- Email: [email protected]
Computer modeling study of ethanol and aldheyde reactivities in Rio de Janeiro urban air
AE International – Central & South America Atmospheric Environment 37 (2003) 1715–1722
Computer modeling study of ethanol and aldheyde reactivities in Rio de Janeiro urban air Eduardo M. Martins, Graciela Arbilla* Departamento de Fisico-Quimica, Universidade Federal do Rio de Janeiro, Instituto de Qu!ımica, Cloco A. Cidade Universitaria, Sala 408, Ilha do Fundao, * 21949-900 Rio de Janeiro, RJ, Brazil Received 2 October 2002; accepted 29 January 2003
Abstract An empirical kinetic modeling approach was used in order to simulate ozone, formaldehyde and acetaldehyde concentrations for an urban downtown area with high vehicular traffic. A base case was designed using experimental data for December 1999 in Presidente Vargas Avenue, Rio de Janeiro, Brazil. The simulated ozone peak was obtained at 3:15 PM (23.00 ppb) in good agreement with experimental results. Calculated acetaldehyde and formaldehyde concentrations peaked at 21–24 and 14–17 ppb, respectively, between 11:00 AM and 13:00 PM, depending on the effective solar flux. Tagged species were used to discriminate between primary and secondary compounds and to follow the reaction paths of individual species. Computational results show that the primary contribution to acetaldehyde and formaldehyde concentrations is higher in the early morning and reaches a maximum, which approximately coincides with the peak automobile traffic. The secondary contribution increases during the morning and reaches a maximum in the early afternoon. As a consequence, total acetaldehyde and formaldehyde levels peak after the primary pollutants’ maximum, obtained in the early morning, and before the secondary pollutants’ maximum as previously noted in experimental determinations. The simulations also confirm the experimental evidence that the high acetaldehyde/ formaldehyde ratios are due to the use of alcohol-based fuels. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Formaldehyde; Acetaldehyde; Vehicular emissions; Ethanol; Air quality simulation
1. Introduction As noted by Nguyen et al. (2001), since the 1980s, there has been an increased interest in the use of ethanol and other alcohol fuels as alternative energy sources. The use of alcohol fuels minimize the country’s dependence on petroleum and lead to elimination of tetraethyl lead, which is an anti-knock additive, and the reduction of CO and hydrocarbon emissions (Howard et al., 1997; Schifter et al., 2001a, b). There is ample evidence that the use of ethanol-based fuels leads to an increase of acetaldehyde levels and higher acetaldehyde-
*Corresponding author. Tel.: +55-21-2562-7755; fax: +5521-2562-7265. E-mail address: [email protected] (G. Arbilla).
to-formaldehyde ratios (Grosjean et al., 1990, 2002; Nguyen et al., 2001; Martins et al., 2003). Acetaldehyde-to-formaldehyde data depend on emissions and on the detailed chemistry. Recently, the use of this data as an anthropogenic-to-biogenic ratio has been discussed (Nguyen et al., 2001). In spite of the experimental evidence that the use of oxygenated fuels can be a potential emission source of aldehydes and secondary pollutants (Gaffney et al., 1997; Tanner et al., 1988), to our knowledge, a detailed mechanistic simulation to assess the relative importance of emissions and chemical transformations has not been accomplished. The main goal of this work is to simulate the atmospheric composition of a typical urban scenario where ethanol-based fuels are used and estimate the contribution of vehicle emissions to the aldehyde’s formation.
1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00081-5
1716
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
The base case was designed on the basis of experimental data obtained at Presidente Vargas Avenue, Rio de Janeiro, Brazil, during December 1999. The metropolitan area of Rio de Janeiro has about 6 million inhabitants distributed over a 6500 km2 area. The main source of emissions in the central area of the city is the vehicular fleet fueled with hydrated ethanol, gasohol (a mixture of 78% gasoline and 22% of anhydrous ethanol), diesel and, in a minor extent, natural gas. Presidente Vargas Avenue traffic includes light-duty vehicles as well as diesel-fueled heavy-duty vehicles, with diurnal fluxes (from Mondays to Fridays) between 6000 and 8000 vehicles/h, depending on the hour of the day. It is estimated to be about 17.5% vehicles using hydrated ethanol and 66.0% using gasohol (Campos et al., 1999; CET-Rio, 1999). Recent experimental data (Martins et al., 2003), obtained in the period from December 1998 to January 2001, gave mean concentration values of 13.69 ppb for formaldehyde and 13.36 ppb for acetaldehyde. The sampling was done at the monitoring station located at 22 540 S and 43 100 W. The high acetaldehyde/ formaldehyde ratio was considered, as in previous reports, a consequence of the use of oxygenated fuels. In this paper, the kinetics processes leading to the production and consumption of acetaldehyde and formaldehyde are analyzed.
2. Methodology Computer simulations were performed using an empirical trajectory model implemented in OZIPR (Gery and Crouse, 1990). The photochemical mechanism was based on the SAPRC model (Carter, 1990). Reactions of ethanol were included and stoichiometric coefficients and rate constants were re-calculated for a volatile organic compound (VOC) mixture characteristic of Brazilian cities (de Oliveira and Arbilla, 1999). Organic compounds were described using the lumpedspecies approach. The lumped species included two groups of alkanes (compounds with rate constants for reaction with OH radical lower and higher than 3 1012 cm3 molecule1 s1), three groups of aromatics (toluene and ethylbenzenes, o-, m- and p-xylenes, and higher molecular weight aromatics), two groups of alkenes (propene and higher terminal olefins and non-terminal acyclic alkenes with CX4) and a group of higher aldehydes (CXC3). Other compounds such as ethylene, acetylene, propane, benzene, ethanol, acrolein, formaldehyde and acetaldehyde, were described explicitly. A base case was designed using experimental data for December 1999. Simulated results were compared with experimental data in order to validate the model (Arbilla et al., 2002). Finally, tagged species were used to discriminate between primary and secondary com-
pounds and to follow the reaction paths of individual species.
3. Description of the base case As previously described, experimental data used to set the model’s initial conditions were collected at Presidente Vargas Avenue, Rio de Janeiro, Brazil. Meteorological data and criteria pollutant concentrations were obtained by FEEMA between 1 December and 29 December 1999 (FEEMA, 1999). Hourly mean values were calculated for the period. Rainy days were not taken into account because, in these conditions, the main effect is the wash-out of the atmosphere. Initial mixing heights were estimated from the atmosphere temperature and pressure profiles available at IAG-USP home page (IAG-USP, 1999). Diurnal variation and final mean values were selected on the basis of simulated data (Maia, 2000), since there were no available experimental data. The main effect of an increase in the mixing height is the dilution of gases. Since, in this work, the absolute values of CO emissions were used as adjustable parameters, an error in the mixing height may be corrected by changing the emissions. Experimental meteorological input parameters as well as estimated mixing heights are presented in Table 1. The initial concentrations of CO (1.7 ppm), NO (0.11 ppm), NO2 (0.05 ppm) and total VOC (0.4 ppmC) were calculated using the average hourly concentrations measured at 8:00 AM. Since, to our knowledge, there is no published information regarding speciated VOC vehicle emissions in Rio de Janeiro, speciated ambient VOC values (samples collected in downtown Porto Alegre, Brazil, between March 1996 and April 1997) were used Table 1 Input parameters used for the simulation of the base case Hour
Relative humidity (%)
Temperature ( C)
Mixing height (m)
8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00
69.18 65.13 63.46 60.80 57.12 54.56 51.93 51.46 50.81 51.62 54.50 57.31
24.74 25.72 26.76 27.54 28.65 29.16 29.50 29.54 29.17 28.59 27.81 27.05
500 500 700 850 1300 1600 2500 1900 1650 1420 1380 1200
Temperature and relative humidity are average experimental values for December 1999. Mixing heights are estimated parameters.
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
ratios between 45.0 and 5.0 ppb, peaking at about 14:00 PM, and with a mean maximum value of 19.1 ppb (Arbilla et al., 2002). The time profile is shown in Fig. 2. The constructed base case was considered representative of average summer time air quality at Presidente Vargas Avenue, Rio de Janeiro. Calculated acetaldehyde and formaldehyde concentrations peaked at 21–24 and 14–17 ppb, respectively between 11:00 AM and 13:00 PM, depending on the effective solar flux. These results are compatible with average values, for the period 8:00 AM–4:00 PM, of 4 December 1998 (21 ppb for acetaldehyde and 19 ppb for formaldehyde). Experimental results are shown in Fig. 3 (Martins et al., 2003). Recent published results report an average ethanol ambient concentration of 176.3 ppb for S*ao Paulo,
Experimental Calculated
3
CO Concetration/ppm
(Grosjean et al., 1998a–c). This approach assumes that ambient VOC compositions are similar for Rio de Janeiro and Porto Alegre, where vehicle emissions are predominant. This may not be rigorously valid but is the only possible approach considering the lack of experimental information. These data are similar to more recent results obtained, in 1999, for Rio de Janeiro (Grosjean, 2000). Acetaldehyde and formaldehyde values were selected using experimental data for Presidente Vargas Avenue (Martins et al., 2003). The data are also consistent with previous results obtained for Rio de Janeiro in 1985 (15.0 and 17.3 ppb for formaldehyde and acetaldehyde, respectively) (Tanner et al., 1988). Ethanol mixing ratio was estimated on the basis of experimental data published by Pereira et al. (1999) for Rio de Janeiro. Values used in the simulation are presented in Table 2. The VOC/NOx/CO emission ratio was calculated in consistence with the vehicle emission inventory for Rio de Janeiro (Campos et al., 1999) and local vehicle flux (VOC/CO=0.173 and NOx/CO=0.157, both on mass basis). This ratio was kept constant during the day and was considered as a non-adjustable parameter. CO emissions were set using measured ambient concentrations (the average hourly values for December 1999) as a rough guide. This approach is valid since CO has the lowest reaction rate in the VOC mixture and its concentration is only slightly affected by OH removal. Fig. 1 shows the experimental and calculated results for CO. The fit is quite good. A further adjustment was not performed since the main goal was to simulate a representative case of downtown Rio de Janeiro, and not to exactly reproduce a particular day and scenario. The simulated ozone peak was obtained at 3:15 PM (23.00 ppb) in good agreement with experimental results for December 1999 that show maximum ozone mixing
1717
2
1
0 8
10
12
14
16
18
20
Time/hours
Fig. 1. Calculated CO time profile for the base case and average experimental values for December 1999, at the FEEMA-Presidente Vargas Avenue monitoring station.
Table 2 Average composition of volatile organic compounds used for the simulation of the base case (units are ppbC in 1 ppbC total) Compound or group
Fraction (on a ppmc basis)
Compound or group
Fraction (on a ppmc basis)
Compound or group
Fraction (on a ppmc basis)
Alkanes 1 (C2–C6)
0.172
0.136
Formaldehyde
0.014
Alkanes 2 (CXC7)
0.111
0.049
Acetaldehyde
0.028
Ethene
0.075
0.049
Non-reactive
0.006
Propene and higher terminal olefins Acyclic alkenes CXC4 (non-terminal)
0.081
Aromatics 2 (toluene and mono-substituted benzenes) Aromatics 2 (xylenes) Aromatics 3 (reactive aromatics of higher molecular weight) Higher aldehydes
0.008
Acroleine
0.011
0.075
Butadiene
0.075
Ethanol
0.062
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
1718 40
Experimental Calculated O3 Concentration/ppb
30
20
10
0
8
10
12
14
16
18
20
Time/hours
Fig. 2. Calculated ozone time profile for the base case and average experimental values for December 1999, at the FEEMA-Presidente Vargas Avenue monitoring station.
Concentration, ppb
30
Formaldehyde
25
Acetaldehyde
Brazil (Nguyen et al., 2001). This value is significantly higher than previous reported data for S*ao Paulo, Rio de Janeiro and Salvador Cities (36.2, 66.4 and 65.4, respectively) (Pereira et al., 1999). In order to estimate the possible impact of ethanol level uncertainties, two simulations were performed: in the first scenario the ethanol fraction in the VOC mixture was set as 0.062 (in a ppbC basis) and the initial concentration of ethanol and VOC (8:00 AM) as 12.4ppb and 0.400 ppmC, respectively. In the second case, the ethanol fraction was increased to 0.57 and the initial concentration to 250 ppb. Initial concentration and emission ratio of total VOC were increased in order to conserve unchanged the concentration (in ppmC) of other VOC. Results for ozone, formaldehyde and acetaldehyde for both scenarios are shown in Table 3. Simulated peak concentrations for the second scenario are 48.6%, 15.8% and 52.2% higher for ozone, formaldehyde and acetaldehyde, respectively. The first scenario with an ethanol fraction of 0.062 is in good agreement with the simulated results of Tanner et al. (1988) and experimental data of Grosjean et al. (1990). The implications of higher ethanol levels will be discussed in the following section.
20 15
4. Results and discussion
10 5 0 08:30
10:30 12:30 Time of day
14:30
Fig. 3. Formaldehyde and acetaldehyde experimental concentrations obtained at FEEMA-Presidente Vargas Avenue monitoring station on December 4, 1998.
Formaldehyde and acetaldehyde isopleths calculated for the base case are displayed in Figs. 4 and 5. The results show that acetaldehyde and formaldehyde concentrations are highly dependent on total VOC concentration. The non-linear dependence is an indication of a further source of aldehydes besides the direct emission of VOC. Also, the slight dependence on NOx
Table 3 Simulated ozone, formaldehyde and acetaldehyde concentrations (in ppb units), and acetaldehyde/formaldehyde ratios for the base case (ethanol fraction=0.062 and initial concentration of 12.4 ppb) and for a second scenario with higher emissions of ethanol (ethanol fraction=0.57 and initial concentration of 250 ppb) (see text for details) Time of the day
8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00
Base case
Scenario with higher ethanol emissions
Ozone
Formaldehyde
Acetaldehyde
Ratio
Ozone
Formaldehyde
Acetaldehyde
Ratio
0 2.712 4.105 7.453 10.24 15.05 21.03 23.01 22.75 18.11 9.324 0.571
5.60 9.292 11.66 14.27 14.23 14.58 11.04 11.04 12.58 13.06 13.56 13.89
5.60 11.29 16.34 21.30 20.38 20.17 14.73 14.73 14.77 14.74 14.78 14.86
1.00 1.22 1.40 1.49 1.43 1.38 1.33 1.33 1.17 1.13 1.09 1.07
0 2.807 4.520 8.722 12.57 19.74 27.90 33.35 34.08 18.86 17.65 4.475
5.60 9.237 11.63 14.39 14.65 15.38 12.00 13.50 14.68 15.69 16.53 17.05
5.60 12.48 20.30 29.55 30.23 32.35 24.69 27.36 29.07 30.15 30.75 31.00
1.00 1.35 1.74 2.05 2.06 2.10 2.05 2.03 1.98 1.92 1.86 1.82
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722 0.40
0.40
0.35
0.35
0.018
0.30
1719
0.01
0.30
0.021
0.01
0.25 0.20
NOx (ppm)
NOx (ppm)
0.014
0.011
0.15
0.004
0.025
0.25
0.02
0.20
0.03 0.04
0.15
0.007 0.10 0.05
0.05
0.00
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
16
28 20
12 0.2
24
32 36
0.1
24
Formaldehyde, ppb
NOx (ppm)
8
0.1
0.2
0.3
0.3
0.4
16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1
0.5
0.6
0.7
Primary Formaldehyde Secondary Formaldehyde Total Formaldehyde
8
0.0 0.0
0.2
0.06
Fig. 6. Ozone isopleths calculated for the base case, Rio de Janeiro, Brazil, December 1999.
0.5
4
0.1
0.09 0.08
VOC (ppmC)
Fig. 4. Formaldehyde isopleths calculated for the base case, Rio de Janeiro, Brazil, December 1999.
0.3
0.07
0.05 0.0
VOC (ppmC)
0.4
0.07
0.10
0.028 0.032 0.025
0.4
0.5
0.6
VOC (ppmC) Fig. 5. Acetaldehyde isopleths calculated for the base case, Rio de Janeiro, Brazil, December 1999.
levels is a consequence of the formaldehyde and acetaldehyde formation in the photooxidation of VOC, through the chain reactions of hydroxyl radicals with organics and the conversion of NO to NO2. For comparison, ozone isopleths are shown in Fig. 6. Figs. 7 and 8 show the formaldehyde and acetaldehyde profiles as well as the primary and secondary contributions to the total concentration. For the purposes of the simulation, the direct emissions and the initial aldehyde fraction in the VOC mixture were considered as primary sources and the ethanol fraction was set as 0.062. The primary contribution is higher in the early morning and reaches a maximum at a time that approximately coincides with peak automobile traffic. Secondary contributions are relatively low in the early morning, increase at about noon and reach a maximum
10
12
14
16
18
20
Time of day
Fig. 7. Primary, secondary and total formaldehyde calculated for the base case.
at about 1:00 PM. The high concentrations of secondary aldehydes after 2:00 PM may be due, in part, to the trapping of the pollutants bellow the inversion layer in the afternoon. At noon, the contributions of secondary formaldehyde and acetaldehyde are 65.5% and 69.4%, respectively. When considering an ethanol fraction of 0.57, these values are 81.81% and 94.69% for formaldehyde and acetaldehyde, respectively. The calculated formaldehyde and acetaldehyde peaks were centered at about noon (12:30 and 11:20 AM, respectively). Both, the experimental and simulated carbon monoxide maximum were obtained at about 9:00 AM and the ozone peak after 2:00 PM. Experimental values for aldehydes obtained at Rio de Janeiro (Martins et al., 2003) and Salvador, Brazil (de Andrade, 2001) also show a peak at about noon. The fact
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
1720
Table 4 Contribution of alkenes, ethanol and other volatile organic compounds to the formation of formaldehyde and acetaldehyde for the conditions of the base case
25 Primary Acetaldehyde Secondary Acetaldehyde Total Acetaldehyde
Acetaldehyde ,ppb
20
Time of day
15
10
5
Alkenes (%)
Ethanol (%)
Other sources (%)
Formaldehyde 10:00 12:00 15:00 17:00
67.40 54.29 51.70 47.86
4.41 6.61 0.73 0.82
28.19 39.61 47.57 51.32
Acetaldehyde 10:00 12:00 15:00 17:00
93.98 91.72 87.90 85.66
3.10 4.07 5.89 6.90
2.92 4.21 6.21 7.44
0
8
10
12
14
16
18
20
Time of day
Fig. 8. Primary, secondary and total acetaldehyde calculated for the base case.
that experimental time–concentration profiles show a maximum which is shifted about 3 h from primary pollutant peaks (mainly CO and NO) was considered as an evidence of both primary and secondary contributions to aldehyde levels (Martins et al., 2003). Figs. 7 and 8 are in good agreement with that experimental evidence. The simulated acetaldehyde/formaldehyde ratio (on a ppb basis) is shown in Table 3. Values higher than 1 are also in agreement with experimental data (Martins et al., 2003). As shown in Table 3, higher emissions of ethanol lead to higher acetaldehyde/ formaldehyde ratios. At noon, the ratio is 44% higher and at evening about 70% higher. This fact may be attributed to the faster photochemical processes consuming formaldehyde. Ethanol and alkene contributions to secondary formaldehyde and acetaldehyde production are displayed in Table 4. Clearly, alkenes are the most reactive species and the main source of aldehydes. Ethanol is much more stable than alkenes with a rate constant for reaction with hydroxyl radical of 3.2 1012 cm3 molecule1 s1, in comparison with rate constants of 3.14 1011 and 6.37 1011 cm3 molecule1 s1 for 1-butene trans-2-butene, respectively. Considering a mixing ratio of 0.062, ethanol contributes with less than 7% of secondary production of acetaldehyde in the evening. This value is increased to 24.38% for a mixing ratio of 0.57. This value may be considered an upper limit to formaldehyde and acetaldehyde production from ethanol oxidation. The formaldehyde and acetaldehyde reactions were also investigated. The four possible facts of aldehydes in urban atmosphere are photolysis and reaction with OH, HO2 and NO3 radicals. The relative contribution of each reaction path, for the conditions of the base case, are shown in Table 5. During daylight hours, photolysis of formaldehyde is comparable to oxidation by OH radical. The rate of
photolysis is higher in the morning and early afternoon while OH reaction becomes more important after 3:00 PM due to increase of pollutant levels and the decrease of solar flux. Both, photodissociation paths: HCHO þ hn-H þ HCO;
ð1Þ
HCO þ O2 -HO2 þ CO
ð2Þ
and HCHO þ hn-H2 þ CO
ð3Þ
and OH attack: HCHO þ OH-HO2 þ HCO; HCO þ O2 -HO2 þ CO
ð4Þ
ð2Þ
lead to the formation of CO, while reactions (1) and (4) produce HO2 radicals which would further react with NO recycling OH radicals and reforming NO2. The main path for acetaldehyde photodecomposition is CH3 CHO þ hn’CH3 þ HCO:
ð5Þ
The radical formation rate from the photolysis of acetaldehyde is much less (approximately and order of magnitude) than that from formaldehyde (see Table 5). The main reaction path for acetaldehyde is hydrogen abstraction by OH radical leading to the formation of acetyl radical followed by the following reactions: CH3 CHO þ O2 -CH3 CðOÞ2 O;
ð6Þ
CH3 CðOÞ2 O þ NO-CH3 COO þ NO2 ;
ð7Þ
CH3 COO-CH3 þ CO2 :
ð8Þ
Methyl radicals, formed in reactions (5) and (8), may be further oxidized to formaldehyde. Reaction (7)
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
1721
Table 5 Relative contribution of each reaction path to the consumption of formaldehyde and acetaldehyde for the conditions of the base case Reaction
HCHO+hn+O2=2HO2+CO HCHO+hn=CO+H2 HCHO+OH+O2=HO2+CO+H2O HCHO+NO3+O2=HNO3+HO2+CO HCHO+HO2=OOCH2OH CH3CHO+O2+hn=CO+HO2+CH3 CH3CHO+OH=CH3CO+H2O CH3CHO+NO3=HNO3+CH3CO
Time of day 10:00
12:00
15:00
17:00
35.11% 45.69% 19.15% 0.00% 0.05% 15.35% 84.64% 0.01%
28.91% 40.58% 30.38% 0.01% 0.12% 7.84% 92.16% 0.00%
10.51% 34.67% 54.44% 0.00% 0.38% 1.45% 98.53% 0.02%
0.46% 1.26% 97.42% 0.03% 0.83% 0.00% 100.00% 0.00%
Table 6 Relative contribution of each reaction path to the lifetimes of formaldehyde and acetaldehyde for the conditions of the base case Reaction
Lifetimes (min)
HCHO+hn+O2=2HO2+CO HCHO+hn=CO+H2 HCHO+Oh+O2=HO2+CO+H2O HCHO+NO3+O2=HNO3+HO2+CO HCHO+HO2=OOCH2OH CH3CHO+O2+hn=CO+HO2+CH3 CH3CHO+Oh=CH3CO+H2O CH3CHO+NO3=HNO3+CH3CO
961 684 909 1.1 108 2.2 105 6666 394 2.45 107
competes with the formation of peroxy acetyl nitrate (PAN; reaction 9), which is thermally unstable and decomposes back into NO2 and peroxyacetyl, which will further react: CH3 CðOÞ2 O þ NO2 3CH3 CðOÞ2 ONO2 :
ð9Þ
As pointed out by Gaffney and Marley (2001), the decomposition of PAN may lead to the formation of formaldehyde and CO as well as convert a number of NO molecules to NO2, which will produce ozone. PAN acts as a reservoir compound and is also an important oxidant. Unfortunately, there are no recent experimental results for PAN concentrations in Rio de Janeiro. The slow decomposition of formaldehyde and acetaldehyde may be one of the reasons for the relatively high concentrations in the afternoon. In Table 6, the lifetimes of formaldehyde and acetaldehyde with OH are compared. The calculations were performed at noon for the conditions of the base case. Lifetimes for formaldehyde and acetaldehyde with respect to oxidation are 15.2 and 6.6 h, respectively. The overall photolytic lifetimes were estimated as 6.7 and 111.1 h for formaldehyde and acetaldehyde, respectively. Overall lifetimes, at noon, are 4.6 and 6.2 h for formaldehyde and acetaldehyde, respectively. HO2 reacts quite readily with formaldehyde forming an intermediate, which can decompose back to
HCOH+HO2. Since the reverse decomposition is also fast, in comparison to alternate reactions of the adduct, little net loss of formaldehyde occurs by this path of reaction. Since oxygenated fuel use is currently being considered in many countries, the potential for the emission of increased aldehydes and the reactivity of the exhausted primary pollutants are of current interest. The use of a separate representation of primary and secondary aldehydes in the model provided some insights about the role of ethanol in the formation of formaldehyde and acetaldehyde. As shown by the calculations, ethanol makes relatively minor contributions to acetaldehyde formation. The main contributions are the primary emissions, due to the incomplete combustion of ethanolbased fuels, and the secondary formation in the photooxidation reactions of alkenes and hydroxyl radicals. It must be noted that the acetaldehyde/ formaldehyde emission ratio is higher than the measured value for no-oxygenated fuels (Martins et al., 2003; Miguel and de Andrade, 1990; Schifter et al., 2001a) and also the acetaldehyde/formaldehyde secondary formation ratio is higher than 1, leading to a higher total acetaldehyde/formaldehyde formation ratio.
5. Conclusions These simulations confirm the experimental evidence that the high acetaldehyde/formaldehyde ratios are due to the use of alcohol-based fuels. Computational results show that the primary contribution to acetaldehyde and formaldehyde concentrations is higher in the early morning and reaches a maximum which approximately coincides with the peak automobile traffic. The secondary contribution increases during the morning and reaches a maximum at noon and early afternoon. As a consequence, total acetaldehyde and formaldehyde levels peak after primary pollutants’ maximum obtained in the early morning (rush hour)
1722
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
and before the secondary pollutants (for example, ozone).
Acknowledgements The authors thank the partial financial support from CNPq, CAPES and FINEP/CTPetro and the kindly collaboration of FEEMA (Rio de Janeiro, Brazil).
References Arbilla, G., Martins, E.M., Moreira, A., Moreira, L.F.R., 2002. Ozone air quality modeling. A case study: a heavily vehicle impacted urban avenue in Rio de Janeiro, Brazil. Journal of the Brazilian Chemical Society 13, 308–317 (http://jbcs.sbq. org.br/). Campos, I.C.B., Pimentel, A.S., Corr#ea, S.M., Arbilla, G., 1999. Simulation of air pollution from mobile source emissions in the City of Rio de Janeiro. Journal of the Brazilian Chemical Society 10, 203–208 (http://jbcs.sbq. qorg.br/). Carter, W.P.L., 1990. A detailed mechanism for the gas-phase atmospheric reactions of organic compounds. Atmospheric Environment 24A, 481–516. CET-Rio, 1999. Companhia de Engenharia de Tr!afego-CETRio. Internal Reports. Private Communication. See also: http://transito.rio.rj.gov.br and http://www.detran.rj.gov. br. de Andrade, M.V.A.S., 2001. Estudo de oxidantes fotoqu!ımicos # no ar: Compostos carbon!ılicos, ozonio e PAN. Ph.D. These, Universidade Federal da Bahia, Brazil. de Oliveira, K.M.P.G., Arbilla, G., 1999. Otimiza@*ao de um mecanismo fotoqu!ımico para a simula@*ao da atmosfera urbana brasileira. Qu!ımica Nova 22, 790–800. FEEMA, 1999. Funda@*ao Estadual de Engenharia do Meio Ambiente, Rio de Janeiro, RJ, Brazil. Internal Reports. Private Communication. Gaffney, J.S., Marley, N.A., 2001. Comments on environmental implications on the oxygenation of gasoline with ethanol in the Metropolitan Area of Mexico City. Environmental Science and Technology 35, 4957–4958. Gaffney, J.S., Marley, N.A., Martin, R.S., Dixon, R.W., Reyes, L.G., Popp, C.J., 1997. Potential air quality effects of using ethanol–gasoline fuel blends: a field study in Albuquerque, New Mexico. Environmental Science and Technology 31, 3053–3061. Gery, M.W., Crouse, R.R., 1990. User’s Guide for Executing OZIPR. US Environmental Protection Agency, Research Triangle Park, NC, EPA-9D2196 NASA 1990. Grosjean, D., 2000. Private Communication. Grosjean, D., Miguel, A.H., Tavares, T., 1990. Urban air pollution in Brazil: acetaldehyde and other carbonyls. Atmospheric Environment 22, 1637–1648.
Grosjean, D., Grosjean, E., Moreira, L.F.R., 2002. Speciated ambient carbonyls in Rio de Janeiro, Brazil. Environmental Science and Technology 36, 1389–1395. Grosjean, E., Grosjean, D., Gunawardena, R., Rasmussen, R.A., 1998a. Ambient concentrations of ethanol and methyl-terc-butyl ether in Porto Alegre, Brazil, 1996 March–1997 April. Environmental Science and Technology 32, 736–742. Grosjean, E., Grosjean, D., Rasmunssen, A.R., 1998b. Ambient concentrations, sources, emission rates and photochemical reactivity of C2–C10 hydrocarbons in Porto Alegre, Brazil. Environnmental Pollution 32, 3371–3379. Grosjean, E., Rasmussen, R.A., Grosjean, D., 1998c. Ambient levels of gas phase pollutants in Porto Alegre, Brazil. Atmospheric Environment 32, 3371–3379. Howard, C., Russell, A., Atkinson, R., Calvert, J., 1997. Air Quality Effects of the Winter Oxyfuel Program. White House Office of Science and Technology Policy, Washington, DC, July 2. IAG-USP, 1999. http://master.iag.usp.br/. Data obtained in December 1999 and January 2000. Maia, L.F.P.G., 2000. Private Communication. Meteorological Department, Instituto de Geoci#encias, Universidade Federal do Rio de Janeiro. Martins, E.M., Corr#ea, S.M., Arbilla, G., 2003. Formaldehyde and acetaldehyde in a high traffic street of Rio de Janeiro, Brazil. Atmospheric Environment 37, 23–29. Miguel, A.H., de Andrade, J.B., 1990. Catalyst and noncatalyst exhaust aldehydes emissions from Brazilian ethanol-fueled vehicles. Journal of the Brazilian Chemical Society 1, 124–127. Nguyen, H.T., Takenaka, N., Bandow, H., Maeda, Y., De Oliva, S.T., Botelho, M.M.F., Tavares, T.M., 2001. Atmospheric alcohols and aldehydes concentrations measured in Osaka, Japan and in S*ao Paulo, Brazil. Atmospheric Environment 35, 3075–3083. Pereira, P.A.P., Santos, E.T.S., Ferreira, T.F., Andrade, J.B., 1999. Determination of methanol and ethanol by gas chromatography following air sampling florisil cartridges and their concentrations at urban sites in the three largest cities in Brazil. Talanta 49, 245–252. Schifter, I., Vera, M., D!ıaz, L., Guzman, E., Ramos, F., LopezSalinas, E., 2001a. Environmental implications on the oxygenation of gasoline with ethanol in the Metropolitan Area of Mexico City. Environmental Science and Technology 35, 1893–1901. ! Schifter, I., Vera, M., D!ıaz, L., Guzm!an, E., Ramos, F., LopesSalinas, E., 2001b. Response to comment on environmental implications on the oxygenation of gasoline with ethanol in the metropolitan area of Mexico City. Environmental Science and Technology 35, 4959–4960. Tanner, R.L., Miguel, A.H., de Andrade, J.B., Gaffney, J.S., Streit, G.E., 1988. Atmospheric chemistry of aldehydes: enhanced peroxyacetyl nitrate formation from ethanolfueled vehicular emission. Environmental Science and Technology 22, 1026–1034.
Computer modeling study of ethanol and aldheyde reactivities in Rio de Janeiro urban air Eduardo M. Martins, Graciela Arbilla* Departamento de Fisico-Quimica, Universidade Federal do Rio de Janeiro, Instituto de Qu!ımica, Cloco A. Cidade Universitaria, Sala 408, Ilha do Fundao, * 21949-900 Rio de Janeiro, RJ, Brazil Received 2 October 2002; accepted 29 January 2003
Abstract An empirical kinetic modeling approach was used in order to simulate ozone, formaldehyde and acetaldehyde concentrations for an urban downtown area with high vehicular traffic. A base case was designed using experimental data for December 1999 in Presidente Vargas Avenue, Rio de Janeiro, Brazil. The simulated ozone peak was obtained at 3:15 PM (23.00 ppb) in good agreement with experimental results. Calculated acetaldehyde and formaldehyde concentrations peaked at 21–24 and 14–17 ppb, respectively, between 11:00 AM and 13:00 PM, depending on the effective solar flux. Tagged species were used to discriminate between primary and secondary compounds and to follow the reaction paths of individual species. Computational results show that the primary contribution to acetaldehyde and formaldehyde concentrations is higher in the early morning and reaches a maximum, which approximately coincides with the peak automobile traffic. The secondary contribution increases during the morning and reaches a maximum in the early afternoon. As a consequence, total acetaldehyde and formaldehyde levels peak after the primary pollutants’ maximum, obtained in the early morning, and before the secondary pollutants’ maximum as previously noted in experimental determinations. The simulations also confirm the experimental evidence that the high acetaldehyde/ formaldehyde ratios are due to the use of alcohol-based fuels. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Formaldehyde; Acetaldehyde; Vehicular emissions; Ethanol; Air quality simulation
1. Introduction As noted by Nguyen et al. (2001), since the 1980s, there has been an increased interest in the use of ethanol and other alcohol fuels as alternative energy sources. The use of alcohol fuels minimize the country’s dependence on petroleum and lead to elimination of tetraethyl lead, which is an anti-knock additive, and the reduction of CO and hydrocarbon emissions (Howard et al., 1997; Schifter et al., 2001a, b). There is ample evidence that the use of ethanol-based fuels leads to an increase of acetaldehyde levels and higher acetaldehyde-
*Corresponding author. Tel.: +55-21-2562-7755; fax: +5521-2562-7265. E-mail address: [email protected] (G. Arbilla).
to-formaldehyde ratios (Grosjean et al., 1990, 2002; Nguyen et al., 2001; Martins et al., 2003). Acetaldehyde-to-formaldehyde data depend on emissions and on the detailed chemistry. Recently, the use of this data as an anthropogenic-to-biogenic ratio has been discussed (Nguyen et al., 2001). In spite of the experimental evidence that the use of oxygenated fuels can be a potential emission source of aldehydes and secondary pollutants (Gaffney et al., 1997; Tanner et al., 1988), to our knowledge, a detailed mechanistic simulation to assess the relative importance of emissions and chemical transformations has not been accomplished. The main goal of this work is to simulate the atmospheric composition of a typical urban scenario where ethanol-based fuels are used and estimate the contribution of vehicle emissions to the aldehyde’s formation.
1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00081-5
1716
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
The base case was designed on the basis of experimental data obtained at Presidente Vargas Avenue, Rio de Janeiro, Brazil, during December 1999. The metropolitan area of Rio de Janeiro has about 6 million inhabitants distributed over a 6500 km2 area. The main source of emissions in the central area of the city is the vehicular fleet fueled with hydrated ethanol, gasohol (a mixture of 78% gasoline and 22% of anhydrous ethanol), diesel and, in a minor extent, natural gas. Presidente Vargas Avenue traffic includes light-duty vehicles as well as diesel-fueled heavy-duty vehicles, with diurnal fluxes (from Mondays to Fridays) between 6000 and 8000 vehicles/h, depending on the hour of the day. It is estimated to be about 17.5% vehicles using hydrated ethanol and 66.0% using gasohol (Campos et al., 1999; CET-Rio, 1999). Recent experimental data (Martins et al., 2003), obtained in the period from December 1998 to January 2001, gave mean concentration values of 13.69 ppb for formaldehyde and 13.36 ppb for acetaldehyde. The sampling was done at the monitoring station located at 22 540 S and 43 100 W. The high acetaldehyde/ formaldehyde ratio was considered, as in previous reports, a consequence of the use of oxygenated fuels. In this paper, the kinetics processes leading to the production and consumption of acetaldehyde and formaldehyde are analyzed.
2. Methodology Computer simulations were performed using an empirical trajectory model implemented in OZIPR (Gery and Crouse, 1990). The photochemical mechanism was based on the SAPRC model (Carter, 1990). Reactions of ethanol were included and stoichiometric coefficients and rate constants were re-calculated for a volatile organic compound (VOC) mixture characteristic of Brazilian cities (de Oliveira and Arbilla, 1999). Organic compounds were described using the lumpedspecies approach. The lumped species included two groups of alkanes (compounds with rate constants for reaction with OH radical lower and higher than 3 1012 cm3 molecule1 s1), three groups of aromatics (toluene and ethylbenzenes, o-, m- and p-xylenes, and higher molecular weight aromatics), two groups of alkenes (propene and higher terminal olefins and non-terminal acyclic alkenes with CX4) and a group of higher aldehydes (CXC3). Other compounds such as ethylene, acetylene, propane, benzene, ethanol, acrolein, formaldehyde and acetaldehyde, were described explicitly. A base case was designed using experimental data for December 1999. Simulated results were compared with experimental data in order to validate the model (Arbilla et al., 2002). Finally, tagged species were used to discriminate between primary and secondary com-
pounds and to follow the reaction paths of individual species.
3. Description of the base case As previously described, experimental data used to set the model’s initial conditions were collected at Presidente Vargas Avenue, Rio de Janeiro, Brazil. Meteorological data and criteria pollutant concentrations were obtained by FEEMA between 1 December and 29 December 1999 (FEEMA, 1999). Hourly mean values were calculated for the period. Rainy days were not taken into account because, in these conditions, the main effect is the wash-out of the atmosphere. Initial mixing heights were estimated from the atmosphere temperature and pressure profiles available at IAG-USP home page (IAG-USP, 1999). Diurnal variation and final mean values were selected on the basis of simulated data (Maia, 2000), since there were no available experimental data. The main effect of an increase in the mixing height is the dilution of gases. Since, in this work, the absolute values of CO emissions were used as adjustable parameters, an error in the mixing height may be corrected by changing the emissions. Experimental meteorological input parameters as well as estimated mixing heights are presented in Table 1. The initial concentrations of CO (1.7 ppm), NO (0.11 ppm), NO2 (0.05 ppm) and total VOC (0.4 ppmC) were calculated using the average hourly concentrations measured at 8:00 AM. Since, to our knowledge, there is no published information regarding speciated VOC vehicle emissions in Rio de Janeiro, speciated ambient VOC values (samples collected in downtown Porto Alegre, Brazil, between March 1996 and April 1997) were used Table 1 Input parameters used for the simulation of the base case Hour
Relative humidity (%)
Temperature ( C)
Mixing height (m)
8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00
69.18 65.13 63.46 60.80 57.12 54.56 51.93 51.46 50.81 51.62 54.50 57.31
24.74 25.72 26.76 27.54 28.65 29.16 29.50 29.54 29.17 28.59 27.81 27.05
500 500 700 850 1300 1600 2500 1900 1650 1420 1380 1200
Temperature and relative humidity are average experimental values for December 1999. Mixing heights are estimated parameters.
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
ratios between 45.0 and 5.0 ppb, peaking at about 14:00 PM, and with a mean maximum value of 19.1 ppb (Arbilla et al., 2002). The time profile is shown in Fig. 2. The constructed base case was considered representative of average summer time air quality at Presidente Vargas Avenue, Rio de Janeiro. Calculated acetaldehyde and formaldehyde concentrations peaked at 21–24 and 14–17 ppb, respectively between 11:00 AM and 13:00 PM, depending on the effective solar flux. These results are compatible with average values, for the period 8:00 AM–4:00 PM, of 4 December 1998 (21 ppb for acetaldehyde and 19 ppb for formaldehyde). Experimental results are shown in Fig. 3 (Martins et al., 2003). Recent published results report an average ethanol ambient concentration of 176.3 ppb for S*ao Paulo,
Experimental Calculated
3
CO Concetration/ppm
(Grosjean et al., 1998a–c). This approach assumes that ambient VOC compositions are similar for Rio de Janeiro and Porto Alegre, where vehicle emissions are predominant. This may not be rigorously valid but is the only possible approach considering the lack of experimental information. These data are similar to more recent results obtained, in 1999, for Rio de Janeiro (Grosjean, 2000). Acetaldehyde and formaldehyde values were selected using experimental data for Presidente Vargas Avenue (Martins et al., 2003). The data are also consistent with previous results obtained for Rio de Janeiro in 1985 (15.0 and 17.3 ppb for formaldehyde and acetaldehyde, respectively) (Tanner et al., 1988). Ethanol mixing ratio was estimated on the basis of experimental data published by Pereira et al. (1999) for Rio de Janeiro. Values used in the simulation are presented in Table 2. The VOC/NOx/CO emission ratio was calculated in consistence with the vehicle emission inventory for Rio de Janeiro (Campos et al., 1999) and local vehicle flux (VOC/CO=0.173 and NOx/CO=0.157, both on mass basis). This ratio was kept constant during the day and was considered as a non-adjustable parameter. CO emissions were set using measured ambient concentrations (the average hourly values for December 1999) as a rough guide. This approach is valid since CO has the lowest reaction rate in the VOC mixture and its concentration is only slightly affected by OH removal. Fig. 1 shows the experimental and calculated results for CO. The fit is quite good. A further adjustment was not performed since the main goal was to simulate a representative case of downtown Rio de Janeiro, and not to exactly reproduce a particular day and scenario. The simulated ozone peak was obtained at 3:15 PM (23.00 ppb) in good agreement with experimental results for December 1999 that show maximum ozone mixing
1717
2
1
0 8
10
12
14
16
18
20
Time/hours
Fig. 1. Calculated CO time profile for the base case and average experimental values for December 1999, at the FEEMA-Presidente Vargas Avenue monitoring station.
Table 2 Average composition of volatile organic compounds used for the simulation of the base case (units are ppbC in 1 ppbC total) Compound or group
Fraction (on a ppmc basis)
Compound or group
Fraction (on a ppmc basis)
Compound or group
Fraction (on a ppmc basis)
Alkanes 1 (C2–C6)
0.172
0.136
Formaldehyde
0.014
Alkanes 2 (CXC7)
0.111
0.049
Acetaldehyde
0.028
Ethene
0.075
0.049
Non-reactive
0.006
Propene and higher terminal olefins Acyclic alkenes CXC4 (non-terminal)
0.081
Aromatics 2 (toluene and mono-substituted benzenes) Aromatics 2 (xylenes) Aromatics 3 (reactive aromatics of higher molecular weight) Higher aldehydes
0.008
Acroleine
0.011
0.075
Butadiene
0.075
Ethanol
0.062
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
1718 40
Experimental Calculated O3 Concentration/ppb
30
20
10
0
8
10
12
14
16
18
20
Time/hours
Fig. 2. Calculated ozone time profile for the base case and average experimental values for December 1999, at the FEEMA-Presidente Vargas Avenue monitoring station.
Concentration, ppb
30
Formaldehyde
25
Acetaldehyde
Brazil (Nguyen et al., 2001). This value is significantly higher than previous reported data for S*ao Paulo, Rio de Janeiro and Salvador Cities (36.2, 66.4 and 65.4, respectively) (Pereira et al., 1999). In order to estimate the possible impact of ethanol level uncertainties, two simulations were performed: in the first scenario the ethanol fraction in the VOC mixture was set as 0.062 (in a ppbC basis) and the initial concentration of ethanol and VOC (8:00 AM) as 12.4ppb and 0.400 ppmC, respectively. In the second case, the ethanol fraction was increased to 0.57 and the initial concentration to 250 ppb. Initial concentration and emission ratio of total VOC were increased in order to conserve unchanged the concentration (in ppmC) of other VOC. Results for ozone, formaldehyde and acetaldehyde for both scenarios are shown in Table 3. Simulated peak concentrations for the second scenario are 48.6%, 15.8% and 52.2% higher for ozone, formaldehyde and acetaldehyde, respectively. The first scenario with an ethanol fraction of 0.062 is in good agreement with the simulated results of Tanner et al. (1988) and experimental data of Grosjean et al. (1990). The implications of higher ethanol levels will be discussed in the following section.
20 15
4. Results and discussion
10 5 0 08:30
10:30 12:30 Time of day
14:30
Fig. 3. Formaldehyde and acetaldehyde experimental concentrations obtained at FEEMA-Presidente Vargas Avenue monitoring station on December 4, 1998.
Formaldehyde and acetaldehyde isopleths calculated for the base case are displayed in Figs. 4 and 5. The results show that acetaldehyde and formaldehyde concentrations are highly dependent on total VOC concentration. The non-linear dependence is an indication of a further source of aldehydes besides the direct emission of VOC. Also, the slight dependence on NOx
Table 3 Simulated ozone, formaldehyde and acetaldehyde concentrations (in ppb units), and acetaldehyde/formaldehyde ratios for the base case (ethanol fraction=0.062 and initial concentration of 12.4 ppb) and for a second scenario with higher emissions of ethanol (ethanol fraction=0.57 and initial concentration of 250 ppb) (see text for details) Time of the day
8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00
Base case
Scenario with higher ethanol emissions
Ozone
Formaldehyde
Acetaldehyde
Ratio
Ozone
Formaldehyde
Acetaldehyde
Ratio
0 2.712 4.105 7.453 10.24 15.05 21.03 23.01 22.75 18.11 9.324 0.571
5.60 9.292 11.66 14.27 14.23 14.58 11.04 11.04 12.58 13.06 13.56 13.89
5.60 11.29 16.34 21.30 20.38 20.17 14.73 14.73 14.77 14.74 14.78 14.86
1.00 1.22 1.40 1.49 1.43 1.38 1.33 1.33 1.17 1.13 1.09 1.07
0 2.807 4.520 8.722 12.57 19.74 27.90 33.35 34.08 18.86 17.65 4.475
5.60 9.237 11.63 14.39 14.65 15.38 12.00 13.50 14.68 15.69 16.53 17.05
5.60 12.48 20.30 29.55 30.23 32.35 24.69 27.36 29.07 30.15 30.75 31.00
1.00 1.35 1.74 2.05 2.06 2.10 2.05 2.03 1.98 1.92 1.86 1.82
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722 0.40
0.40
0.35
0.35
0.018
0.30
1719
0.01
0.30
0.021
0.01
0.25 0.20
NOx (ppm)
NOx (ppm)
0.014
0.011
0.15
0.004
0.025
0.25
0.02
0.20
0.03 0.04
0.15
0.007 0.10 0.05
0.05
0.00
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
16
28 20
12 0.2
24
32 36
0.1
24
Formaldehyde, ppb
NOx (ppm)
8
0.1
0.2
0.3
0.3
0.4
16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1
0.5
0.6
0.7
Primary Formaldehyde Secondary Formaldehyde Total Formaldehyde
8
0.0 0.0
0.2
0.06
Fig. 6. Ozone isopleths calculated for the base case, Rio de Janeiro, Brazil, December 1999.
0.5
4
0.1
0.09 0.08
VOC (ppmC)
Fig. 4. Formaldehyde isopleths calculated for the base case, Rio de Janeiro, Brazil, December 1999.
0.3
0.07
0.05 0.0
VOC (ppmC)
0.4
0.07
0.10
0.028 0.032 0.025
0.4
0.5
0.6
VOC (ppmC) Fig. 5. Acetaldehyde isopleths calculated for the base case, Rio de Janeiro, Brazil, December 1999.
levels is a consequence of the formaldehyde and acetaldehyde formation in the photooxidation of VOC, through the chain reactions of hydroxyl radicals with organics and the conversion of NO to NO2. For comparison, ozone isopleths are shown in Fig. 6. Figs. 7 and 8 show the formaldehyde and acetaldehyde profiles as well as the primary and secondary contributions to the total concentration. For the purposes of the simulation, the direct emissions and the initial aldehyde fraction in the VOC mixture were considered as primary sources and the ethanol fraction was set as 0.062. The primary contribution is higher in the early morning and reaches a maximum at a time that approximately coincides with peak automobile traffic. Secondary contributions are relatively low in the early morning, increase at about noon and reach a maximum
10
12
14
16
18
20
Time of day
Fig. 7. Primary, secondary and total formaldehyde calculated for the base case.
at about 1:00 PM. The high concentrations of secondary aldehydes after 2:00 PM may be due, in part, to the trapping of the pollutants bellow the inversion layer in the afternoon. At noon, the contributions of secondary formaldehyde and acetaldehyde are 65.5% and 69.4%, respectively. When considering an ethanol fraction of 0.57, these values are 81.81% and 94.69% for formaldehyde and acetaldehyde, respectively. The calculated formaldehyde and acetaldehyde peaks were centered at about noon (12:30 and 11:20 AM, respectively). Both, the experimental and simulated carbon monoxide maximum were obtained at about 9:00 AM and the ozone peak after 2:00 PM. Experimental values for aldehydes obtained at Rio de Janeiro (Martins et al., 2003) and Salvador, Brazil (de Andrade, 2001) also show a peak at about noon. The fact
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
1720
Table 4 Contribution of alkenes, ethanol and other volatile organic compounds to the formation of formaldehyde and acetaldehyde for the conditions of the base case
25 Primary Acetaldehyde Secondary Acetaldehyde Total Acetaldehyde
Acetaldehyde ,ppb
20
Time of day
15
10
5
Alkenes (%)
Ethanol (%)
Other sources (%)
Formaldehyde 10:00 12:00 15:00 17:00
67.40 54.29 51.70 47.86
4.41 6.61 0.73 0.82
28.19 39.61 47.57 51.32
Acetaldehyde 10:00 12:00 15:00 17:00
93.98 91.72 87.90 85.66
3.10 4.07 5.89 6.90
2.92 4.21 6.21 7.44
0
8
10
12
14
16
18
20
Time of day
Fig. 8. Primary, secondary and total acetaldehyde calculated for the base case.
that experimental time–concentration profiles show a maximum which is shifted about 3 h from primary pollutant peaks (mainly CO and NO) was considered as an evidence of both primary and secondary contributions to aldehyde levels (Martins et al., 2003). Figs. 7 and 8 are in good agreement with that experimental evidence. The simulated acetaldehyde/formaldehyde ratio (on a ppb basis) is shown in Table 3. Values higher than 1 are also in agreement with experimental data (Martins et al., 2003). As shown in Table 3, higher emissions of ethanol lead to higher acetaldehyde/ formaldehyde ratios. At noon, the ratio is 44% higher and at evening about 70% higher. This fact may be attributed to the faster photochemical processes consuming formaldehyde. Ethanol and alkene contributions to secondary formaldehyde and acetaldehyde production are displayed in Table 4. Clearly, alkenes are the most reactive species and the main source of aldehydes. Ethanol is much more stable than alkenes with a rate constant for reaction with hydroxyl radical of 3.2 1012 cm3 molecule1 s1, in comparison with rate constants of 3.14 1011 and 6.37 1011 cm3 molecule1 s1 for 1-butene trans-2-butene, respectively. Considering a mixing ratio of 0.062, ethanol contributes with less than 7% of secondary production of acetaldehyde in the evening. This value is increased to 24.38% for a mixing ratio of 0.57. This value may be considered an upper limit to formaldehyde and acetaldehyde production from ethanol oxidation. The formaldehyde and acetaldehyde reactions were also investigated. The four possible facts of aldehydes in urban atmosphere are photolysis and reaction with OH, HO2 and NO3 radicals. The relative contribution of each reaction path, for the conditions of the base case, are shown in Table 5. During daylight hours, photolysis of formaldehyde is comparable to oxidation by OH radical. The rate of
photolysis is higher in the morning and early afternoon while OH reaction becomes more important after 3:00 PM due to increase of pollutant levels and the decrease of solar flux. Both, photodissociation paths: HCHO þ hn-H þ HCO;
ð1Þ
HCO þ O2 -HO2 þ CO
ð2Þ
and HCHO þ hn-H2 þ CO
ð3Þ
and OH attack: HCHO þ OH-HO2 þ HCO; HCO þ O2 -HO2 þ CO
ð4Þ
ð2Þ
lead to the formation of CO, while reactions (1) and (4) produce HO2 radicals which would further react with NO recycling OH radicals and reforming NO2. The main path for acetaldehyde photodecomposition is CH3 CHO þ hn’CH3 þ HCO:
ð5Þ
The radical formation rate from the photolysis of acetaldehyde is much less (approximately and order of magnitude) than that from formaldehyde (see Table 5). The main reaction path for acetaldehyde is hydrogen abstraction by OH radical leading to the formation of acetyl radical followed by the following reactions: CH3 CHO þ O2 -CH3 CðOÞ2 O;
ð6Þ
CH3 CðOÞ2 O þ NO-CH3 COO þ NO2 ;
ð7Þ
CH3 COO-CH3 þ CO2 :
ð8Þ
Methyl radicals, formed in reactions (5) and (8), may be further oxidized to formaldehyde. Reaction (7)
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
1721
Table 5 Relative contribution of each reaction path to the consumption of formaldehyde and acetaldehyde for the conditions of the base case Reaction
HCHO+hn+O2=2HO2+CO HCHO+hn=CO+H2 HCHO+OH+O2=HO2+CO+H2O HCHO+NO3+O2=HNO3+HO2+CO HCHO+HO2=OOCH2OH CH3CHO+O2+hn=CO+HO2+CH3 CH3CHO+OH=CH3CO+H2O CH3CHO+NO3=HNO3+CH3CO
Time of day 10:00
12:00
15:00
17:00
35.11% 45.69% 19.15% 0.00% 0.05% 15.35% 84.64% 0.01%
28.91% 40.58% 30.38% 0.01% 0.12% 7.84% 92.16% 0.00%
10.51% 34.67% 54.44% 0.00% 0.38% 1.45% 98.53% 0.02%
0.46% 1.26% 97.42% 0.03% 0.83% 0.00% 100.00% 0.00%
Table 6 Relative contribution of each reaction path to the lifetimes of formaldehyde and acetaldehyde for the conditions of the base case Reaction
Lifetimes (min)
HCHO+hn+O2=2HO2+CO HCHO+hn=CO+H2 HCHO+Oh+O2=HO2+CO+H2O HCHO+NO3+O2=HNO3+HO2+CO HCHO+HO2=OOCH2OH CH3CHO+O2+hn=CO+HO2+CH3 CH3CHO+Oh=CH3CO+H2O CH3CHO+NO3=HNO3+CH3CO
961 684 909 1.1 108 2.2 105 6666 394 2.45 107
competes with the formation of peroxy acetyl nitrate (PAN; reaction 9), which is thermally unstable and decomposes back into NO2 and peroxyacetyl, which will further react: CH3 CðOÞ2 O þ NO2 3CH3 CðOÞ2 ONO2 :
ð9Þ
As pointed out by Gaffney and Marley (2001), the decomposition of PAN may lead to the formation of formaldehyde and CO as well as convert a number of NO molecules to NO2, which will produce ozone. PAN acts as a reservoir compound and is also an important oxidant. Unfortunately, there are no recent experimental results for PAN concentrations in Rio de Janeiro. The slow decomposition of formaldehyde and acetaldehyde may be one of the reasons for the relatively high concentrations in the afternoon. In Table 6, the lifetimes of formaldehyde and acetaldehyde with OH are compared. The calculations were performed at noon for the conditions of the base case. Lifetimes for formaldehyde and acetaldehyde with respect to oxidation are 15.2 and 6.6 h, respectively. The overall photolytic lifetimes were estimated as 6.7 and 111.1 h for formaldehyde and acetaldehyde, respectively. Overall lifetimes, at noon, are 4.6 and 6.2 h for formaldehyde and acetaldehyde, respectively. HO2 reacts quite readily with formaldehyde forming an intermediate, which can decompose back to
HCOH+HO2. Since the reverse decomposition is also fast, in comparison to alternate reactions of the adduct, little net loss of formaldehyde occurs by this path of reaction. Since oxygenated fuel use is currently being considered in many countries, the potential for the emission of increased aldehydes and the reactivity of the exhausted primary pollutants are of current interest. The use of a separate representation of primary and secondary aldehydes in the model provided some insights about the role of ethanol in the formation of formaldehyde and acetaldehyde. As shown by the calculations, ethanol makes relatively minor contributions to acetaldehyde formation. The main contributions are the primary emissions, due to the incomplete combustion of ethanolbased fuels, and the secondary formation in the photooxidation reactions of alkenes and hydroxyl radicals. It must be noted that the acetaldehyde/ formaldehyde emission ratio is higher than the measured value for no-oxygenated fuels (Martins et al., 2003; Miguel and de Andrade, 1990; Schifter et al., 2001a) and also the acetaldehyde/formaldehyde secondary formation ratio is higher than 1, leading to a higher total acetaldehyde/formaldehyde formation ratio.
5. Conclusions These simulations confirm the experimental evidence that the high acetaldehyde/formaldehyde ratios are due to the use of alcohol-based fuels. Computational results show that the primary contribution to acetaldehyde and formaldehyde concentrations is higher in the early morning and reaches a maximum which approximately coincides with the peak automobile traffic. The secondary contribution increases during the morning and reaches a maximum at noon and early afternoon. As a consequence, total acetaldehyde and formaldehyde levels peak after primary pollutants’ maximum obtained in the early morning (rush hour)
1722
E.M. Martins, G. Arbilla / Atmospheric Environment 37 (2003) 1715–1722
and before the secondary pollutants (for example, ozone).
Acknowledgements The authors thank the partial financial support from CNPq, CAPES and FINEP/CTPetro and the kindly collaboration of FEEMA (Rio de Janeiro, Brazil).
References Arbilla, G., Martins, E.M., Moreira, A., Moreira, L.F.R., 2002. Ozone air quality modeling. A case study: a heavily vehicle impacted urban avenue in Rio de Janeiro, Brazil. Journal of the Brazilian Chemical Society 13, 308–317 (http://jbcs.sbq. org.br/). Campos, I.C.B., Pimentel, A.S., Corr#ea, S.M., Arbilla, G., 1999. Simulation of air pollution from mobile source emissions in the City of Rio de Janeiro. Journal of the Brazilian Chemical Society 10, 203–208 (http://jbcs.sbq. qorg.br/). Carter, W.P.L., 1990. A detailed mechanism for the gas-phase atmospheric reactions of organic compounds. Atmospheric Environment 24A, 481–516. CET-Rio, 1999. Companhia de Engenharia de Tr!afego-CETRio. Internal Reports. Private Communication. See also: http://transito.rio.rj.gov.br and http://www.detran.rj.gov. br. de Andrade, M.V.A.S., 2001. Estudo de oxidantes fotoqu!ımicos # no ar: Compostos carbon!ılicos, ozonio e PAN. Ph.D. These, Universidade Federal da Bahia, Brazil. de Oliveira, K.M.P.G., Arbilla, G., 1999. Otimiza@*ao de um mecanismo fotoqu!ımico para a simula@*ao da atmosfera urbana brasileira. Qu!ımica Nova 22, 790–800. FEEMA, 1999. Funda@*ao Estadual de Engenharia do Meio Ambiente, Rio de Janeiro, RJ, Brazil. Internal Reports. Private Communication. Gaffney, J.S., Marley, N.A., 2001. Comments on environmental implications on the oxygenation of gasoline with ethanol in the Metropolitan Area of Mexico City. Environmental Science and Technology 35, 4957–4958. Gaffney, J.S., Marley, N.A., Martin, R.S., Dixon, R.W., Reyes, L.G., Popp, C.J., 1997. Potential air quality effects of using ethanol–gasoline fuel blends: a field study in Albuquerque, New Mexico. Environmental Science and Technology 31, 3053–3061. Gery, M.W., Crouse, R.R., 1990. User’s Guide for Executing OZIPR. US Environmental Protection Agency, Research Triangle Park, NC, EPA-9D2196 NASA 1990. Grosjean, D., 2000. Private Communication. Grosjean, D., Miguel, A.H., Tavares, T., 1990. Urban air pollution in Brazil: acetaldehyde and other carbonyls. Atmospheric Environment 22, 1637–1648.
Grosjean, D., Grosjean, E., Moreira, L.F.R., 2002. Speciated ambient carbonyls in Rio de Janeiro, Brazil. Environmental Science and Technology 36, 1389–1395. Grosjean, E., Grosjean, D., Gunawardena, R., Rasmussen, R.A., 1998a. Ambient concentrations of ethanol and methyl-terc-butyl ether in Porto Alegre, Brazil, 1996 March–1997 April. Environmental Science and Technology 32, 736–742. Grosjean, E., Grosjean, D., Rasmunssen, A.R., 1998b. Ambient concentrations, sources, emission rates and photochemical reactivity of C2–C10 hydrocarbons in Porto Alegre, Brazil. Environnmental Pollution 32, 3371–3379. Grosjean, E., Rasmussen, R.A., Grosjean, D., 1998c. Ambient levels of gas phase pollutants in Porto Alegre, Brazil. Atmospheric Environment 32, 3371–3379. Howard, C., Russell, A., Atkinson, R., Calvert, J., 1997. Air Quality Effects of the Winter Oxyfuel Program. White House Office of Science and Technology Policy, Washington, DC, July 2. IAG-USP, 1999. http://master.iag.usp.br/. Data obtained in December 1999 and January 2000. Maia, L.F.P.G., 2000. Private Communication. Meteorological Department, Instituto de Geoci#encias, Universidade Federal do Rio de Janeiro. Martins, E.M., Corr#ea, S.M., Arbilla, G., 2003. Formaldehyde and acetaldehyde in a high traffic street of Rio de Janeiro, Brazil. Atmospheric Environment 37, 23–29. Miguel, A.H., de Andrade, J.B., 1990. Catalyst and noncatalyst exhaust aldehydes emissions from Brazilian ethanol-fueled vehicles. Journal of the Brazilian Chemical Society 1, 124–127. Nguyen, H.T., Takenaka, N., Bandow, H., Maeda, Y., De Oliva, S.T., Botelho, M.M.F., Tavares, T.M., 2001. Atmospheric alcohols and aldehydes concentrations measured in Osaka, Japan and in S*ao Paulo, Brazil. Atmospheric Environment 35, 3075–3083. Pereira, P.A.P., Santos, E.T.S., Ferreira, T.F., Andrade, J.B., 1999. Determination of methanol and ethanol by gas chromatography following air sampling florisil cartridges and their concentrations at urban sites in the three largest cities in Brazil. Talanta 49, 245–252. Schifter, I., Vera, M., D!ıaz, L., Guzman, E., Ramos, F., LopezSalinas, E., 2001a. Environmental implications on the oxygenation of gasoline with ethanol in the Metropolitan Area of Mexico City. Environmental Science and Technology 35, 1893–1901. ! Schifter, I., Vera, M., D!ıaz, L., Guzm!an, E., Ramos, F., LopesSalinas, E., 2001b. Response to comment on environmental implications on the oxygenation of gasoline with ethanol in the metropolitan area of Mexico City. Environmental Science and Technology 35, 4959–4960. Tanner, R.L., Miguel, A.H., de Andrade, J.B., Gaffney, J.S., Streit, G.E., 1988. Atmospheric chemistry of aldehydes: enhanced peroxyacetyl nitrate formation from ethanolfueled vehicular emission. Environmental Science and Technology 22, 1026–1034.