to which they scatter and absorb different wavelengths of light depends on the shape, size, composition, and structure of the particles. Of particular interest here, the chemical composition is directly connected to the complex refractive index ( m ), which has real ( n ) and imaginary ( k ) parts, m = n + ik . 36 − 40 Measurements of the type described above provided a comprehensive set of values for the real components of the refractive index at CRDS wavelengths of 405 and 532 nm of aqueous droplets containing various inorganic salts found in atmospheric aerosol particles. 40 In addition, the wavelength dispersions of these refractive indices were quantified by refractive index retrievals from phase function measurements with laser excitation at 633 and 473 nm. Regardless of the above limitations, the accessible size and absorption windows offer scope for wide-ranging measurements on particles of atmospheric importance, such as brown carbon. Excluding mineral dusts and cloud droplets, atmospheric aerosols are overwhelmingly measured to have particles diameters below 1 μm. 83 Such sizes are compatible with our demonstrated cross-section measurements on particles with radii as small as ∼300 nm. 4 Summary and Outlook In this Feature Article, we have underscored the unique ability of single-particle CRDS to provide extinction cross-section measurements to a level of sensitivity and accuracy that enables the robust retrieval of aerosol physicochemical information, including complex refractive index and particle morphology. This technique has been applied to different particle types found in the atmosphere, including spherical and nonspherical particles with a range of absorption strengths. It now offers opportunities to address several outstanding challenges in atmospheric chemistry.