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6246lect05_S10

6246lect05_S10 - Advanced Environmental Geochemistry GLY...

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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 1 LECTURE 5 - Rainwater and Aerosols Introduction The long cycle component of the hydrologic cycle involves seawater evaporation and subsequent precipitation of part of this water over the continents. Besides transporting water to the continents, rainwater also transports other substances dissolved in the rainwater. Aerosol transport also moves substances from one place to another, including from the oceans onto the surface of the land. To evaluate the extent of this atmospheric transport we must look at the chemistry of rainwater and aerosols. This chemistry is also important when we look at the weathering of rocks. The chemistry of rainwater affects weathering processes. Andreae and Crutzen (1997) state, "Atmospheric aerosols play important roles in climate and atmospheric chemistry. They scatter sunlight, provide condensation nuclei for cloud droplets, and participate in heterogeneous chemical reactions." In addition, if we are to evaluate weathering contributions to the geochemical cycle, we must be able to correct for inputs from rainwater. The natural chemistry of rainwater also affects the way in which atmospheric pollutants are cleansed from the atmosphere. Formation of atmospheric precipitates The terms vapor pressure, relative humidity, and dew point were discussed in Lecture 3. We need to add the term absolute humidity to this list. Absolute humidity is the mass of water vapor in the atmosphere per unit volume. The units are often expressed as grams/meter 3 . The absolute humidity varies over time in any given place. It also varies in a systematic way with altitude. As an air mass rises, it experiences less pressure and it expands. This leads to a condition known as adiabatic cooling. (Adiabatic refers to a change in which the total heat content is constant). The absolute humidity stays the same, but the relative humidity increases as the temperature drops. This process effectively ends at the tropopause, where the absolute humidity is reduced near to zero. Above the tropopause, temperature increases and the relative humidity drops.
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