Raman lidar observations of aged Siberian and Canadian forest fire smoke in the free troposphere over Germany in 2003: Microphysical particle characterization

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Abstract

[1] Dual-wavelength Raman lidar observations were regularly carried out at Leipzig (51.3°N, 12.4°E) from May to August 2003. The measurements showed that particle backscatter and extinction coefficients in the free troposphere were higher compared to values in 2000–2002. Backward dispersion modeling indicates that intense forest fires that occurred in Siberia and Canada in spring/summer 2003 were the main cause of these free tropospheric haze layers. Measurements on 3 days were selected for an optical and microphysical particle characterization of these well-aged particle plumes. Particle lidar ratios measured at 532 nm wavelength were higher than at 355 nm. This property seems to be a characteristic feature of aged biomass-burning particles observed over central Germany. Mean particle Ångström exponents calculated for the wavelength range from 355 to 532 nm varied from 0 to 1.3. Particle effective radii varied between 0.24 and 0.41 μm. Pollution advected from North America on 25 August 2003, in contrast, was characterized by considerably smaller particles. Mean effective radii were ≤0.2 μm, and Ångström exponents were 1.8–2.1. Lidar ratios in that case were lower at 532 nm compared to those at 355 nm. Such signatures are characteristic for anthropogenic particles. At the moment, however, it cannot be completely ruled out that extremely hot forest fires in western areas of Canada generated comparably small particles. Except for this specific case the forest fire particles were considerably larger than what is usually reported from in situ observations of biomass-burning smoke. Possible explanations for this difference could be the kind of burning process, which could generate much larger particles in the source region, condensation of organic vapors on existing particles, and coagulation processes during the long transport time of more than a week. Relative humidity measured in these layers was very low. Hygroscopic growth of the particles therefore seemed to have little influence on the size of the particles. The forest fire smoke consisted of moderately absorbing material. Real parts of the complex refractive index of the particles were mostly <1.5, and imaginary parts were <0.01i. Single-scattering albedo in all cases varied between 0.9 and 0.98 at 532 nm.

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