The tropospheric degradation of isoprene: an updated module for the regional atmospheric chemistry mechanism

Atmospheric Environment 37 (2003) 1503–1519

The tropospheric degradation of isoprene: an updated module for the regional atmospheric chemistry mechanism Harald Geiger*, Ian Barnes, Iustinian Bejan, Thorsten Benter, Markus Spittler Bergische Universitat . Gesamthochschule Wuppertal, Fachbereich 9—Physikalische Chemie, D-42097 Wuppertal, Germany Received 12 April 2002; received in revised form 2 October 2002; accepted 25 November 2002

Abstract A highly condensed reaction scheme for the tropospheric oxidation of isoprene is presented. This mechanism was implemented into the regional atmospheric chemistry mechanism (RACM), which is an established chemical module for regional air quality modelling but contains an isoprene chemistry which is no longer state-of-the-art. The reaction scheme developed here is based on the recently published Mainz isoprene mechanism (MIM) that has been constructed for application in global chemistry transport models. The MIM code was reduced to a size suitable for use in regional atmospheric chemistry models. Redundant reactions were identified and removed from the reaction scheme by means of sensitivity analyses. The revised mechanism was successfully tested against the results of smog chamber experiments carried out in the European photoreactor EUPHORE. A model intercomparison between both the original and the updated RACM mechanism was performed for a number of well-defined scenarios employing conditions ranging from very clean to highly polluted air masses. The calculations revealed large deviations in the concentration–time profiles for key species of the isoprene degradation, particularly under ‘‘low-NOx’’ conditions. The new isoprene chemistry requires only a few additional reactants (7) and chemical reactions (7) and, therefore, offers the possibility for the successful application of the revised reaction scheme in chemistry-transport models (CTM) without an excessive increase in computational efforts. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Tropospheric chemistry; Biogenic VOC; Chemical box model; Smog chamber study; Condensed mechanisms

1. Introduction Isoprene (2-methyl-1,3-butadiene) plays an important role in tropospheric chemistry. Although large uncertainties still exist in biogenic hydrocarbon emission inventories (Simpson et al., 1999), it is assumed that on a global scale almost 50% of all biogenic non-methane hydrocarbon (NMHC) emissions are due to isoprene (Guenther et al., 1995). Even in regions that are strongly influenced by anthropogenic emission sources, biogenic isoprene emissions can reach comparable magnitudes (Lamb et al., 1993; Simpson et al., 1995). Very recently, *Corresponding author. Tel.: +49-202-439-3832; fax: +49202-439-2757. E-mail address: [email protected] (H. Geiger).

Shallcross et al. (2001) reported that isoprene emissions can play a significant role in photosmog formation even in urban and suburban environments. In the free troposphere, isoprene mixing ratios can reach values of up to 1 ppbV (Warneke et al., 2001), whereas above forest canopies, isoprene mixing ratios of about 4 ppbV and more have been detected (Apel et al., 2002; Rinne et al., 2002). Isoprene is by far the most important biogenic NMHC in the troposphere with the largest flux of any single volatile organic compound (VOC) apart from methane. In addition, isoprene shows a much higher reactivity towards OH, NO3 and O3 than most other anthropogenic VOC (Atkinson, 1994). Due to the high emission rates of isoprene in combination with its high reactivity towards tropospheric oxidants, the degradation mechanism of

1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(02)01047-6

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isoprene in the atmosphere is of immense interest, particularly for chemistry-transport model (CTM) applications performed on regional and global scales (von Kuhlmann et al., 1999; von Kuhlmann, 2001). The chemistry modules used in CTM are usually characterised by a high level of mechanism condensation. In order to provide processing of complicated models even on modern computers, the chemical code has to be minimised. In particular defining classes of compound instead of treating all VOC explicitly reduces the number of organic chemistry reactions and reactants quite considerably. Only a few VOC, which exert extensive influence on tropospheric chemistry, e.g. methane, ethene and isoprene, remain in the models as individual VOC, although even in these cases the oxidation pathways are also reduced to a minimum of reaction steps. Unfortunately, most chemical modules currently established in CT modelling do (i) not reflect the latest laboratory results on tropospheric isoprene degradation and (ii) often lead to very different modelling results . when applied to similar scenarios (Poschl et al., 2000; Geiger et al., 2002a). In recent times, one of the most widely applied chemical mechanisms in CTM has been the regional acid deposition model (RADM2) by Stockwell et al. (1990). This mechanism has found wide application in regional ozone prognosis and air quality modelling. Also in the RADM2 mechanism, isoprene was defined as an individual compound. However, its degradation was treated more like the oxidation of a ‘‘simple’’ alkene with respect to the notation of the RADM2 surrogate compounds and no typical isoprene oxidation products were defined in the mechanism. In order to gain better modelling results with regard to the isoprene chemistry, Zimmermann and Poppe (1996) developed a suitable supplement for RADM2. Seven years after the publication of RADM2, Stockwell et al. (1997) presented a revised and much more detailed version, the regional atmospheric chemistry mechanism (RACM), which better reflected the results of the laboratory studies at that time and included a number of additional surrogate compounds. In the RACM mechanism, the degradation scheme of isoprene is more comprehensive than that in RADM2. In particular, for the representation of the primary products of its OH-initiated oxidation, an individual surrogate species was defined. In the intervening period since the publication of RACM, numerous laboratory studies on the tropospheric oxidation of isoprene (e.g. Lei et al., 2001; McGivern et al., 2000; Ruppert and Becker, 2000; Benkelberg et al., 2000; Paulson et al., 1999; Orlando et al., 1999; Jenkin et al., 1998; Chen et al., 1998) have appeared rendering the isoprene chemistry in RACM no longer state-of-theart. Since isoprene is highly relevant in tropospheric photochemistry, frequent updates of its oxidation

mechanism based on new mechanistic developments are essential. The master chemical mechanism (MCM) (Jenkin et al., 1997; Saunders et al., 1997) is by far the most detailed tropospheric chemistry model. This explicit mechanism describes the degradation of more than 120 VOC. It consists of more than 10,000 elementary reactions and more than 3000 individual species. Since the MCM is periodically updated on the basis of newly published laboratory results (Pilling et al., 2002), it is always more state-of-the-art than other tropospheric chemistry mechanisms. However, due to its size the application of the MCM in CT models is not practicable. Nevertheless, the MCM is an excellent database, which can be used for the construction of condensed reaction mechanisms. . Very recently, Poschl et al. (2000) reported the development of the Mainz isoprene mechanism (MIM), a condensed isoprene degradation mechanism constructed for application in global atmospheric modelling. Their reaction scheme was directly derived from the MCM. In an extensive box model intercomparison for a couple of well-defined scenarios, the authors showed that their mechanism reproduces MCM calculations well with a high grade of accuracy, whereas other chemical modules established in CTM applications show large deviations due to their inadequate isoprene chemistry. Although the MIM represents, to our knowledge, the most actual condensed tropospheric isoprene degradation scheme, its still comparatively large size of 44 elementary reactions and 16 additional species does not make its direct and successful adaptation into an established chemical module, such as the RACM mechanism, an easily amenable task. Accordingly, the MIM scheme was reworked in order to facilitate its implementation in CTMs. In the present study, the MIM was implemented into the RACM Version 1.3 scheme (Stockwell et al., 1997), followed by further simplifications made on the basis of sensitivity analysis results. The revised mechanism was applied to environmental chamber experiments for validation. Furthermore, an extensive box model intercomparison for both the original and updated RACM code applied to a number of well-defined scenarios meeting tropospheric conditions ranging from very clean to polluted air masses was performed.

2. Experimental and modelling techniques and scenarios 2.1. Smog chamber experiments The smog chamber experiments were performed in the European photoreactor, EUPHORE, in Valencia, Spain. Set-up and operation procedures of this

H. Geiger et al. / Atmospheric Environment 37 (2003) 1503–1519

large-volume outdoor smog chamber have been described in detail previously (Siese et al., 2001 and references therein). Briefly, the chamber consists of a half-spherical FEP bag (fluorine–ethene–propene, foil thickness 127 mm) with a volume of about 187 m3 and a base diameter of 9.2 m. Purified and dried ambient air is used to fill the chamber and flush it between the experiments. The air purification system consists of a compressor, a condensate trap and an absorption dryer filled with a suitable molecular sieve, leading to a dew point below 213 K. In addition, a special charcoal adsorber unit eliminates NOx and reduces oil vapour and non-methane hydrocarbons below 0.3 mg m
3. Rapid mixing of the reactants (o1.5 min) is achieved by two fans with a nominal throughput of 8000 m3 h
1 housed in the chamber. The humidity in the chamber is measured by a dew point hygrometer (Walz, model TS-2 TSM2), the temperature is monitored by several thermocouples (PT100) located in different positions inside the chamber. For the present study, isoprene was oxidised in the presence of NOx in NMHC mixtures containing fractions of n-butane, ethane and toluene. The following analytical instruments were used to record the concentration–time profiles of selected reactants: FTIR spectroscopy (Nicolet Magna 550): ethene, NO2, O3, HCHO, HNO3, PAN GC-ECD (Schmitt/Glasheutten); GC-FID (Hewlett-Packard HP 6890): isoprene, toluene; TGA/GC-FID (Fisons Instruments trace gas analyser): n-butane, methacrolein, methylvinylketone; NOx monitor (Monitor Labs ML 9841 A): NO; Ozone monitor (Monitor Labs ML 9810): O3. J(NO2) was measured by means of either a filter radiometer (Meteorologie Consult) or a spectral radiantmeter (Gigahertz Optics). The chamber leakage was calculated from the added constant dilution flow. The compounds used in the experiments had the following purities: Isoprene (Aldrich, >99%), n-butane (Messer Griesheim, 99.95%), ethene (Messer Griesheim, 99.95%), toluene (Aldrich, >99.9%), NO (Messer Griesheim, X99.8%). 2.2. Box model calculations and sensitivity analyses All the computer simulations and sensitivity analyses were carried out using the box model SBOX (Seefeld and Stockwell, 1999). This FORTRAN 77 program incorporating the Gear algorithm (Gear, 1971) was operated on an SGI OCTANE workstation (Silicon Graphics) running under IRIX 6.5. The program uses the public domain library VODE (Brown et al., 1989) to integrate the ordinary differential equations. The generation of the photolysis frequencies for all photosensitive processes is described in detail by Kuhn et al. (1998). The photolysis frequencies were calculated using the program ‘‘photoRACM’’ (Seefeld and Stockwell, 1999), which employs the algorithm of Madronich (1987). For

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simulating the smog chamber experiments, the calculated values of J(NO2) were fit to the experimental filter radiometer or spectral radiantmeter data using scaling factors. All other J values were then calculated relative to J(NO2). Sensitivity analyses were performed using the direct decoupled method of Dunker (1984). The sensitivities of selected reactants towards all the rate coefficients in the chemical mechanism were calculated. For this purpose, rate parameter sensitivity coefficients, SK ; were determined as described by Stockwell et al. (1995). These rate parameter sensitivity coefficients depend on the concentrations of all reactants and, therefore, on reaction time. In order to obtain relative sensitivities for each elementary reaction, the SK values were first summarised and time-averaged. Relative sensitivity coefficients, SR ; were then calculated following the procedure of Stockwell et al. (1995) using equation: SA;i ; SR;i ¼ P j SA;j

ð1Þ

where i; j is the number of reactions and SA the timeaveraged normalised sensitivity coefficients, obtained by averaging the absolute values of the normalised time dependent sensitivity coefficients SK over the simulation time period. 2.3. Scenarios The present work reports modelling results for four well-defined scenarios with tropospheric conditions ranging from very clean to highly polluted air masses. These scenarios are taken from a paper of Poppe et al. (2001), which provides a variety of fixed conditions for tropospheric modelling applications. The scenarios FREE, LAND, PLUME and URBAN reflect the free troposphere, the remote planetary boundary layer and two different cases with relatively high anthropogenic VOC and NOx contents, respectively. Table 1 gives details of the modelling conditions. In addition to the definitions given by Poppe et al. (2001), an isoprene emission rate of 4.6 ppbV/day (in accordance with . Poschl et al., 2000) was added for the cases URBAN, PLUME and LAND. For the FREE scenario, this rate was divided by a factor of 5 in order to meet the lower VOC burden of the free troposphere. For simplification, the emission rates were kept constant over each simulation, which was run for 5 days starting at noon with photolysis conditions corresponding to 1 July, 451 north. A number of other scenarios with different initial concentrations and emission rates for VOC and NOx were also treated in the same manner described above. The results are not given here, since they showed that the cases FREE, LAND, PLUME and URBAN give a comprehensive and representative picture of

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Table 1 Model parameters for the computer simulations according to Poppe et al. (2001). All runs were performed for a time period of 5 days Model parameter

Scenario FREE

LAND

Initial mixing ratios (ppbV) Ozone 100 30 NO 0.05 0.1 NO2 0.05 0.1 HNO3 0.1 0.1 CO 100 100 1700 1700 CH4 H2 500 500 H2O2 2 2 HCHO — 1 Isoprene — — H2O (%) 0.05 1 Emission rates (ppbV/day) Isoprene 0.92 4.60 SNMHC — — NOx — —

PLUME URBAN 50 0.2 0.5 0.1 200 1700 500 2 — — 1

4.60 18.87 3.86

30 0.1 0.1 1.5 100 1700 500 2 1 — 1

4.60 18.87 19.32

Temperature (K) 236.21 288.15 288.15 298.00 total pressure (Torr) 267.4 760 760 760 Photolysis conditions July 1; 451 north; start: 12:00 GMT

tropospheric chemistry where isoprene degradation plays a role. The calculations for the additional scenarios did not lead to any information that could not be obtained from the four ‘‘base’’ cases.

3. Results and discussion 3.1. Implementation of the MIM isoprene chemistry into RACM . The MIM by Poschl et al., 2000 was implemented into the RACM by Stockwell et al. (1997). The isoprene chemistry of the original RACM scheme was completely removed and replaced by the corresponding MIM reactions. The MIM consists of 34 elementary chemical reactions and 10 photolysis processes and includes 16 reactants that are directly related to the isoprene degradation. Since a number of MIM reactions, particularly those of secondary degradation steps, already exist in a very similar form in RACM, the total number of MIM chemical reactions to be transferred to the RACM code was only 22. Seven photolysis processes defined in the MIM scheme had to be added to the full revised RACM mechanism. The implementation of the MIM scheme did not require large modifications regarding the notation of the

Table 2 Species relevant for the isoprene oxidation in the revised RACM scheme in alphabetical order Species

Definition

Origin

ACO3 HACE ISHP

Acyl peroxy radicals (CX2) Hydroxyacetone and other C3 ketones b-hydroxy hydroperoxides from ISOP+HO2 Isoprene b-hydroxyalkylnitrates from ISOP+NO and alkylnitrates from ISO+NO3 Peroxy radicals from ISO+OH Peroxy radicals from MACR+OH Methacrolein, methylvinylketone and other unsaturated carbonyls Hydroperoxides from MACP+HO2 Methylglyoxal and other a-carbonyl aldehydes Methyl peroxy radical Peroxymethacryloylnitrate and other higher peroxyacylnitrates from isoprene oxidation Nitrooxyacetaldehyde Internal alkenes Terminal alkenes Formic acid Higher organic acids (CX2)

RACM MIM MIM

ISO ISON

ISOP MACP MACR MAHP MGLY MO2 MPAN

NALD OLI OLT ORA1 ORA2

RACM MIM

RACM MIM RACM MIM RACM RACM MIM

MIM RACM RACM RACM RACM

reactants. Most species defined in MIM are similarly notated in RACM. Seven new reactants had to be defined in the revised RACM scheme. The reactants relevant to isoprene oxidation in the updated RACM mechanism are listed in Table 2. The Tables 3a and b give an overview of the reactions that were implemented into the RACM scheme from MIM. While the original RACM includes 77 reactants and 237 elementary reactions, the full updated scheme consists of 84 species and 252 reactions (see also Table 4). 3.2. Reduction of the new isoprene chemistry As already mentioned above, chemical modules applied in CT models have to be as compact as possible in order to minimise processing time and enable the simulation of scenarios with high numbers of grid points. Accordingly, an appraisal has been performed to determine whether all the reactions defined in MIM are necessary for the description of different tropospheric chemical systems or if selected reactions might be removed from the full updated RACM scheme without loosing significant accuracy of the modelling results for representative scenarios. For this purpose, sensitivity analyses were performed for the scenarios FREE, LAND, PLUME and URBAN with the conditions summarised in Table 1. Time-averaged relative

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Table 3 (a) Reactions adapted from the MIM isoprene degradation scheme, which were implemented in the updated RACM schemea No. Reaction Rate coefficientb - ISOP - 0.65MACR+0.58HCHO+0.1MACP +0.1ACO3+0.08MO2+0.28ORA1+0.14CO +0.09H2O2+0.25HO2+0.25OH R3 ISO+NO3 - ISON R4 ISOP+NO - MACR+NO2+HCHO+HO2+0.046ISON - ISHP R5 ISOP+HO2 R6 ISOP+ISOP - 2MACR+HCHO+HO2 R7 ISHP+OH - MACR+OH R8 ISON+OH - HACE+NALD R9 MACR+OH - MACP R10 MACR+O3 - 0.9MGLY+0.45ORA1+0.32HO2+0.22CO +0.19OH+0.1ACO3 R11 MACP+NO - NO2+0.25HACE+0.25CO+0.25ACO3 +0.5MGLY+0.75HCHO+0.75HO2 R12 MACP+HO2 - MAHP R13 MACP+MACP - HACE+MGLY+0.5HCHO+0.5CO+HO2 R14 MACP+NO2 - MPAN R1 R2

ISO+OH ISO+O3

R15 R16 R17 R18 R19

MPAN MPAN+OH MAHP+OH HACE+OH NALD+OH

-

MACP+NO2 HACE+NO2 MACP MGLY+HO2 HCHO+CO+NO2

2.54E
11 exp(410/T) 7.86E
15 exp(
1913/T)

3.03E
12 exp(
446/T) 2.43E
12 exp(360/T) 2.05E
13 exp(1300/T) 2.0E
12 1.0E
10 1.3E
11 0.5  (4.13E
12 exp(452/T)+1.86E
11 exp(175/T)) 0.5  (1.36E
15 exp(
2112/T)+7.51E
16 exp(
1521/T)) 2.54E
12 exp(360/T) 1.82E
13 exp(1300/T) 2.0E
12 k0 =9.7E
29(T/300)
5.6 kN=9.3E
12(T/300)
1.5 Fc =0.6 kR15=kR14/(9.0E
19 exp(14000/T)) 3.6E
12 3.0E
11 3.0E
12 5.6E
12 exp(270/T)

(b) Reactions adapted from the MIM isoprene degradation scheme neglected after sensitivity analysis towards MACR, MAHP, HACE, NALD, ISON, ISHP, MPAN and OHc No. R20 R21 R22 R23 R24 R25

Reaction NALD+hn PAN+hn MPAN+hn HACE+hn ISHP+hn MAHP+hn

R26 ISON+hn R27 ORA2+OH

-

HCHO+CO+NO2+HO2 ACO3+NO2 MACP+NO2 ACO3+HCHO+HO2 MACR+OH+HCHO+HO2 0.5HACE+0.5CO+0.5MGLY +0.5HCHO+OH+HO2 - MACR+HCHO+NO2+HO2 - MO2

Rate coefficientd . see Poschl et al. (2000) . see Poschl et al. (2000) . see Poschl et al. (2000) . see Poschl et al. (2000) . see Poschl et al. (2000) . see Poschl et al. (2000) . see Poschl et al. (2000) 4.0E
13 exp(200/T)

a

All reactants are given in RACM notation. Corresponding reactions of the original RACM were replaced by those given here. . In molecular units, according to Poschl et al. (2000). c All reactants are given in RACM notation. d In molecular units. b

Table 4 Numbers of species and reactions of the mechanisms applied in the present work Mechanism

Species

Chemical reactions

Photolysis reactions

RACM 1.3a Full updated RACM Reduced updated RACM

77 84 84

214 222 221

23 30 23

a

Stockwell et al. (1997).

sensitivities (see above) were calculated for all the elementary reactions towards MACR, MAHP, ISON, ISHP, HACE, MPAN and NALD. These reactants are direct or indirect reaction products of the isoprene oxidation defined in the mechanism under scrutiny (see Table 2). Elementary reactions of the isoprene chemistry that are not significantly sensitive towards their concentration can be removed from the mechanism. In order to maintain the overall radical balance of the system, the sensitivities towards OH radicals were also

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calculated and considered. For the reactions summarised in Table 3b, the time-averaged sensitivities obtained for all scenarios were always less than 1.5%. These are all photolysis reactions adapted from the MIM scheme (R20–R26) as well as the reaction of acetic and higher acids (ORA2) with OH radicals (R27). The model calculations described below were always performed with three different chemical codes whose general parameters are summarised in Table 4: (i) the original RACM 1.3, (ii) the RACM 1.3 code with implemented full MIM reactions displayed in the Tables 3a and b (full updated RACM) and (iii) the RACM 1.3 code only updated by the reactions of Table 3a (reduced updated RACM). 3.3. Simulation of the smog chamber experiments Environmental chamber experiments usually do not exactly meet atmospheric conditions, because the reactant concentrations are relatively high and wall effects as well as uncertainties in photolysis rates might disturb the systems’ chemistry. Accordingly, the usefulness of chamber studies is often limited by these factors. However, such experiments allow an atmospheric chemical mechanism to be tested as a complete system (Stockwell et al., 1997). From the problems mentioned above, Stockwell et al. (1990) derived that the accuracy of e.g. predicted tropospheric ozone concentrations by chemical models could not be better than 730%. Nevertheless, smog chamber experiments are practically the only means of checking a new chemical code before it is applied in tropospheric chemistry modelling. Therefore, the present study includes the application of the revised isoprene degradation scheme to suitable environmental chamber data. To this end, a number of experiments were carried out in the European photoreactor EUPHORE in Valencia, Spain. Isoprene was oxidised in simple mixtures of NMHC containing n-butane, ethene and toluene in the presence of NOx. The composition of the NMHC mixtures used was chosen to reach a split of alkanes, alkenes and aromatics representative for the polluted troposphere. A very similar NMHC mixture has already been applied in previous studies (Geiger et al., 2002b). The experiments were modelled by all three mechanisms summarised in Table 4. Whereas both the full and the reduced updated RACM code computed similar concentration–time profiles, the calculations using the original RACM 1.3 scheme yielded significantly different results. This is illustrated in Figs. 1 and 2 showing selected c=t profiles for chamber experiments the conditions of which are summarised in Table 5. The two example runs chosen here for discussion are characterised by very different NMHC/NOx ratios of 9.5 (high NOx) and 53.0 ppbC/ppbV (low NOx), respectively. In general, the revised isoprene code leads

to a very good description of the chamber data, whereas application of the original RACM shows significant disparities. The deviations between the original and the revised RACM results are generally higher for ‘‘low NOx’’ (Fig. 2) than for ‘‘high NOx’’ conditions (Fig. 1). For all mechanisms and both runs, the results for nbutane and toluene are quite similar and the agreement with the chamber data is good. In contrast, the original RACM calculations yield systematic deviations from the experimental data for isoprene, NO and NO2, which is much more pronounced for the low NOx than for the high NOx case. The isoprene mixing ratio is overestimated and the NO/NO2 crossing point is significantly shifted to larger reaction times, indicating that the radical budget in the original RACM calculations is too low. This is also supported by the fact that the formation of HNO3, whose only source is the reaction of OH radicals with NO2, is underestimated in the initial phase of the experiment (only high NOx case, since HNO3 data are not available for the other experiment). In contrast, the revised RACM code leads to a much better agreement of the isoprene, ethene, NO and NO2 profiles (as well as HNO3 in the initial phase in the high NOx case). While the calculated ozone profile in the high NOx case is not significantly affected by the type of mechanism used, the disparities at low NOx conditions are much higher. Here, the original RACM leads to a systematic underestimation of ozone over nearly the whole run. Only in the late afternoon does the calculated ozone concentration reach the measured level. The revised RACM code leads to ozone mixing ratios that are about 10% higher than those obtained in the experiment, which is still within the uncertainties of the analytical device and the model. For PAN (peroxy acetyl nitrate), the disparities between modelled and measured concentrations are generally small. Except for the calculation for the low NOx case by the revised RACM, which shows very good agreement, the experimental data are always slightly underestimated. The HCHO concentration is better described by the original RACM scheme at high NOx levels. However, the HCHO mixing ratio calculated by the updated RACM for this case is still within the experimental error of the HCHO measurements of about 720%. In the case of low NOx, the modelling results are reversed. Here, the revised RACM scheme predicts an HCHO concentration that is in very good agreement with the experimental data, whereas the measured concentrations are clearly underestimated by the original RACM code. Both models overestimate the HNO3 concentration at higher reaction times, which might be due to the increasing significance of HNO3 wall loss when the NO2 and OH concentrations are decreasing at higher reaction times. Another explanation of the experimental HNO3 behaviour may be the formation of

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Fig. 1. Comparison of experimental and simulated (RACM 1.3 and reduced updated RACM) concentration–time profiles for an example run in the EUPHORE chamber (high NOx conditions): (K) isoprene; (J) toluene; (m) n-butane; (W) ethene; (.) NO; (,) NO2; (~) O3/2; (}) HCHO; (’) HNO3; (&) PAN; (2) MACR. Initial conditions are given in Table 5.

aerosols in the toluene oxidation when the NO concentration becomes nearly zero (Mart!ın et al., 2002). These aerosols may then serve as a significant sink for nitric acid. It is worthy of note that for both chamber runs the original RACM clearly underestimates the mixing ratio of MACR, whereas the updated version calculates MACR values that are significantly

higher than the experimental data. This observation is probably due to the fact that the experiments only represent the sum of methacrolein and methylvinylketone. In contrast, in the notation of both RACM codes, MACR is a surrogate compound for all unsaturated C4 carbonyls. Accordingly, the original RACM clearly underestimates the MACR formation

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Fig. 2. Comparison of experimental and simulated (RACM 1.3 and reduced updated RACM) concentration–time profiles for an example run in the EUPHORE chamber (low NOx conditions): (K) isoprene  2; (J) toluene  2; (m) n-butane; (W) ethene; (.) NO; (X) NO2; (~) O3/2; (}) HCHO; (&) PAN; (2) MACR. Initial conditions are given in Table 5.

due to a lack of mechanistic information, whereas the overestimation observed for the revised RACM scheme is probably caused by incomplete experimental data, since not all unsaturated carbonyls were measured. In conclusion, it can be stated that the reduced updated RACM scheme incorporating the revised isoprene chemistry leads to a significantly better description of environmental chamber data than the original RACM 1.3 code. Therefore, it is recommended

to use the revised RACM mechanism for future model applications instead of the former code. 3.4. Model intercomparison In the light of the smog chamber modelling results discussed above, it is obvious that the use of the revised isoprene chemistry will also significantly influence the results of chemical models applied to conditions closer

H. Geiger et al. / Atmospheric Environment 37 (2003) 1503–1519 Table 5 Initial conditions for the EUPHORE smog chamber experiments and the corresponding computer simulations the results of which are displayed in Figs. 1 and 2 Model parameter

High NOx case

Initial mixing ratios (ppbV) Isoprene 58 n-butane 195 Ethene 198 Toluene 61 NO 158 NO2 41

Low NOx case 47 245 262 77 38 5

NMHC/NOx (ppbC/ppbV)

9.5

Temperature (K) Total pressure (Torr) Chamber leakage (h
1) Duration of experiment Date of experiment

30471 29271 751 760 2  10
2 2  10
2 8:45–16:45 GMT10:10–15:20 GMT July 8, 1998 November 20, 2001

53.0

to the ‘‘real’’ atmosphere than environmental chamber studies. In order to check this assumption, box model calculations were performed for the cases FREE, LAND, PLUME and URBAN with the parameters summarised in Table 1. Corresponding concentrationtime profiles for selected reactants are displayed in Figs. 3–6. The data were calculated using all three chemical codes listed in Table 4. Total PAN was obtained by adding up the mixing ratios of the surrogate species PAN (alkylperoxyacetylnitrates) and TPAN (unsaturated peroxyacetylnitrates) for the original RACM scheme. For the updated mechanism, also the compound MPAN (peroxyacetylnitrates from isoprene oxidation) had to be considered. The original RACM reflects organic nitrate only by the single surrogate ONIT, whereas it was determined for the revised RACM scheme by adding up the concentrations of ONIT, ISON and NALD (see Table 2). 3.4.1. Comparison of original and revised RACM results Calculations with the revised RACM scheme lead to very different disparities for a number of reactants in comparison to the original RACM mechanism, as illustrated in Figs. 3–6. Regarding the updated RACM codes, the comparative model calculations for all scenarios revealed that removal of the reactions R20–R27 does not cause any significant disparities in the modelling results, as had already been indicated by the results of the sensitivity studies. The deviations calculated for all relevant concentrations were always less than 2% and are in most cases not shown in the figures. Consequently, reactions R20–R27 can be removed from

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the full revised isoprene scheme without loosing any significant information in the simulations. In addition, the reaction of O(3P) with isoprene, which is part of the original RACM mechanism, was removed for the . present update. Poschl et al. (2000) also did not consider this reaction in MIM. It could be shown that, under the various tropospheric conditions considered here, this reaction has negligible influence on the model calculations. While the description of ozone is very similar in all cases, the new isoprene chemistry leads to much higher organic nitrate yields and much lower concentrations of . total PAN, which confirms the observations of Poschl et al. (2000). The only exception is the mixing ratio of total PAN for the URBAN scenario, which is more or less similarly described by both the original and the revised RACM mechanism. For a more detailed analysis of the profiles, the lowNOx cases FREE and LAND must be viewed separately from the high-NOx cases URBAN and PLUME. For the FREE and the LAND scenario (see Figs. 3 and 4, respectively), the original RACM systematically overestimates the concentrations of ozone, NOx and OH radicals, while the isoprene and MACR mixing ratios are always much lower than obtained for the updated mechanism. For MHP, no uniform trend is observed. In the FREE scenario, both mechanisms yield very similar MHP concentrations for the first 3 days. After day 3, the MHP mixing ratio for the original RACM becomes significantly higher. In the LAND case, the original RACM yields systematically lower MHP values, probably due to the different VOC/NOx ratio. In the PLUME scenario, both the original and updated RACM lead to more or less similar mixing ratios for OH, ozone, NOx, and isoprene (see Fig. 5). Except for isoprene, the deviations are somewhat higher for the URBAN case (see Fig. 6). For both scenarios, the MACR mixing ratio strongly oscillates in the original RACM calculation, whereas the oscillation is less pronounced in the simulations using the updated scheme. At each noon, the MACR mixing ratio calculated with the original RACM mechanism goes down to nearly zero, whereas the revised scheme yields concentrations that are much higher. The maximum MACR concentrations at night increase during the PLUME simulation, while they continuously decrease in the URBAN case. The MHP mixing ratios remain similar for the first 24 h for the PLUME as well as the URBAN case. After day 1, the original RACM yields systematically higher MHP values. Their absolute deviations increase with reaction time. The relative disparities are very similar in both scenarios. The sometimes very high disparities between the modelling results for the original and the revised RACM are clearly caused by the very different isoprene degradation schemes. Besides its very high actuality

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Fig. 3. Mixing ratios of selected reactants vs. reaction time calculated for the FREE case: (- - - - -) original RACM scheme; (     ) full updated RACM scheme; (——) reduced updated RACM scheme.

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Fig. 4. Mixing ratios of selected reactants vs. reaction time calculated for the LAND case: (- - - - -) original RACM scheme; (     ) full updated RACM scheme; (——) reduced updated RACM scheme.

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Fig. 5. Mixing ratios of selected vs. versus reaction time calculated for the PLUME case: (- - - - -) original RACM scheme; (     ) full updated RACM scheme; (——) reduced updated RACM scheme.

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Fig. 6. Mixing ratios of selected vs. versus reaction time calculated for the URBAN case: (- - - - -) original RACM scheme; (     ) full updated RACM scheme; (——) reduced updated RACM scheme.

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with regard to laboratory studies the MIM chemistry considers (i) a much higher number of isoprene reaction products and (ii) self and cross-reactions of peroxy radicals relevant to isoprene oxidation. The original RACM only provides a very rudimentary description of isoprene degradation. In scenarios where isoprene is not the dominant reactant with regard to the overall VOC reactivity, the RACM isoprene chemistry might be sufficient for a description of the chemical processes. However, in very clean areas with a high burden of biogenic VOC as well as polluted regions influenced by large amounts of isoprene, a more detailed and actual mechanism is definitely necessary. Consequently, it is recommended to update chemical modules of CT models based on the RACM scheme by the modified MIM chemistry summarised in Table 3a. 3.4.2. Influence of peroxy radical cross-reactions The updated RACM mechanism includes two peroxy radical self-reactions ISOP+ISOP (R8) and MACP+ MACP (R15) that were adapted from the MIM scheme. These reactions will play a significant role in isoprene degradation when the NOx concentration is sufficiently low, as, e.g., in the cases FREE and LAND. Also peroxy radical cross-reactions such as, e.g., ISOP+MACP might have some influence under low NOx conditions. However, the mechanistic knowledge on such reactions is generally poor. Four different peroxy radicals, ISOP, MACP, ACO3 and MO2, are relevant to the revised RACM isoprene chemistry, resulting in five possible peroxy radical cross-reactions (not considering

ACO3+MO2, whose chemistry is well established). Although the consideration of these cross-reactions should lead to an increase in modelling accuracy, the lack of information about the reaction products of these steps may otherwise affect the uncertainty of the calculations. In addition, the influence of peroxy radical cross-reactions on the model description of isoprene degradation might be weaker than expected on first sight, since at low NOx conditions, the concentration of hydrogen peroxy radicals (HO2) should be much higher than that of RO2 and the reaction with HO2 will be the major sink of peroxy radicals. In order to check whether the consideration of peroxy radical cross-reactions might be negligible with respect to the isoprene degradation, a further intercomparison was performed between the revised RACM mechanism and another scheme that was enhanced by the reactions ISOP+ MO2, ISOP+ACO3 (part of the original RACM mechanism proposed by Stockwell et al., 1997) and . ISOP+MACP (derived from Poschl et al., 2000). The reactions MACP+MO2 and MACP+ACO3 were not considered, since no information about the reaction products is available. The simulations showed that the influence of RO2 cross-reactions is fairly low. As expected, no deviations were observed for the PLUME and the URBAN cases. Even for the scenarios FREE and LAND only small deviations occur, as displayed in Figs. 7 and 8. The influence on ozone is practically zero. For other concentrations relevant to isoprene degradation, the disparities are always less than 3%. Only the MHP concentration is calculated with deviations of not more than
12% if RO2 cross-reactions are considered.

Fig. 7. Percent deviation between concentration–time profiles of selected reactants calculated for the updated RACM scheme when peroxy radical cross-reactions are considered (FREE case).

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Fig. 8. Percent deviation between concentration–time profiles of selected reactants calculated for the updated RACM scheme when peroxy radical cross-reactions are considered (LAND case).

Accordingly, these reactions can be neglected without loosing too much modelling accuracy.

4. Summary and conclusions A chemical module describing the tropospheric degradation of isoprene has been derived from the . Mainz isoprene mechanism (MIM, Poschl et al., 2000) and was implemented into the regional atmospheric chemistry mechanism (RACM, Stockwell et al., 1997), which is established in many CT models but includes an isoprene chemistry that is no longer state-of-the-art. The MIM code was derived from the highly explicit and actual master chemical mechanism (MCM) and includes a number of specific isoprene reaction products and peroxy–peroxy self-reactions that were not considered in the original RACM scheme. The code of the ‘‘new’’ isoprene chemistry was reduced on the basis of sensitivity analysis results. It was shown that this reduction did not cause any significant disparities of modelling results for both full and reduced updated RACM schemes. Peroxy radical cross-reactions were not considered, since they increase the model uncertainty due to missing knowledge on their reaction products. This additional uncertainty will probably compensate a possible increase of accuracy caused by the inclusion of these reactions into the mechanism. The final revised RACM scheme is characterised by seven additional species and seven additional reactions compared to the original RACM. The new mechanism was

tested at different VOC/NOx ratios against smog chamber data generated in the European photoreactor EUPHORE. These studies yielded a much better description of the experimental concentration-time profiles by the revised code in comparison to the original RACM 1.3 scheme. Box model intercomparison studies for a number of well-defined scenarios representing different tropospheric conditions exhibited strong systematic deviations for a couple of reactants, due to the very different isoprene oxidation pathways in both mechanisms. These calculations demonstrate that particularly for clean areas controlled by biogenic VOC emissions, the isoprene chemistry of the original RACM is too general to be able to give a representative picture of tropospheric photochemistry processes. Accordingly, it is recommended here to update the original RACM mechanism by the reactions summarised in Table 3a. For the CT modellers’ community, particularly the numbers of seven additional species might still appear fairly high with regard to implementation of the revised RACM scheme into CT models. However, from the chemical point of view, the mechanism provided here seems to be the most reasonable solution providing a maximum of both detail and modelling accuracy with respect to tropospheric isoprene degradation. Nevertheless, potential applicants of the revised isoprene scheme might decide to implement also some of the elementary reactions identified as redundant for the conditions here into their models if necessary.

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Acknowledgements . The authors thank U. Poschl, Technical University Munich, R. Forkel, Research Centre Karlsruhe and H.-P. Dorn, Research Centre Julich, . for very helpful discussions and L. Ruppert, Audi Motor Company, as well as K. Wirz, CEAM Valencia, for the supply of experimental data. The financial support by the German Department of Education and Research (BMBF) within the German Atmospheric Research Programme ‘‘AFO 2000’’ is gratefully acknowledged.

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