Chuong Hoa Hoc khi quyen (1) potx

1 1 Chapter 9 THE ATMOSPHERE AND ATMOSPHERIC CHEMISTRY Environmental Chemistry, 9th Edition Stanley E. Manahan Taylor and Francis/CRC Press 2010 9.1 The Atmosphere and Atmospheric Chemistry The atmosphere consists of the following (on a dry basis): • 78.1% N2 • 21.0% O2 • 0.9% Ar • 0.04% CO2 • Low levels of noble gas helium, neon, krypton, xenon • Trace gases (see Table 9.1) Most of the mass of the atmosphere is very close to Earth’s surface relative to Earth’s diameter • If Earth were a classroom globe, virtually all air would be in a layer the thickness of the coat of varnish on the globe! 2 3 Photochemistry and Some Important Terms Photochemical reactions occur in the atmosphere when molecules absorb energy in the form of photons • Mostly in ultraviolet region of spectrum • E = hν A chemical species in an excited (energized) state is designated with an asterisk, * The photochemical reaction of stratospheric ozone: • O3+ hν(λ< 420nm) → O*+ O2 •The O3 absorbs a photon of energy hν • The O3 undergoes photodissociation • The oxygen atom product is excited denoted O* Free radicals with unpaired electrons shown with a dot, • •H2O2+ hν→ HO• + HO• Energy-absorbing third body, M, usually N2 molecule •O + O2 + M → O3+ M 4 Gaseous Oxides in the Atmosphere Low and variable levels of C, N, and S oxides • Pollutants at elevated levels • Discussed in Chapters 11 and 14 Atmospheric methane • From anoxic bacteria and underground sources • Significant greenhouse gas • Influences levels of hydroxyl radical (HO•), ozone, stratospheric water vapor Hydrocarbons and photochemical smog • Hydrocarbons required for photochemical smog formation (see Chapters 13 and 14) Particulate matter (see Chapter 10) Primary pollutants emitted directly Secondary pollutants formed from reactions of primary pollutants 5 9.2 Importance of the Atmosphere Protective function • Filters out harmful radiation • Stabilizes temperature Part of hydrologic cycle Source of CO2 for plant photosynthesis Source of N for plant growth, industrial chemicals Variation of pressure and density with altitude • Pressure and density decrease rapidly with increasing altitude Figure 9.1 Variation of air pressure and temperature with altitude 6 7 Stratification of the Atmosphere • Lower atmosphere, the troposphere 8 Stratosphere and Upper Atmosphere 9 9.4 Energy Transfer in the Atmosphere Solar Flux 10 Earth’s Atmospheric Heat Balance See detail of Earth’s atmospheric heat balance in text Figure 9.4 Re-absorption of outbound infrared stabilizes atmospheric temperature