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Weizmann Institute of Science (2014)

Aerosol exchange processes between the atmosphere and a semi-arid pine forest : laboratory and field measurements

Avi Lavi

Titre : Aerosol exchange processes between the atmosphere and a semi-arid pine forest : laboratory and field measurements

Auteur : Avi Lavi

Etablissement de soutenance : Weizmann Institute of Science

Grade : Doctor of Philosophy (PhD) 2014

Résumé
Two-way atmosphere-ecosystem interactions through accumulation mode aerosols can have important consequences to ecosystem health as well as to the formation of atmospheric particles that have significant climatic and air quality effects. Despite the large global distribution of semi-arid forests and the projections for their extension due to climatic changes, measurements and knowledge on aerosol exchange processes in these areas are scarce. In this thesis we conducted first aerosol flux measurements in a semi-arid forest (Yatir forest, a pine forest in southern Israel). Both downward and upward fluxes were observed, with increasing upward flux at the larger aerosol size bins. The upward fluxes were not associated with a local particle source, but were rather associated with the wind direction and with the micrometeorological conditions. To test the hypothesis that the apparent fluxes were not caused by particle evaporation, as was previously suggested in the literature, we studied the thermal properties of common organic aerosol components, namely alkyl amminium salts that from in aged biogenic aerosols, and were recently identified in many forested environments. Specifically we studied the physiochemical properties of alkylaminium sulfate, monocarboxylate and dicarboxylate salts and assessed their possible contributions to the observed upward fluxes in Yatir forest by gas/particle interconversion. Our measurements indicate that the saturation vapor pressure of alkylaminium sulfate salts are orders of magnitude lower than that of ammonium sulfate (AS) and estimated that the saturation vapor pressure of monocarboxylate salts is comparable to that of AS . Our measurements suggest that these salts are not purely ionic compounds but a mixture of ionic and non-ionic precursors and behave as poor ionic liquids at room temperature. The vapor pressure of dicarboxylate salts is slightly lower than that of their dicarboxylic acid precursors. Using the calculated vapor pressure and enthalpy of vaporization values, we suggest that these salts are unlikely to result in the observed apparent upward fluxs in Yatir forest, and hence suggest that the hilly terrain and patchy fetch around Yatir forest result in the apparent upward flux.

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