Ultraviolet refractive index values of organic aerosol extracted from deciduous forestry, urban and marine environments

Abstract

The refractive index values of atmospheric aerosols are required to address the large uncertainties in the magnitude of atmospheric radiative forcing and measurements of the refractive index dispersion with wavelength of particulate matter sampled from the atmosphere are rare over ultraviolet wavelengths. An ultraviolet-optimized spectroscopic system illuminates optically-trapped single particles from a range of tropospheric environments to determine the particle's optical properties. Aerosol from remote marine, polluted urban, and forestry environments is collected on quartz filters, and the organic fraction is extracted and nebulized to form micron-sized spherical particles. The radius and the real component of refractive index dispersion with wavelength of the optically trapped particles are determined to a precision of 0.001 μm and 0.002 respectively over a near-ultraviolet-visible wavelength range of 0.320–0.480 μm. Remote marine aerosol is observed to have the lowest refractive index (n = 1.442 (λ = 0.350 μm)), with above-canopy rural forestry aerosol (n = 1.462–1.481 (λ = 0.350 μm)) and polluted urban aerosol (n = 1.444–1.485 (λ = 0.350 μm)) showing similar refractive index dispersions with wavelength. In-canopy rural forestry aerosol is observed to have the highest refractive index value (n = 1.508 (λ = 0.350 μm)). The study presents the first single particle measurements of the dispersion of refractive index with wavelength of atmospheric aerosol samples below wavelengths of 0.350 μm. The Cauchy dispersion equation, commonly used to describe the visible refractive index variation of aerosol particles, is demonstrated to extend to ultraviolet wavelengths below 0.350 μm for the urban, forestry, and atmospheric aerosol water-insoluble extracts from these environments. A 1D radiative-transfer calculation of the difference in top-of-the-atmosphere albedo between atmospheric core–shell mineral aerosol with and without films of this material demonstrates the importance of organic films forming on mineral aerosol.