Environ. Sci. Technol. 1991, 25, 415-420
(13) Wallington, T. J.; Gierczak, C.A,; Ball, J. C.; Japar, S. M. Int. J . Chem. Kinet. 1989,21, 1077. (14) Atkinson, R.; Baulch, D. L.;Cox,R. A.; Hampson, R. F.; Kerr, J. A. J . Phys. Chem. Ref. Data 1989, 18, 881. (15) Wallington, T. J.; Dagaut, P.; Liu, R.; Kurylo, M. J. Int. J . Chem. Kinet. 1988, 20, 177.
(16) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 492.
Received for review May 11, 1990. Revised manuscript received August 15, 1990. Accepted August 23, 1990.
Ozone-Forming Potential of a Series of Oxygenated Organic Compounds Steven M. Japar,” Timothy J. Wallington,” Sara J. Rudy, and Tai Y. Chang” Research Staff, Ford Motor Company, Dearborn, Michigan 48121
An incremental reactivity approach has been used to assess the relative ozone-forming potentials of various important oxygenated fuels/fuel additives, i.e., tert-butyl alcohol (TBA), dimethyl ether (DME), diethyl ether (DEE),methyl tert-butyl ether (MTBE), and ethyl tertbutyl ether (ETBE), in a variety of environments. Calculations were performed using a single-cell trajectory model, combined with the Lurmann-Carter-Coyner chemical mechanism, with [NMOC]/ [NO,] ratios ranging from 4 to 20. This work provides the first quantitative assessment of the air quality impact of release of these important oxygenated compounds. ETBE and DEE are the two most reactive compounds on a per carbon equivalent basis, while TBA is the least reactive species. At a [NMOC]/[NO,] ratio of 8, which is generally typical of polluted urban areas in the United States, TBA, DME, MTBE, and ETBE all have incremental reactivities less than or equal to that of the “urban NMHC mix”. Thus, use of these additives in fuels may have a beneficial impact on urban ozone levels. Introduction Highly branched oxygenated hydrocarbons such as tert-butyl alcohol (TBA), methyl tert-butyl ether (MTBE), and ethyl tert-butyl ether (ETBE) can be added to gasolines to boost octane and to lower tailpipe CO emissions ( I ) . Because of these characteristics, such compounds have a significant role to play in the ongoing research effort within the petroleum and automotive industries aimed at formulating new gasoline mixtures to decrease the atmospheric reactivity of automotive emissions. A necessary prerequisite to the design of new gasoline blends is a clear understanding of the atmospheric reactivity of the individual fuel components that constitute the blend. This in turn translates to a need for accurate kinetic and mechanistic data concerning the atmospheric oxidation of these oxygenated fuel components. In our laboratory and elsewhere, this need has led to the generation of a significant data base concerning the kinetics and mechanisms of the atmospheric reactions of oxygenated species that are either currently in use (e.g., TBA and MTBE) or have been proposed for use in new fuels (e.g., ETBE). The purpose of this work is to quantify the relative impact that release of a variety of oxygenates would have in terms of the generation of urban ozone. To accomplish this, we have combined kinetic and new experimental mechanistic information with a computer model that simulates urban air chemistry to yield quantitative assessments of the relative ozone-forming potentials of a series of important oxygenated species. The modeling approach we have used is based upon that described by Carter and Atkinson (2) in which the “incremental hydrocarbon 0013-936X/91/0925-0415$02.50/0
Table I. OH Rate Constants for Selected Oxygenated Hydrocarbons compound
KOH (298 K), 10” cm3 molecule-’ s-l
0.90 3.4
methanol ethanol
tert-butyl alcohol dimethyl ether diethyl ether methyl tert-butyl ether ethyl tert-butyl ether 2-methoxy-2-methylpropanal 2-ethoxy-2-methylpropanal
1.1 2.5 14 3.2 8.5 30n 40”
ref
19 17 8 6 6 8, 9 9 11
Estimated; see text.
reactivity” associated with the release of individual organic compounds into the atmosphere is estimated. We have applied a variation of this modeling approach, described by Chang and Rudy (3, 4 ) , to calculate “relative incremental reactivities” for TBA, dimethyl ether (DME), diethyl ether (DEE), MTBE and ETBE. Atmospheric Chemistry of Oxygenated Fuel Additives The atmospheric fate of simple aliphatic alcohols and ethers is largely controlled by their reaction with the OH radical (5),since photolysis and reactions with O3 and the NO3 radical are negligibly slow. Rate constants for the reaction of OH with TBA, DME, DEE, MTBE, and ETBE are available from recent studies in our laboratory and elsewhere (6-9). These rate constants (at 298 K) are presented in Table I, along with others pertinent to the following discussion. Little has been known about the mechanisms of the atmospheric oxidations of the oxygenated species under discussion. Recently, however, we have supplemented the qualitative results of Cox and Goldstone (10) with detailed studies of the mechanisms of the OH-initiated oxidation of DME, DEE, TBA, MTBE, and ETBE (11,121. The major features of these derived mechanisms are presented in Table 11, while the results for the individual oxygenated compounds are summarized below. Dimethyl Ether (DME) (11). The consumption of DME is accompanied by the formation of methyl formate, with a yield of 90 f 8%. There is no evidence for the formation of any other carbon-containing product (an upper limit of 5% for HzCO yield has been established). The mechanism for the atmospheric oxidation of DME is, therefore, relatively simple. Hydroxyl radicals abstract hydrogen from DME, and the alkyl radical so formed adds O2 to yield an alkylperoxy radical. In an urban airshed, the alkylperoxy radical most likely reacts with NO to form an alkoxy radical. This alkoxy radical reacts with O2 to
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 3, 1991
415
Table 11. OH-Initiated Oxidation Mechanisms
-
dimethyl ether CH3OCH3 + NO + OH HCOOCH, + NO, + HOz diethvl ether CzH,50C,H, + 2 N 0 + OH C2H,0C(0)H + HCHO + 2N0, + HO, tert-butyl alcohol (CHJ3COH + NO + OH H&O + (CHJpCO + HOz + NO2 methyl tert-butyl ether CH,OC(CH,), + NO + OH O.GHCOOC(CH,), + 0.4CH30C(CH3)2CHO + HO2 + NO, CH,OC(CH,)*CHO + OH + 3 N 0 COZ + HzCO + CH30C(O)CH, + HOz + 3NO2 ethyl tert-butyl ether CzH50C(CH,), + 1.8NO + OH 0.8HCOOC(CH,), + O.ICZH,OC(CH,)zCHO + 0.8HCHO C2HSOC(CH,),CHO + OH + 3 N 0 COZ + HCHO + CzH,OC(O)CH, + Hop + 3N02 methanol CHSOH + OH HCHO + HOp ethanol" (20) CZHbOH + O.INO + OH 0.922CH3CHO = 0.156HCHO + HO, + O.INO,
- +
-+
" 0.922CH3CH0
=
(5)
(3) (6)
+ HOZ + 1.8NO2
O.SOOCH,CHO
(4) (7)
(8)
-
(9)
+ 0.022HOCH2CHO.
yield methyl formate and the H 0 2 radical. Therefore, in urban airsheds the atmospheric oxidation of DME can be represented by reaction 1. CH3OCH3 + NO + OH HCOOCH, + NO, HO2 (1) Diethyl Ether (DEE) (12).The major product of the OH-initiated oxidation of DEE is ethyl formate, with a yield of 92 f 6%. If the oxidation is initiated by C1 (in the presence of NO) it is possible to identify other carbon-containing products, including H2C0, methyl nitrite, and methyl nitrate. The sum of the product yields of HCHO + C H 3 0 N 0 CH30N02,87 f 6%, is indistinguishable from that of ethyl formate. The mechanism of the oxidation of DEE is more complex than that for DME because attack can occur at either of the two different carbons. However, the experimental results indicate that attack at either carbon results in the decomposition of the resulting alkoxy radicals by C-C bond scission, to form ethyl formate and either a methyl radical (if the original attack is on the CH, group) or H2C0 (if the attack is on the CH3 group). The observation of methyl nitrite and methyl nitrate formation in the C1-initiated oxidation of DEE provides evidence for the generation of methyl radicals, which react with O2 to form methylperoxy radicals. CH302radicals are rapidly converted to CH30 radicals, and the formation of C H 3 0 N 0 and CH30N02 is then explained by the competition between NO, NO2, and 0, for the available CH30 radicals. No evidence has been found for the presence of likely nondecomposition products of the alkoxy radicals such as ethoxyacetaldehyde and ethyl acetate, or for products from an alkoxy radical decomposition involving C-0 bond scission, such as acetaldehyde (upper limits of 5% can be ascribed to the yield of each of these compounds). The high yield of ethyl formate, 92 f 670, indicates that alkoxy radical isomerization analogous to the 1,5 H shift seen for alkoxy radicals produced in the oxidation of long-chain alkanes (13) is not a significant reaction pathway in the DME system. The atmospheric oxidation can be represented by C2H,OC,H, 2NO + OH -.+ C,H,OC(O)H + HCHO + 2N02 + HOt (2) +
+
+
+
Methyl tert-Butyl Ether (MTBE). There have been two product studies of the oxidation of MTBE under simulated atmospheric conditions. The first, by Cox and Goldstone ( I O ) , employed the photolysis of MTBEHONO-air mixtures and found tert-butyl formate (TBF) as the major product together with a minor product tentatively identified as acetone. In the second, more corn418
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Environ. Sci. Technol., Vol. 25, No. 3, 1991
prehensive study performed by Japar et al. ( I I ) , three different chemical systems were investigated. These systems were the photolyses of (i) MTBE-CH30NO-NO-air, (ii) MTBE-C12-NO-air, and (iii) MTBE-C12-air mixtures. Consistent with the earlier study ( I O ) , tert-butyl formate was observed to be the major product in all three systems. In the MTBE-CH30NO-NO-air mixtures the yield of TBF is 62 f 570,so that the majority of the OH attack occurs at the methoxy group, leading to the formation of an alkoxy radical, which reacts with molecular oxygen to give the formate and an HOz radical. This is consistent with the observation (11) of essentially 100% methyl formate yield from the oxidation of dimethyl ether under simulated atmospheric conditions. Variation of the oxygen concentration from 140 to 700 Torr had no effect on the observed TBF yield, suggesting that under atmospheric conditions (160 Torr 0,) other loss processes for the alkoxy radical, e.g., decomposition or isomerization via a 1,5 H shift, are minor. The TBF yield (62 f 5 % ) thus provides a direct measure of the fraction of OH radicals that attack the methoxy group. The remaining 38 f 5% of OH radical attack presumably occurs at the tert-butoxy group. While no products associated with attack of OH radicals on the tert-butoxy group have been positively identified (11), circumstantial evidence leads to the speculation that such attack yields 2-methoxy-2-methylpropanal (MMP) + H 0 2 as products (11). The evidence comes from the observed product yields in the irradiation of MTBE-Clz-air mixtures with and without NO added. TBF was observed as the major product in both systems, with an yield of 60 f 10%. However, whereas HCHO was observed (26 f 5% yield) in the presence of NO, it was absent when NO was absent. This observation suggests that the alkoxy radical formed following attack of C1 atoms on the tert-butoxy group, CH30C(CH3),CH20,reacts with 0, to form MMP and does not significantly decompose to yield HCHO. The HCHO observed in the irradiation of MTBE-Cl,-NO-air mixtures can be ascribed to attack by OH radicals (formed by the reaction of HO, radicals with NO) on MMP. Based on the above discussions, the mechanism for the atmospheric oxidation of MTBE can be represented as CH30C(CH3)3 + NO + OH O.GHCOOC(CH,),+ 0.4CH30C(CHJZCHO + HO2 + NO, (3) +
Ethyl tert-Butyl Ether (ETBE). The major product of the OH-initiated oxidation of ETBE is tert-butyl formate, with a yield of 76 f 6% (12). The mechanism for the atmospheric oxidation of ETBE is complex. By analogy with the oxidation of DEE, it is expected that OH attack on the ethoxy group of ETBE should result exclu-
sively in the formation of TBF. One can compare the observed TBF product yield, 7670, to the expected percentage of OH attack on the ethoxy group of ETBE. Kinetic studies of OH-ether reactions have indicated that the rate of attack of the OH radical on the two alkyl groups in the ether is independent and additive (7). Thus, reactivities of C(CH,), and C2H5groups in ethers of 1.6 X 10-l2and 6.8 X cm3 molecule-'^-^, respectively, have been derived (7). From these values, the OH radical would be expected to attack approximately 80% of the time at the ethoxy group. The measured TBF product yield is therefore indistinguishable, within the experimental errors, from the fraction of OH radicals expected to react with the ethoxy group in ETBE. No evidence has been found for the formation of nondecomposition products such as tert-butoxyacetaldehyde or tert-butyl acetate, or for products due to C-0 bond scission, such as acetaldehyde (an upper limit of 15% for the yield of any of these species). By analogy to the diethyl ether oxidation, it is expected that for each molecule of TBF that is formed there will be one molecule of either HCHO or CH302 formed. As in the case of DEE, C1-initiated oxidation was used to look for formaldehyde and related products (121, and the product yield sum ( H 2 C 0 + C H 3 0 N 0 + CH3ONO2)is significantly greater than that of TBF, especially at high conversions. This can be accounted for through attack by OH radicals (produced in the chamber by reaction of H 0 2 radicals with NO) on one or more of the initial reaction products. The 20% of OH attack on ETBE that proceeds via reaction with the tert-butoxy group, by analogy to MTBE, might lead to the formation of 2-ethoxy-2-methylpropanal. OH attack at the ethoxy group will yield an alkoxy radical capable of undergoing a 1,5 H shift. Since the TBF yield resulting from attack on the ethyl group is the same as the fraction of OH attack at that group, based on kinetic arguments (see above), it appears that the 1,5 H shift is not an important consequence of reaction at the ethoxy group. However, there is no evidence available concerning the possibility of intramolecular hydrogen abstraction by peroxy or alkoxy radicals based on the tert-butoxy group. On the basis of the experimental results and the assumptions discussed above, the atmospheric oxidation of ETBE can be represented by
-
+ + 1.8N02
ETBE + OH + 1.8NO 0.8HCOOC(CH3), 0.2C2H,OC(CHJ2CHO + HO2 + 0.8HCHO
(4) tert-Butyl Alcohol (TBA) ( 1 0 , I I ) . H2C0 and acetone are the sole products of the atmospheric oxidation of TBA. The mechanism for the oxidation of TBA is uncomplicated. It forms an alkoxy radical, which does not react with 02,but decomposes to yield HCHO and acetone.
Model Description In order to evaluate relative reactivities of organic compounds, a single-cell trajectory model, the Ozone Isopleth Plotting Package with Optional Mechanism (OZIPM) (14, was used for one-day simulations. The simulation conditions for this model are shown in Table 111. The simulations were started by assuming an initial concentration of non-methane organic compounds (NMOC) in the center of an urban area at 0800 LDT to be 0.7 ppm C. The initial NO, concentration was varied depending on the NMOC/NO, ratio used in the simulation. NMOC and NO, were emitted to the air parcel as a fraction of the initial concentrations, as shown in Table 111. We chose to use two dilution scenarios: a “low-dilution” case (the
Table 111. Trajectory Model (OZIPM)Simulation Conditions latitude date tema. K RH; k simultn period initial concn NMOC, ppm C NO, ([NO,]/[NO,] =
34.1’ N June 21 303
50 0800-2100 LDT 0.7
variable depending on NMOC/NO, ratio
0.253
CO, ppm CH,, PPm
1.2 2.0
time (LDT) 0800-1300 1300-1800
aloft concn NMOC, ppm C 0%PPm CO, ppm CHI, PPm
emission sched” NMOC NO, CO 0.15 0.05
0.25 0.08
0.10 0.03
0.04 0.06
0.5 1.6
mixina height,b m initial at final at 0800 (LDT) 1500 (LDT) low dilution high dilution
speciesd ALK4 ALK7 ETHE PRPE TBUT TOLU XYLE TMBZ HCHO ALD2 NR
300 300
600 1500
NMOC comoositionsC initial aloft carbon fractns (:arbon fractns 0.214 0.280 0.037 0.048 0.029 0.080 0.079 0.042 0.020 0.030 0.141
0.363 0.151 0.034 0.010 0.014 0.042 0.024 0.000 0.070 0.037 0.255
Fraction of initial concentration emitted per hour. *Mixing height varied with time according to the characteristic curve recommended by Hog0 and Gery (14). cInitial and aloft NMOC compositions taken from Jeffries et al. (18). dAbbreviations for species are defined in Lurmann et al. (15). (I
mixing height rising from 300 to 600 m), which is typical of high-ozone days in the South Coast Air Basin of California, and a “high-dilution” case (the mixing height rising from 300 to 1500 m), which is typical of high-ozone days in urban areas in the eastern United States. The OZIPM model allows the input of an optional mechanism. In the present work, the Lurmann-CarterCoyner (LCC) OZIPM chemical mechanism (15) was used. The kinetic and mechanistic data describing the reaction of OH radicals with the series of oxygenates studied were included, as summarized in Tables I, 11, and IV. For simplicity, the chemistry of product formates and acetates was neglected, since they react much more slowly with OH than do the parent ethers (16). The OH rate constants for 2-methoxy- and 2-ethoxy-2-methylpropanal (MMP and EMP) were estimated from rate constants available for other aldehydes (17). PAN analogue formation from these two aldehydes, their photolyses and reactions with NO3, and the reactions of the resulting RC03 radicals with other peroxy radicals are also included, with the appropriate rate constants set equal to those for the 1-propanal system. For the sake of comparison we have also included in our calculations the chemistry of methanol and ethanol, as indicated in Tables I, 11, and IV. Environ. Sci. Technol., Vol. 25, No. 3, 1991 417
Table IV. Reactions Added to the LCC Chemical Mechanism" dimethyl ether CH3OCH3 + OH = ROzR + ROp (+ HCOOCH3) diethyl ether CzH,OC,HS + OH = ROZR + R202 + 2R02 + HCHO (+ HCOOCZH,) tert-butyl alcohol (CH,),COH + OH = ROzR + RO, + HCHO + (CH,),CO methyl tert-butyl etherb CH3OC(CH,), + OH = ROPR + ROp 0.4CH,OC(CH,),CHO {MCHO)(+ O.GHCOOC(CH,),) MCHO + OH = MCO, + RCO, MCO3 + NO = NO, + HCHO + ROzR + Rz02 + 2R02 (+ CH,OC(O)CHJ MC03 + NO, = MCO3NOz MCO3NOz = MCO, + NOp + RCO, ethyl tert-butvl etherb C2H50C(CH3),+ OH = R0,R + 0.8R202+ 1.8RO2+ 0.8HCHO + 0.2CzH50C(CH3),CH0{ECHO)(+ 0.8HCOOC(CH3),) ECHO + OH = ECO, + RCO, ECO, + NO = NO2 t HCHO + ROzR + RzO, + 2R02 (+ C,H,OC(O)CHS) ECO:, + NO, = ECO,NOZ EC03NOz = ECO, + NO, + RCO, Methanol CH30H + OH = HCHO + HOZ ethanol CzH50H + OH = 0.922CH3CHO + 0.9H02 + 0.156HCHO + O.lR02R + 0.1R02
+
"ROzR, R,O,, ROZ, and RC03 are defined in ref 15. Compounds in parentheses are relatively unreactive in an urban environment and are subsequently ignored. *For the aldehydes CH30C(CH3)zCH0and C2H6OC(CH3)&HO,photolysis and reactions with NO3 and the reactions of resulting R C 0 3 radicals with other peroxy radicals are also included. Photolysis rates and rate constants relevant to these aldehydes are taken to be the same as those relevant to 1-Dronanal. 2.0
I
I
Table V. Relative Atmospheric Reactivities of Various Organic Compounds" 4
I 4
0
12
16
Flgure 1. Relative incremental reactivities of alcohols and ethers as a function of initial [NMOC]/[NO,]ratio. Lowdilution case (see text).
The LCC chemical mechanism requires the lumping of NMOC components into structurally similar groups. The composition of the initial and aloft NMOC mixtures used in the present study was derived by Jeffries et al. (18) as shown in Table 111. The NMHC composition in the initial NMOC was derived based on NMHC measurements in a number of urban areas and is taken to be the base NMHC mixture. The relative ozone-forming potentials of the various compounds have been calculated by using the incremental reactivity approach developed by Carter and Atkinson (2), as modified by Chang and Rudy (3, 4 ) . Incremental reactivity (IR) for a NMOC species or mixture is defined as IR(a) = [ ~ ( +b Au) - z(b)]/Aa where z ( b) is the maximum ozone concentration calculated with the base NMHC mixture, z ( b + Aa) is the maximum ozone concentration calculated with the test NMOC species added to the base mixture, and Aa is the incremental concentration, in terms of carbon, of the test NMOC species that would be present at the end of the simulation if there were no chemical reactions. The calculated incremental reactivities are reported relative to the incremental reactivity of the base NMHC mixture. Since 418
Environ. Sci. Technol., Vol. 25,
No. 3, 1991
[NMoCl/[No,lb 8 10 14
20
methanol ethanol tert-butyl alcohol dimethyl ether diethyl ether methyl tert-butyl ether ethyl tert-butyl ether
Low Dilution 0.30 0.38 0.55 0.64 0.10 0.16 0.30 0.30 1.61 1.46 0.22 0.26 0.76 0.79
0.51 0.64 0.82 1.18 0.72 0.75 0.65 0.44 0.23 0.32 0.45 0.75 0.40 0.80 1.34 2.31 1.64 2.02 2.77 4.83 0.30 0.48 0.73 1.20 0.88 1.16 1.62 2.93
methanol ethanol tert-butyl alcohol dimethyl ether diethyl ether methyl tert-butyl ether ethyl tert-butyl ether
High Dilution 0.33 0.49 0.60 0.78 0.13 0.21 0.30 0.39 1.49 1.58 0.23 0.32 0.74 0.84
0.56 0.84 0.26 0.64 1.70 0.47 1.00
20
[NMOCI/"l
6
0.64 0.88 0.34 0.90 1.97 0.58 1.20
0.65 0.84 0.48 1.09 2.22 0.70 1.34
0.95 0.78 0.49 1.92 4.03 0.99 2.31
Incremental reactivity relative to that of an average urban NMHC mixture. Initial ratio. 2.0 x c >
.-.c
0
E -
0
.I-
C
-
1.5
0
ETBE
A
Ethanol Methanol
A
MTBE
. 0
/
0-•
TBA
!i
E
.-C W .-
.I-
o W
E
0.0 T
4
i 8
12
16
20
[NMOCI/[NO,]
Figure 2. Relative incremental reactivities of alcohols and ethers as a function of initial [NMOC]/[NO,] ratio. Highdilution case (see text).
incremental reactivities are intended for small changes of the maximum O3 concentration, no more than 5% of the
Table VI. Atmospheric Reactivity of Various Organic Compounds koHlnC,n W2cm3 molecule-’ tert-butyl alcohol methyl tert-butyl ether methanol dimethyl ether ethyl tert-butyl ether ethanol diethyl ether formaldehyde
0.28 0.64 0.9
1.25 1.4 1.7
3.5 11
re1 reactivb [NMOCI / [NO,] = 8 0.23 0.30 0.51 0.40 0.88 0.72 1.64 5.82 (4)
Onc, number of carbon atoms per molecule. *Incremental reactivity relative to that of an average urban NMHC mixture; lowdilution case (see text).
test NMOC is added in the present simulations. Results The results are presented in Table V and Figures 1 and 2 for the range of values of initial [NMOC]/[NO,] ratio between 4 and 20. Table V presents per carbon incremental reactivities (relative to that of the base NMHC mixture) derived for both low- and high-dilution cases over the full NMOC/NO, ratio range. From Table V and Figures 1 and 2, several interesting points emerge. First, in all cases, ethyl tert-butyl ether and diethyl ether are the two most reactive compounds in terms of ozone formation per ppm of carbon equivalent. Second, the least reactive species in all but one case, (low dilution, [NMOC]/[NO,] = 20) is tert-butyl alcohol. Third, the reactivities of methanol, dimethyl ether, and MTBE are similar, with the relative ordering of these compounds being dependent on the conditions. (The incremental reactivities shown in Table V and Figures 1 and 2 are relative to those for the base NMHC mix at each NMOC/NO, ratio. Absolute incremental reactivities for the base NMHC are high a t low and intermediate NMOC/NO, ratios and low at high NMOC/NO, ratios (2, 4 ) . In Table VI we compare the results from Table V a t [NMOC]/[NO,] = 8 with the OH rate constants, per carbon atom, for the ethers and the alcohols. The rate constants are quoted on a per carbon basis because the relative reactivities are calculated on that basis. It can be seen that the relative incremental reactivities of the oxygenates do not increase monotonically with the rate constant for their reaction with OH radicals. For example, while dimethyl ether is significantly more reactive than methanol in terms of OH rate constant, the incremental reactivity of methanol is higher than that of dimethyl ether. This behavior can be understood in terms of the products formed following the attack of OH on methanol and dimethyl ether. Atmospheric oxidation of methanol forms formaldehyde as the sole carbon-containing product. Formaldehyde is very reactive in the atmosphere and promotes the formation of tropospheric ozone. The sole carbon-containing product of dimethyl ether oxidation is methyl formate. In marked contrast to formaldehyde, methyl formate (because of its small OH rate constant) is essentially unreactive in terms of ozone formation in urban areas. Similarly, ethanol is more reactive toward OH than ETBE and yet ETBE has an incremental reactivity slightly higher than that of ethanol. Clearly, in assessments of the impact upon urban ozone of emissions of various organic compounds both kinetic and mechanistic information need to be considered ( 2 ) .
Conclusions Atmospheric modeling calculations have been performed that enable various oxygenated organic compounds to be ranked in terms of their ozone-forming potential, per ppm of carbon equivalent, in a variety of environments. A t an initial [NMOC]/[NO,] ratio of 8, which is generally typical of polluted urban areas in the United States, tert-butyl alcohol, dimethyl ether, methyl tert-butyl ether, and ethyl tert-butyl ether all have incremental reactivities less than or equal to that of the “urban NMHC mix”. The urban NMHC mix is assumed to be similar to the emissions mixture of current gasoline-fueled vehicles. Thus, use of these additives may have a beneficial effect on urban ozone levels. The present work provides the first quantitative assessment of the air quality impact of release of these various important oxygenated fuels/fuel additives. However, limitations of the present work need to be recognized. Specifically, in our calculations we evaluate reactivities, in terms of ppm of carbon, of each material. Final assessment of the impact of these materials on urban ozone levels requires the determination of their vehicle emission rates under realistic operating conditions. It should also be noted that while ozone is a very important secondary pollutant, it is not the sole measure of air quality. Other secondary products, such as peroxyacetyl nitrate (PAN), need to be considered. As shown in the present paper, use of ethanol in environments with [NMOC]/[NO,] > 10 is attractive in terms of reducing ozone formation; however, use of significant amounts of ethanol will lead to higher PAN levels. Another limitation of the present study is that the reactivity derivations are based on one-day simulations. For many urban areas, one-day simulations are useful for understanding ozone formation. However, the highest ozone concentrations are often observed after more than one day of hot, sunny, and stagnant conditions. Consequently, multiday simulations are important to further evaluate reactivities of organic species such as the oxygenated fuel additives discussed here. Further research is required to define in-use emissions for automobiles powered with fuels containing oxygenated additives. Currently such an evaluation is being carried out through an Auto/Oil Air Quality Improvement Research Program. R e g i s t r y No. DME, 115-10-6;DEE, 60-29-7; TBA, 75-65-0; MTBE, 1634-04-4; ETBE, 637-92-3; 03, 10028-15-6; NO,, 11104-93-1.
Literature Cited (1) Guidance on Estimating Motor Vehicle Emission Reductions from the Use of Alternative Fuels and Fuel Blends. U.S. EPA, Report No. EPA-AA-TSS-PA-87-4, Ann Arbor, MI, 1988. (2) Carter, W. P. L.; Atkinson, R. Enuiron. Sci. Technol. 1989, 23, 864. (3) Chang, T. Y.; Rudy, S. J. Atmos. Enuiron. 1990,24A, 2421. (4) Chang, T. Y.; Rudy, S. J. Impact of Organic Emissions from Alternative-Fueled Vehicles on Urban Ozone Air Quality. Proceedings of the International Specialty Conference on Tropospheric Ozone and the Environment, Air and Waste Management Association, Los Angeles, CA, March 1990. ( 5 ) Atkinson, R. Atmos. Enuiron. 1990, 24A, 1. (6) Wallington, T. J.; Liu, R.; Dagaut, P.; Kurylo, M. J. Znt. J. Chem. Kinet. 1988,20, 41. (7) Wallington, T. J.; Dagaut, P.; Liu, R.; Kurylo, M. J. Int. J. Chem. Kinet. 1988,20, 541. (8) Wallington, T. J.; Dagaut, P.; Liu, R.; Kurylo, M. J. Enuiron. Sci. Technol. 1988, 22, 842. (9) Wallington, T. J.; Andino, J. M.; Skewes, L. M.; Siegl, W. 0.;Japar, S. M. Znt. J. Chem. Kinet. 1989,21, 993. Environ. Sci. Technol., Vol. 25,
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Symposium on the Physic0 Chemical Behavior of Atmospheric Pollutants; D. Riedel Publishing Co.: Dordrecht, Holland, 1982; pp 112-119. Japar, S. M.; Wallington, T. J.; Richert, J. F. 0.; Ball, J. C. Int. J . Chem. Kinet. 1990, 22, 1257. Wallington, T. J.; Japar, S. M. Environ. Sci. Technol., preceding article in this issue. Carter, W. P. L.; Atkinson, R. J. Atmos. Chem. 1985,3,377. Hogo, H.; Gery, M. W. User's guide for executing OZIPM-4 with CRM-IV or optional mechanism; U.S. Environmental Protection Agency Report No. EPA/600/8-88/073b; U S . EPA: Research Triangle Park, NC, 1988. Lurmann, F. W.; Carter, W. P. L.; Coyner, L. A. A surrogate
species chemical reaction mechanism f o r urban-scale air quality simulation models. Adaption of the mechanism; U.S. Environmental Protection Agency Report No.
EPA/600/3-87/014a; U.S. EPA: Research Triangle Park, NC. 1987: Vol. I. (16) Wallington, T. J.; Dagaut, P.; Liu, R.; Kurylo, M. J. Znt. J . Chem. Kinet. 1988, 20, 177. (17) Atkinson, R. Chem. Rev. 1986, 86, 69. (18) Jeffries, H. E.; Sexton, K. G.; Arnold, J. R. Validation
testing o f new mechanisms with outdoor chamber data. Analysis of VOC data for the CB4 and CAL photochemical mechanisms; U S . Environmental Protection Agency cooperative agreement No. CR-813107; U S . EPA: Research Triangle Park, NC, 1989; Vol. 2. (19) Wallington, T. J.; Kurylo, M. J. Int. J. Chem. Kinet. 1987, 19, 1015. (20) Atkinson, R., private communication, 1989.
Received f o r review June 8, 1990. Revised manuscript received October 16, 1990. Accepted October 24, 1990.
Biogeochemistry of Arsenic in Natural Waters: The Importance of Methylated Species Linda C. D. Anderson* and Kenneth W. Bruland Earth Sciences Board and Institute of Marine Sciences, University of California at Santa Cruz, Santa Cruz, California 95064
Water samples from a number of lakes and estuaries, mostly in California, showed measurable concentrations of methylated arsenic (equivalent to 1-59% of total As) with the exception of one highly alkaline lake. Neither depleted phosphate concentrations nor high dissolved salts correlated with the appearance of methylated forms of As. A temporal study of As speciation in Davis Creek Reservoir, a seasonally anoxic lake in northern California, demonstrated that dimethylarsinic acid increased sufficiently to become the dominant form of dissolved As within the surface photic zone during late summer and fall. Methylated forms decreased while arsenate increased when the lake over-turned in early December, which suggested a degradation of dimethylarsinic acid to arsenate.
Introduction Dissolved arsenic can occur in natural waters in both inorganic and organic forms. Arsenic's inorganic forms include formal oxidation states As(V), arsenate, and As(111),arsenite, with primary aqueous species at natural pHs being anionic in arsenate (H2As0,- and HAsOZ-) or neutral for arsenite (As(OH),O). The location of As on the periodic table directly below phosphorus predicts an analogous chemical behavior for arsenate and phosphate including incorporation into organic molecules. However, As has no recognized use in enzymatic systems and could potentially interfere with numerous biological mechanisms normally dependent on phosphorus ( I ) . It has been suggested that organisms have developed mechanisms to isolate and detoxify As by producing organoarsenicals (2). In addition, incorporation of As into arsonium zwitterions such as arsenobetaine and arsenocholine may serve dual purposes of detoxification and osmoregulation analogous to some sulfur compounds ( 3 ) . Identification of organoarsenicals produced during culture experiments with bacteria, fungi, and algae demonstrate that biosynthesis of methylated arsenicals is common ( 4 , 5). Dissolved As compounds measured in culture exudates include arsine (ASH,), monomethylarsonic 420
Environ. Sci. Technol.,Vol. 25, No. 3, 1991
Table I. Chemical Forms of Arsenic Observed in Water Samples, Culture Exudates, or Tissue Extractions name
chem formula
Water Samples or Culture Exudates arsenate HZASO, arsenite As(OH)3 arsine ASH^ monomethylarsonate CH~ASO~OH(MMAA) dimethylarsenate (DMAA) (CH,),AsOOdimethylarsine (DMA) (CH,),AsH trimethylarsine (TMA) (CH,),As trimethylarsine oxide (CHJ3AsO (TMAO) arsenobetaine arsenocholine arsenoribosides (e.g.)
Tissue Culture Extracts (CH,),As+CH,COOH (CHjj,As+(CH2),0H
YY
(CHd2h-CHz
arsenophospholipids (e.g.)
OH
OCHpCHOHCHpR
OH
CHp-
I I
0
CH
I
CH-O-P-O-CH,CH~~S+(CH& ~
b~~~~~
~
acid (MMAA), dimethylarsinic acid (DMAA), dimethylarsine (DMA), trimethylarsine oxide (TMAO), and trimethylamine (TMA) (Table I). At natural pHs, MMAA and DMAA occur as anionic monomethylarsenate (CH3As020H-) and dimethylarsinate ((CH,),AsOO-j, respectively. Confusion exists in the literature over whether the formal oxidation state of As in TMAO, DMAA, and MMAA is As(II1) or As(V). A recent review by Cullen and Reimer (5) states that these compounds all contain As(V). In addition, a large diversity of more complex organic compounds has been measured in tissue extractions, including arsenobetaine, arsenocholine, arsenoribosides, and arsenophospholipids (Table I). The importance of biotic transformations of metal(1oid)s is becoming increasingly recognized. Natural occurrences
0013-936X/91/0925-0420$02.50/0
0 1991 American Chemical Society
