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Unformatted text preview: Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 1 LECTURE 6 - RAINWATER CHEMISTRY AND THE SULFUR CYCLE Rainwater Chemistry Most rainwater is not pure, although it is initially the product of natural distillation. Rainwater gathers impurities in several processes. It nucleates on aerosol particles many of which dissolve in the water thus altering the chemistry. As rain begins to form, it interacts with various substances, dissolving some of them. As we have seen, the condensation and subsequent interaction processes are called rainout . The rainout process produces a near constant chemistry in a given area at a given time. The other process that affects rainwater chemistry is washout . As the rain falls, it carries many particles to the ground with it. Most washout occurs early in the rainstorm. Washout affects the chemistry of the rainwater and is usually the dominant process at the beginning of the storm. As the storm continues, the air becomes cleaner and washout affects the chemistry of the rainwater less and less. Junge (1963) gives one set of data for a study in Frankfurt am Main, Germany in 1956-57. The data is summarized in Table 6-1. Lect06, slide 2 here While the data in this table represent only one place at one time, they do give us an idea of how much material can be removed from the air by precipitation. Washout removes ions derived from marine aerosols (Na + , Mg 2+ , and Cl- ) near the coast. Terrestrial ions (K + , Ca 2+ , NO 3- ) are quickly removed by washout from the lower atmosphere where they concentrate near the ground (Horne, 1978, 376). The form of the precipitation also matters. Neumann et al . (1959) found that the amount of organic matter removed by rainfall over Sweden was nearly twice as much as that removed by snowfall. Rainwater is responsible for removing most radioactive substances from the air. Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 2 CONTAMINANT % DECREASE AFTER RAINFALL Aerosols NH 4 + 30 NO 3- 73 SO 4 2- 42 Gases NH 3 49 NO 2 24 SO 2 35 Table 6-1 Three studies showed that precipitation removed 80-90% of the radioactivity in the air (Collins and Holden, 1958; Martell, 1959; and Small, 1960). Pollution can increase the number of sub-micrometer sized particles in the atmosphere. Since each cloud droplet nucleates on a particle, this increases the number of droplets that form. Temperature and atmospheric motion drive cloud formation. If more particles form, they will use up the available moisture, and each droplet will have a smaller size. Clouds containing smaller particles have larger droplet surface area than clouds with big particles, for the same amount of condensed water (Toon, 2000). This increases the reflectivity of the cloud formed from pollution versus natural clouds. Figure 6-1 shows the affect....
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This note was uploaded on 05/04/2011 for the course GLY 6246 taught by Professor Warburton during the Spring '11 term at FAU.
- Spring '11