Environ. Sci. Technol. 2004, 38, 5540-5547
Box Model Investigation of the Effect of Soot Particles on Ozone Downwind from an Urban Area through Heterogeneous Reactions YAYNE-ABEBA AKLILU AND DIANE V. MICHELANGELI* Earth and Space Science, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada
Soot can provide additional surface area where heterogeneous reactions can take place in the atmosphere. These reactions are dependent on the number of reactive sites on the soot surface rather than the soot surface area per se. A box model, MOCCA, is used to investigate the effects of introducing heterogeneous reactions on soot into an air parcel passing over an urban area and traveling downwind. The model was run at two soot mass concentrations of 2 µg/m3 and 20 µg/m3 with a surface density of n-hexane and decane. Signifcant change in gasphase concentration was only observed for the higher soot concentration. Due to the noncatalytic nature of the heterogeneous reactions, soot sites are rapidly consumed, and soot site concentrations are greatly reduced shortly after emissions are turned off. Notable changes in gaseous concentrations due to the introduction of heterogeneous reactions are not observed in the urban setting. The impact of heterogeneous reactions is more evident after emissions are turned off (i.e. downwind from the urban center). These changes are minimal for the condition that used n-hexane surface density. For conditions that used decane soot, NOx concentrations showed a slight increase, with NO being higher in the day time and NO2 at night. The maximum O3 reduction observed when using the higher soot concentration is 7 ppb, downwind of the urban center. Change in O3 concentration was less than 1 ppb when using the lower soot loading. The observed effects of heterogeneous reactions on soot decrease with time.
Introduction Soot (elemental or black carbon) particles are produced primarily as a result of fossil fuel combustion and biomass burning. Although rare in some remote areas, soot particles are abundant throughout the atmosphere. Global annual black carbon emission estimates can range from 13 Tg (1) to 24 Tg (2). In addition to carbon, soot can contain varying amounts of oxygen, hydrogen, and nitrogen. The physical and chemical properties of soot can vary depending on the fuel burned and on the combustion process (3); on the whole soot particles form agglomerates and can have specific surface area as high as 368 m2 g-1 (4). As a result, these particles can provide a large surface area on which heterogeneous reactions may take place. * Corresponding author fax: (416)736-5817; e-mail:
[email protected]. 5540
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Physisorption and reaction on soot particle surfaces are two independent or combined pathways for the uptake of trace gases (3). Gases that are observed to undergo heterogeneous reactions on soot surfaces include O3, NO2, HNO3, NO3, and N2O5. O3 formation occurs through the combination of an oxygen molecule and oxygen atom. In the troposphere the production of oxygen atoms results mainly through NO2 photolysis (a small amount is contributed from NO3 photolysis). The introduction of a heterogeneous reaction might reduce the level of O3, either via O3 decomposition on the soot surface or via destruction due to secondary processes. O3 decomposition producing CO, CO2, and O2 are suggested by Smith and Caughtai (5) and Kamm et al. (6). Two reaction pathways have been suggested for the reaction of NO2 on soot surfaces. Heterogeneous reaction of NO2 is observed to produce HONO (7-10). Kotamarthi et al. (9) suggest that this reaction would lead to increased night time concentrations of HONO, with early morning photolysis of HONO increasing NO and OH concentrations. The increase in OH concentration will increase day time O3 levels. The reduction of NO2 to NO is also noted (11). This reaction would result in the daytime increase of NO. Daytime increase in NO will counteract the effect of an increase in OH by leading to the destruction of O3. In addition, the reduction of NO2 slows down the production of O3. The release of NOx from the reservoir and/or sink species HNO3 is also suggested to take place on soot surfaces (12, 4, 13). This reaction would produce NO2 and would be competing with the reduction of NO2 mentioned above in altering the levels of NO2 and O3. Others (14) have only observed the physorption of HNO3 on soot with no NOx production. Laboratory studies so far have mainly investigated the reaction of a single gas species on soot. How these reactions are affected by competition with other gases, and the relevance of these reactions in comparison to other tropospheric reactions needs further investigation. The purpose of this work was to include soot heterogeneous reactions into a chemical box model that already contained all the relevant urban tropospheric gas-phase reactions. Since the aim was to investigate the relevance of heterogeneous reactions, box model results are considered adequate. Conservative assumptions were made regarding the number of surface sites available on the soot and the reactivity of the gases in order to investigate the “worst case” or upper limit of the potential impacts on gas-phase species. This work was exploratory in nature, and therefore, the results cannot be considered quantitatively definitive. The model used was “Model of Chemistry Considering Aerosol” (MOCCA). It is a box model designed for tropospheric multiphase gas-particle chemistry (15, 16). The existing parametrization within MOCCA allows heterogeneous reactions on the surface of particles. The model was designed to simulate gas-phase and aqueous-phase chemistry. The mass transfer is determined by the gas-phase diffusion, mass accommodation, and the liquid water content which is held constant. There is no aqueous phase associated with the soot in these investigations. The noncatalytic nature of heterogeneous reactions on soot particles cannot be adequately represented by the aqueous-phase chemical mechanism in MOCCA. A number of recent laboratory studies have investigated the interaction of atmospheric gases with soot particles (for example refs 10, 13, 6). These studies have shown that the saturation of the soot surface occurs shortly after the start of the experiment, indicating that heterogeneous reactions on soot surfaces are noncatalytic. The consumption of reactive sites on the soot surface with heterogeneous reactions 10.1021/es035079x CCC: $27.50
2004 American Chemical Society Published on Web 09/24/2004
explains these observations. Kamm et al. (6) and Kirchner et al. (12) suggested that when considering heterogeneous reactions on soot, the reactive sites be considered as reactive species that can be consumed. Within MOCCA, this is the best method to represent the heterogeneous reactions on soot. In addition to the rapid consumption of these reactive sites, a slow regeneration of some consumed sites allowed some reactions to take place following the initial rapid uptake (6). Wei et al. (17), Aumont et al. (18), and Kamm et al. (6) have modeled heterogeneous reactions on soot by tracking reactive sites. Kamm et al. (6) have examined the uptake of O3 on soot surface, Wei et al. (17) investigated the effects of aircraft emitted soot in the upper troposphere and lower stratosphere, while Aumont et al. (18) explored the effect of NO2 and soot heterogeneous reactions on gas-phase chemistry. All three have determined that the regeneration of soot site is an important factor in the significance of heterogeneous reactions on soot surface sites. In addition to heterogeneous reactions of O3 on soot surfaces, a number of reaction mechanisms for gases involved in the production and loss of O3 are included in the model. Furthermore, laboratory studies with soot have determined the possibility of a number of different soot reactive sites. Kamm et al. (6) showed that the reactive sites involved in the decomposition of O3 are not consumed by the exposure to HNO3. Aubin and Abbatt (14) showed that the uptake of HNO3 on soot is not affected by the exposure of the soot to O3. Disselkamp et al. (13) suggest that sites that are involved in physisorption of HNO3 may differ from sites where HNO3 is reduced to NO2. As a result, three different soot reactive sites are used. To compare the impact of soot heterogeneous reactions on O3 with gaseous chemistry, the aqueous phase of the model has been switched off.
Modeling Methodology The site where a gaseous species absorbs and/or reacts on a soot particle is referred to as the reactive site; these are commonly represented as number of sites per soot surface area. The number of reactive sites per unit area is determined using the mass concentration of soot, specific surface area, and concentration changes in the gas species (13). These sites for the most part are consumed during heterogeneous reactions, allowing a single reaction per site. Because of the need for keeping track of these reactive sites (17, 18), reactive sites are represented as reactive species within the model. Studies have either looked at the interaction of a limited number of gaseous species with soot particles (18) or the reactions were performed under upper tropospheric conditions (17). The work of Kamm et al. (6), Disselkamp et al. (13), and Aubin and Abbatt (14) suggest that there is more than one type of soot active sites. Their findings indicate there are sites where decomposition of O3 takes place, HNO3 is reduced, and where physisorption of HNO3 takes place. In this work, three types of soot sites are used. Decomposition of O3, reduction of NO2, and hydrolysis of N2O5 are represented as taking place on the first type of site referred to as SS1. Physisorption of HNO3 and NO3 take place on the second type of soot site, SS2. Reduction of HNO3 takes place on a last type of site, SS3. Reactions included in the model as well as their reaction probabilities are shown below. All reaction probabilities are taken from published laboratory experiments. It should be noted that these experiments were not all performed under the same experimental conditions nor were the same soot types used. This can significantly alter the reaction probabilities. There is a lack of a reasonable, consistent experimental database for use in modeling. Variables such as the method of soot production, the soot surface area used in
determining the reaction probability, and the temperature of experiments can all significantly affect the results obtained. This being said, the reactions shown below provide an upper limit to heterogeneous reactions on soot reactive sites. Significant changes in ozone concentration due to these heterogeneous reactions would indicate the need for a good surrogate for atmospheric soot particles to be used in laboratory experiments. On the other hand, a small or insignificant change in ozone concentration would question the importance and relevance of these heterogeneous reactions in the atmosphere. Reactions 1 to 4 show the interaction of O3 with soot and are adopted from Kamm et al. (6). Their experiment used graphite spark generated soot in an aerosol chamber, a drawback in the experiment. This type of soot contains relatively small amounts of hydrogen and oxygen and form a poor surrogate for atmospheric soot particles (3). A fast consumption of active sites (reaction 1) is followed by a slow regeneration (reactions 2 and 3) or passivation (reaction 4). The passivation of a site removes it from any further heterogeneous reactions.
SS1 + O3 f SSO + O2 SSO f SS1 +CO
(19)
k ) 4.5 × 10-5 s-1
SSO + O3 f SS1 + 2O2 SSO f SSP
γ)0.003
(1)
(6)
(2)
γ ) 1 × 10-7 (6)
(3)
k ) 2.3 × 10-5 s-1 (6)
(4)
A wide range of results were found for the interaction of HNO3 with soot particles. The probability of the existence of two types of sites, one for HNO3 reduction and the other for physisorption is discussed by Disselkamp et al. (13). Aubin and Abbott (14) found the uptake of HNO3 onto soot surfaces to be nonreactive and reversible. Staathoff et al. (20) found a small time average uptake probability shown in reaction 5; no significant production of NO2 would suggest that this process is possibly due to physisorption. Note that reaction 5 may be reversible as observed by Aubin and Abbott (14) but was not represented as such in the model. Rogaski et al. (21) found much higher reaction probability shown in reaction 6; this experiment was done with a higher HNO3 concentration. SS2OH and SS3HNO3 represent inactive sites. The heterogeneous interaction of HNO3 with soot does not have a reactivation process for the consumed site (13).
SS2 + HNO3 f SS2HNO3
γ ) 3 × 10-7 (20)
SS3 + HNO3 f SS3OH + NO2
γ ) 0.038
(5)
(21) (6)
Reactions 7 and 8 show heterogeneous reactions of NO2. These reactions have been represented to take place on SS1. The probability of NO2 reducing to form NO and/or HONO will depend on the type of soot. Stadler and Rossi (22) have found that soot from a lean flame will produce both HONO and NO, while soot from a rich flame produces only HONO. Both of these reactions were included in the model. An initial fast uptake followed by a much slower uptake similar to what is observed for O3 is also observed for NO2 (22).
SS1 + NO2 f SSO + NO SS1 + NO2 f SSO + HONO
γ ) 0.06
(22)
γ ) 0.011
(7)
(7)
(8)
Reactions 9 and 10 are influenced by the presence of water vapor. The reaction probabilities (γ) used in the model are those determined for 50% relative humidity. They are found VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Mixing ratios of SS1 and SS2 active soot sites as a function of time. Scale for SS1 is on the left. Scale of SS2 is on the right. to be higher than the reaction probabilities observed in experiments that are carried out in dry air (20).
N2O5 + SS1 f HNO3 + SSO NO3 + SS2 f SS2NO3
γ ) 2 × 10-4 (20) γ ) 1 × 10-3 (20)
(9) (10)
Reaction probabilities (γ) obtained from literature are converted to first-order rate constants, k, using mean molecular speed of the reacting gas molecule 〈c〉 and the surface site density Fs.
〈c〉 1 k)γ 4 Fs
(1a)
Surface density of active sites where O3 decomposition takes place is about 6.5 × 1018 m-2 of soot (6). This value is used to represent the surface density of the site type SS1. Disselkamp et al. (13) found the surface density of active sites where HNO3 is reduced to be 3.3 × 1017 m-2 and 5.7 × 1016 m-2 where physisorption of HNO3 occurs. These results are used as surface densities of SS3 and SS2, respectively. Surface density is also used to determine the abundance of SS1, SS2, and SS3 sites. To convert mass concentration of soot into active site concentration, specific surface area representing atmospheric soot particles had to be chosen. The surface area of soot, determined using the BET (Burnauer, Emmett, and Teller) method, can lead to a significant range of values (4). Sander et al. (3) suggest that organic combustion generated soot is a good surrogate for atmospheric soot. Soot surface area can vary depending on the combustion material and combustion method. As surface area directly influences the number of active sites available for reaction, the model was run using two surface areas: one based on n-hexane and the other on decane combustion. BET surface area for n-hexane soot is 46 m2g-1 (4). The surface area for decane soot produced from a lean fire is 218 m2g-1 (22). Conversion of mass concentration into site concentration is done using equation 2a
( )
site conc
# ) Fs × A × Csoot m3
(2a)
where Fs is surface site density (m-2), A is the specific soot surface area (m2g-1), and Csoot is the soot concentration in the air (gm-3 air). To investigate the upper limit of the effect of the heterogeneous reactions, the model was run with a 5542
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high soot loading. Emissions for soot were set to 0.02 and 0.2 ng cm-2 s-1. This provided a 24-hour average soot concentration of about 2 and 20 µg m-3 for the first day. The lower soot loading is more typical of the atmosphere, while the high soot loading could correspond to air directly downwind from a combustion source. The deposition rate for all the sites is set to 0.1 cm s-1 (1). The model is run for a total of 7 days. The first day is run under urban conditions where emissions of gaseous species and soot are turned on from 06:00 to 20:00. The rest of the 7 days are run under rural conditions with no emissions. Gas-phase deposition for both urban and rural conditions are considered the same. The model did not consider loss of active sites by methods other than the above-discussed heterogeneous reactions and deposition. Further details on the box model can be found in refs 15 and 16.
Results and Discussion Model results of active and passive soot site concentrations were as expected. To illustrate the evolution of the soot sites concentration from the higher soot loading are used. Slow regeneration of SSO sites to SS1 sites allowed the continuation of heterogeneous reactions on these sites for about 72 h. Figures 1 to 3 show soot site concentrations. Significant changes in fresh soot site concentrations (SS1 and SS2) are associated with the switching on and off of emissions. These sites were consumed rapidly shortly after turning off emissions (Figure 1). SS2 sites are not regenerated; therefore, by the end of the first day they are depleted. On the other hand, SS1 sites are present up to the third day. Figures 2 and 3 show the buildup of the inactive sites (SSHNO3, SSNO3, and SSP). There is an overall rapid increase in the number of inactive sites during the first day. The increase in SSNO3 concentration slows down in the middle of the day, possibly due to the low NO3 concentration. A decrease in the concentration of these sites, following the first day, is due to deposition. Figure 3 shows that SSP concentration continues to rise until the end of the second day due to the presence of SSO sites (reactions 1-4). SSO decreases at a higher rate, because two loss processes are involved: deposition and regeneration to SS1. The addition of heterogeneous reactions to the model does not significantly affect concentrations of gas species while emissions are turned on (the first day). Figures 4 to 10 show that when using soot surface area of 46 m2 g-1 (from n-hexane) very little difference in gas-phase concentration is observed. Increasing soot surface area to 218 m2 g-1 (from decane) provided a more notable, albeit small, difference.
FIGURE 2. Mixing ratios of inactive sites that result from physisorption of HNO3 and NO3 as a function of time. Scales for SSHNO3 and SSNO3 are on the left and right sides, respectively.
FIGURE 3. Mixing ratios of SSO and SSP sites as a function of time. The larger surface area provides a greater soot site concentration; therefore, these observations indicate that within the model conditions, heterogeneous reactions on soot are limited by the number of soot sites. The introduction of reaction 6 (i.e. heterogeneous reaction of HNO3) did not significantly alter the model results. The largest change was observed for HNO3, for which the concentration was only reduced by 3%. For this reason reaction 6 was not included in subsequent model runs. For the first 20 h, HNO3 concentration is not significantly affected (Figure 4). It was during this time that physorption of HNO3 on soot sites would have taken place, as these sites are not regenerated and would have been rapidly depleted shortly after emissions are turned off (20:00). The larger perturbation in HNO3 concentration, a 3 ppb reduction when using the higher soot loading, is observed after the consumption of SS2. When using the lower soot loading the maximum HNO3 reduction was 0.24 ppb, also observed after the consumption of SS2. Therefore the observed reduction in concentration must be due to a secondary effect, a result of perturbations in the concentrations of other gases. Decrease in HNO3 concentration due to physisorption would have been counteracted by reaction 9, hydrolysis of N2O5. Figure 5 shows that for the second and third days, the
N2O5 concentration is greatly reduced for the model condition that used a surface area of 218 m2/g and soot concentration of 20 µg/m3. All three runs showed the increase in concentration observed during the first 6 h; note that no soot was added in the model at that time. For the higher soot loading a maximum reduction of 4 ppb is observed at hour 29, and this reduction was only 0.56 ppb for the lower soot loading. HONO rapidly photolyzes during the day to give NO and OH. Unless the rate of reaction 8 is higher than the photolysis rate of HONO, no significant contribution from this reaction is expected during the day. This is observed in Figure 6. The incorporation of heterogeneous reactions increased the concentration of HONO during the night; an effect observed for the first three nights. The maximum increase in HONO observed is 0.9 ppb and 0.093ppb for the higher and lower soot loading, respectively. With the complete consumption of regenerated sites (day 3), nighttime HONO levels return to base levels. Increase in nighttime HONO concentration will have an impact on morning NO concentrations. Relative to NO concentration, this impact would be significant only for the second morning. NO2 and NO concentrations are shown in Figures 7 and 8, respectively. The introduction of heterogeneous reactions increased the concentration of NOx VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. HNO3 mixing ratios with and without heterogeneous reactions as a function of time, with a soot surface area of 46 m2/g and 218 m2/g and soot concentration of 20 µg/m3.
FIGURE 5. N2O5 mixing ratios with and without heterogeneous reactions as a function of time, with a soot surface area of 46 m2/g and 218 m2/g and soot concentration of 20 µg/m3.
FIGURE 6. HONO mixing ratios with and without heterogeneous reactions as a function of time, with a soot surface area of 46 m2/g and 218 m2/g and soot concentration of 20 µg/m3. species. Diurnal cycling of NOx from NO to NO2 is evident. As in the case of HNO3, change due to the introduction of heterogeneous reactions occurs after emissions are turned off. NO concentrations are higher during the day and NO2 5544
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is higher during the night. When using the higher soot loading of 20 µg/m3, the maximum NO increase of 3 ppbv was observed at 40 h (12:00 of the second day). The maximum increase of NO2 concentration observed at hour 53 (05:00 on
FIGURE 7. NO2 mixing ratios with and without heterogeneous reactions as a function of time, with a soot surface area of 46 m2/g and 218 m2/g and soot concentration of 20 µg/m3.
FIGURE 8. NO mixing ratios with and without heterogeneous reactions as a function of time, with a soot surface area of 46 m2/g and 218 m2/g and soot concentration of 20 µg/m3.
FIGURE 9. NO3 mixing ratios with and without heterogeneous reactions as a function of time, with a soot surface area of 46 m2/g and 218 m2/g and soot concentration of 20 µg/m3. the third day) was 11 ppb. Maximum change in NO and NO2 concentrations for the lower soot loading were 0.33 and 1.3 ppb, respectively.
NO3 reduction during the first three nights shown in Figure 9 cannot be explained by the physisorption process shown in reaction 10, because SS2 sites are close to being depleted VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 10. Ozone mixing ratios with and without heterogeneous reactions as a function of time, with a soot surface area of 46 m2/g and 218 m2/g and soot concentration of 20 µg/m3. by hour 24. Running the model without reaction 10 gave comparable concentration of NO3. It must then be assumed that changes in NO3 concentrations must have resulted from perturbations in other gases. During the first day, when emissions are turned on, O3 concentration decreases due to high NOx levels, as shown in Figure 10. It is important to note here that MOCCA is only a box model, and while it was initialized with actual field data from Southern Ontario taken in July 1999, it cannot represent the true evaluation of the chemistry in the air parcel downwind from the source. In reality, the trajectory that the parcel may take can take it over the high emission urban region of Toronto or it can follow a cleaner path over Lake Ontario. In addition, the complexities involved with the lake breeze circulation acting on air parcels on a daily basis will have an impact on the emissions and deposition from the air parcel. These factors make it difficult, if not impossible to compare the box model result to the ambient concentrations measured in Southern Ontario. However, we note that the observations do show a similar diurnal patter as obtained by the model, with O3 mixing ratios varying from 10 to 30 pbb during nonsmog episode periods in the urban regions of Southern Ontario. Nevertheless, the box model is useful to investigate the potential importance of the detailed chemical processes in the atmosphere. The effect of introducing soot heterogeneous reactions is minimal for the lower soot loading. Heterogeneous effects are also small for the first day. For the remaining time, O3 concentration shows a typical diurnal cycle, and a small effect from heterogeneous reactions is observed. The higher ozone reduction is observed when using the surface area for decane soot (218 m2/g). As illustrated in Figure 10, even with the high soot loading of 20 µg/m3, ozone reduction is negligible when using the surface area for n-hexane soot. The largest O3 reduction of 7.5 pbb is observed for hours 20 and 43, when maximum NO2 concentrations are observed. A maximum reduction of 0.99 ppb is observed when using the lower soot loading of 2 µg/m3. The lowering of O3 concentrations with the introduction of heterogeneous reactions was further reduced with increased time. This work did not consider the loss of active sites due to the condensation of semivolatile gases and/or coagulation with other types of particles. Po¨schl et al. (2001) for example have looked at the effect of soot bound benzo[a]pyrene on the uptake of O3. Neither did we consider activation of sites as a result of thermal desorption. In addition, only one type of soot is used for each of the model runs. In the real atmosphere, soot composition would be more complex and 5546
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the observed reduction in O3 concentration due to heterogeneous reactions on soot surfaces would most probably be even lower than results estimated in this work. Based on these results, we do not see a necessity of including such reactions in 3-D models of atmosphere at this time.
Acknowledgments The authors would like to thank Dr. Rolf Sander for making MOCCA available for this work and Surandokht Nikzad for her help with the MOCCA urban model. Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
Literature Cited (1) Cooke, W. F.; Wilson, J. N. J. Geophys. Res. 1996, 101, 1939519409. (2) Penner, J. E.; Eddleman, H.; Novakov, T. Atmos. Environ. 1993, 72A, 1277-1295. (3) Sander, R.; Ravishankara, A. R.; Golden, D. M.; Kolb, C. E.; Kurylo, M. J.; Molina, M. J.; Moortgat, G. K.; Finlayson-Pitts, B. J. Chemical kinetics and photochemical data for use in atmospheric studies, Evaluation 14, California Institute of Technology, 2002. (4) Choi, W.; Leu, M. T. J. Phys. Chem. 1998, 102, 7618-7630. (5) Smith, D. M.; Chughtai, A. R. J. Atmos. Chem. 1997, 26, 77-91. (6) Kamm, S.; Mohler, O.; Naumann, K.-H.; Saathoff, H.; Schurath, U. Atmos. Environ. 1999, 33, 4651-4661. (7) Ammann, M.; Kalberer, M.; Jost, D. T.; Tobler, L.; Rossler, E.; Piguet, D.; Gaggeler, W.; Baltensperger, U. Nature 1998, 395, 157-160. (8) Kleffmann, J.; Becker, K. H.; Lackhoff, M.; Wiesen, P. Phys. Chem. Chem. Phys. 1999, 1, 5443-5450. (9) Kotamarthi, V. R.; Gaffney, J. S.; Marley, N. A.; Doskey, P. V. Atmos. Environ. 2001, 35, 4489-4498. (10) Salgado, M. S.; Rossi, M. J. Intl. J. Chem. Kinet. 2002, 34, 620631. (11) Kalberer, M.; Ammann, M.; Parrat, Y.; Weingartner, E.; Baltensperger, U. J. Phys. Chem. 1996, 100, 15487-15493. (12) Kirchner, U.; Scheer, V.; Vogt, R. J. Phys. Chem. 2000, 89088915. (13) Disselkamp, R. S.; Carpenter, M. A.; Cowin, J. P. J. Atmos. Chem. 2000, 37, 113-123. (14) Aubin, D. G.; Abbott, J. P. J. Phys. Chem. A 2003, 107, 1103011037. (15) Sander, R.; Crutzen, P. J. J. Geophys. Res. 1996, 101, 9129-9138. (16) Autho In Centre for Research in Earth and Space Science; York University: Toronto, 2004. (17) Wei, C.-F.; Larson, S. M.; Pattern, K. O.; Wuebbles, D. J. Atmos. Environ. 2001, 35, 6167-6180. (18) Aumont, B.; Madronich, S.; Ammann, M.; Kalberer, M.; Baltensperger, U.; Hauglustaine, D.; Brocheton J. Geophys. Res. 1999, 104, 1729-1736.
(19) Fendel, W.; Matter, D.; Burtscher, H.; Schmidt-Ott, A. Atmos. Environ. 1995, 29, 967-973. (20) Saathoff, H.; Naumann, K.-H.; Reimer, N.; Kamm, S.; Mohler, O.; Schurath, U.; Vogel, H.; Vogel, B. Geophys. Res. Lett. 2001. (21) Rogaski, C. A.; Golden, D. M.; Williams, L. R. Geophys. Res. Lett. 1997, 24, 381-384.
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Received for review September 30, 2003. Revised manuscript received July 14, 2004. Accepted August 11, 2004. ES035079X
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