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1、二、光化学photochemistry反应基础,1、光子photon的能量 爱因斯坦-普朗克（Einstein-Planck）关系式： = h = hc/ (J) E= N0 h = N0 hc/ (KJ/mol) 式中hPlanck常数，6.621034J.s； 光子的频率，Hz； c光速，2.9981010cm/s； 光子的波长，cm； N0阿佛加德罗常数，6.021023/mol。,1,特制内容,随着波长的增加，光子的能量减小。,不同波长光的能量,高能光，能引起光化学反应（光离解）,低能光，不能引起光化学反应,例1计算波长为200nm紫外光的能量。,一般化学鍵的键能大于167.44KJ/
2、mol,2,特制内容,2、光化学反应原理 光化学反应：由一个原子、分子、自由基或离子吸收一个光子后所引发的反应。 初级过程： 引发：A(分子)hA* (激发态分子) 次级过程： 离解：A* C+ 与其他分子反应生成新的物种：A*+BD+ 与惰性inertia分子碰撞失活(返回基态)：A*+MA+M 发光而失活(返回基态)：A*Ah,3,特制内容,3、大气中重要吸光物质的光离解 吸光物质 高层大气：N2、O2、O3 （290nm） 低层大气：NO2、SO2等,4,特制内容,氧和氮的光离解 氧分子键能：493.8kJ/mol（240nm以下紫外光可引发）： O2h O+O 氮分子键能：939.4k
3、J/mol（127nm，仅限于臭氧层以上）： N2h N+N,当入射波长低于79.6nm (1391kJ/mol) 时，N2将电离成N2。,5,特制内容,臭氧和过氧化物的光离解 臭氧分子键能：101.2kJ/mol 220290nm 强吸收（254nm最强吸收） 290320nm 少量吸收 450700nm 微弱吸收 离解反应： O3hO2+O 烷基过氧化物在300700nm范围有微弱吸收，光离解反应： ROORhRO+RO,6,特制内容,NO2的光离解 键能：300.5 kJ/mol（300400nm吸光），光离解反应： NO2hNO+O O2+OMO3M,NO2是污染大气中最重要的吸光物质
4、,在低层大气中可以吸收全部来自太阳的紫外光和部分可见光。,O3的唯一人为来源,7,特制内容,OXIDIZING NATURE OF THE ATMOSPHERE,Earths atmosphere is oxidizing due to presence of O2 SO2 - SO42-; CH4 - CO; CO - CO2; NO2 - HNO3 Radicals are oxidizing agents in the atmosphere OH is the cleansing agent of the atmosphere Key to understanding atmospher
5、ic oxidant chemistry Understand radical cycling but radical cycle is intimately connected to oxidant chemistry of other trace compounds including O3 Start with O3 O3 is important from chemical, climate, and health perspectives,8,特制内容,ATMOSPHERIC O3 A BRIEF HISTORY,1840: Ozone discovered in 1840 by C
6、. 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: Odling suggested that ozone was O3 1930: Chemical mechanism for O3 layer postulated 1952: O3 identified as component of chem
7、ical smog,Christian Frederich Schnbein,9,特制内容,OZONE 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. O3,Source: Stratospheric Ozone, NASA/GSFC,10,特制内容,OZONE AND STRATOSPHERIC TEMPERATURE,Local heating of the stratosphere due to
8、 UV absorption by O3 Tropospheric O3 is also an important greenhouse gas,Source: Stratospheric Ozone, NASA/GSFC,Source: Environmental Science, Cunningham, P. W. and B. W. Saigo, 2001,11,特制内容,OZONE AND ATMOSPHERIC CHEMISTRY,O3 is the primary source of tropospheric OH OH is atmospheric detergent,12,特制
9、内容,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 hole,Dobson units 1 DU = 2.69 x 1016 molecules O3 cm-2,Source: Stratospheric Ozone, NASA/GSFC,13,特制内容,STRATOSPHE
10、RIC O3 CHEMISTRY,14,特制内容,THE CHAPMAN MECHANISM FOR STRATOSPHERIC O3,Cycling between O, O2, and O3,Source: Stratospheric Ozone, NASA/GSFC,15,特制内容,MISSING CHEMISTRY IN CHAPMAN MECHANISM,Global O3 production rate = 5 times destruction rate Imbalance suggests overest. of prodn. or underest. of loss O3 p
11、roduction 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,measured,calculated,Source: Stratospheric Ozone, NASA/GSFC,16,特制内容,CATALYTIC OX DESTRUCTION IN THE STRATOSPHERE,Radical ch
12、ain 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 precursors,Source: Stratospheric Ozone, NASA/GSFC,17,特制内容,Column
13、 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 Antarctica,O3,Source: Stratospheric Ozone, NASA/GSFC,Source: Farmann et al., Nature, v. 315, May 1985,18,特制内容,ALTITUDE DEPENDE
14、NCE OF ANTARCTIC O3 DECREASE,Strong depletion between 12 and 20 km Gas phase chemistry predicted decrease near 40 km,Source: Stratospheric Ozone, NASA/GSFC,19,特制内容,TEMPORAL DEPENDENCE OF ANTARCTIC O3 DECREASE,Depletion begins around Sep 1. & minimum is reached around Oct 1,Source: NOAA/CMDL,20,特制内容,
15、REACTIONS ON POLAR STRATOSPHERIC CLOUDS,Conversion of inactive Cl to active Cl and removal of NOx,Source: Stratospheric Ozone, NASA/GSFC,21,特制内容,ROLE OF METEOROLOGY,Low temps. PSC formation release of active Cl and removal of NOx Strong vortex Isolates air from mid-lats. prevents high O3 air influx,
16、Figure 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 stratosphere,Source: Stratospheric Ozone, NASA/GSFC,22,特制内容,NORTHERN vs SOUTHERN HEMISPHERE O3 TRENDS,Vortex not as strong and temps. not as lo
17、w in NH,Source: Stratospheric Ozone, NASA/GSFC,23,特制内容,PROJECTED 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,24,特制内容,WMO 1998 Scientific Assessment of Ozone
18、 Depletion Ozone depletion in 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 mi
19、dlatitudes in the Southern Hemisphere 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 CONTROLS,25,特制内容,TROPOSPHERIC O3 CHEMISTRY,Source: EPA,26,特制内容,Tropospheric O
20、3 generally less than 100 ppb away from urban areas,TROPOSPHERIC O3,Source: Wang et al., 1998,27,特制内容,O3 chemical production in stratosphere followed by downward transport to the troposphere,O2,O(3P),Solar radiation, ( 240 nm),O3,O2,Solar radiation (320 nm), M,STRATOSPHERIC SOURCE OF TROPOSPHERIC O3
21、,Strat. chem. destruction by HOx, NOx, Clx,Transport to trop.,28,特制内容,NO2,NO or O3,OH,HO2,CO,O3,O2,solar radiation, O2,Net: CO + 2O2 - CO2 + O3 Catalytic role of NOx (NO + NO2) in recycling HO2 to OH Coupling between OH and HO2 (HOx) via NO,CO OXIDATION CYCLE O3 PRODUCTION,CO2,O3,29,特制内容,NO or O3,OH
22、,HO2,CO,O2,Net: CO + O3 - CO2 + O2 Chemical O3 destruction Coupling between OH and HO2 (HOx) via O3,CO OXIDATION CYCLE O3 DESTRUCTION,CO2,O3,2O2,30,特制内容,O3 + hv O2 + O(1D) 2. O(1D) + M O + M 3. H2O + O(1D) 2OH 4. RH + OH RO2 + H2O 5. RO2 + NO RO + NO2 6. RO + O2 RCHO + HO2 7. HO2 + NO OH + NO2 8. HO
23、2 + HO2 H2O2 + O2 9. OH + NO2 + M HNO3 + M,SCHEMATIC OF HYDROCARBON CHEMISRY,O2,Net rxns 1-7: RH + 4O2 RCHO + 2O3 + H2O,Source: Introduction to Atmospheric Chemistry, Jacob, D. J., 1999,can produce more O3,31,特制内容,ROLE OF NOX IN O3 CHEMICAL PRODUCTION,Cycling of HOx (OH + HO2) by NOx vs. radical ter
24、mination reactions Too little NOx: Radical termination (e.g. HO2 + HO2) rather than radical cycling (e.g. HO2 + NO) leading to O3 chemical destruction Too much NOx: Radical termination by alternate route (e.g. OH + NO2) as well as short-term O3 destruction by NO + O3 - NO2 = implications for O3 peak
25、 downwind of strong NOX sources,32,特制内容,NOx- AND HYDROCARBON-LIMITED REGIMES,NOx limited,Hydrocarbon limited,Complications: Natural emissions of hydrocarbons are important Transport of pollutants into and out of region,Source: Introduction to Atmospheric Chemistry, Jacob, D. J., 1999,33,特制内容,Questio
26、ns: NOx or HC emission controls or combination Degree of emission controls Uncertainties Reliability of emission inventories清单(e.g. natural hydrocarbon inventories) Reliability of air quality models (e.g. local vs transported NOx/HC/O3),ISSUES IN O3 POLLUTION CONTROL,34,特制内容,1998 MEASURED SURFACE OZ
27、ONE CONCENTRATIONS,2nd highest daily max 1-hr (ppb),65,65-124,125-164,65-84,205-404,Source: 1998 EPA National Trends Report,118,153,169,36,141,155,167,165-204,4th highest daily max 8-hr (ppb),65,85-104,105-124,125-374,35,特制内容,ESTIMATED GLOBAL EXPOSURE STATISTICS,Population in areas with max. monthly
28、-mean O3 conc. above a given value,Crops in areas with growing season mean O3 conc above a given value,Exposure to O3 pollution: 40-60% of population in areas with max. monthly-mean O3 50 ppbv and 10-20% of crops in areas with growing-season mean O3 50 ppbv Potentially large impact in future years:Y
29、ear 2100 IPCC scenario from HARVARD model gives 50% of population in areas with max. monthly-mean O3 85 ppbv, and 50% of crops in areas with growing season mean O3 70 ppbv,36,特制内容,ATMOSPHERIC AEROSOLS AND ACID RAIN,Combustion generated,Aerosols and acid rain can effect natural & managed ecosystems,3
30、7,特制内容,硝酸和烷基硝酸酯的光离解 RO-NO2键能：199.4kJ/mol（吸收120335nm）： 硝酸：HNO3（HONO2）hHO + NO2 烷基硝酸酯：RONO2h RO + NO2,对于300nm以上的光的吸收速度很小,38,特制内容,亚硝酸和烷基亚硝酸酯的光离解 HO-NO键能：201.1kJ/mol H-ONO键能：324.0kJ/mol HNO2对200400nm的光有吸收，发生光离解： HONOhHO+NO HNO2hH+NO2 RONOh NO+RO,仅次于NO2光离解的最重要的光离解初级反应。,39,特制内容,醛的光离解（CH2O和CH3CHO ） H-CHO键能
31、：365.5kJ/mol（吸收240360nm），光离解反应： 甲醛H2COhH+HCO H2COhH2+CO 乙醛CH3CHOhH+ CH3CO CH3CHOhCH3+HCO CH3CHOhCH4+CO,40,特制内容,卤代烃的光离解 卤代甲烷的光解： CH3XhCH3+X 式中X代表Cl, Br, I, F。,键强顺序：CH3FCH3HCH3ClCH3BrCH3I,41,特制内容,SO2的光吸收 SO2键能：545.1 kJ/mol（200nm），吸收三个波段： 340400nm（极弱） 240330nm（较强） 280240nm（很强） SO2不能光离解，只能形成激发态分子： SO2hS
32、O2,活性粒子：HO、RO、RO2、H、HCO、CH3、CH3CO等自由基被称为大气中的“活性粒子”，它们性质特别活泼，能够引发一系列反应，参与很多的污染物的化学转化过程，导致生成各种各样的二次污染物。,42,特制内容,三、大气中重要自由基的来源,键的断裂与自由基（free radical）的形成： 不对称裂解形成正、负离子； 对称裂解形成自由基。 自由基具有强烈的夺取电子倾向和结合力。 自由基具有很强的氧化能力和化学活性。,43,特制内容,1、HO的来源,HO基的形成途径 （1）HONOHONO （400nm） （2）H2O22HO （300nm） （3）OH2O2HO （O来自O3的光离解
33、） （4）HO2NOHONO2 （HO2来自HCHO光离解， 产生的H与O2作用）,HO基的 形成途径,44,特制内容,大气中HO的浓度测算： 用数学模拟算出大气中HO基的全球平均浓度约为7105个分子/cm3； 用激光共振荧光光谱法测定HO基的浓度范围为35104个分子/cm3， 浓度数值随纬度、高度及地区的不同而变化，与季节有关。,45,特制内容,HO在对流层中随高度和纬度的分布,HO最高浓度出现在热带（温度高，太阳辐射强）；在两半球间分布不对称。,46,特制内容,2、HO2的来源,由CH2O、CH3ONO以及H2O2形成： （1）HCHOH HCO （313nm） H O2HO2 HCO
34、 O2HO2 CO （2）CH3ONONO CH3O （300 400nm） CH3O O2HO2 CH2O （3）H2O22HO （370nm） 2HO 2H2O22HO2 2H2O 2HO 2CO2CO22H 2H O22HO2 ,47,特制内容,3、CH3 、CH3O、RO2的来源,CH3 ： CH3CHOh CH3+HCO CH3COCH3h CH3+ CH3CO ,CH3O： CH3ONOhCH3O +NO,CH3O2 和RO2： CH3+ O2 CH3O2 R O2RO2 ,48,特制内容,四、氮氧化物的转化transform,1、NOx的光化学反应 NO (nitric oxid
35、e)、NO2 (nitrogen dioxide)与O3 (ozone)之间存在着的化学循环chemical cycling是大气光化学过程的基础。 当阳光照射到含有NO和NO2的空气时，反应： NO2hNO+O O2+OMO3M O3NONO2+O2,49,特制内容,2、NOx的气相转化,N2O 光解photolysis：N2OhN2+O 清除elimination：N2O ON2+O2 N2O ONO+NO,50,特制内容,2、NOx的气相转化,NO NO O3 NO2+O2 HO + NO HONO RO + NO RONO RO2 + NO RO + NO2 HO2 + NO HO +
36、 NO2 RCOO2 + NO RO + CO + NO2,51,特制内容,NO2 HO + NO2 HNO3 O3 + NO2 NO3 + O2 NO3可以和NO反应或光解作用再生成NO2，或者再与NO2进一步反应生成N2O5。 N2O5与H2O作用形成HNO3。,PAN（过氧乙酰基硝酸酯 peroxy acetyl nitrate） CH3CO + O2 CH3COO2 CH3COO2NO2 CH3COO2NO2 PAN具有热不稳定性，遇热会分解回到CH3COO2和NO2。,52,特制内容,3、NOx的液相转化,NOx可溶于大气的水中，构成一个液相平衡体系。 NOx的液相平衡 NO(g) NO(aq) aquatic NO2(g) NO2(aq) 2NO2(aq) N2O4 NO(aq)+ NO2(aq) N2O8(aq) 对于NO-NO2体系，存在液相平衡： 2 NO2(g) + H2O 2H+ + NO2- + NO3- NO(g) + NO2(g) + H2O 2H+ + 2NO2-,53,特制内容,NH3和HNO3的液相平衡aquatic balance NH3(g) + H2O NH3 H2O NH3 H2O NH4+ + OH- HNO3(g) + H2O HNO3 H2O HNO3 H2O H+NO3-,54,特制内容,