CAPRAM 2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D14, 4426, doi: /2002jd002202, 2003 CAPRAM 2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application B. Ervens, 1 C. George, 2 J. E. Williams, 3,4 G. V. Buxton, 5 G. A. Salmon, 5 M. Bydder, 5 F. Wilkinson, 5 F. Dentener, 6 P. Mirabel, 7 R. Wolke, 1 and H. Herrmann 1,8 Received 14 February 2002; revised 3 May 2002; accepted 14 June 2002; published 29 July [1] A detailed and extended chemical mechanism describing tropospheric aqueous phase chemistry (147 species and 438 reactions) is presented here as Chemical Aqueous Phase Radical Mechanism (CAPRAM) 2.4 (MODAC mechanism). The mechanism based on the former version 2.3 [Herrmann et al., 2000] contains extended organic and transition metal chemistry and is formulated more explicitly based on a critical review of the literature. The aqueous chemistry has been coupled to the gas phase mechanism Regional Atmospheric Chemistry Modeling (RACM) [Stockwell et al., 1997], and phase exchange accounted for using the resistance model of Schwartz [1986]. A method for estimating mass accommodation coefficients (a) is described, which accounts for functional groups contained in a particular compound. A condensed version has also been developed to allow the use of CAPRAM 2.4 (MODAC mechanism) in higher-scale models. Here the reproducibility of the concentration levels of selected target species (i.e., NO x, S(IV), H 2 O 2,NO 3, OH, O 3, and H + ) within the limits of ± 5% was used as a goal for eliminating insignificant reactions from the complete CAPRAM 2.4 (MODAC mechanism). This has been done using a range of initial conditions chosen to represent different atmospheric scenarios, and this produces a robust and concise set of reactions. The most interesting results are obtained using atmospheric conditions typical for an urban scenario, and the effects introduced by updating the aqueous phase chemistry are highlighted, in particular, with regard to radicals, redox cycling of transition metal ions and organic compounds. Finally, the reduced scheme has been incorporated into a one-dimensional (1-D) marine cloud model to demonstrate the applicability of this mechanism. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0317 Atmospheric Composition and Structure: Chemical kinetic and photochemical properties; 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0345 Atmospheric Composition and Structure: Pollution urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere composition and chemistry; KEYWORDS: Multiphase mechanism, cloud chemistry, aerosol chemistry Citation: Ervens, B., et al., CAPRAM 2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application, J. Geophys. Res., 108(D14), 4426, doi: /2002jd002202, Introduction 1 Institut für Troposphärenforschung, Leipzig, Germany. 2 Laboratoire d Application de la Chimie à l Environnement (LACE), CNRS, Domaine Scientifique de la Doua-Bâtiment J. Raulin, Villeurbanne Cedex, France. 3 Institute For Atomic and Molecular Physics, Amsterdam, Netherlands. 4 Institute for Marine and Atmospheric Research, University of Utrecht, Utrecht, Netherlands. 5 School of Chemistry, University of Leeds, Leeds, USA. 6 Joint Research Centre, Institute for Environment and Sustainability, Ispra, Verona, Italy. 7 Equipe de Physico-Chimie de l Atmosphere, Université Louis Pasteur, Strasbourg Cedex, France. 8 Corresponding author. Copyright 2003 by the American Geophysical Union /03/2002JD [2] A wide variety of experimental field studies as well as modeling studies indicate that one of the largest uncertainties in the current understanding of tropospheric chemistry is the role of aqueous phase particles, that is, droplets of clouds, fog and rain as well as aqueous (deliquescent) aerosol particles. For the latter, recent efforts were able to deepen our knowledge of halogen release from sea-salt, but the impact of chemical conversions caused by the chemically much more complex rural continental or even urban aerosol is largely unresolved. The situation for cloud and fog chemistry is somewhat better as here chemistry concept for dilute aqueous solutions can be applied. A variety of modeling studies [e.g., Jacob, 1986; Jacob et al., 1989; Matthijsen et al., 1995; Jacobson, 1997] exists, but organic chemistry, when included, has always been restricted to molecules with only one carbon atom. This is not a realistic situation especially in continental regions. Therefore, in previous studies as an intermediate step the chemical aqueous phase mechanism (Chemical Aqueous Phase Radical Mechanism (CAPRAM) 2.3) was developed to cover AAC 12-1

2 AAC 12-2 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) C 2 organic chemistry to a large extent [Herrmann et al., 2000]. [3] CAPRAM 2.3 has been further developed as all chemical processes included have been broken down into their elementary steps. A thorough literature review and data evaluation by the authors has been also performed. After detailed discussion, the set of uptake parameters and reaction rate constants was updated. This differs in some cases from the set used in CAPRAM 2.3. Furthermore, extra organic reactions were added to the scheme, for example, oxalate production further completing the organic chemistry to difunctional organics. Additionally the inorganic chemistry, especially the interactions between HO x and transition metal ions, was revised and extended due to the availability of recent data from laboratory studies. As the aqueous phase chemistry is treated using elementary reactions of organics, CAPRAM 2.4 comprises far more species and reactions compared with CAPRAM 2.3. Therefore, to allow an application of the full mechanism in the current generation of higher order models, a reduction of the mechanism has been performed using three independent methods with CAPRAM 2.4 (MODAC mechanism) as reference to satisfactorily reproduce the results of the full mechanism with respect to the diurnal variation of selected target species. The condensed mechanism (CAPRAM 2.4 (MODAC mechanism) condensed) is about half the size of the full scheme and contains about 20% fewer species. It may be recommended that the reduced mechanism CAPRAM 2.4 (MODAC mechanism)-condensed may be used in higher-scale tropospheric models. As a first step in this direction, the reduced scheme has been applied in a one-dimensional (1-D) marine cloud model (section 5). In view of the complexity of an applied model system, the interested user might decide to either to employ CAPRAM 2.3 in which reactions are not all broken down to the elementary level, or CAPRAM 2.4 (MODAC mechanism) in its full version or the condensed version of the latter. These three CAPRAM schemes are available as supporting e-material within the AGU/JGR framework. 1 The schemes as well as comments, extensions and supporting material can also be found at tropos.de/chemie/multimod/capram/capram.html. 2. Model Description 2.1. Details of the Box Model [4] Mechanism development, testing and reduction were all performed in a zero-dimensional box model representing the first 1 km of the tropospheric boundary layer. A cloud with a monodisperse droplet distribution (r = 10 mm) was assumed throughout the box. The liquid water content (LWC) of the cloud was fixed at 0.3 g m 3 which corresponds to 70 droplets cm 3 (g). All simulations were performed for a latitude of 51 N during midsummer (i.e., on 21 June) over a period of 3 days so as to show maximal effects due to the photolysis processes. The temperature and pressure were constant at 288 K and 1 atm, respectively. Most of the simulations were performed assuming the presence of a permanent cloud. More realistically, a number of simulations was also performed using a shorter cloud duration of 2 hours introduced in the box at 2 p.m. each day. 1 Supporting materials are available at ftp://agu.org/apend/jd/ 2002JD This shorter period is thought to be more representative of the mean lifetime of low tropospheric clouds [Lelieveld and Crutzen, 1991]. The residence time of a single air parcel within a cloud is much shorter (some minutes). Therefore, the consideration of several hours in the model studies corresponds to the assumption that an air parcel crosses a convective cloud several times. For all simulations in the zero-dimensional model, cloud formation and dissipation were assumed to be instantaneous, that is, temporal evolution of the cloud was not considered. [5] The calculations with the zero-dimensional box model were performed both on a UNIX workstation using software developed by Wolke and Knoth [1996], based on a modified version of the LSODE solver [Hindmarsh, 1980], and on a PC using the commercial FACSIMILE software package [Curtis and Sweetenham, 1987] Emission and Deposition [6] In order to allow a meaningful simulation to be performed over a 3 day period it was necessary to include both emission and dry deposition velocities for chemical species into the box. Accurate estimates for the emission rates for each of the different scenarios were obtained from databases. Anthropogenic emissions were taken from the EDGAR 1-1 database of Olivier et al. [1996], and biogenic emissions from the global database of Guenther et al. [1995]. From these the maximum, median and minimum values for each species were calculated and used for the emission rates for the urban, rural and marine scenarios, respectively. In the marine case, emission rates for certain species, such as isoprene, were set to zero. Dry deposition velocities for the most important gas phase species were taken from Ganzeveld et al. [1998]. Although the heterogeneity of emissions and depositions is very large on a European scale and dependent on many meteorological, biological, and chemical parameters, it was thought that the approach adopted here results in a fairly representative set of scenarios. This is supported by the fact that the combination of values chosen resulted in calculated concentrations for O 3,HO x, and NO x, which are within realistic limits for each scenario. For the specific details regarding the values used the reader is referred to the CAPRAM home page at CAPRAM/capram.html Gas Phase Chemistry (RADM2 to RACM) [7] Several gas phase mechanisms have been developed over the last decade for the purpose of simulating tropospheric chemical processes. A number of these have already been coupled to complex multiphase chemical schemes, for example, RADM/RADM2 [Stockwell, 1986; Stockwell et al., 1990] with CAPRAM 2.3 [Herrmann et al., 2000]. Recent progress concerning the understanding of gas phase chemical processes has resulted in the insertion of new data into existing codes. One example is the upgrade of RADM/ RADM2 to Regional Atmospheric Chemistry Modeling (RACM) [Stockwell et al., 1997]. This code is a versatile condensed chemical mechanism which has been tested using a wide range of scenarios, and this is why it was chosen here for the development of CAPRAM 2.4 (MODAC mechanism). RACM includes 237 reactions involving 17 inorganic and 32 organic stable species. The main changes from RADM2 were in the treatment of

3 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) AAC 12-3 alkanes, alkenes, and aromatics and also a better description of organic peroxyl radicals. Moreover, the description of the chemistry of the most important biogenic species, namely isoprene, a-pinene, and limonene, was substantially improved. Another feature of RACM is the lumping of species into groups, which dramatically reduces the overall size of the mechanism Uptake of Gases/Estimation Method [8] A central point for modeling tropospheric multiphase chemistry is the phase transfer. This process may act as both a source and a sink for gaseous oxidants and, therefore, should be treated explicitly by numerical models. In fact, phase equilibrium is not necessarily achieved under real conditions due to various competing transformations as has been demonstrated before [e.g., Audiffren et al., 1998; Herrmann et al., 2000]. The rate of uptake of a species by a liquid is a multistep process, which is related to the fundamental properties of the gas, the interface, and the condensed phase. These include the mass accommodation coefficient (a), solubility and chemical reactivity. The overall uptake process may be summarized as follows: (1) diffusion of the molecules in the gas phase to the liquid interface, (2) transfer across the interface (accommodation process), and (3) reaction in the condensed phase. The rate at which a trace gas molecule is transferred from a wellmixed gas phase at a given concentration (n mixed ) into the condensed phase is obtained from the kinetic theory of gases. The mass accommodation coefficient (a) represents the probability that a molecule impinging on the interface will be transferred into the condensed phase, that is, a 1.0. [9] However, not all air masses are well mixed and certainly not close to a liquid droplet. In this case, gas phase diffusion should also be taken into account. An effective parametrization for the uptake process is given by Schwartz [1986]: k mt ¼ r2 þ 4r 1 ð1þ 3D g 3hcia where is the radius of the droplet, D g the gas phase diffusion coefficient and hci the mean velocity of the gas molecules. To apply the above equation, one has to know both the diffusion and the mass accommodation coefficients. Several semiempirical techniques [e.g., Reid et al., 1986] have been developed over recent years to estimate values of D g. [10] The data available concerning mass accommodation coefficients are sparser and, to date, no satisfactory estimation techniques have been reported. Mass accommodation coefficients used in this study were taken from experimental studies or estimated, which leads to values between 10 2 and 1. A way of estimating a, developed here, is based on experimental data and a modeling approach initially derived by Davidovits et al. [1995]. [11] In order to explain the negative temperature dependence of a, Davidovits et al. [1995], developed a model, later modified by Nathanson et al. [1996], for the description of the dynamics at the interface. They considered that mass accommodation is a multistep process where the trace gas first thermally accommodates on the droplet surface, with unit probability, and either undergoes a further step into the liquid or is released back to the gas phase. The penetration into the liquid is described as a continuous nucleation process at the interface where only clusters reaching a critical size are taken up by the liquid phase. The critical size is defined as N*, that is, the number of molecules in the cluster or, more precisely, the number of hydrogen bonds used to form the cluster by condensation. In this theory, there is a direct relationship between H obs and S obs governed by N*. The changes of entropy S obs and enthalpy H obs can then be determined from N* by using the equation reported by Nathanson et al. [1996] H obs ¼ 10 N * 1 þ 7:53 N *2=3 1 0:1 10 S obs ¼ 13 N * 1 19 N * 1 0:1 13 kcal M 1 ð2þ þ 9:21 N *2=3 1 cal M 1 K 1 ð3þ Consequently, the Gibbs free energy G* can be determined from S obs and H obs. The mass accommodation coefficient a is then calculated using:! a G* ¼ exp 1 a RT N* is related to the chemical structure of the incoming gas [Duan et al., 1993]. Therefore, for methyl hydroperoxide, CH 3 OOH, H obs and S obs fall between those reported values for methanol and hydrogen peroxide, indicating quite clearly that N* is related to the chemical structure of the gas. Therefore, it should be possible to calculate mass accommodation coefficients as a function of temperature with a certain degree of accuracy from existing data. The following assumption is made: N* is the sum of the contributions of the functional groups in a molecule (Table 1), viz.: N * ¼ X i where N* i is the contribution of the ith chemical group in a given molecule. For example, N* for OH is half that of H 2 O 2, although this does not take care of the peroxy function, and it is possible to determine the contribution of the OH function, given by: N i * N * OH ¼ N* H 2O 2 =2 Likewise, it is possible to calculate the contribution N* of the methyl, ethyl and peroxy groups from N* reported for methanol, ethanol and methyl hydroperoxide. These are listed in Table 1. In fact, for ethyl hydroperoxide, N* can be calculated according to: N * EtOOH ¼ N* CH 3CH 2 þ N * OOH ¼ 1:67 þ 0:66 ¼ 2:33 ð7þ ð4þ ð5þ ð6þ

4 AAC 12-4 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) Table 1. Group Contribution to N* Species Group Contribution to N* OH 0.88 CH CH 3 CH C(O) OOH 0.66 H 1.52 COOH 0.84 Table 2. Comparison Between Experimentally Determined and Estimated Mass Accommodation Coefficients a a exp at 288 K Calculated N* a Estimated at 288 K Ethylene glycol Propanol EtOOH CH 2 (OH)OOH a Measured values are taken from Nathanson et al. [1996]. At the current state of knowledge, such estimates should be used with caution and experimental values should be preferred when available. As shown in Table 2, estimated values for a are relatively close to the measured ones, but since the current database is small, the estimation method described above is not well founded. However, this simple approach can be expected to demonstrate the effect of chemical structure on a and to predict correctly the trends between various gases Aqueous Reaction Scheme [12] CAPRAM 2.4 (MODAC mechanism) explicitly describes all reactions involving OH, HO 2, NO 3, SO 4, Cl 2,Br 2 and CO 3 with inorganic (TMIs, NO 3,Cl,Br ) and organic (C 1 /C 2 ) reactants in the tropospheric aqueous phase. Moreover, both radical and redox reaction pathways are included for the oxidation of N(III) and S(IV). The complete aqueous phase scheme can be found at the CAPRAM home page. This page also contains details of the previous mechanism (CAPRAM 2.3) [Herrmann et al., 2000] and updates/extensions of the mechanism used in this paper. Both versions of the mechanism are downloadable in pdf and ascii format for easy reference. The CAPRAM 2.4 (MODAC mechanism) includes a total of 153 species that are involved in a total of 439 chemical processes. Phase transfer for 34 species is also included (see section 2.4). More details of the numbers of species and reactions for versions 2.3 and 2.4 are given in Table 3. As can be seen, the aqueous phase mechanism is divided into nine subsystems. In the following the most essential extensions and differences between CAPRAM 2.3 and CAPRAM 2.4 (MODAC mechanism) are specified; further marginal changes, such as the update of selected kinetic data, in the mechanism are not discussed in detail here Chemical Subsystems in CAPRAM O 3 +HO 2 [13] The reduction of ozone by the hydroperoxyl radical anion, O 2, reaction (R1), has been split into elementary steps using the multistep mechanism suggested by Staehelin and Hoigné [1984] in which the O 3 radical is formed as an intermediate, see reactions (R1) (R4): ðr1þ ðr2þ ðr3þ ðr4þ O 2 þ O 3 þ H þ! 2O 2 þ OH O 2 þ O 3! O 3 þ O 2 O 3 þ Hþ! HO 3 HO 3! OH þ O 2 [14] The temperature dependence of reactions (R2) and (R4), were recently investigated [Hesper and Herrmann, 2000] and the results are included in the mechanism. Although no competing reactions for HO 3 are included in CAPRAM 2.4 the influence of ph on the rate of O 3 depletion is represented more accurately by splitting up the net reaction (R1). This also has consequences for the OH (aq) budget. The effect is most important at ph 6, for example, in regions of high NH 3 emissions, as O 3 can act as a reservoir species under such conditions (pk a (HO 3 )=8.3 [Bühler et al., 1984]). Although such a high ph is not relevant to the simulations (see section 3.2) this modification will make the complete mechanism robust enough to perform well under such conditions FeO 2+ [15] Various reactions of the ferryl ion, FeO 2+, have been added based on the recent laboratory investigations of Jacobsen et al. [1997, 1998] who have measured rate constants as a function of temperature for this intermediate. In aqueous solution FeO 2+ is potentially important due to its high oxidation capacity. It is predominantly formed via the reaction of Fe 2+ with O 3, so that it may also contribute to the dynamic redox cycling of iron in the aqueous phase Organic Chemistry, TMI Complexes [16] Further revisions have been made to the organic chemistry of C 1 /C 2 compounds, where the number of reactions has been increased from 50 to 110. First, the oxidation of these compounds by OH, NO 3,SO 4,SO 5, Cl 2,Br 2, and CO 3 takes into account the formation and reactions of all alkyl and peroxyl radical intermediates. Second, reactions of difunctional C 2 -organic compounds, Table 3. Numbers of Processes and Species in the Three Versions of CAPRAM: 2.3, 2.4 (MODAC mechanism) and 2.4 (MODAC mechanism)-reduced CAPRAM 2.3 CAPRAM 2.4 (MODAC Mechanism) CAPRAM Reduced 2.4 (MODAC Mechanism) HO x and TMI Nitrogen Sulfur Organics Chlorine Bromine Carbonate Equilibria 31(2) 57(2) 39(2) Photolysis Sum Phase transfers Species number

5 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) AAC 12-5 that is, (CH(OH) 2 ) 2 (glyoxal), CH(OH) 2 COOH (glyoxylic acid), and H 2 C 2 O 4 (oxalic acid) have been added. This results in the aqueous phase chemistry being closely linked to RACM and provides additional sinks for glyoxal in the gas phase. Thirdly, the formation and photolysis processes of the iron-oxalato-complexes are included which provide an efficient pathway for the reduction for Fe(III) via: ðr5þ Fe III ðc 2 O 4 3 2n Þ n ð Þ! Fe 2þ þ ðn 1ÞC 2 O 2 4 þ CO 2 þ CO 2 n ¼ 1; 2; N 2 O 5 [17] Whereas in CAPRAM 2.3 the hydrolysis of N 2 O 5 was written as leading directly to the formation of HNO 3, the formation of the nitryl ion (NO 2 + ) is now included to take into account of recent experimental results [e.g., Behnke et al., 1997]. ðr6þ ðr7þ N 2 O 5! NO þ 2 þ NO 3 NO þ 2 þ H 2O! NO 3 þ 2Hþ Although NO 2 + reacts predominantly with water to form NO 3 by reaction (R7), it can also react with halides to form nitryl halides, BrNO 2 and ClNO 2, especially in deliquescent aerosol. Further reactions of BrNO 2 and ClNO 2 with halide ions lead to halogens and nitrite, reactions (R8) (R10): ðr8þ ðr9þ ðr10þ BrNO 2 þ Br! Br 2 þ NO 2 BrNO 2 þ Cl! BrCl þ NO 2 ClNO 2 þ Br! BrCl þ NO 2 [18] These reactions may play a role in the activation of halogens in aqueous particles under polluted, that is, NO x rich, conditions (H. Herrmann et al., Halogen production from aqueous tropospheric particles, submitted to Chemosphere, 2002, hereinafter referred to as Herrmann et al., submitted manuscript) NO 3 [19] A further fundamental update is the inclusion of the back reactions of the following processes: ðr11þ ðr12þ NO 3 þ Cl! NO 3 þ Cl SO 4 þ Cl! SO 2 4 þ Cl [20] Recent kinetic studies indicate that these processes are not only sinks but also sources for SO 4 and NO 3 [Buxton et al., 1999a, 1999b]. It has to be noted, however, that the currently available kinetic data show considerable scatter [Buxton et al., 1999a] (see Herrmann and Zellner, 1998, for an overview). Further experimental studies on equilibria (R11) and (R12) are in progress. 3. Model Results for Selected Species 3.1. Inorganic Radicals (OH and NO 3 ) [21] Due to the continuous emission of NO into the box, the mean NO 2 concentration is nearly constant ( cm 3 ) over the simulation time. In the former calculations with CAPRAM 2.3 no emissions were taken into account. There, the importance of NO x was decreased with time due to the lack of any further renewal NO x in the box. A constant high concentration of NO x in the box causes two effects: (1) The OH radical is influenced by the higher NO x concentration in the multiphase system. Its maximum concentration reaches about M (see Figure 1). (2) Due to the efficient uptake of HNO 3 by atmospheric droplets, nitrate concentrations of 10 mm or even higher can be reached in the aqueous phase being comparable with results form field measurements [e.g., Erel et al., 1993]. [22] In these circumstances, the photolysis of nitrate is an effective source (11% at noon of the second day) of OH via: ðr13þ NO 3 þ hn þ Hþ! NO 2 þ OH In Figure 2 the most important sinks and sources of the OH radical in the aqueous phase are shown. It becomes evident that OH is also formed by the photo-reduction of the iron(iii)-monohydroxo complex (8%) and by the photolysis of hydrogen peroxide (12%). Although in general the iron(ii) concentration is low (see section 3.3) the Fenton reaction and the oxidation of copper(i) by H 2 O 2 are important sources of the OH radical in the aqueous phase (46% and 13%, respectively). This emphasizes the influence of transition metal ions on the HO x cycle in the aqueous phase. Unlike the case of the NO 3 radical transport from the gas phase plays only a minor role for OH (9%), due to the efficient in-situ production within the aqueous phase (see Figure 2). [23] The main sink for OH in the aqueous phase is its reaction by hydrated formaldehyde (74%). However, other organics such as ethanol (5%), glyoxal (4%), and glyoxylic acid (3%) also contribute to its loss. The contributions of the last two species to the loss of OH show that the oxidation of difunctional organic compounds is not insignificant. The potential importance of these compounds is discussed in section 3.4. [24] In the less polluted remote and marine environments, the importance of the organics sinks for the OH concentration level is decreased, as can be seen from Figure 2. The corresponding maximum concentration of OH at noon are higher by factors of 2 in the remote case ( M) and 4.5 in the marine case ( M), due to lower concentrations of sink species, that is, organics. In fact, in the marine case about half of the OH is consumed by H 2 O 2, HMS, and Br whereas the reaction with CH 2 (OH) 2 contributes only 20%. The most important source of OH in the marine environment is HO 3, via the reaction of

6 AAC 12-6 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) Figure 1. Concentrations of OH and NO in the aqueous phase (urban conditions). ozone with O 2 (see section 2.6.1), because the other species, such as transition metal ions, that contribute to OH production in the other cases are present only in small concentrations. [25] The maximum concentrations of NO 3 predicted by CAPRAM 2.4 (MODAC mechanism), cm 3 in the gas phase and M in the aqueous phase, see Figure 1, are higher than those obtained with CAPRAM 2.3. The only effective source of NO 3 in the aqueous phase is its transport from the gas phase. Its main sinks are the reactions with inorganic anions such as chloride (55% of the loss at t = 48 h of simulation), bromide (38%), and sulfate (3%). Figure 2. Relative contributions [%] of sinks and sources of OH in the aqueous phase & urban, 2 remote and 5 marine.

7 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) AAC 12-7 Further discussion of NO 3 chemistry both in the gas and aqueous phase, for example, the influence to the gas phase concentration level, was given by Herrmann et al. [2000] HONO Production in Clouds [26] The highly dynamic interactions between the gas and aqueous phases with respect to NO x chemistry lead to another important sequence of processes, (R14) (R19), as suggested by Georgii and Warneck [1999]: ðr14þ ðr15þ ðr16þ ðr17þ ðr18þ ðr19þ ðr20þ Figure 3. X : Evolution of the ph over the simulation time; urban, remote and ----marine. NO 2g ðþ þ HO 2g ðþ! HNO 4g ðþ HNO 4g ðþ! HNO 4aq ð HNO 4aq ð Þ! H þ þ NO 4 NO 4! NO 2 þ O 2 H þ þ NO! 2 HONO ðaqþ HONO ðaqþ! HONO ðþ g cloud chemistry NO2ðgÞ þ HO 2g ðþ!hono ðþ g þ O 2 Þ and only 5% of the HNO 4 is transported into the aqueous phase. In the marine and the remote cases where the ph is higher (see Figure 3), the uptake of HNO 4 into the aqueous phase increases in importance, amounting to 76% in the marine case and 78% in the remote case. [28] In addition to equilibrium (R16), HNO 4 in solution may dissociate into radicals, via reaction (R21): ðr21þ HNO 4aq ð Þ! NO 2aq ð Þ þ HO 2aq ð [29] This pathway is favored if the ph value is lower than pka (HNO 4 ) which is 5 [Lammel et al., 1990]. In this case (R21) represents the dominant sink for the acid and the acid-base dissociation, to a lesser degree. In the less polluted cases HNO 4 dissociates to NO 4 via reaction (R16), which decays to NO 2, reaction (R17). In the urban scenario the ph is about 2.3 (at noon of the second day) the small concentration of NO 2 formed in (R17) protonates rapidly to form HONO (pka (HONO) = 3.28 [Park and Lee, 1988]). At the higher ph of 5.4 in the remote case and ph = 6 in the marine case (see Figure 3), NO 2 can be destroyed in (R22). However, it should be noted that (R22) is not an important sink for OH (see section 3.1). Þ [27] In the gas phase, HNO 4 (pernitric acid) represents a reservoir species for both HO x and NO x and is formed by the recombination of HO 2 and NO 2 in (R14). In the urban case where the ph is low (see Figure 3) equilibrium (R16) and hence equilibria (R15) and (R14) are shifted to the left ðr22þ NO 2 þ OH=Br 2! NO 2 þ OH =2Br [30] The resulting rates of transfer of HONO into the gas phase amount to cm 3 s 1 (urban), cm 3 s 1 (remote) and cm 3 s 1 (marine). These

8 AAC 12-8 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) Figure 4. Concentrations of HONO(g) in the urban case with, without and temporary (1 h) aqueous phase chemistry, in the marine case with, without 444 and temporary (1 h) 555 aqueous phase. correspond to 87%, 89%, and 81%, respectively, of the total HONO production (gas and aqueous phase) at noon (t = 36 h). In summary, it is concluded that chemical processes within the tropospheric aqueous phase play an important role for the conversion of NO x as written in the overall reaction (R20). The impact of aqueous phase chemistry of HNO 4 on tropospheric chemistry is further discussed by Dentener et al. [2002] in more detail. [31] The efficiency of the HONO formation in clouds relative to that in the gas phase can be highlighted by comparing the concentration profiles of HONO (g) in the presence and absence of the aqueous phase. Figure 4 shows that, in the urban case especially, both the maximum concentration and the temporal behavior of the concentration profile change in the presence of clouds. Whereas in a cloudless environment the maximum concentration ( cm 3 ), is reached in the morning in the cloudy case HONO accumulates during night (up to cm 3 ) because its main sink is the photolysis process leading to OH and NO radicals being an important OH source in the morning. Even if only a short cloud period of 2 hours, between 1400 and 1600 LT is considered (see Figure 4) the HONO production in the aqueous phase leads to significantly higher concentrations in the gas phase. In the remote and marine cases the HONO concentration is increased by a factor of five to cm 3 and cm 3, respectively. [32] Even when only a small fraction of HNO 4 is dissociated, Figure 4 shows that HONO production is effective in presence of clouds at low ph values when only a small fraction of HNO 4 is dissociated. The rapid decomposition of NO 4 in (R17) causes a continuous shift in the equilibria (R14) (R16). [33] The evaporation of HONO from the clouds represents the main flux of nitrogen species from the aqueous phase. Even the most effective reaction producing NO 2 within clouds (R13), contribute 1% to the total NO 2(g) production in all three cases Chemistry of Transition Metal Ions (TMI) [34] Differences exist between the results obtained by earlier reaction schemes for the concentration of the transition metal ions. CAPRAM 2.3 and other models [e.g., Warneck, 1999] predict that Fe(III) is reduced by Cu(I) in the first few seconds of the simulation due to reaction (R23): ðr23þ Fe 3þ þ Cu þ! Fe 2þ þ Cu 2þ [35] In the scheme presented here the concentration of the ions show a dynamic time-dependent redox cycling in

9 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) AAC 12-9 Figure 5. Concentrations of Fe(II)- and Fe(III)-species (urban case) [Fe(OH)] 2+ ; Fe 2+ ; [Fe(C 2 O 4 )] +. all scenarios considered. However, for sake of clarity, the following description is restricted to the urban scenario where the influence of TMIs will be most important due to their high concentrations. Nevertheless, qualitatively similar behavior has been found to occur in the unpolluted scenarios, indicating that ph in the droplets does not influence the TMI redox cycling to any great degree. Figure 5 shows the concentrations of Fe 2+, [Fe(OH)] 2+ and Fe(C 2 O 4 ) + for the urban scenario. It is evident that the concentration of Fe(III) is an order of magnitude greater than that of Fe(II). [36] This ratio has been also found in field measurements focused on oxidation states of TMIs in clouds and fog sampled in urban locations [Erel et al., 1993]. The photoreduction of the Fe(III)-oxalato- and hydroxo-complexes leads directly to the formation of Fe 2+. Hence, the highest concentrations of Fe(II) are found during the day. Whereas the concentration of [Fe(OH)] 2+ is more or less constant over the entire simulation time, the high concentration of oxalate (C 2 O 4 2 ) and, subsequently, the Fe(III)-oxalatocomplexes, controls the redox cycling of iron in the aqueous phase which explains the lack of any ph dependence. Only the photo-reduction of the di- and tri-oxalatocomplexes [Fe(C 2 O 4 ) 2 ] and [Fe(C 2 O 4 ) 3 ] 3 are included in the mechanism because the corresponding photolysis rate of the mono-oxalato-complex [Fe(C 2 O 4 )] + is unknown. Therefore, in the model, the concentration of this complex accumulates with increasing C 2 O 4 2 and represents about 40% of the total iron(iii), see Figure 5. [37] The initial step of the redox cycling is the oxidation of Cu + by molecular oxygen, reaction (R24): ðr24þ Cu þ þ O 2! Cu 2þ þ O 2 This process is very efficient due to the high oxygen concentration present in solution ( M) and it diminishes the extent of reaction (R23). As a result of reactions (R23) and (R24), nearly 100% of the copper (0.3 mm) is present as Cu(II). The main fraction of iron(iii) exists as either the hydroxo- or oxalato-complexes, which are photolyzed during day time. In summary, it can be stated that TMI chemistry in the aqueous phase mechanism presented here reproduces the highly dynamic redox cycling of iron, which is thought to be an important photochemical oxidation cycle in clouds [e.g., Sedlak and Hoigné, 1993; Faust, 1994; Brandt et al., 1994; Warneck, 1996]. However, it must be noted that large uncertainties still exist about the reactivity of the TMI-oxalato complexes with free-radical oxidants such as OH. Because of this, such processes are currently omitted but studies should be undertaken to obtain a more comprehensive understanding of redox cycling Difunctional C 2 Organics [38] In RACM the removal of (CHO) 2 (glyoxal) is restricted to its photolytic dissociation and the reaction with OH and NO 3. The rate of the reaction of NO 3 with glyoxal is relatively slow (k gas = cm 3 s 1 ) so that the most efficient removal from the gas phase occurs during

10 AAC ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) Figure 6. Concentrations of glyoxal in the gas phase with and without -----aqueous phase chemistry (urban case). daytime. As a result, glyoxal accumulates up to cm 3 during the night. This is four times higher than the maximum concentration achieved during the daytime (Figure 6). [39] The same effect is also found in the other two scenarios, where the maximum concentrations of glyoxal in the gas phase are of the order of 10 7 molecules cm 3 (remote scenario) and 10 6 molecules cm 3 (marine scenario). If multiphase chemistry is considered, the nighttime maxima are decreased by about one order of magnitude. The oxidation of glyoxal to glyoxylic acid and finally to oxalate, causes an increase in oxalate of 6.6 mm in the urban case after 3 days of simulation. Two thirds of the total oxalate (4.4 mm) is bound in the iron(iii)-monooxalatocomplex. It should be noted that this concentration is independent of whether or not oxalate is initially present in the aqueous phase (5 mm in the urban case). The resulting oxalate concentration corresponds to a mass concentration of 180 ng m g 3, which is of the same order of magnitude as that found in aerosol samples from field measurements made in polluted environments [Beck, 1998; Neusüß et al., 2000]. The final concentrations predicted in the other scenarios (0.3 mm for remote and 0.9 mm for marine, respectively) are about one order of magnitude lower than for the urban case. [40] Figure 7 shows that the variation of the total oxalate concentration in the remote case is similar to the urban case, whereas in the marine case a continual increase is calculated as explained in detail below. In the remote case 50% of the oxalate is present in chemical complexes. However, due to the smaller iron concentrations initially present in the marine case, the iron-monooxalato complex only accounts for 5% of the total oxalate concentration. A rough comparison of the rates of removal of oxalate shows that photolysis of the iron-dioxalato-complex is much more effective than oxidation of oxalate by OH (k OH = s 1 ; J Fe(Ox)2 = s 1 ). Due to the low TMI concentration for the marine case, the rate of depletion of oxalate via photodissociation is lower than for either the remote or urban cases. This results in an oxalate concentration in the marine case which exceeds that for the remote scenario after 3 days simulation. [41] Oxidation of glyoxylic acid represents the only source of oxalate, that is, there are no emissions. In the continental cases the precursor glyoxal is mainly formed from aromatics (to more than 80%), whereas in the marine case small alkanes ( HC3 ) represent the most important precursors of glyoxal due to the emission fluxes of aromatics being much smaller. The results show that, in continental scenarios, the oxidation of aromatics in the gas phase may contribute indirectly to oxalate formation in clouds due to the formation of glyoxal as a ring cleavage product. Ultimately, it leads to oxalate in residual particles as a result of reactions taking place in clouds. Oxalate concentrations in the particle phase are probably underestimated by CAPRAM 2.4 (MODAC mechanism) as recent findings from field measurements and laboratory studies suggest that several other pathways for the forma-

11 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) AAC Figure 7. Concentrations of the total oxalate concentrations in the aqueous phase urban, remote, marine. tion of oxalic acid may be active in both the gaseous and aqueous phase [e.g., Beck, 1998; Baboukas et al., 2000]. Moreover, the direct emission of either oxalic acid or higher dicarboxylic acids is not considered in the current model, because the emission source strengths are very uncertain. This is important since it has been suggested by Kawamura and Ikushima [1993], that higher dicarboxylic acids, may act as precursors for the smaller dicarboxylic acids (i.e., oxalic and malonic acid) in the gas phase and hence contribute a significant amount in urban areas. [42] In a more recent study Behnke et al. [1999] also found oxalic acid to be produced in highly concentrated halide solutions in the presence of alkanes. However, under atmospheric conditions this source is considered to be fairly negligible due to the low solubility of alkanes. In summary, this discussion shows that the organic chemistry accounted for in current aqueous phase mechanisms may still be inadequate for simulating the production of important particulate phase species such as oxalate. Therefore, more experimental studies should be performed to determine the important oxidation mechanisms for atmospheric organic species. Such studies should allow future models to also include more complex oxygenated organic compounds Peroxyl Radicals (HO 2 and RO 2 ) [43] The chemistry of the organic peroxyl radicals CH 3 O 2, ETHP (ethyl peroxyl radical) and ACO 3 (acetyl peroxyl radical) in the aqueous phase was significantly expanded. These organic radicals enter the aqueous phase predominantly via phase transfer. Whereas in CAPRAM 2.3 the only sink for ACO 3(aq) was phase transfer, leading to an apparent peroxyl radical production in the presence of clouds, more numerous loss processes are taken into account in the present mechanism. Besides the recombination of ACO 3, leading to decarboxylation, the reaction with O 2 forming peroxy acetate has also been included. In all three scenarios the aqueous phase represents an important sink for CH 3 O 2, ETHP and ACO 3. A comparison between the results from RACM and from the multiphase scheme shows that the concentrations of the three organic peroxyl radicals are lowered by about 40% in the urban case and up to 70% in both less polluted cases when aqueous phase processes taken into account (Table 4). By recombination of the ACO 3 radical with NO 2 peroxy acetyl nitrate (PAN) is formed. Therefore, the decrease of the ACO 3 radical concentration in presence of clouds leads to lower PAN concentrations in the gas phase. A comparison of the concentration levels of the two cases (cloudy/cloudless) shows, that in all three scenarios considered the PAN concentration in the gas phase is reduced by exactly the same factors as found for the ACO 3 radical concentration (Table 4). [44] The maximum HO 2 concentration in the urban case, as predicted by RACM, is 10 9 molecules cm 3. When multiphase chemistry is active the maximum concentration at noon falls by 50% to cm 3. Even larger

12 AAC ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) Table 4. Maximum Concentrations (Noon, t = 36 h) of the Peroxyl Radicals (HO 2 and RO 2 ) in the Presence and Absence of Cloud Chemistry Urban Remote Marine Species Gas Phase Multiphase, % Gas Phase Multiphase, % Gas Phase Multiphase, % HO CH 3 O a ACO C 2 H 5 O a In the aqueous phase it is defined as P (O 2 CH 2 COOH + CH 3 COCOO). decreases (about 86%) are found for remote and marine cases (Table 4). [45] Qualitatively these results are in agreement with those of Cantrell et al. [1996a, 1996b]. Their model, which was set up to explain peroxyl radical measurements in the gas phase, overestimates the concentrations of RO 2 by between 10% and 40%, depending on the amount of aerosols present. However, in the studies by Cantrell et al. the chemistry of the particulates is restricted to a simple parametrization for reactive uptake processes at surfaces of condensation nuclei. The calculations performed, using CAPRAM 2.4 (MODAC mechanism), give a more detailed insight of the loss processes for small organic peroxyl radicals in the tropospheric aqueous phase. [46] For example, three of the four different pathways for the recombination of the acetate peroxyl radical (R25) to (R28) [Schuchmann et al., 1985] lead to the production of glyoxylic and glycolic acids. For simplification, glycolic acid is lumped with acetate. ðr25þ 2O 2 CH 2 COO 2O 2 CH 2 COO ðr26þ! 2H2O 2CHðOHÞ 2 COO þ H 2 O 2 2H2O! 2 HCHO þ H 2 O 2 þ 2OH þ 2CO 2 2O 2 CH 2 COO! H2O CHðOHÞ 2 COO þ CH 2 OHCOO þ O 2 ðr27þ 2O 2 CH 2 COO 2OH =O2! 2O 2 ðr28þ þ 2CH ð OH Þ 2 COO þ 2H 2 O [47] In the urban case most of the acetate peroxyl radical will be protonated forming ACO 3 (more than 90%). The pka value of this radical is assumed to be equal to that of acetic acid. It can be expected that even if the pka differs by ± unit from this value the general behavior will be unchanged. In general, this mechanism represents an additional pathway for the functionalization of organics (section 3.4). The reactions (R25), (R27), and (R28) represent the only in-situ source of glyoxylate in the mechanism but contribute only about 0.1% to the total glyoxylate/glyoxylic acid. At the relatively high ph of the remote and marine scenarios, the dissociation of ACO 3 to O 2 CH 2 COO represents a significant sink for ACO 3. In fact, in the marine scenario the total rate of production of glyoxylate at t = 36 h is M s 1, but at this time the production rate of glyoxylate from glyoxal is Ms 1. [48] Some peroxyl radicals decay by eliminating HO 2 and by other pathways; these have also been included in the model. In some cases the reaction rates and branching ratios are not available and hence were estimated. 4. Mechanism Reduction [49] For nearly all chemical modeling studies one of the defining parameters which governs the run-time efficacy of a model is the number of species and chemical processes which it has to accommodate. State-of-the-art regional and global chemistry transport models (CTMs) still have severe computational limitations. Therefore, to facilitate the incorporation of CAPRAM 2.4 in higherscale models, it was deemed necessary to reduce the complete aqueous phase scheme from 439 to less than 220 reactions, including phase transfer. This reduction was achieved by analyzing the concentration versus time profiles of a number of the following chemical species: OH (g) /OH (aq), NO 3(g) /NO 3(aq), SIV (aq), NO x(g) (= NO + NO 2 ), H 2 O 2(aq), O 3(g), and H +, hereafter referred to as the target species. [50] These species were chosen due to their dominant role in tropospheric chemistry in all scenarios considered in the present study. Reactions were removed within the restriction that the results from the reduced and complete schemes for all target species should differ by <5%. Three different nonnumerical methods were applied to obtain a general reduced scheme, namely analysis of sources and sinks, integrated chemical rates and instantaneous chemical rates. The results for each method are summarized below. It is evident that the methods applied here for the reduction are time and labor intensive but currently for such complex systems no appropriate numerical reduction method is available. The application of three different nonnumerical methods seemed to be necessary to avoid the over- or underestimation of certain processes by using only one of these methods Analysis of Sources and Sinks [51] For this approach all chemical reactions which made contributions of less than 1% to the total rate of production or loss of the selected target species at simulation times of t = 36:00 h (noon of the second day) and t = 48:00 h (high noon of the second day) considering both day- and nighttime chemistry, respectively, were omitted from the chem-

13 ERVENS ET AL.: CAPRAM 2.4 (MODAC MECHANISM) AAC ical scheme. For example, the overall rate of loss for OH (aq) may be defined as: doh ½ Š dt ¼ ðk 1 ½OH Š½CH 3 OHŠþk 2 ½OHŠ½Cu þ Š fflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflffl} <1% of doh ½ Š dt þ...k n ½OHŠ½XŠÞ [52] All processes contributing <1% to the total production/destruction rate were omitted. Exceptions were made (1) for acid dissociation reactions, that represent a unique sink thus preventing the accumulation of species and (2) reactions involved in the formation of RO 2 during the oxidation of C 1 or C 2 organic species. This approach resulted in a total of 215 chemical reactions being identified as insignificant with respect to the target species Analysis of Integrated Reaction Rates [53] This technique involved the introduction of artificial aqueous phase tracers (e.g., Dh1i, Dh2i,.Dhii) as exemplified below, H 2 O 2 þ Fe 2þ! OH þ OH þ Fe 3þ þ Dh1i ð8þ ð9þ SO 4 þ Br! SO 2 4 þ Br þ Dh2i ð10þ [54] The concentration of each tracer accumulates over the simulation time and represents the integrated rate of the corresponding reaction. The concentrations of all tracers were compared at t = 24:00 (nighttime) and t = 72:00 (daytime) and those processes were removed having the smallest values of Dhii. This procedure was repeated several times, which resulted in 199 processes having identified an insignificant effect on the concentrations of the target species Analysis of Instantaneous Reaction Rates [55] For the third approach the instantaneous rate for every chemical reaction at t = 24:00 (nighttime) and t = 72:00 (daytime) was determined, with the rate defined as: dc ½ Š dt dc ½ Š dt ¼ k 1 ½H 2 O 2 Š Fe 2þ ð11þ ¼ k 2 SO 4 Br ½ Š ð12þ [56] This method identified 212 insignificant reactions in the full mechanism Synthesis of a Final Scheme [57] Due to the fact that the number of insignificant reactions identified by each reduction method was slightly different, three separate reduced aqueous phase schemes were obtained. Comparison of these schemes revealed that 89 chemical processes had been identified for removal by all three methods and 126 chemical processes by at least two of them. After some further iterative tests applying the different reduction methods a total of 256 reactions were found to have minimal influence on the concentrations of the target species. This result shows that by combination of all three methods a higher number of reactions could be identified as insignificant as compared to the efficiency of only one of the techniques. The resulting scheme includes 183 important chemical processes. This final set of reactions was tested over the three environmental scenarios using two different sets of initial conditions. For details of the input parameters, the reader is referred to the CAPRAM web site ( html). Although the target species, listed above, were chosen to be representative of the most important oxidation steps in the tropospheric multiphase system, it is acknowledged that if different target species are selected, or more localized scenarios chosen (e.g., high halide concentrations), a different reduced mechanism may result. However, given the quality of the input data (initial concentrations and emissions) it is evident that this reduced mechanism is still comprehensive and robust for a wide range of atmospheric conditions List of Retained Reactions [58] Comparison of the processes in the full and reduced versions of CAPRAM 2.4 (MODAC mechanism) shows that the majority of the reactions removed from the scheme pertain to TMI and organic chemistry (37 and 74 processes, respectively) Phase Transfers [59] Phase transfer of peroxy acetic acid, CH 3 C(O)OOH, from the gas phase was removed from the scheme because all aqueous phase reactions of this species, that is, acid dissociation and oxidation of S(IV) oxidation, are neglected in the reduced version HO x and TMI Chemistry [60] In the analysis of the TMI chemistry it was found that neither OH nor H 2 O 2 concentration is influenced if all processes involving Mn(II), Mn(III) and Mn(IV) are neglected. This is due to the fact that although iron and manganese show very similar chemical behavior, the reactions of Mn with the relevant target species are generally slower. This finding is in contrast to results from several laboratory studies which show a synergistic interaction between the oxidation Fe and Mn with respect to the rate of S(IV) [e.g., Grgić etal., 1992; Rao and Collett, 1998]. However, the interaction between Fe and Cu was found to be influential in the catalytic TMI oxidation cycle. The importance of the redox cycling of iron has been highlighted in section 3.3. However, analysis of the processes involving FeO + 2 has shown that its contribution to the overall redox cycling of Fe is relatively small. Accordingly, only reactions of FeO + 2 with species present in significant concentrations, namely Fe(II) and Cl, are retained in the reduced scheme Nitrogen Chemistry [61] The important sinks for the nitrate radical, NO 3,are its reactions with HSO 3,Cl and SO 2 4. Reactions of NO 2 leading to NO 2 only influence the total NO 2 budget by a few percent due to the low solubility of NO 2. The decay of N 2 O 5 and the subsequent reactions with halides leading to