Application of Photochemical Trajectory Model toEvaluate Ozone Formation by AtmosphericConcentrations and Emissions of Propane andButane in Mexico City Metropolitan Zone
FRANCISCO HERNÁNDEZ1 and ELBA ORTIZ2
Gerencia de Ciencias del Ambiente, Instituto Mexicano del Petróleo, México D.F.
Accepted 18 May 2000
Abstract. Propane and butane have very low reaction velocity and are abundant in the atmosphereof the Mexico City Metropolitan Zone (MCMZ). An important contribution of those alkane to ozoneformation is observed. The present work studies to what extent the atmospheric concentrations andemissions of propane and butane contribute to the ozone formation. This evaluation was realizedwith the trajectory photochemical model, emissions and meteorological data of the MCMZ. Propanecontributes to the maximum of ozone concentration with 0.013 ppm (7.78% of the total) and bu-tane with 0.027 ppm (15.34% of the total). The total contribution of propane and butane to ozoneformation can be split, respectively, in 0.001 ppm and 0.013 ppm of ozone produced by emission,and 0.012 ppm, and 0.014 ppm of ozone produced by atmospheric concentrations from the initialconditions. Propane and butane have a total contribution to a maximum of ozone concentration of23.12%. The maximum concentration of propane has a decrement of 68.62% in the photochemicalmodeling.
Key words: Ozone, photochemical modeling, propane, butane and Mexico.
Introduction
In the Mexico City Metropolitan Zone (MCMZ) the ozone keeps being the mostimportant air pollutant. It is a photo-oxidant agent not emitted directly to the atmo-sphere. Ozone is formed by photochemical reactions of volatile organic compounds(VOC) with nitrogen oxides (NOx). In studies of smog chambers in Mexico City, aphotochemical regimen limited to NOx had been studied [1a]. In the 1994 officialemissions inventory, NOx and hydrocarbon emissions reached 451,618 ton/yearand 1,025,759 ton/year, respectively [1b]. Those precursors are therefore abundantin the metropolitan zone.
Environmental measurements of VOC, reported by Arriagaet al. [3] in theMCMZ, showed that VOC are constituted by more than 200 chemical species,being propane and butane among the most abundant species, both compoundsrepresent between 21% to 29% of the total environmental VOC in Mexico City.It is also important to remark, that the reaction rates are low [4]. As a result, bothcompounds remain for a long time in the urban atmosphere. The emissions sourcesof these precursors in the MCMZ are landfills, the burning of garbage, the exhaustand evaporative emissions of fuels and storage charge, and the distribution andconsumption of liquefied petroleum gas (LP gas) [5]). These emissions are morethan 90% constituted by propane and butane [6]. The emissions of LP gas areestimated in 76,413.54 ton/year [7].
The contribution of VOC to ozone formation can be evaluated by application ofphotochemical modeling. The use of a photochemical model allows us to simulatethe dispersion characteristics and environment conditions of the MCMZ. The pur-pose of this work is to evaluate the contribution of propane and butane to the ozoneatmospheric concentration in the MCMZ by applying the CIT photochemical tra-jectory model [8]. We use emissions and meteorological representative parametersof the MCMZ, in order to mimic the ozone concentration and VOC characteristics.
PHOTOCHEMICAL TRAJECTORY MODEL
The photochemical modeling is one of the most important tools for ozone controland other secondary pollutants. The results of this model has been used to evaluatestrategies as the impact of the use of a new fuel in order to help the governmentdecision-makers.
The model is used to simulate the photochemical pollutant concentration andreaction on the air during the transport, dispersion and accumulation of primarypollutants. The inputs for the model are a data set that characterizes emissions,topography and meteorology of the region.
The photochemical trajectory CIT model is based on the atmospheric diffusionequation to describe the pollutants transport. The CIT model represents a columnon the urban zone surface, which follows the dominant wind direction, and simu-lates the physical and chemical processes that influence the pollutant concentration.The modeled column is bounded at the bottom by the ground surface and at theupper level by the top of the mixing layer. Starting at an initial place, the air parceldisplaces under the dominant wind flow; passing over the source emissions, whereprimary pollutants are incorporated, for further chemical reaction in the column.The column can be split in several air layers in order to simulate the vertical mix-ing between the lower upper cells (Figure 1). The trajectory model is applied tosimulate a base case, using adverse conditions to pollutant dispersion, in order tohave the worst case as a reference.
The trajectory model requires a chemical mechanism, which describes thechemical processes in the atmosphere. In this work we use the Lurmann–Carter–
APPLICATION OF PHOTOCHEMICAL TRAJECTORY MODEL 449
Figure 1. Visualization of photochemical trajectory model and the processes that are realizein the air column.
Coyner (LCC) mechanism. This mechanism models the ethane and formaldehydechemistry, and uses a specific molecular treatment for the others organic com-pounds, according to their chemical character. Therefore, a prototype specierepresents the chemistry of the whole class. For example, the monoalkyl benzeniccompounds are included in the TOL class. The AROM class includes the polyalkylbenzenic compounds. The alkane compounds in the ALKA class [4]. The originalLCC mechanism was modified, because we need to consider separately the initialconditions and emissions of propane and butane from the other alkanes. Thereforethe following chemical reactions to propane and butane are included explicitly inthe LCC mechanism:
being KPROPANE= (1.5E+6 ppm−1 min−1/TEMP)EXP(−324/TEMP en◦K)
BUTANO + OH K— 0.07 RO2N + 0.93RO2R + 0.40R2RO2
+ 0.57CH3CHO + 0.52MEK + 0.15RCHO [4]
being KBUTANO = (2.28951E + 4 ppm−1 min−1)EXP(−353/TEMP en◦K)
being respectively: OH, hydroxyl radical; MEK, methyl-ethyl-ketone; ALD2, largealdehydes such as acetaldehyde; HCHO, formaldehyde; R2O2, RO2, RO2R andR2RO2, peroxy radicals; RO2N, alkyl nitrate; CH3CHO, acetaldehyde; RCHO,propanaldehyde (both aldehydes were included in ALD2 class). The propane and
450 FRANCISCO HERNANDEZ AND ELBA ORTIZ
Figure 2. Meteorological parameters used in the base case.
butane reactions are only useful in the scenario with propane or butane, not in thereference case.
Method
To analyze the ozone formation with and without propane (PROP) or butane(BUTA) by applying the trajectory model, it is necessary to build several hypothet-ical scenarios for the MCMZ. This is performed by modifying some parameters inthe base case, as shown in Table 1.
The base case reproduces a day with typical environmental conditions, that canbe represented by photochemical modeling. The input data and the informationused in the model was obtained by experimental and fieldwork. In the present study,partial information of the base case is taken from the MARI report [10]. In thisaim, we use the meteorological and air quality information of 22 February 1991(Figure 2). The pollution condition during this day is representative of a pollutionepisode with very bad conditions for the dispersion of atmospheric pollutants andhigh concentration in ozone. The maximum concentration occurs in the southwest
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Table I. Principals parameters included in the study scenarios
Scenario Reaction of propane Propane to initial Emissions of Reaction of butane Butane to initial Emissions of butane
conditions propane conditions
A∗ Does not included Does not included Do not included
B Included Included Do not included
C Included Included Do not Included
D∗ Does not included Does not included Do not included
E Included Included Do not included
F Included Included Included
∗Scenarios utilized as reference.
452 FRANCISCO HERNANDEZ AND ELBA ORTIZ
of the city. Respect to the trajectory (Northeast to southwest) of the column in themodeling, this is defined by taking into account the direction of the dominant windin the MCMZ during the period which corresponds to the modeling day.
In the simulation of the trajectory model a grid of 14 for 16 cells is used.Each cell represents 25 km2 (5 km by 5 km). The spatial distribution of emissioninventory in the grid permits us to have a representation and localization of theplaces where emissions of pollutants occur. For this purpose, it was necessary todistribute spatially every type of source of the inventory in the simulation grid, andtemporary patterns of emissions are assigned for the spatial source distribution. Asa result, a spatial and temporary distribution of emissions in the MCMZ is simu-lated. The LCC mechanism is used for the ALKA, ALKE, AROM, HCHO, ALD2,TOLU, ETHE, MEK, PROP and BUTA classes. For this reason, the emissions areincorporated to the column when it passes over the points where the emissions arespilled to atmosphere.
Discussion of Results
TEMPORARY PATTERNS OF ALKANE
Figure 3. shows the variation of alkane (ALKA) and ozone during the simulationperiod. In this figure, the typical pattern of alkane (including propane and butane),in the MCMZ can be seen. The highest modeled concentration occurs during theperiod from 7 h to 9 h, however, the highest emissions of these species wereobserved from 10 h to 12 h.
The alkane has the highest decrement in its concentration between 9 h and 13 h.This is produced first of all by an increment in the height of the mixing layer.Whenever the decrement of alkane is analyzed with and without change in mixinglayer; a large decrement of alkane is observed from 9 h to 13 h with change, as itcan be seen in Figure 4. On the contrary, if the height of the mixing layer remainsfixed at 109 m, the alkane shows only a slight decrement due chemical reaction.For propane there is no change in its concentration in the period from 12 h to15 h, but at 16 h a new decrement is observed. Finally the concentration does notchange after of 18 h (Figure 3). On the other hand, after 13 h the change in theconcentration of butane is very small, and there is only a slight increment at 16 h,and after 17 h, it does not change significantly. It is important to point out that thetemporary patterns of alkane as propane and butane agree with those measured byGaffney [11] for the environmental air of Mexico City.
ROLE OF PROPANE IN THE OZONE FORMATION
The role of propane in the atmospheric chemistry starts with the extraction of ahydrogen atom by an OH radical. If the extraction occurs on a primary H atom,the principal product is propanaldehyde [12], however, the principal extractionmechanism by OH takes place on a secondary H atom. The oxidation then proceeds
APPLICATION OF PHOTOCHEMICAL TRAJECTORY MODEL 453
Figure 3. Temporary pattern of alkane, O3 and alkyl nitrate.
Figure 4. Effect of variation in height of mixing layer on alkane concentrations. With alkaneonly in initial conditions.
454 FRANCISCO HERNANDEZ AND ELBA ORTIZ
to produce RO2 radical (peroxy). The peroxy radical proceeding from propaneoxidation oxidizes NO to NO2 and the RO radical forms dimethyl-ketone as theprincipal reaction product.
Even in the afternoon, the propane continues participating in the alkyl radicalproduction, but this radical can undergo an additional reaction with NO, after beingoxidized to alkyl peroxy radical, and produces terminal compounds as alkyl nitrate.This reaction could be prevailing during the second decrement of propane concen-tration in the period 16 h to 18 h (Figure 3) and this fall down could eliminate apropane portion in the afternoon. As shown in Figure 3, the highest decrement ofpropane occurs during the daytime (from 9 h to 13 h) when the OH and NO radicalsare present. These species allow the VOC oxidation and the NO2 formation, thenNO2 can participate in the formation of ozone.
The most important decrement of propane is observed from 9 h to 11 h (with achange of 0.18 ppm to 0.0683 ppm), period during which a decrement of 62% withrespect to the maximum concentration of propane (0.18 ppm) at 9 h is seen. After11 h, the variation is small and reaches minimum concentration of 0.0565 ppmat 18 h. This last concentration (which corresponds to 31.38% with respect to theconcentration at 9 h) represents a portion of propane which is added to the VOCof the next day as part of the initial conditions (environmental concentration in themorning), prevailing when a new photochemical cycle starts. Therefore, during apollution episode in the MCMZ, with chemical reaction, dispersion and dilution ofpollutants, only 68.62% of the total propane is eliminated during this period fromthe maximum propane concentration.
If we compare case B (with propane in initial conditions) with respect to case A(without propane), we can see that, the initial conditions with propane produce anincrement in the maximum ozone concentration of 0.012 ppm, which correspondsto an increment of 7.18%; and a change in the ozone concentration of 0.167 ppm(case A) to 0.179 ppm (case B) (Figure 5).
The case (C) with propane emission and at the initial conditions, as in the Bcase, shows also ozone concentrations higher than case A. The comparison ofcase C with case A, shows that in the case C, a maximum ozone concentrationof 0.18 ppm or 7.78% higher than in case A (with zero propane) is observed.
EFFECT OF BUTANE IN THE OZONE FORMATION
In the butane case, the extraction of a primary H by OH radical starts the reactionto produce an alkyl radical (R). This process continues with the addition of O2,and the R radical passes to form a RO2 radical which can initially achieve a firstoxidation of NO to NO2, producing as subproducts an alcoxy radical (RO). Whenthis reacts with O2, it produces ethyl-methyl-ketone and HO2 (this can oxides NOto NO2). In addition, it yields acetaldehyde and ethyl peroxy. The ethyl peroxy canrealize another oxidation of NO, which produces another acetaldehyde moleculeand an additional oxidation of NO to NO2 [12].
APPLICATION OF PHOTOCHEMICAL TRAJECTORY MODEL 455
Figure 5. Differences in ozone concentrations of cases B and C respect to reference case A.
Briefly, we can say that the oxidation of butane is reflected in an importantproduction of peroxy radical. Since its oxidation produces a CH3CH2C(O)CH3,two HO2 and CH3CHO. The butane oxidation trajectory described involves twooxidation processes of NO to NO2 and produces two HO2 radicals, very reactivespecies that has great influence in the oxidation of NO and VOC. It measures theimportance of the butane in ozone accumulation.
In the analysis of butane development during the higher photolytic activityperiod (from 12 h to 13 h), it is observed that this alkane has a important decrement,as other VOC, when the largest rate of decrement in NO concentration is observed(9 h to 15 h). Accordingly, it is very probable that alkyl peroxy radical, produced bybutane oxidation, also participates in the NO oxidation. As in the propane case, thescenario for the ozone concentrations in butane obtained in the reference case (D),with butane in the initial condition (E), and with butane in the initial conditions andemissions (F), are described in Table 1.
With respect to the contribution of butane to the maximum ozone concentration,in the reference case (D) a maximum of ozone of 0.176 ppm is obtained. In thecase E, a maximum ozone of 0.19 ppm are found. Therefore, the maximum ozoneconcentration was incremented by 7.9% if butane is present when the photolyticcycle starts. On the other hand, if we analyze the case F (with butane in initialconditions and emissions), the maximum of ozone concentration is seen to reach0.203 ppm. If this value is compared with the 0.176 ppm of reference case (D),one can observe an increment of 15.34% (Figure 6), i.e., the butane emissionsare almost equivalent to the contribution of ozone formation reached by butane inthe initial conditions. We can see that the butane contribution (0.027 ppm) to themaximum ozone concentration practically duplicates the contribution produced bypropane (0.013 ppm).
456 FRANCISCO HERNANDEZ AND ELBA ORTIZ
Figure 6. Differences in ozone concentrations of cases E and F respect to reference case D.
The analysis of results shows that butane and propane have a considerablecontribution on ozone formation. Nevertheless, their theoretical half life time ina urban atmosphere (considering [OH]=5E6 radicals/cc, KOHbutano= 2.55E–12ccmolec/seg and KOHpropano = 1.18E–12cc molec/seg [12]) of 0.9 and 1.96 days,respectively, indicates that they have a low reactivity. It was observed also that thepresence of both alkanes in the initial conditions induces a higher contribution inthe ozone concentration than their presence in the emissions. Finally, we may con-clude that a part of these two alkanes could be related with the reaction trajectoriesthat give rise to alkyl nitrates compounds., during the afternoon chemistry. Thosereactions eliminate only a small part of alkane. These could be associated with theafternoon decrement in ALKA, PROP and BUTA concentration after 16 h (Figure3).
Conclusions
The results of photochemical modeling showed that propane is responsible for anincrement of 7.78% in the maximum ozone concentration. That is, if all propane iseliminated in the photochemical simulation, the maximum ozone concentration islowered down to 0.013 ppm. The daily emissions are also observed to contributewith 0.001 ppm of this increment on ozone concentration. The initial propaneconditions (with residual species of previous days) have a higher contribution(0.012 ppm) than the propane emissions to ozone accumulation.
The butane shows a larger participation than propane in the ozone formation.Its contribution to the maximum of ozone concentration reaches 15.34%, i.e.,0.027 ppm. In butane, the initial condition contributions reach almost half of the
APPLICATION OF PHOTOCHEMICAL TRAJECTORY MODEL 457
total increment. If we consider only butane in the initial conditions, the maximumozone concentration is incremented in 7.9% (0.014 ppm). The daily emissions con-tribute with 7.44% (0.013 ppm). In the butane case, the initial conditions determine,in a large extent, the contribution of butane to the ozone formation.
Propane and butane contribute (with a ratio VOC/NOx = 14.8) with 23.12% tothe maximum of ozone concentration in the MCMZ, percentage lower than thatof 30% calculated by Blake and Rowland [13] for the contribution of propane,butane and isobutane. We may conclude that, with the purpose of obtaining apreliminary evaluation, the product of the maximum increment of reactivity [14]with the environmental concentration of every hydrocarbon, allows us to have anapproximate idea over the contribution of every VOC or chemical species relatedto the increment in the ozone concentration.
The most important increment in ozone concentration is observed in the period(from 9 h to 15 h). During this period, the highest decrement (from 9 h to 13 h)in ethylene, alkene, formaldehyde and toluene concentrations are also given. In thesame period, this decrement is also observed with a change in the mixing layerfrom 523 m (10 h) to 2,226 m (14 h).
The alkane concentrations, including propane and butane, have their maximumat 9 h; and continue to decrease up to 13 h, keeping without any change until16 h, when they present a new decrement, afterwards the alkane concentrationremains without any variation. This result is in agreement with the pattern obtainedexperimentally by Gaffney in 1997 [11] from measurements made in Mexico City.
The decrement in alkane concentration during the day is determined principallyby the change in the height of mixing layer and to less extent, by chemical reac-tions. While the decrement of alkane concentration from 16 h to 18 h (period withminimum variation in height of mixing layer), is observed simultaneously with themaximum alkyl nitrate concentration. For this reason, the variations (from 16 h to18 h) in alkane and alkyl nitrate concentrations could be related.
The maximum propane concentration in the model (0.18 ppm at 9 h) presents adecrement of 68.62% during the hours when photochemical processes occur. Whenthis process finalizes, 31.38% (0.0565 ppm) of propane remains, that forms part ofVOC that will participate in the photochemical cycle of the next day.
Acknowledgments
We appreciated the details and valuable comments of the referees. Also we wishto extend special thanks to the Institute Mexican of Petroleum for supporting thepresent work and for the fellowship granted to F. Hernández to development thisproject.
458 FRANCISCO HERNANDEZ AND ELBA ORTIZ
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