9.volatility.volatilization.10

9.volatility.volatilization.10 - Atmosphere, Volatility and...

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Unformatted text preview: Atmosphere, Volatility and Volatilization Introduction Volatility and Volatilization Important Definitions Volatilization from Surfaces Volatilization from Water (Henry's law) Summary and Conclusions I. Introduction Atmosphere Temperature, Pressure and Altitude As expected, pressure declines as altitude increases. However, temperature fluctuates with altitude due to the stratospheric ozone layer! Ozone converts UV radiation to infrared (heat). Composition and Air Pollutants Overview of Chemodynamics Revisited Partitioning to air can occur from both water and solid surfaces (soils, leaves, etc.). Processes governed by different properties (H versus P). II. Volatility and Volatilization Most important chemical dissipation route. Volatility is reflected by the vapor pressure (P) of a chemical. In the environment, volatilized chemicals behave like ideal gases thus the universal gas law applies. P is influenced by temperature and atmospheric pressure must be measured under standard conditions (25 C, 1 atm). atm Effect of Temperature on Vapor Pressure III. Important Definitions Vapor pressure (P) = the pressure exerted by the vapor of a substance on its own solid or liquid surface at equilibrium. Reported in: 1 atm = 760 torr (mmHg) 1 atm = 1.013 x 105 Pa 1 Pa = 0.0075 torr Boiling point (bp) = temperature where the vapor pressure of a liquid equals the atm atmospheric pressure (P = P ) constant at a IV. Volatilization from Surfaces Universal gas law: PV = nRT P = vapor pressure, V = gas volume, n = moles of gas, R = universal gas constant, T = temperature (K) For surfaces: P = nRT / V, measured at STP (25 C, 1 atm) Can re-write as P = RT n/V Since R = 0.082 L atm/mol and T = 298K: RT = 24.4 L atm/mol Thus, simplified to P = 24.4 n/V; n/V is in mol/L Vapor Pressure is Commonly Reported V. Volatilization from Water Subject to Henry's law = at equilibrium, concentration of a chemical in the vapor phase bears a constant relationship to its aqueous concentration. In essence influenced by both S and P. Dimensionless Henry's law a w constant: H' = C / C H' can be measured in the laboratory using the same Gas Stripper Dimensional Henry's Law Constant Dimensional Henry's law constant (H) can be calculated from the universal gas law (PV = nRT). H = vapor pressure / water solubility. Mathematically written as H = (n / V) (1 / S) Since P / RT = n / V, substitute P / RT for n / V Thus, H = (P / RT) (1 / S) Drop RT since at STP they are constant values. Thus, H = P / S (in atm mol/L or atm m3/mol) General Rules Henry's Law Constant It gives an indication of partitioning between air and water at the air-water interface. Dimensional H = P / S (atm/mol/L or atm m3/mol) a w Dimensionless H' = C / C Low values indicate low P or high S chemical favors water. High values indicate high P or low S chemical favors air. Constants, Molecular Mass and Volatility Considering neutral organics can provide a useful example. Chemicals with larger molecular masses possess lower P values. However, they also possess lower S values. Thus, to estimate partitioning Henry's values must be obtained. In general, chemicals with Remember Influence of Salinity on + Solubility Presence of dissociated Na and Cl . Increased "ionic strength" reduces ability to accommodate dissolution of organic molecules. fluence of Water Salinity on Henry's Constan The reduced S for organics in seawater increases their Henry's law constant values (think H = P /S). Henry's Constants are Commonly Reported VI. Summary and Conclusions The atmosphere consists of four general regions. It contains numerous gasses including pollutants. Chemical volatility is influenced by both P and atm P . atm When P = P , the boiling point is attained. Volatilization from surfaces is dictated by P, and can be characterized by the universal gas law. ...
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This note was uploaded on 06/29/2011 for the course GENETICS 101 taught by Professor Debrakimbrell during the Summer '09 term at UC Davis.

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9.volatility.volatilization.10 - Atmosphere, Volatility and...

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