Dana L. Ranschaert, Nicholas J. Schneider, and Matthew J. Elrod*

Transcription

1 5758 J. Phys. Chem. A 2000, 104, Kinetics of the C 2 H 5 O 2 + NO x Rections: Temperture Dependence of the Overll Rte Constnt nd the C 2 H 5 ONO 2 Brnching Chnnel of C 2 H 5 O 2 + NO Dn L. Rnschert, Nichols J. Schneider, nd Mtthew J. Elrod* Deprtment of Chemistry, Hope College, Hollnd, Michign ReceiVed: Jnury 28, 2000; In Finl Form: April 12, 2000 The temperture dependence of the overll rte constnt for the C 2 H 5 O 2 + NO rection nd the rte constnt for the minor brnching chnnel resulting in the production of C 2 H 5 ONO 2 hve been mesured using the turbulent flow technique with high-pressure chemicl ioniztion mss spectrometry for the detection of rectnts nd products. The temperture dependence of the overll rte constnt for the C 2 H 5 O 2 + NO rection ws investigted between 299 nd 213 K t 100 Torr pressure, nd the dt ws fit to the following Arrhenius expression (with 2 stndrd devition error limits indicted): exp[(290 ( 110)/T] cm 3 molecule -1 s -1. The temperture dependence of the overll rte constnt grees well with the current recommendtion for tmospheric modeling. The minor rection chnnel C 2 H 5 O 2 + NO f C 2 H 5 ONO 2 ws directly observed for the first time, nd the temperture dependence of the rte constnt for this chnnel ws investigted between 298 nd 213 K t 100 Torr pressure. The following Arrhenius expression ws determined for the minor chnnel: exp[(1160 ( 310)/T] cm 3 molecule -1 s -1. The Arrhenius expressions for the overll rte nd the C 2 H 5 ONO 2 producing chnnel indicte brnching rtio of bout t 298 K nd 0.02 t 213 K t 100 Torr pressure. The temperture dependence of the overll rte constnt for the C 2 H 5 O 2 + NO 2 rection ws lso investigted between 299 nd 213 K t 100 Torr pressure, nd the dt ws fit to the following Arrhenius expression: exp[(620 ( 89)/T] cm 3 molecule -1 s -1. Introduction Alkyl peroxy rdicls (RO 2 ) re importnt intermedite species formed in the oxidtion of lknes in the tmosphere. 1 RH + OH f R + H 2 O (1) R + O 2 + M f RO 2 + M (2) Ozone levels in the tmosphere re directly ffected by RO 2 rections, which themselves re dependent on the levels of the nitrogen oxides (NO x ). Under high NO x conditions (generlly, lower tropospheric urbn conditions), RO 2 rections led to the production of ozone, the most deleterious constituent of photochemicl smog. RO 2 + NO f RO + NO 2 (3) NO 2 + hν (λ < 380 nm) f NO + O (4) O + O 2 + M f O 3 + M (5) RO 2 cn lso be temporrily removed from the ozone production cycles by the formtion of reservoir species RO 2 + NO + M f RONO 2 + M (3b) RO 2 + NO 2 + M T RO 2 NO 2 + M (6) Becuse rection 3b nd the forwrd rection 6 remove the rdicl chin crrier RO 2 from the ozone production cycle, they hve direct effect on the ozone-producing efficiency of the * To whom correspondence should be ddressed. E-mil: elrod@hope.edu. Telephone: (616) FAX: (616) relevnt RH/NO x system. Atkinson nd co-workers hve developed n empiricl reltionship for the yield of orgnic nitrtes from rection 3b from the results of environmentl chmber photolysis studies of C 3 -C 8 n-lknes. 2 The brnching rtio [k 3b /(k 3 + k 3b )] is predicted to rise from for the methyl cse to 0.32 for the n-octyl cse t 298 K nd 760 Torr, thus indicting the importnce of the ssocition chnnel 3b for the lrger lkyl peroxy rdicls. Bertmn et l. hve proposed tht the lkyl nitrte brnching rtios cn be used in regionl tmospheric models tht determine the ir mss ge from observtions of the reltive concentrtion of lkyl nitrte to the prent lkne. This mesurement cn serve s photochemicl clock since the rtio of lkyl nitrte to the prent lkne increses with ir mss ge. Using this ir mss ge, in ddition to meteorologicl bck trjectories, cn help to identify specific regionl ozone production events. 3 Although their method ws found to be self-consistent for the secondry C 4 nd C 5 nitrtes, Bertmn et l. found tht their model systemticlly underpredicted mesured ethyl nitrte levels by substntil mount. They speculted tht either the brnching rtio they hd used for ethyl nitrte (0.014, n upper limit estimted erlier for 760 Torr nd 298 K by Atkinson et l. 4 ) in their model ws too low or tht there ws n ethyl nitrte source other thn rection 3b in the photochemicl system. Therefore, it is of interest to directly mesure the ethyl nitrte brnching rtio under tmospheric conditions. There hve been severl previous kinetics investigtions of the rte of rection of C 2 H 5 O 2 + NO. 5-9 It hs been well estblished tht rection 7 C 2 H 5 O 2 + NO f C 2 H 5 O + NO 2 is the predominnt chnnel. Becuse C 2 H 5 ONO /jp000353k CCC: $ Americn Chemicl Society Published on Web 05/23/2000 (7) hs been

2 Kinetics of the C 2 H 5 O 2 + NO x Rections J. Phys. Chem. A, Vol. 104, No. 24, Figure 1. Experimentl pprtus. detected in the tmosphere, 3 it is ssumed tht rection 7b C 2 H 5 O 2 + NO + M f C 2 H 5 ONO 2 + M (7b) is lso kineticlly ccessible, lthough no direct lbortory detection of rection 7b hs been previously reported. In this rticle we describe our investigtion of the kinetics of the C 2 H 5 O 2 + NO conducted t pressures ner 100 Torr nd t rnge of tempertures extending to those found in the lower strtosphere using turbulent flow (TF) tube coupled to high pressure chemicl ioniztion mss spectrometer (CIMS). It hs been previously shown tht the TF technique cn be used to ccurtely determine the rte constnts of rections t pressures rnging from 50 to 760 Torr nd t tempertures s low s 180 K. 10 As in previous kinetics studies of HO 2 + NO, 11 HO 2 + BrO, 12 ClO + NO 13 2 OH + ClO, 14,15 nd CH 3 O 2 + NO 16 using the coupled TF-CIMS pproch, we re ble to directly ccess tmospheric pressure nd temperture conditions nd sensitively monitor mny of the relevnt rectnts nd products for the C 2 H 5 O 2 + NO rection. In the CH 3 O 2 + NO work, TF-CIMS kinetics technique ws used in the determintion of the temperture-dependent overll rte constnt nd n upper limit for the CH 3 ONO 2 producing chnnel of the CH 3 O 2 + NO rection for pressures nd tempertures representtive of the upper troposphere. 16 In this rticle we describe similr temperture-dependent kinetics investigtion of the overll rte (k 7 ) nd the first mesurement of the C 2 H 5 ONO 2 producing chnnel rte (k 7b ) of the C 2 H 5 O 2 + NO rection, s well s the first mesurement of the temperture dependence of the rte constnt for the rection of C 2 H 5 O 2 with NO 2 C 2 H 5 O 2 + NO 2 + M f C 2 H 5 O 2 NO 2 + M (8) Experimentl Section Turbulent Fst Flow Kinetics. A schemtic of the experimentl pprtus is presented in Figure 1 nd is similr to tht used in our previous study of CH 3 O 2 + NO (ref 16). The flow tube ws constructed with 2.2 cm i.d. Pyrex tubing nd ws 100 cm in totl length. A lrge flow of nitrogen crrier gs (pproximtely 30 STP L min -1 ) ws injected t the rer of the flow tube. The gses necessry to generte C 2 H 5 O 2 were introduced through 10 cm long 12.5 mm dimeter siderm locted t the rer of the flow tube. NO ws dded vi n encsed movble injector. The encsement (mde from corrugted Teflon tubing) ws used so tht the injector could be moved to vrious injector positions without breking ny vcuum sels, s well s to prevent mbient gses from condensing on cold portions of the injector. A fn-shped Teflon device ws plced t the end of the injector in order to enhnce turbulent mixing. The polonium-210 lph-emitting ioniztion source ws plced between the temperture regulted flow tube nd the inlet to the CIMS. Most of the flow tube gses were removed t the CIMS inlet by 31 L s -1 roughing pump. All gs flows were monitored with clibrted mss flow meters. The flow tube pressure ws mesured upstrem of the ioniztion source using Torr cpcitnce mnometer. The temperture ws determined t both the entrnce nd exit points of the temperture regulted region of the flow tube using Cuconstntn thermocouples. Rectnt Preprtion. C 2 H 5 O 2 ws generted using the following rections: C 2 H 6 + Cl f C 2 H 5 + HCl (9) C 2 H 5 + O 2 + M f C 2 H 5 O 2 + M (10) (k 9 ) cm 3 molecule -1 s -1 nd k 10 ) cm 3 molecule -1 s -1 t 100 Torr; ll rte constnt vlues quoted re for 298 K vlues unless otherwise indicted). 17 Chlorine toms were produced by pssing dilute Cl 2 /He mixture through microwve dischrge produced by Beenkker cvity operting t 50 W. The dilute Cl 2 /He mixture ws obtined by combining 2.0 STP L min -1 flow of helium (99.999%), which hd pssed through silic gel trp immersed in liquid nitrogen, with STP ml min -1 flow of 1% Cl 2 (99.9%)/He mixture. To generte C 2 H 5, the chlorine toms were then injected into siderm nd mixed with n excess of C 2 H 6 (CP grde, molecules cm -3 ) in order to ensure tht no chlorine toms were introduced into the min flow. C 2 H 5 O 2 ws then produced by the ddition of lrge excess of O 2 (99.995%; > molecules cm -3 ) just downstrem of the production of C 2 H 5. Absolute C 2 H 5 O 2 concentrtions (needed to ensure pseudo firstorder kinetics conditions nd for brnching rtio modeling) were determined using two independent methods. The first method involved the mesurement of the HCl produced from rection 9, nd the determintion of the bsolute HCl concentrtion vi clibrtion of the mss spectrometer signl using bubbler contining 20 wt % HCl/H 2 O solution kept t 0 C(p HCl ) Torr). 18 The C 2 H 5 O 2 concentrtion ws then ssumed to be the sme s the mesured HCl concentrtion. This p-

3 5760 J. Phys. Chem. A, Vol. 104, No. 24, 2000 Rnschert et l. proximtion is expected to be vlid, s the high concentrtion of O 2 gurntees n extremely fst conversion of C 2 H 5 to C 2 H 5 O 2 vi rection 10, nd C 2 H 5 O 2 rects reltively slowly with itself. The second method involved the titrtion of C 2 H 5 O 2 with NO in the presence of O 2 to produce CH 3 CHO: C 2 H 5 O 2 + NO f C 2 H 5 O + NO 2 (11) C 2 H 5 O + O 2 f CH 3 CHO + HO 2 (12) (k 11 ) cm 3 molecule -1 s -1 nd k 12 ) cm 3 molecule -1 s -1 ) 17 nd subsequent clibrtion of the CH 3 - CHO mss spectrometer signl. Stndrd smples were prepred by drwing n pproprite mount of CH 3 CHO vpor from pure liquid smple, followed by mixing with N 2 to mke 1% CH 3 CHO mixtures pproprite for mss spectrometric clibrtion. Computer modeling of these titrtion rections indicted tht not ll of the C 2 H 5 O 2 initilly present ws converted to the stble species, CH 3 CHO. This is minly due to rections of C 2 H 5 O with other species in the chemicl system (which will be ddressed in the discussion section). Therefore, the C 2 H 5 O 2 concentrtions were clculted from the CH 3 CHO concentrtions by explicitly determining the conversion fctor (usully 0.8) from computer modeling using the specific experimentl conditions. The NO (CP grde) used in rection 11 ws purified by severl freeze/pump/thw cycles to remove NO 2 impurities. The NO 2 /NO rtio of the purified smple ws typiclly less thn 1% s determined from CIMS NO 2 detection nd clibrtion methods. For this study, C 2 H 5 O 2 concentrtions rnged from 0.5 to molecules cm -3. For the overll rte determintion for the C 2 H 5 O 2 + NO rection, NO ws purified before use s described bove nd ws dded to the flow rector s 10% mixture in N 2 through the movble injector. For the overll rte determintion for the C 2 H 5 O 2 + NO 2 rection, the NO 2 ws used without further purifiction nd dded s 10% mixture in N 2. NO nd NO 2 concentrtions used in this study rnged from 0.5 to molecules cm -3. To ensure pseudo first-order kinetics conditions, NO nd NO 2 were kept t levels t lest 10 times lrger thn the C 2 H 5 O 2 levels. For the low-temperture studies, liquid nitrogen cooled silicone oil ws used s the coolnt for the jcketed flow tube. Nitrogen crrier gs ws precooled by pssing it through copper coil immersed in liquid N 2 reservoir followed by resistive heting. The temperture ws controlled in the rection region to within 1 K. The pressure ws mintined t pproximtely 100 Torr in order to chieve optimum instrument performnce. Pressures below 100 Torr were found to lower the ioniztion efficiency nd pressures bove 100 Torr were found to lower the rdicl production efficiency. C 2 H 5 ONO 2 Brnching Chnnel Mesurements. In these studies, the production of C 2 H 5 ONO 2 from rection 7b ws monitored directly over rection time of 25 ms. Computer modeling ws used to extrct the rte constnt k 7b from the observed production of C 2 H 5 ONO 2 nd the initil concentrtions of ll relevnt chemicl species. As will be discussed in the Results nd Discussion section, both the time profile of the C 2 H 5 - ONO 2 production nd the bsolute mount of C 2 H 5 ONO 2 produced over the longest rection time were used s constrints on the determintion of k 7b, therefore it ws criticl tht the bsolute C 2 H 5 ONO 2 concentrtions were determined ccurtely. Stndrd smples of C 2 H 5 ONO 2 for mss spectrometric clibrtion were prepred s follows. C 2 H 5 ONO 2 ws prepred by dropwise ddition of solution of 2 ml of 95% H 2 SO 4 nd 6 ml of 100% C 2 H 5 OH to n ice-bth cooled solution contining 15 ml of 95% H 2 SO 4,15mLof70%HNO 3, nd 0.5 g of ure. The solution ws stirred for 5 min. The upper orgnic lyer (contining C 2 H 5 ONO 2 ) ws pipetted off nd wshed with 20 ml of ice-cold distilled wter. A seprtory funnel ws used to drin the lower queous lyer, nd the wshing process ws repeted severl times. The orgnic lyer ws then wshed with 80 ml of ice-cold 22% NCl solution nd ws drined nd stored in volumetric flsk t -10 C. The high purity of the C 2 H 5 ONO 2 smples ws confirmed by 1 H NMR spectroscopy (δ CH2 ) 4.44 ppm, nd δ CH3 ) 1.30 ppm) in CDCl 3 solvent. The smples did not undergo degrdtion over the course of severl weeks. Stndrd smples were prepred by drwing n pproprite mount of C 2 H 5 ONO 2 vpor from the liquid smple, followed by mixing with N 2 to mke 1% C 2 H 5 ONO 2 mixtures pproprite for mss spectrometric clibrtion. To id in the detection of the C 2 H 5 ONO 2 product, higher C 2 H 5 O 2 levels were used for the brnching rtio mesurements thn in the determintion of the bimoleculr rte constnt (becuse C 2 H 5 O 2 is the limiting regent). The mjor compliction in the brnching rtio determintion is the side rection resulting from the rection of the products of the min chnnel (rection 7) of rection 7 C 2 H 5 O + NO 2 + M f C 2 H 5 ONO 2 + M (13) (k 13 ) cm 3 molecule -1 s -1 t 100 Torr). 17 Rection 13 is therefore potentil source of C 2 H 5 ONO 2, nd experimentl conditions must be designed to minimize this reltively fst rection. By keeping O 2 concentrtions very high, computer modeling shows tht rection 12 will consume most of the C 2 H 5 O produced by rection 7 nd minimize production of C 2 H 5 ONO 2 by rection 13. To drive the production of C 2 H 5 - ONO 2 from rection 13 down to very low levels, O 2 ws used s the crrier gs for the brnching rtio experiments, so tht the oxygen concentrtion ws nerly equl to the totl moleculr density (for experiments t 100 Torr totl pressure, [O 2 ] > molecules cm -3 ). Chemicl Ioniztion Mss Spectrometric Detection. Positive ion chemicl ioniztion schemes (with H 3 O + s the regent ion) were used to detect C 2 H 5 O 2,CH 3 CHO, nd C 2 H 5 ONO 2, nd negtive ion chemicl ioniztion schemes (with SF 6 - s the regent ion) were used to detect HCl nd NO 2 with the qudrupole mss spectrometer. H 3 O + ws produced in the ion source by pssing lrge O 2 flow (10 STP L min -1 ) through the polonium-210 lph-emitting ioniztion source (with H 2 O impurities being sufficiently bundnt to produce dequte quntities of regent ions). SF 6- ws produced in the ion source by pssing lrge N 2 flow (10 STP L min -1 ) nd 0.1 STP ml min -1 of SF 6 through the ioniztion source. The commercil ioniztion source consisted of hollow cylindricl (69 by 12.7 mm) luminum body with 10 mcurie ( disintegrtions s -1 ) of polonium-210 coted on the interior wlls. Ions were detected with qudrupole mss spectrometer housed in two-stge differentilly pumped vcuum chmber. Flow tube gses (neutrls nd ions) were drwn into the front chmber through 0.1 mm perture, which ws held t potentil of (210 V. The ions were focused by three lenses constructed from 3.8 cm i.d., 4.8 cm o.d luminum gskets. The front chmber ws pumped by 6 in L s -1 diffusion pump. The gses entered the rer chmber through skimmer cone with 1.0 mm orifice (held t (130 V) which ws plced pproximtely 5 cm from the front perture. The rer chmber ws pumped by 250 L s -1 turbomoleculr pump. Once the ions pssed through the skimmer cone, they were mss filtered nd detected with qudrupole mss spectrometer.

4 Kinetics of the C 2 H 5 O 2 + NO x Rections J. Phys. Chem. A, Vol. 104, No. 24, Chemicl Ioniztion Schemes To perform pseudo-first-order rte kinetics studies of the overll rte of rection 7, chemicl ioniztion scheme for C 2 H 5 O 2 is required. In our previous study of the CH 3 O 2 + NO rection, 16 we developed the following chemicl ioniztion detection scheme for CH 3 O 2 bsed on the computtionl thermochemicl prediction 19 tht CH 3 O 2 hs higher proton ffinity thn H 2 O: CH 3 O 2 + H 3 O + f CH 3 OOH + + H 2 O (14) Becuse the proton ffinity of C 2 H 5 O 2 is expected to be very similr to CH 3 O 2, we successfully ttempted to detect C 2 H 5 O 2 with the nlogous rection C 2 H 5 O 2 + H 3 O + f C 2 H 5 OOH + + H 2 O (15) Depending on the mount of the wter in the flow system, C 2 H 5 O 2 ws detected s C 2 H 5 OOH + (H 2 O) n, where n ws typiclly 2 or 3. To perform brnching rtio mesurements for rection 7b, chemicl ioniztion schemes re needed for CH 3 CHO, HCl (for the two C 2 H 5 O 2 bsolute concentrtion determintion methods described bove), NO 2 (to ssess the purity of the NO used nd ny potentil interference from side rection 13), nd for the product C 2 H 5 ONO 2. In previous study, cetldehyde ws found to rect quickly with H 3 O + Figure 2. Pseudo first-order C 2H 5O 2 decy curves for the C 2H 5O 2 + NO rection t 100 Torr, 253 K, nd 1070 cm s -1 velocity. CH 3 CHO + H 3 O + f CH 3 CHOH + + H 2 O (16) (k 16 ) cm 3 molecule -1 s -1 ). 20 Agin, depending on the mount of the wter in the flow system, CH 3 CHO ws detected s CH 3 CHOH + (H 2 O) n, where n ws typiclly 2 or 3. HCl nd NO 2 were detected using previously reported SF 6 - chemicl ioniztion schemes HCl + SF 6 - f F - HCl + SF 5 (17) NO 2 + SF 6 - f NO SF 6 (18) (k 17 ) cm 3 molecule -1 s -1 nd k 18 ) cm 3 molecule -1 s -1 ). 21 Previously, we found tht CH 3 ONO 2 rected with H 3 O +, but t rte tht ws too slow for sensitive mss spectrometric detection. 16 In this study, it ws determined tht C 2 H 5 ONO 2 rects very quickly with H 3 O + C 2 H 5 ONO 2 + H 3 O + f C 2 H 5 ONO 2 H + + H 2 O (19) such tht very high sensitivity could be chieved for the detection of this importnt product. Agin, depending on the mount of the wter in the flow system, C 2 H 5 ONO 2 ws detected s C 2 H 5 ONO 2 + (H 2 O) n, where n ws typiclly 2 or 3. The detection sensitivity for C 2 H 5 ONO 2 ws estimted to be bout 100 ppt t 100 Torr. Results nd Discussion Overll Rte Constnt Determintion. Bimoleculr rte constnts were obtined vi the usul pseudo first-order pproximtion method, using NO or NO 2 s the excess regent. Typicl C 2 H 5 O 2 decy curves s function of injector distnce re shown in Figure 2 for the NO kinetics mesurements. The first-order rte constnts obtined from fitting the C 2 H 5 O 2 decy curves were plotted ginst [NO] in order to determine the bimoleculr rte constnt, s shown in Figure 3. This pproch for determining bimoleculr rte constnts ssumes tht devitions from the plug flow pproximtion (moleculr velocities Figure 3. Determintion of the overll bimoleculr rte constnt for the C 2H 5O 2 + NO rection from dt in Figure 2. re equl to the bulk flow velocity) re negligible. Under the conditions present in our turbulent flow tube (Reynolds no. > 2000), Seeley et l. estimted tht these devitions result in pprent rte constnts which re t most 8% below the ctul vlues. 22 Hence, the flow corrections were neglected s they re smller thn the sum of the other likely systemtic errors in the mesurements of gs flows, temperture, detector signl, nd pressure. Considering such sources of error, we estimte tht rte constnts cn be determined with n ccurcy of (30% (2σ). We performed severl determintions of the rte constnt t 100 Torr nd 299 K for the C 2 H 5 O 2 + NO rection (see Tble 1 for complete list of experimentl conditions nd mesured rte constnts) nd rrived t the men vlue of k ) (10.1 ( 0.9) cm 3 molecule -1 s -1 ; the uncertinty represents the two stndrd devition sttisticl error in the dt nd is not n estimte of systemtic errors. Our room-temperture rte constnt is in excellent greement with previous studies 5-9 nd thus with the JPL recommended vlue for tmospheric modeling. 17 Temperture Dependence of the Overll Rte Constnt (k 7 ) for C 2 H 5 O 2 + NO. We performed severl mesurements t 100 Torr pressure nd t tempertures between 299 nd 213 K in order to estblish the temperture dependence of the rte constnt for conditions relevnt to the upper troposphere (nd lower strtosphere). The rte constnt incresed by pproxi-

5 5762 J. Phys. Chem. A, Vol. 104, No. 24, 2000 Rnschert et l. TABLE 1: Overll Rte Constnt Dt for the C 2 H 5 O 2 + NO Rection t 100 Torr Pressure temperture (K) velocity (cm s -1 ) Reynolds no. k 7 (10-12 cm 3 molecule -1 s -1 ) ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 1.2 Stted error is 2σ. TABLE 2: Overll Rte Constnt Dt for the C 2 H 5 O 2 + NO 2 Rection t 100 Torr Pressure temperture (K) velocity (cm s -1 ) Reynolds no. k 8 (10-12 cm 3 molecule -1 s -1 ) ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 0.79 Stted error is 2σ. Figure 4. Arrhenius plot of the temperture dependence of the overll rte constnt for the C 2H 5O 2 + NO rection. mtely 50% s the temperture ws lowered over this rnge. From the dt listed in Tble 1 nd plotted in Figure 4, we obtined the Arrhenius expression k(t) ) exp[(290 ( 110)/T]cm 3 molecule -1 s -1. Our results re in good greement with the two previous temperture dependence studies 5,6 nd the JPL recommended vlue for tmosphere modeling. 17 Temperture Dependence of the Overll Rte Constnt (k 8 ) for C 2 H 5 O 2 + NO 2. In mnner similr to our mesurement for the C 2 H 5 O 2 + NO rection, we performed severl mesurements t 100 Torr pressure nd t tempertures between 299 nd 213 K in order to detil the temperture dependence of the overll rte constnt for C 2 H 5 O 2 + NO 2. The rte constnt incresed pproximtely 3-fold s the temperture ws lowered over this rnge. From the dt listed in Tble 2 nd plotted in Figure 5, we obtin the Arrhenius expression k(t) ) exp[(620 ( 89)/T]cm 3 molecule -1 s -1. To our knowledge, there hs been only one previous estimte of the pressure dependence of this rte constnt nd no temperture dependence study for this rection. Elfers et l. 23 mesured the rte of the C 2 H 5 O 2 + NO 2 rection reltive to the C 2 H 5 O 2 + NO rection t 254 K nd found tht the rection rte incresed from 0.21 k 7 t 7.5 Torr, to 0.50k 7 t 75 Torr to 0.81k 7 t 746 Torr. Using our vlue for k 7 t 254 K s clculted from the Arrhenius Figure 5. Arrhenius plot of the temperture dependence of the overll rte constnt for the C 2H 5O 2 + NO 2 rection. expression, the reltive rte constnts of Elfers et l. cn be converted to the following bsolute rte constnts: k 8 (7.5 Torr) ) cm 3 molecule -1 s -1, k 8 (75 Torr) ) cm 3 molecule -1 s -1, nd k 8 (750 Torr) ) cm 3 molecule -1 s -1. Our vlue for k 8 t 100 Torr nd 254 K (s clculted from Arrhenius expression bove) is cm 3 molecule -1 s -1, which is resonble greement with the mesurement of Elfers et l. t 75 Torr. We ttempted to detect the C 2 H 5 O 2 NO 2 product from rection 8 using the H + (H 2 O) n chemicl ioniztion proton trnsfer rection successfully used for C 2 H 5 O 2,CH 3 CHO nd C 2 H 5 ONO 2 detection. However, we did not observe the ion corresponding to the expected proton trnsfer product. This result most likely indictes tht C 2 H 5 O 2 - NO 2 hs lower proton ffinity thn wter, rther thn indicting tht C 2 H 5 O 2 NO 2 is not product of rection 8. C 2 H 5 ONO 2 Brnching Rtio Determintion. We were ble to observe the production of very smll concentrtions of C 2 H 5 - ONO 2 ( molecules cm -3 ) over the rection time ( 25 ms), which we hve positively identified s coming from rection 7b. In Figure 6, the C 2 H 5 ONO 2 rise (squres) hs been overlid on the bckground signl (circles) to demonstrte tht the C 2 H 5 ONO 2 rise is significntly lrger thn the (2σ level of the bckground signl. Under the optiml experimentl

6 Kinetics of the C 2 H 5 O 2 + NO x Rections J. Phys. Chem. A, Vol. 104, No. 24, TABLE 3: Kinetics Prmeters for Brnching Rtio Determintion A rection (cm 3 s -1 molecule -1 ) E /R (K) k o 300 (cm 6 s -1 molecule -2 ) fvor the following rection which effectively scvenges C 2 H 5 O from the system. Although the HO 2 product formed from this rection could conceivbly led to the reforming of C 2 H 5 O 2 (nd thus C 2 H 5 O) in the system vi the following rections m k 300 (cm 3 s -1 molecule -1 ) C 2H 5O 2 + NO f products (this work) C 2H 5O 2 f products C 2H 5O 2 + NO 2 f C 2H 5O 2NO (IUPAC) C 2H 5O 2 + HO 2 f CH 3CHO + H 2O + O (IUPAC) C 2H 5 + O 2 f C 2H 5O C 2H 5O + O 2 f CH 3CHO + HO (IUPAC) C 2H 5O + NO f C 2H 5ONO C 2H 5O + NO 2 f C 2H 5ONO HO 2 + NO f OH + NO HO 2 + OH f H 2O + O OH + OH f H 2O OH + NO f HONO OH + NO 2 f HNO OH + CH 3CHO f CH 3CO + H 2O OH + C 2H 6 f C 2H 5 + H 2O All dt from the JPL compiltion (DeMore, W.B.; Snder, S. P.; Howrd, C. J; Rvishnkr, A. R.; Golden, D. M.; Kolb, C. E.; Hmpson, R. F; Kurylo, M. J.; Molin, M. J. Chemicl Kinetics nd Photochemicl Dt for Use in Strtospheric Modeling; JPL Publiction 97-4; Jet Propulsion Lbortory: Psden, CA, 1997), unless specificlly indicted s from the IUPAC compiltion (Atkinson, R.; Bulch, D. L.; Cox, R. A.; Hmpson, R. F.; Kerr, J. A.; Rossi, M. J.; Troe, J. Summry of EVluted Kinetic Dt for Atmospheric Chemistry; IUPAC Subcommittee on Gs Kinetic Dt Evlution for Atmospheric Chemistry: Cmbridge, UK, 1999 ( Pressure-independent rte constnts clculted from k(t) ) Ae -E /RT. Pressure-dependent rte constnts clculted from k([m],t) ) ((k o(t)[m])/(1 + k o(t)[m]/ k (T)))0.6 (1+(log 10 ((k o (T)[M])/(k (T))))2 ) -1 where k o(t) ) k o 300 (T/300) -n nd k (T) ) k 300 (T/300) -m. C 2 H 5 O + O 2 f CH 3 CHO + HO 2 (12) HO 2 + NO f OH + NO 2 (20) OH + C 2 H 6 f C 2 H 5 + H 2 O (21) C 2 H 5 + O 2 + M f C 2 H 5 O 2 + M (10) n Figure 6. Observed production of C 2H 5ONO 2 from rection 7b (squres) Above the C 2H 5ONO 2 Bckground level (circles) s function of injector distnce. A lest-squres fit to the bckground level ws performed, nd the dotted lines represent the 2σ level. This dt set ws obtined under the following conditions: P ) 100 Torr, T ) 223 K, velocity ) 925 cm s -1, Reynolds no. ) 2820, [C 2H 5O 2] 0 ) molecules cm -3, [NO] 0 ) molecules cm -3. conditions ([C 2 H 5 O 2 ] molecules cm -3, [NO] molecules cm -3, nd [O 2 ] molecules cm -3 ) kinetics modeling shows tht side rections cn produce concentrtions of C 2 H 5 ONO 2 tht re on the order of the detection level of the instrument ( molecules cm -3 ) nd re significntly less thn the ctul C 2 H 5 ONO 2 observed. Tble 3 contins list of the rections used in the modeling. An inspection of Tble 3 indictes tht only one side rection directly produces C 2 H 5 ONO 2 : C 2 H 5 O + NO 2 + M f C 2 H 5 ONO 2 + M (13) However, this rection is significnt in tht the rectnts re the products of dominnt rection chnnel for C 2 H 5 O 2 + NO. As mentioned in the Experimentl Section, oxygen ws used s the crrier gs in the brnching rtio experiments in order to chieve very high O 2 concentrtions. The high O 2 concentrtions (k 20 ) cm 3 molecule -1 s -1 nd k 21 ) cm 3 molecule -1 s -1 t 100 Torr), computer modeling shows tht the OH product from rection 20 primrily rects with NO nd NO 2 (rther thn C 2 H 6 ), to form HONO nd HNO 3, respectively, which re reltively unrective under the flow tube conditions. Computer modeling ws used to extrct rte constnt for rection 7b by fitting the observed C 2 H 5 ONO 2 production. The ctul time profile of the C 2 H 5 ONO 2 production, s well s the totl production of C 2 H 5 ONO 2 over the totl rection time were used s constrints in the fitting process. The ltter constrint is useful in tht the competing side rection 13 pproches completion fster thn the desired rection 7b, such tht the proportion of C 2 H 5 ONO 2 tht hs been produced by rection 7b increses s longer rection times re chieved. The model input included the concentrtions of C 2 H 5 O 2, NO, nd ll precursors. Tble 4 contins list of the initil conditions nd clculted rte constnts (k 7b ) for the brnching rtio experiments. Figure 7 demonstrtes the sensitivity of the fitted k 7b vlue to the ctul kinetics dt collected. The fitted k 7b vlue is subject to error rising from experimentl uncertinties, s well s uncertinties in the rte constnts used in the modeling process. As stted erlier, we believe tht the relevnt experimentl uncertinty for rte constnt determintion with this pprtus is on the order of 30%, lthough the uncertinty my be somewht higher due to the fct tht the brnching rtio

7 5764 J. Phys. Chem. A, Vol. 104, No. 24, 2000 Rnschert et l. TABLE 4: Brnching Rtio Dt for the C 2 H 5 O 2 + NO Rection t 100 Torr Pressure T (K) [C 2H 5O 2] 0 (10 11 molecule cm -3 ) [NO] 0 (10 11 molecule cm -3 ) k 7b (10-14 cm 3 molecule -1 s -1 ) brnching rtio k 7b/(k 7+k 7b) Figure 7. Sensitivity of fitted vlue of k 7b to brnching rtio dt (given in Figure 6). mesurements re being mde very close to the detection level of the instrument. In ddition, the brnching rtio method requires the dded step of titrtion nd clibrtion experiments needed to determine [C 2 H 5 O 2 ] 0. The uncertinty in the rte constnt for rection 13 is expected to ply the lrgest role in the propgtion of rte constnt uncertinties through our fitting process. According to the JPL dtbse, 17 the uncertinty in k 13 is only 15% (becuse we re operting t the high-pressure limit for this rection). Propgting this vlue through our model indictes tht this uncertinty hs minor effect (less thn 10%) on our fitted vlue for k 7b. Therefore, we estimte the totl error to be pproximtely 50% for the brnching rtio mesurements. We performed severl mesurements of the brnching rtio for C 2 H 5 O 2 + NO t tempertures between 213 nd 298 K in order to estblish the temperture dependence of the rte constnt k 7b for conditions relevnt to the upper troposphere. From the dt listed in Tble 4 nd plotted in Figure 8, we obtined the Arrhenius expression k 7b (T) ) exp[(1160 ( 310)/T]cm 3 molecule -1 s -1. Agin, the uncertinty represents the two stndrd devition sttisticl error in the dt nd is not n estimte of systemtic errors (which re discussed bove). The brnching rtios [k 7b /(k 7 + k 7b )] reported in Tble 4 were clculted using the mesured Arrhenius expression for the overll rte constnt k 7 (k 7 + k 7b ). The brnching rtios rnge from bout t 298 K to 0.02 t 213 K, indicting the incresed importnce of the C 2 H 5 ONO 2 -producing chnnel t lower tempertures. Becuse the C 2 H 5 ONO 2 -producing chnnel is lmost certinly pressure-dependent, it would be of interest to ttempt to estblish the pressure dependence of the Figure 8. Arrhenius plot of the temperture dependence of the rte constnt of the C 2H 5ONO 2 brnching chnnel (squres nd solid line) nd prediction from Atkinson model 2 (dshed line) t 100 Torr pressure. rte constnt. However, becuse the mesurements mde t 100 Torr were brely within the detection limit of the instrument, it ws not surprising tht we were unble to obtin sufficient sensitivity to pursue experiments t pressures other thn 100 Torr. There hve been no previous mesurements of the rte constnt for rection 7b. However, ll but one of the rte constnt determintions previously reported in the literture were obtined by following the production of NO 2 (refs 6, 8, nd 9) or C 2 H 5 O (ref 7) nd thus were mesurements of k 7, rther thn the overll rte constnt k 7. The fct tht these mesurements of k 7 re in good greement with the only previous mesurement of k 7 (ref 5) nd the present result for k 7 indictes tht the vlue k 7b must be quite smll, which is consistent with the present results. The model of Atkinson nd co-workers, 2 which is bsed on mesured C 3 -C 8 lkyl nitrte brnching rtios, cn be used to predict vlue for k 7b. The results from this model re plotted with the experimentl results in Figure 8. Although the temperture dependence for k 7b is well predicted by the model, the bsolute experimentl rte constnts re consistently higher thn the model predictions. As discussed in the Introduction, the ethyl nitrte-bsed ir mss ge model of Bertmn et l. 3 used vlue for k 7b derived from the Atkinson work. 4 Therefore, our finding tht k 7b is somewht lrger thn predicted from the Atkinson model could potentilly provide n explntion for the Bertmn et l. result tht their ir mss ge model systemticlly underpredicted C 2 H 5 ONO 2 levels in the tmosphere. However, if one ssumes

8 Kinetics of the C 2 H 5 O 2 + NO x Rections J. Phys. Chem. A, Vol. 104, No. 24, tht rection 7b is the only source of C 2 H 5 ONO 2 in the tmosphere, mesured [C 2 H 5 ONO 2 ]/[C 2 H 6 ] rtios cn be used (vi the Bertmn et l. model) to predict the brnching rtio (β ) k 7b /k 7 ) for rection 7b: where k A is the pseudo first-order rte constnt for the rtelimiting OH + C 2 H 6 step in the mechnism tht eventully leds to C 2 H 5 ONO 2 production ( s -1 ), k B is the pseudo first-order rte constnt for the conversion of C 2 H 5 ONO 2 into other products ( s -1 ), nd t is the ir mss ge. Using eq 22, [C 2 H 5 ONO 2 ]/[C 2 H 6 ] vlue of nd n ir mss ge of 0.5 dys (bsed on mesurements mde by Bertmn et l. t Kinterbish, Albm) we clculte n tmosphericlly inferred brnching rtio of Therefore, lthough our lbortory findings indicte higher brnching rtio (β ) t 100 Torr nd 298 K) thn indicted by the empiricl reltionship of Atkinson et l. (β ) t 100 Torr nd β ) t 760 Torr nd 298 K), our vlue is still much lower thn the vlue inferred from tmospheric mesurements nd the Bertmn et l. model (β 0.17 t 760 Torr nd 298 K) which ssumes C 2 H 5 ONO 2 production from rection 7b only. This result suggests tht ethyl nitrte sources other thn rection 7b re opertive in the tmosphere. Conclusions β ) [C 2 H 5 ONO 2 ] tm [C 2 H 6 ] tm k B - k A k A 1 (1 - e (k A-k B )t ) (22) The results presented here represent the first mesurement ofc 2 H 5 ONO 2 producing chnnel from the C 2 H 5 O 2 + NO rection, s well s mesurements of the temperture dependence of the overll rte constnt for the rections of C 2 H 5 O 2 with NO nd NO 2. The brnching rtio [k 7b /(k 7 + k 7b )] t 100 Torr pressure for C 2 H 5 ONO 2 formtion ws determined to be bout t 298 K nd ws found to increse to bout 0.02 t 213 K. Although the temperture dependence of the rte constnt k 7b ws firly ccurtely predicted by extrpolting from model bsed on orgnic nitrte formtion from C 3 -C 8 hydrocrbon systems, 2 our bsolute rte constnts re systemticlly higher thn the model results. Our experimentl results should improve the relibility of models 3 tht use C 2 H 5 ONO 2 formtion s mesure of ir mss ge in the trcking of ozone-forming pollution sources. Our mesurement of the temperture dependence of the overll rte constnt for the C 2 H 5 O 2 + NO rection ws found to be in good greement with the JPL recommendtion. 17 We lso performed the first thorough temperture dependence (t 100 Torr pressure) study of the overll rte constnt for the C 2 H 5 O 2 + NO 2 rection, nd resonble greement ws found with n erlier pressure dependence study performed t 254 K. 23 This work should help improve modeling of ozone formtion rising from ethne/no x chemistry by plcing more stringent constrints on the formtion of C 2 H 5 - ONO 2 nd by helping to estblish temperture-dependent rte constnts for the rections of C 2 H 5 O 2 with NO nd NO 2 for tropospheric pressure nd temperture conditions. Acknowledgment. This reserch ws funded by grnts from the Cmille nd Henry Dreyfus Foundtion, Americn Chemicl SocietysPetroleum Reserch Fund, Reserch Corportion, nd the Ntionl Science Foundtion (Grnt ATM ). References nd Notes (1) Brsseur, G. P.; Orlndo, J. J.; Tyndll, G. S. Atmospheric Chemistry nd Globl Chnge; Oxford University Press: New York, (2) Atkinson, R.; Crter, W. P. L.; Winer, A. M. J. Phys. Chem. 1983, 87, (3) Bertmn, S. B.; Roberts, J. M.; Prrish, D. D.; Buhr, M. P.; Goldn, P. D.; Kuster, W. C.; Fehsenfeld, F. C.; Montzk, S. A.; Westberg, H. J. Geophys. Res. 1995, 100, (4) Atkinson, R.; Aschmnn, S. M.; Crter, W. P. L.; Winer, A. M.; Pitts, J. N. J. Phys. Chem. 1982, 86, (5) Eberhrd, J.; Howrd, C. J. Int. J. Chem. Kinet. 1996, 28, 731. (6) Mricq, M. M.; Szente, J. J. J. Phys. Chem. 1996, 100, (7) Dele, V.; Ry, A.; Vsslli, I.; Poulet, G.; Le Brs, G. Int. J. Chem. Kinet. 1995, 27, (8) Sehested, J.; Nielsen, O. J.; Wllington, T. J. Chem. Phys. Lett. 1993, 213, 457. (9) Plumb, I. C.; Ryn, K. R.; Steven, J. R.; Mulchy, M. F. R. Int. J. Chem. Kinet. 1982, 14, 183. (10) Seeley, J. V.; Jyne, J. T.; Molin, M. J. Int. J. Chem. Kinet. 1993, 25, 571. (11) Seeley, J. V.; Meds, R. F.; Elrod, M. J.; Molin, M. J. J. Phys. Chem. 1996, 100, (12) Elrod, M. 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