环境化学课件：光化学反应基础

1、波长（波长（nm）能量（能量（kJ/mol）光的区域光的区域200300597.2398.4紫外光紫外光400700298.9170.8可见光可见光2000500059.723.9红外线红外线不同波长光的能量不同波长光的能量 高能光，能引起光化高能光，能引起光化学反应（光离解）学反应（光离解） 低能光，不能引起低能光，不能引起光化学反应光化学反应 OXIDIZING NATURE OF THE ATMOSPHERE Earths atmosphere is oxidizing due to presence of O2SO2 - SO42-; CH4 - CO; CO - CO2; NO2 -

2、 HNO3 Radicals are oxidizing agents in the atmosphere OH is the cleansing agent of the atmosphere Key to understanding atmospheric oxidant chemistry Understand radical cycling but radical cycle is intimately connected to oxidant chemistry of other trace compounds including O3 Start with O3O3 is impo

3、rtant from chemical, climate, and health perspectivesATMOSPHERIC O3 A BRIEF HISTORY 1840: Ozone discovered in 1840 by C. F. Schnbein thought it was made up of oxygen and hydrogen 1848: Systematic measurement attempts curiousity, growing interest in env., health effects, economy of nature 1861: Odlin

4、g suggested that ozone was O3 1930: Chemical mechanism for O3 layer postulated 1952: O3 identified as component of chemical smogChristian Frederich SchnbeinOZONE AND HEALTH 90% of O3 is in the stratosphere; O3 layer with max 9 ppm Absorption of = 200-320 nm (UV-B and UV-C) by strat. O3Source: Strato

5、spheric Ozone, NASA/GSFC Local heating of the stratosphere due to UV absorption by O3 Tropospheric O3 is also an important greenhouse gasSource: Stratospheric Ozone, NASA/GSFCSource: Environmental Science, Cunningham, P. W. and B. W. Saigo, 2001O3O(1D)O(3P)2OHSolar radiation, wavelength 290-320 nmH2

6、OO2, N2O2 O3 is the primary source of tropospheric OH OH is atmospheric detergent LATITUDINAL & TEMPORAL VARIATION OF TOTAL O3 Total O3 in range of 300-400 DU Patterns due to stratospheric circulation Low total O3 at high southern lat in southern spring due to ozone holeDobson units 1 DU = 2.69

7、x 1016 molecules O3 cm-2Source: Stratospheric Ozone, NASA/GSFCSTRATOSPHERIC O3 CHEMISTRYTHE CHAPMAN MECHANISM FOR STRATOSPHERIC O3 Cycling between O, O2, and O3 Source: Stratospheric Ozone, NASA/GSFCMISSING CHEMISTRY IN CHAPMAN MECHANISM Global O3 production rate = 5 times destruction rate Imbalance

8、 suggests overest. of prodn. or underest. of loss O3 production well constrained by good spectroscopic data Implies missing chemical sinks for Ox Reactions of radicals with O and/or O3 But radicals will also be consumed by reaction measuredcalculatedSource: Stratospheric Ozone, NASA/GSFCCATALYTIC OX

9、 DESTRUCTION IN THE STRATOSPHERE Radical chain reactions X + O3 XO + O2 XO + O X + O2 Net: O + O3 2O2 X in the stratosphere H, OH, NO, Cl HOx, NOx, and Clx HOx = H + OH + HO2 NOx = NO + NO2 Clx = Cl + ClO Reservoirs tie up active radicals e.g. ClO + NO2 ClONO2 Stratospheric Clx precursorsSource: Str

10、atospheric Ozone, NASA/GSFCColumn O3 (DU) ANTARCTIC TOTAL OZONE DECREASE Depletion of total column O3 starting in mid- to late-70s during SH spring Gas-phase chemistry predicted smaller decreases & not over AntarcticaO3Source: Stratospheric Ozone, NASA/GSFCSource: Farmann et al., Nature, v. 315,

11、 May 1985 ALTITUDE DEPENDENCE OF ANTARCTIC O3 DECREASE Strong depletion between 12 and 20 km Gas phase chemistry predicted decrease near 40 kmSource: Stratospheric Ozone, NASA/GSFC TEMPORAL DEPENDENCE OF ANTARCTIC O3 DECREASE Depletion begins around Sep 1. & minimum is reached around Oct 1Source

12、: NOAA/CMDL REACTIONS ON POLAR STRATOSPHERIC CLOUDS Conversion of inactive Cl to active Cl and removal of NOxSource: Stratospheric Ozone, NASA/GSFCROLE OF METEOROLOGY Low temps.PSC formation release of active Cl and removal of NOxStrong vortexIsolates air from mid-lats. prevents high O3 air influxFi

13、gure shows strong polar vortex旋涡(as shown by size of wind vectors) & low polar temps. (as shown by colors) at various altitudes in the southern hemisphere stratosphereSource: Stratospheric Ozone, NASA/GSFCNORTHERN vs SOUTHERN HEMISPHERE O3 TRENDS Vortex not as strong and temps. not as low in NHS

14、ource: Stratospheric Ozone, NASA/GSFCPROJECTED CHANGES IN STRATOSPHERIC Clx Montreal Protocol and subsequent amendments will have signifcant impacts on projected Clx loading of stratosphere(ppb)Source: Stratospheric Ozone, NASA/GSFC WMO 1998 Scientific Assessment of Ozone Depletion Ozone depletion i

15、n 2050 would be at least 50% at midlatitudes in the Northern Hemisphere and 70% at midlatitudes in the Southern Hemisphere, about 10 times larger than today Surface UV-B radiation in 2050 would at least double at midlatitudes in the Northern Hemisphere and quadruple at midlatitudes in the Southern H

16、emisphere compared with an unperturbed atmosphere. This compares to the current increases of 5% and 8% in the Northern and Southern Hemispheres, respectively, since 1980 ESTIMATED IMPACTS OF Clx CONTROLSTROPOSPHERIC O3 CHEMISTRYSource: EPA Tropospheric O3 generally less than 100 ppb away from urban

17、areasTROPOSPHERIC O3Source: Wang et al., 1998O2O(3P)Solar radiation, ( 240 nm)O3O2Solar radiation ( CO2 + O3 Catalytic role of NOx (NO + NO2) in recycling HO2 to OH Coupling between OH and HO2 (HOx) via NOCO OXIDATION CYCLE O3 PRODUCTIONCO2O3NO or O3 OHHO2COO2Net: CO + O3 - CO2 + O2 Chemical O3 dest

18、ruction Coupling between OH and HO2 (HOx) via O3CO OXIDATION CYCLE O3 DESTRUCTIONCO2O32O21. O3 + hv O2 + O(1D)2. O(1D) + M O + M3. H2O + O(1D) 2OH4. RH + OH RO2 + H2O5. RO2 + NO RO + NO26. RO + O2 RCHO + HO27. HO2 + NO OH + NO28. HO2 + HO2 H2O2 + O29. OH + NO2 + M HNO3 + MSCHEMATIC OF HYDROCARBON CH

19、EMISRYO2Net rxns 1-7:RH + 4O2 RCHO + 2O3 + H2OSource: Introduction to Atmospheric Chemistry, Jacob, D. J., 1999can produce more O3NOx- AND HYDROCARBON-LIMITED REGIMESNOx limitedHydrocarbon limitedComplications:Natural emissions of hydrocarbons are importantTransport of pollutants into and out of regionSource: Introduction to Atmospheric Chemistry, Jacob, D. J., 1999ISSUES IN O3 POLLUTION CONTROL1998 MEASURED SURFACE OZONE CONCENTRATIONS2nd highest