The scattering and absorption of light by aerosol particles: Measurement and modelling.

摘要

In July, 2002, atmospheric aerosol measurements were conducted over the northeast Pacific Ocean. The size distribution and size-segregated chemical composition of the aerosols were measured, along with scattering and backscattering coefficients at three wavelengths, and the absorption coefficient at one wavelength. Using Mie theory to calculate the aerosol scattering and absorption coefficients from the size distribution and chemical measurements, closure with the measured scattering coefficients is not attained. Discrepancies exist between the calculated and measured scattering and backscattering coefficients because the nephelometer measures scattering only between 7°and 170°, largely missing the prevalent forward-scatter lobe from coarse aerosols. During this study the majority of the total scattering and backscattering in the marine boundary layer of this region was from coarse particles consisting mostly of sea salt. To examine scattering at higher altitudes in the marine environment, another closure study was carried out based upon measurements taken during five flights in October, 2003, over the waters surrounding Nova Scotia. The same quantities were measured, with the exception of the absorption coefficient. In this case closure is attained for more than 70% of the total scattering measurements, but is not attained for the backscattering coefficients. Coarse particles are found to account for roughly half of the total scattering and 70% of the backscattering for altitudes up to ∼1000 m. The contribution of coarse particles calculated from on-board measurements is underestimated due to substantial inlet losses for particles larger than 2 mum. The final part of the thesis involves the modelling of absorption by black carbon (BC) particles within cloud droplets. Based upon the volume fraction of activated BC, the effective radius of the cloud, and the wavelength band, the single-scattering albedo of the cloud is calculated. The Maxwell-Garnett mixing rule is used to calculate the index of refraction of the composite droplet. These results are compiled in a look-up table of representative values, which is tested using a 1-D radiation model.