Environmental Environmental Acid rain Acid rain Chemistry - PDF document

Environmental Environmental Acid rain Acid rain Chemistry Chemistry APCH211 APCH211 Dr PG Ndungu Dr PG Ndungu Acid Rain Acid Rain Acidic Deposition Acidic Deposition Dry deposition � “Acid cid rai rain" – " – General eral term appl rm applied ied t to an any f form o rm of wet Wet deposition precipitation, usua prec usually in in the the tr trop oposphere, w with a acid idic ic spec species � In areas where the weather is dry, the � refers to acidic rain, fog and snow. str stronger tha than CO CO 2 acid chemicals may become incorporated into dust or smoke and � If the acidic chemicals in the air are � Includes r es rain, s sleet, s , snow ow, , fog, o or dew fall to the ground through dry blown into areas where the weather deposition sticking to the ground, is wet, the acids can fall to the � Na Natu tural ral pH pH of ra rain in water r ~ ~ 5.6 5.6 (f (from d m dissolved CO CO 2 ) buildings, homes, cars, and trees. ground in the form of rain, snow, Later, when moisture content � Can ha n have deposi deposition on of of dr dry y gases and gases and com compou ounds – – so o calle called dr dry y fog, or mist. g i increases, acid solution is produced. id l ti i d d deposition dep � Aqueo eous us and and dr dry dep deposi sition ar are e col collectively t termed aci med acid dep deposi sition � The e precur ursors rs or chemical f al forerunne rerunners o rs of ac acid r rain f formation rmation result fr res from om both th na natur tural sour sources, such such a as volc lcanoes a and deca caying vege getation a and ma man- n-made sour sources, prim primarily emi emissions of sulf of sulfur dioxide (S di e (SO 2 ) an and d nitrog ogen en ox oxid ides (N es (NO x ) r ) resulti sulting f ng from fossil ssil fuel comb mbust ustio ion. n. � Acid id ra rain is in is pa partic icularly d dama magi ging t to la lakes, str streams, a and fore rests an and t d the pl e plan ants ts an and d an animals t that l at live i in these ese e ecosys osystems ms. . Main Culprits in Acidic Precipitation Main Culprits in Acidic Precipitation Atmosph Atmosph mospheric mospheric c Pr c Pr Production Production oduction Of Nitric A oduction Of Nitric A Of Nitric Acid Of Nitric Acid id id The principle reaction sequence Nitrogen spec Nit species: Sul Sulfur Spec Species contributing to production of nitric acid starts with nitric oxide, NO from combustion processes. � NOx NOx � SO SO 2 � Sou Sources incl include b burn rning of of � Foss ssil f il fuels & els & sulfur o fur ore s e smeltin elting Nitric tric oxide ide Chemis hemistry: Dayti time me fo fossil f fuels, s, b biomas ass, s, e etc � H 2 S, S, & & CS CS 2 : � NO is oxidized by O 2 , O 3 or ROO – e.g. � Natural S tural Sour urce ces; s; A Anorexic ic NO + O 3 � NO 2 + O 2 � Wetlands a � Wetlands a and sub and sub submerged soi submerged soi soils soils 3 wa waters, t t ers, so soil ils, il t i s, etc � This NO 2 radical can then contribute to ozone and OH radical production i.e. � Dim Dimethylsulfide, ( (CH 3 ) 2 S, c S, carbon onyl yl � NH NH 3 plays a role in smog formation sul sulfi fide, COS, COS, me methyl mer mercaptan, Short lifetime, thus smog events don’t last, � � Animal exc excret eta, a, fertiliz izer ers a and CH CH 3 SH a and Dime Dimethyl disu isulfide, & not that frequent micr crobio obiolo logical r gical release ase CH CH 3 SS SSCH CH 3 . . Main removal sequence for NO is via � catalyst (M), OH rxns � Ocean an Ocean and d soils. ils. NO 2 + OH + M � HNO 3 + M 1

Nitric Nitric acid Nitric Nitric acid acid Product acid Product Production at Production at on at Night on at Night Night Night Reactions of Reactions of actions of NO actions of NO NO NO 3 with h with hydroca with h with hydroca drocarbo drocarbo bons bons ns ns Easily removes an H from alkanes Key species is the nitrate radical o � (NO 3 ). NO 3 + RH � R + HNO 3 Formed via O 3 & NO 2 The R radical can then react with O 2 to form peroxyl radicals � o With alkenes the NO 3 radical reacts via an addition mechanism producing nitro-oxy Atmospheric production of � o substituted organic radicals which can regenerate NO 2 , or relatively stable organic nitrate dinitrogen pentoxide (N 2 O 5 ) occurs compounds (see - Paul S. Monks. Gas-phase radical chemistry in the troposphere. Ch Chem. when NO 3 reacts with NO 2 is the Soc. Soc. Rev ., 2005, 34, 376-395, for e.g. with propene) only way to form in the atmosphere only way to form in the atmosphere. NO 3 + C n H 2n � C n H 2n NO 3 N 2 O 5 is a store of NO 3 . � With aldehydes, typically form nitric acid and the corresponding radical o Can decompose back to NO 3 � NO 3 + RCHO � RCO + HNO 3 and NO 2 Overall nighttime chemistry of NO 3 can recycle NO x , or form HNO 3 , depending on the mix o of hydrocarbons Can react with water to form � nitric acid Can be a nighttime source of OH radicals o Easily zapped by the rising sun! ( λ ~ 600 -700nm) Remo Remo moval of moval of l of Nitric l of Nitric Nitric Acid Nitric Acid Acid Acid Atmos Atmos mospheric Pr mospheric Pr Produ Produ oducti oducti tion of tion of of Sulfuri of Sulfuri Sulfuric A Sulfuric A Acid Acid id id Ox Oxida idation of reduc educed ed sulf sulfur ur spec specie ies • Production of sulfuric acid is more complex than that of nitric acid as the starting materials cover a wide range of reduced � Removal is accomplished by either wet or dry sulfur and partially oxidized deposition sulfur compounds. � One of the main contributors to acid precipitation. • These include hydrogen sulfide, carbon disulfide, carbonyl sulfide, methyl mercaptan, di dimethyl disulfide, and dimethyl th l di lfid d di th l � Nitric acid can react with ammonia: sulfide. All these compounds contain sulfur in its oxidation NH 3 + HNO 3 � NH 4 NO 3 state (-2). Mostly from natural sources • � The ammonium nitrate, NH 4 NO 3 can act as a condensation nucleus for the formation of a water droplet or it can be deposited as part of the solid aerosol S H C S O C S S H Se Se Sequence o Sequence o ence of Reaction ence of Reaction tions o tions o s of Sul s of Sul Sulfur Sulfur ur ur Cmpds Cmpds Cmpds Cmpds More About SO More About SO x … Further oxidation of thionyl • Once sulfur compounds are radical eventually produces in the air, a sequence of sulfur dioxide: • Dimethyl sulfide is produced by reaction begin as follows: phytoplankton living in surface waters of the SH + O 2 � SO + OH H 2 S + OH � H 2 O + SH SH + O 3 � SHO + O 2 ocean. It is oxidized by hydroxyl radical (OH) CS 2 + OH � COS + SH SHO + O 2 � SO + HOO with a final product being sulfuric acid. COS + OH � CO 2 + SH The SO radical can then react with either O 2 , O 3 or NO 2 to give NB: NB SO 2 and other products. The above reactions release � • Sulfur dioxide SO 2 is also released in large thionyl radical, SH as the initial 2SO + O 2 � 2SO 2 product. quantities directly into the atmosphere from SO + O 3 � SO 2 + O 2 Importance of OH! � sulfide ore smelting and fossil-fuel SO + NO 2 � SO 2 + NO Hydrogen sulfide and carbon � combustion. NB. NB disulfide unlike carbonyl sulfide, SO 2 is ultimately converted to sulfuric are very reactive and therefore are quickly consumed acid, H 2 SO 4 . Where is the OH coming from? 2