Lecture 13: Gas Phase rganic- N x + UV Reactions II Required Reading: FP&P Chapter 6 (except as noted next) Additional Reading: S&P Chapter 5 Catching-Up Reading: Jacob Chapters 11 & 12 (free online) Atmospheric Chemistry CHEM-5151 / ATC-5151 Spring 2005 Prof Jose-Luis Jimenez All figures from F-P&P utline of Lecture The big picture of atmospheric oxidation A xidants B Lifetimes of rganics C Reactions of Alkanes D of R, R, and R 2 Radicals E of Alkenes & Biogenics F of Alkynes G of Aromatics H of -Containing rganics I of N-Containing rganics J Chemistry of Remote Regions K Atm Chem & Biomass Burning Last Lecture Today 1
Decomp Isom Decomp Isom Decomp Isom H 2, N, (R 2 ) H 2, N, (R 2 ) D Summary of Alkane xidation Chemistry is all about electrons Species with unpaired electrons (radical) doing most of the work D Big Picture of rganic xidation Parent (primary) species 1 st generation stable products 2 nd generation stable products VC+oxidant VC+oxidant VC+oxidant H N 3 3 hν H N 3 3 hν H N 3 3 hν Peroxy (R 2 ) N N 3, (R 2 ) Peroxy (R 2 ) N N 3, (R 2 ) Peroxy (R 2 ) N N 3, (R 2 ) C 2 Alkoxy (R) 2 Alkoxy (R) 2 Alkoxy (R) Aumont, Laval, and Madronich, accepted in ACPD, 2004 2
D xidation of CH 4 : High N x Case k = 11 10-11 e -1440/T k(298 K) = 10 10-13 cm 3 /#/s [Cl] max = 10 4 #/cm 3 τ CH4 =(k[cl]) -1 = 32 years + Cl Consumed: 1 H; 1 N Produced: 1 N 2 ; 3 H 2 Followed by: N 2 +hv (+ 2 ) N+ 3 - HCl CH 4 + H -H 2 + 2 M + N 2 -N 2 + 2 -H 2 k = 245 10-12 e -1775/T k(298 K) = 63 10-15 cm 3 /#/s [H] max = 10 6 #/cm 3 τ CH4 =(k[h]) -1 = 5 years From S Nizkorodov H 2 + 2 -C CH 2 hv HC + H or H 2 + C + 2 M H 2 D xidation of CH 4 : Low N x Case + H + CH 4 CH 2 3 -H 2 M + H 2 2 + 2 H + CH 2 + 2 or + + 2 CH 2 + H 2 + 2 or H + 2 + 2 -H 2 CH 2 We already know what happens to CH 2 (it is converted to H 2, H 2 and C) How about H and H? H 2 droplets + H -H 2 2 H 2 droplets + H -H 2 H From S Nizkorodov hv + H + H -H 2 CH 2 H CH 2 + H H + H -H 2 CH 2 H + 2 -H 2 CH 2 3
B Lifetimes of rganics As always: enormous number of possibilities, but what is important? rg + X Products (X is an oxidant) d[rg]/dt = -k[x][rg] ; lifetime: τ = 1/k[X] From F-P&P E Reactions of Alkenes Eg HC 3 -CH=CH-, 2-butene Double bond adds reactivity For alkanes H could abstract any H No strong preference for reaction site The double bond has extra electron density Attacked by electrophilic radicals: H, 3, N 3, Cl The double bond gets the whole molecule in trouble Alkenes are more reactive than alkanes 4
E Alkenes + H Remember that collision rate ~25 x 10-10 Very fast reactions, faster for larger alkenes Pressure dep, negative T dep Suports importance of addition to double b Compare H + Propane: 1 x 10-12 Propene: 26 x 10-12 Heptane: 7 x 10-12 Heptene: 40 x 10-12 E What happens after H addition? Hydroxy group + alkyl radical Alkyl radical Peroxy radical Peroxy radical Alkoxy radical or (stable) nitrate Alkoxy radical reaction with 2, decomposition, isomerization 5
E Example of β-hydroxyalkyl Isomerization As for alkanes, larger alkoxy radicals isomerize: Solve in class: What products are expected here? E 3 + Alkenes Remember that collision rate ~25 x 10-10 Much slower reactions than for H Compare H +Propene: 26 x 10-11 3 + Propene: 1 x 10-17 But remember: d[rg]/dt = - k[xidant][rg] H: 01 ppt 3 : 100 ppb So although ozonolysis of alkenes is a slow process, it is important in the atmosphere because of the large concentrations of 3 6
E Mechanism of 3 + Alkenes: First steps 3 adds across the double bond The primary ozonide is not stable and breaks c breaks, a or b break E Fate of Excited Criegee Intermediates Contain excess energy (from broken bonds) Stabilized by collision Decompose in various ways Some to radicals and some to stable products Example of Criegee from 1-propene + 3 Stabilized Criegee Intermediate 7
E Fate of Stabilized Criegee Intermediates React with H 2, S 2, N, N 2, C, aldehydes, and ketones All reactions lead to stable products Reaction with H 2 dominates thers more uncertain, S 2 & N may be important in urban atmospheres E Importance of H generation H can react with all organics, not just alkenes! Specially important at night because no photolytic H sources 8
E N 3 adds to double bond Excited adduct can: Form epoxide Stabilize, form peroxy radical, blah blah N 3 + Alkenes N 3 Reaction Rates E Remember that collision rate ~25 x 10-10 Reactions are quite fast for biogenic alkenes Comparable rates to H d[rg]/dt = - k[xidant][rg] N 3 : 50 ppt @ night H: 01 ppt @ day N 3 reactions with biogenic alkenes @ night are very important Biogenics 9
F,G,H,I What about other organics? Similar types of radical chemistries Aromatics: H-addition Aldehydes: aldehydic H-abstraction Ketones and alcohols: alkyl chain H-abstraction Carboxylic acids: H-addition or H-abstraction Similar types of downstream chemistries Gets really complicated quickly You should be able to understand it from what we have covered If need to know for your research: See the book for introduction Then search the literature J Biogenic VCs Biogenic VCs dominate globally Lots of double bonds Large products to aerosols 10
J Mechanisms of VC emission from plants Ray Fall in CU Bio J Isoprene vs T & Light Very strong function of both Not stored, emitted as it is made Enzyme + precursor Both depend on light Enzyme more active as T, degrades if T 11
J Biogenic Emissions of xygenates Many different species Can be a large fraction of total emitted C J Chemistry of Remote Regions Low N x (R 2 reactions) destruction of 3, formation of peroxides 12
K Biomass Burning Emissions Very important source of many species Being studied intensively (NCAR) 13