Hygroscopic Properties of Aerosols from Open-Air Burning and Controlled Combustion of Biomass
Abstract: The major uncertainty in predicting the influence of human activities on global climate change is the effect of aerosols. Two physical properties of aerosols largely controlling their influence on climate are the particle number size distribution and the interaction of the particles with the surrounding water vapor. The work presented in this thesis provides new information on the interaction between atmospheric water vapor and sub-micrometer aerosol particles resulting from the combustion of biomass. The investigations extend from smoldering, slash and burn fires, to almost complete combustion. The ability of particles to interact with water vapor determines the ambient size of the particles and consequently their ability to scatter light, as well as their ability to act as condensation nuclei for cloud droplets. Their interaction with water vapor is also important regarding the influence of aerosols on human health. The dry number size distributions of the aerosols are presented and discussed, as are particle mixing status and morphology. The instruments used include a hygroscopic differential mobility analyzer (H-TDMA) for measuring the particle water uptake at subsaturation and determining mixing status, a differential mobility particle sizer (DMPS) for measuring dry number size distributions, and a cloud condensation nuclei (CCN) counter for measuring CCN concentrations. Several experiments were performed in order to study the physical properties of aerosols resulting from various kinds of biomass combustion. The results from these studies are presented together with some modeling work. Two of the experiments were performed in the Amazon basin in Brazil. In these studies the aerosol properties of the regional haze resulting from slashing and burning were studied ? both the fresh and aged biomass-burning aerosol ? and compared to the wet-season Amazonian background aerosol. The interaction between water vapor and aerosol particles containing inorganic and organic mixtures was studied in the laboratory, and water uptake was linked to chemical composition. Finally, the characteristics of particle emissions from two commercial moving-grate boilers in Sweden, operating on forest residues, were studied. The particles ranged from nearly hydrophobic to very hygroscopic, with hygroscopic diameter growth factors ranging between ~1.1 for the smoldering fires and ~ 1.8 for controlled combustion (at 90% RH). This is explained by the fact that the chemical composition of the aerosol particles is highly dependent on the combustion conditions. The components formed during complete combustion (mainly inorganic salts) and incomplete combustion (dominated by organic compounds) exhibit very different hygroscopic behavior. A model for the prediction of the CCN concentration as a function of water vapor supersaturation was developed using number size distributions and the particle diameter growth at 90% RH as input data. The model proved to reproduce the measured CCN concentrations well. The CCN spectra for the various air masses studied in the Amazon basin were parameterized and can be used for input to, and validation of, models on various temporal and spatial scales, incorporating the description of cloud formation processes. Furthermore, the Zdanovskii-Stokes-Robinson mixing rule was successfully used to predict the hygroscopic diameter growth of mixtures containing inorganic and organic compounds.
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