環(huán)境化學(xué)課件【專業(yè)研究】

1、二、光化學(xué)photochemistry反應(yīng)基礎(chǔ),1、光子photon的能量 愛(ài)因斯坦-普朗克（Einstein-Planck）關(guān)系式： = h = hc/ (J) E= N0 h = N0 hc/ (KJ/mol) 式中hPlanck常數(shù)，6.621034J.s； 光子的頻率，Hz； c光速，2.9981010cm/s； 光子的波長(zhǎng)，cm； N0阿佛加德羅常數(shù)，6.021023/mol。,1,特制內(nèi)容,隨著波長(zhǎng)的增加，光子的能量減小。,不同波長(zhǎng)光的能量,高能光，能引起光化學(xué)反應(yīng)（光離解）,低能光，不能引起光化學(xué)反應(yīng),例1計(jì)算波長(zhǎng)為200nm紫外光的能量。,一般化學(xué)鍵的鍵能大于167.44KJ/

2、mol,2,特制內(nèi)容,2、光化學(xué)反應(yīng)原理 光化學(xué)反應(yīng)：由一個(gè)原子、分子、自由基或離子吸收一個(gè)光子后所引發(fā)的反應(yīng)。 初級(jí)過(guò)程： 引發(fā)：A(分子)hA* (激發(fā)態(tài)分子) 次級(jí)過(guò)程： 離解：A* C+ 與其他分子反應(yīng)生成新的物種：A*+BD+ 與惰性inertia分子碰撞失活(返回基態(tài))：A*+MA+M 發(fā)光而失活(返回基態(tài))：A*Ah,3,特制內(nèi)容,3、大氣中重要吸光物質(zhì)的光離解 吸光物質(zhì) 高層大氣：N2、O2、O3 （290nm） 低層大氣：NO2、SO2等,4,特制內(nèi)容,氧和氮的光離解 氧分子鍵能：493.8kJ/mol（240nm以下紫外光可引發(fā)）： O2h O+O 氮分子鍵能：939.4k

3、J/mol（127nm，僅限于臭氧層以上）： N2h N+N,當(dāng)入射波長(zhǎng)低于79.6nm (1391kJ/mol) 時(shí)，N2將電離成N2。,5,特制內(nèi)容,臭氧和過(guò)氧化物的光離解 臭氧分子鍵能：101.2kJ/mol 220290nm 強(qiáng)吸收（254nm最強(qiáng)吸收） 290320nm 少量吸收 450700nm 微弱吸收 離解反應(yīng)： O3hO2+O 烷基過(guò)氧化物在300700nm范圍有微弱吸收，光離解反應(yīng)： ROORhRO+RO,6,特制內(nèi)容,NO2的光離解 鍵能：300.5 kJ/mol（300400nm吸光），光離解反應(yīng)： NO2hNO+O O2+OMO3M,NO2是污染大氣中最重要的吸光物質(zhì)

4、,在低層大氣中可以吸收全部來(lái)自太陽(yáng)的紫外光和部分可見(jiàn)光。,O3的唯一人為來(lái)源,7,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)容,OZONE AND ATMOSPHERIC CHEMISTRY,O3 is the primary source of tropospheric OH OH is atmospheric detergent,12,特制

9、內(nèi)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,特制內(nèi)容,STRATOSPHE

10、RIC O3 CHEMISTRY,14,特制內(nèi)容,THE CHAPMAN MECHANISM FOR STRATOSPHERIC O3,Cycling between O, O2, and O3,Source: Stratospheric Ozone, NASA/GSFC,15,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)容,TEMPORAL DEPENDENCE OF ANTARCTIC O3 DECREASE,Depletion begins around Sep 1. & minimum is reached around Oct 1,Source: NOAA/CMDL,20,特制內(nèi)容,

15、REACTIONS ON POLAR STRATOSPHERIC CLOUDS,Conversion of inactive Cl to active Cl and removal of NOx,Source: Stratospheric Ozone, NASA/GSFC,21,特制內(nèi)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旋渦(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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)容,TROPOSPHERIC O3 CHEMISTRY,Source: EPA,26,特制內(nèi)容,Tropospheric O

20、3 generally less than 100 ppb away from urban areas,TROPOSPHERIC O3,Source: Wang et al., 1998,27,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)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,特制內(nèi)容,Questio

26、ns: NOx or HC emission controls or combination Degree of emission controls Uncertainties Reliability of emission inventories清單(e.g. natural hydrocarbon inventories) Reliability of air quality models (e.g. local vs transported NOx/HC/O3),ISSUES IN O3 POLLUTION CONTROL,34,特制內(nèi)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,特制內(nèi)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,特制內(nèi)容,ATMOSPHERIC AEROSOLS AND ACID RAIN,Combustion generated,Aerosols and acid rain can effect natural & managed ecosystems,3

30、7,特制內(nèi)容,硝酸和烷基硝酸酯的光離解 RO-NO2鍵能：199.4kJ/mol（吸收120335nm）： 硝酸：HNO3（HONO2）hHO + NO2 烷基硝酸酯：RONO2h RO + NO2,對(duì)于300nm以上的光的吸收速度很小,38,特制內(nèi)容,亞硝酸和烷基亞硝酸酯的光離解 HO-NO鍵能：201.1kJ/mol H-ONO鍵能：324.0kJ/mol HNO2對(duì)200400nm的光有吸收，發(fā)生光離解： HONOhHO+NO HNO2hH+NO2 RONOh NO+RO,僅次于NO2光離解的最重要的光離解初級(jí)反應(yīng)。,39,特制內(nèi)容,醛的光離解（CH2O和CH3CHO ） H-CHO鍵能

31、：365.5kJ/mol（吸收240360nm），光離解反應(yīng)： 甲醛H2COhH+HCO H2COhH2+CO 乙醛CH3CHOhH+ CH3CO CH3CHOhCH3+HCO CH3CHOhCH4+CO,40,特制內(nèi)容,鹵代烴的光離解 鹵代甲烷的光解： CH3XhCH3+X 式中X代表Cl, Br, I, F。,鍵強(qiáng)順序：CH3FCH3HCH3ClCH3BrCH3I,41,特制內(nèi)容,SO2的光吸收 SO2鍵能：545.1 kJ/mol（200nm），吸收三個(gè)波段： 340400nm（極弱） 240330nm（較強(qiáng)） 280240nm（很強(qiáng)） SO2不能光離解，只能形成激發(fā)態(tài)分子： SO2hS

32、O2,活性粒子：HO、RO、RO2、H、HCO、CH3、CH3CO等自由基被稱為大氣中的“活性粒子”，它們性質(zhì)特別活潑，能夠引發(fā)一系列反應(yīng)，參與很多的污染物的化學(xué)轉(zhuǎn)化過(guò)程，導(dǎo)致生成各種各樣的二次污染物。,42,特制內(nèi)容,三、大氣中重要自由基的來(lái)源,鍵的斷裂與自由基（free radical）的形成： 不對(duì)稱裂解形成正、負(fù)離子； 對(duì)稱裂解形成自由基。 自由基具有強(qiáng)烈的奪取電子傾向和結(jié)合力。 自由基具有很強(qiáng)的氧化能力和化學(xué)活性。,43,特制內(nèi)容,1、HO的來(lái)源,HO基的形成途徑 （1）HONOHONO （400nm） （2）H2O22HO （300nm） （3）OH2O2HO （O來(lái)自O(shè)3的光離解

33、） （4）HO2NOHONO2 （HO2來(lái)自HCHO光離解， 產(chǎn)生的H與O2作用）,HO基的 形成途徑,44,特制內(nèi)容,大氣中HO的濃度測(cè)算： 用數(shù)學(xué)模擬算出大氣中HO基的全球平均濃度約為7105個(gè)分子/cm3； 用激光共振熒光光譜法測(cè)定HO基的濃度范圍為35104個(gè)分子/cm3， 濃度數(shù)值隨緯度、高度及地區(qū)的不同而變化，與季節(jié)有關(guān)。,45,特制內(nèi)容,HO在對(duì)流層中隨高度和緯度的分布,HO最高濃度出現(xiàn)在熱帶（溫度高，太陽(yáng)輻射強(qiáng)）；在兩半球間分布不對(duì)稱。,46,特制內(nèi)容,2、HO2的來(lái)源,由CH2O、CH3ONO以及H2O2形成： （1）HCHOH HCO （313nm） H O2HO2 HCO O2HO2 CO （2）CH3ONONO CH3O （300 400nm） CH3O O2HO2 CH2O （3）H2O22HO （370n