环境化学完整(4)ppt课件

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 N0h N0hc KJ mol 式中h Planck常数 6 62 1034J s 光子的频率 Hz c 光速 2 998 1010cm s 光子的波长 cm N0 阿佛加德罗常数 6 02 1023 mol 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 mol 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 h A 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 B D 与惰性inertia分子碰撞失活 返回基态 A M A M发光而失活 返回基态 A A h 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 氧和氮的光离解氧分子键能 493 8kJ mol 240nm以下紫外光可引发 O2 h O O氮分子键能 939 4kJ mol 127nm 仅限于臭氧层以上 N2 h N N 当入射波长低于79 6nm 1391kJ mol 时 N2将电离成N2 臭氧和过氧化物的光离解臭氧分子键能 101 2kJ mol220 290nm强吸收 254nm最强吸收 290 320nm少量吸收450 700nm微弱吸收离解反应 O3 h O2 O烷基过氧化物在300 700nm范围有微弱吸收 光离解反应 ROOR h RO R O NO2的光离解键能 300 5kJ mol 300 400nm吸光 光离解反应 NO2 h NO OO2 O M O3 M NO2是污染大气中最重要的吸光物质 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 OXIDIZINGNATUREOFTHEATMOSPHERE Earth satmosphereisoxidizingduetopresenceofO2SO2 SO42 CH4 CO CO CO2 NO2 HNO3RadicalsareoxidizingagentsintheatmosphereOHisthecleansingagentoftheatmosphereKeytounderstandingatmosphericoxidantchemistryUnderstandradicalcycling butradicalcycleisintimatelyconnectedtooxidantchemistryofothertracecompoundsincludingO3StartwithO3O3isimportantfromchemical climate andhealthperspectives ATMOSPHERICO3 ABRIEFHISTORY 1840 Ozonediscoveredin1840byC F Sch nbein thoughtitwasmadeupofoxygenandhydrogen1848 Systematicmeasurementattempts curiousity growinginterestinenv healtheffects economyofnature 1861 OdlingsuggestedthatozonewasO31930 ChemicalmechanismforO3layerpostulated1952 O3identifiedascomponentofchemicalsmog ChristianFrederichSch nbein OZONEANDHEALTH 90 ofO3isinthestratosphere O3layerwithmax 9ppmAbsorptionof 200 320nm UV BandUV C bystrat O3 Source StratosphericOzone NASA GSFC OZONEANDSTRATOSPHERICTEMPERATURE LocalheatingofthestratosphereduetoUVabsorptionbyO3TroposphericO3isalsoanimportantgreenhousegas Source StratosphericOzone NASA GSFC Source EnvironmentalScience Cunningham P W andB W Saigo 2001 OZONEANDATMOSPHERICCHEMISTRY O3istheprimarysourceoftroposphericOH OHisatmospheric detergent LATITUDINAL TEMPORALVARIATIONOFTOTALO3 TotalO3inrangeof300 400DUPatternsduetostratosphericcirculationLowtotalO3athighsouthernlatinsouthernspringdueto ozonehole Dobsonunits1DU 2 69x1016moleculesO3cm 2 Source StratosphericOzone NASA GSFC STRATOSPHERICO3CHEMISTRY THECHAPMANMECHANISMFORSTRATOSPHERICO3 CyclingbetweenO O2 andO3 Source StratosphericOzone NASA GSFC MISSINGCHEMISTRYINCHAPMANMECHANISM GlobalO3productionrate 5timesdestructionrateImbalancesuggestsoverest ofprodn orunderest oflossO3productionwellconstrainedbygoodspectroscopicdataImpliesmissingchemicalsinksforOxReactionsofradicalswithOand orO3Butradicalswillalsobeconsumedbyreaction measured calculated Source StratosphericOzone NASA GSFC CATALYTICOXDESTRUCTIONINTHESTRATOSPHERE RadicalchainreactionsX O3 XO O2XO O X O2Net O O3 2O2 X inthestratosphereH OH NO ClHOx NOx andClxHOx H OH HO2NOx NO NO2Clx Cl ClOReservoirstieupactiveradicalse g ClO NO2 ClONO2 StratosphericClxprecursors Source StratosphericOzone NASA GSFC ColumnO3 DU ANTARCTICTOTALOZONEDECREASE DepletionoftotalcolumnO3startinginmid tolate 70 sduringSHspringGas phasechemistrypredictedsmallerdecreases notoverAntarctica O3 Source StratosphericOzone NASA GSFC Source Farmannetal Nature v 315 May1985 ALTITUDEDEPENDENCEOFANTARCTICO3DECREASE Strongdepletionbetween12and20kmGasphasechemistrypredicteddecreasenear40km Source StratosphericOzone NASA GSFC TEMPORALDEPENDENCEOFANTARCTICO3DECREASE DepletionbeginsaroundSep1 minimumisreachedaroundOct1 Source NOAA CMDL REACTIONSONPOLARSTRATOSPHERICCLOUDS ConversionofinactiveCltoactiveClandremovalofNOx Source StratosphericOzone NASA GSFC ROLEOFMETEOROLOGY Lowtemps PSCformationreleaseofactiveClandremovalofNOxStrongvortexIsolatesairfrommid lats preventshighO3airinflux Figureshowsstrongpolarvortex旋涡 asshownbysizeofwindvectors lowpolartemps asshownbycolors atvariousaltitudesinthesouthernhemispherestratosphere Source StratosphericOzone NASA GSFC NORTHERNvsSOUTHERNHEMISPHEREO3TRENDS Vortexnotasstrongandtemps notaslowinNH Source StratosphericOzone NASA GSFC PROJECTEDCHANGESINSTRATOSPHERICClx MontrealProtocolandsubsequentamendmentswillhavesignifcantimpactsonprojectedClxloadingofstratosphere ppb Source StratosphericOzone NASA GSFC WMO1998ScientificAssessmentofOzoneDepletionOzonedepletionin2050wouldbeatleast50 atmidlatitudesintheNorthernHemisphereand70 atmidlatitudesintheSouthernHemisphere about10timeslargerthantodaySurfaceUV Bradiationin2050wouldatleastdoubleatmidlatitudesintheNorthernHemisphereandquadrupleatmidlatitudesintheSouthernHemispherecomparedwithanunperturbedatmosphere Thiscomparestothecurrentincreasesof5 and8 intheNorthernandSouthernHemispheres respectively since1980 ESTIMATEDIMPACTSOFClxCONTROLS TROPOSPHERICO3CHEMISTRY Source EPA TroposphericO3generallylessthan100ppbawayfromurbanareas TROPOSPHERICO3 Source Wangetal 1998 O3chemicalproductioninstratospherefollowedbydownwardtransporttothetroposphere O2 O 3P Solarradiation 240nm O3 O2 Solarradiation 320nm M STRATOSPHERICSOURCEOFTROPOSPHERICO3 Strat chem destructionbyHOx NOx Clx Transporttotrop NO2 NOorO3 OH HO2 CO O3 O2 solarradiation O2 Net CO 2O2 CO2 O3CatalyticroleofNOx NO NO2 inrecyclingHO2toOHCouplingbetweenOHandHO2 HOx viaNO COOXIDATIONCYCLE O3PRODUCTION CO2 O3 NOorO3 OH HO2 CO O2 Net CO O3 CO2 O2ChemicalO3destructionCouplingbetweenOHandHO2 HOx viaO3 COOXIDATIONCYCLE O3DESTRUCTION CO2 O3 2O2 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 R CHO HO27 HO2 NO OH NO28 HO2 HO2 H2O2 O29 OH NO2 M HNO3 M SCHEMATICOFHYDROCARBONCHEMISRY O2 Netrxns1 7 RH 4O2 R CHO 2O3 H2O Source IntroductiontoAtmosphericChemistry Jacob D J 1999 canproducemoreO3 ROLEOFNOXINO3CHEMICALPRODUCTION CyclingofHOx OH HO2 byNOxvs radicalterminationreactionsToolittleNOx Radicaltermination e g HO2 HO2 ratherthanradicalcycling e g HO2 NO leadingtoO3chemicaldestructionToomuchNOx Radicalterminationbyalternateroute e g OH NO2 aswellasshort termO3destructionbyNO O3 NO2 implicationsforO3peakdownwindofstrongNOXsources NOx ANDHYDROCARBON LIMITEDREGIMES NOxlimited Hydrocarbonlimited Complications NaturalemissionsofhydrocarbonsareimportantTransportofpollutantsintoandoutofregion Source IntroductiontoAtmosphericChemistry Jacob D J 1999 Questions NOxorHCemissioncontrolsorcombinationDegreeofemissioncontrolsUncertaintiesReliabilityofemissioninventories清单 e g naturalhydrocarboninventories Reliabilityofairqualitymodels e g localvstransportedNOx HC O3 ISSUESINO3POLLUTIONCONTROL 1998MEASUREDSURFACEOZONECONCENTRATIONS 2ndhighestdailymax1 hr ppb 65 65 124 125 164 65 84 205 404 Source 1998EPANationalTrendsReport 118 153 169 36 141 155 167 165 204 4thhighestdailymax8 hr ppb 65 85 104 105 124 125 374 ESTIMATEDGLOBALEXPOSURESTATISTICS Populationinareaswithmax monthly meanO3conc aboveagivenvalue CropsinareaswithgrowingseasonmeanO3concaboveagivenvalue ExposuretoO3pollution 40 60 ofpopulationinareaswithmax monthly meanO3 50ppbvand10 20 ofcropsinareaswithgrowing seasonmeanO3 50ppbvPotentiallylargeimpactinfutureyears Year2100IPCCscenariofromHARVARDmodelgives50 ofpopulationinareaswithmax monthly meanO3 85ppbv and50 ofcropsinareaswithgrowingseasonmeanO3 70ppbv ATMOSPHERICAEROSOLSANDACIDRAIN Combustiongenerated Aerosolsandacidraincaneffectnatural managedecosystems 硝酸和烷基硝酸酯的光离解RO NO2键能 199 4kJ mol 吸收120 335nm 硝酸 HNO3 HONO2 h HO NO2烷基硝酸酯 RONO2 h RO NO2 对于300nm以上的光的吸收速度很小 亚硝酸和烷基亚硝酸酯的光离解HO NO键能 201 1kJ molH ONO键能 324 0kJ molHNO2对200 400nm的光有吸收 发生光离解 HONO h HO NOHNO2 h H NO2RONO h NO RO 仅次于NO2光离解的最重要的光离解初级反应 醛的光离解 CH2O和CH3CHO H CHO键能 365 5kJ mol 吸收240 360nm 光离解反应 甲醛H2CO h H HCOH2CO h H2 CO乙醛CH3CHO h H CH3COCH3CHO h CH3 HCOCH3CHO h CH4 CO 卤代烃的光离解卤代甲烷的光解 CH3X h CH3 X式中X代表Cl Br I F 键强顺序 CH3 F CH3 H CH3 Cl CH3 Br CH3 I SO2的光吸收SO2键能 545 1kJ mol 200nm 吸收三个波段 340 400nm 极弱 240 330nm 较强 280 240nm 很强 SO2不能光离解 只能形成激发态分子 SO2 h SO2 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 三 大气中重要自由基的来源 键的断裂与自由基 freeradical 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 1 HO的来源 HO基的形成途径 1 HONO HO NO 400nm 2 H2O2 2HO 300nm 3 O H2O 2HO O来自O3的光离解 4 HO2 NO HO NO2 HO2来自HCHO光离解 产生的H与O2作用 HO基的形成途径 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7 105个分子 cm3 用激光共振荧光光谱法测定HO基的浓度范围为3 5 104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 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 2 HO2的来源 由CH2O CH3ONO以及H2O2形成 1 HCHO H HCO 313nm H O2 HO2 HCO O2 HO2 CO 2 CH3ONO NO CH3O 300 400nm CH3O O2 HO2 CH2O 3 H2O2 2HO 370nm