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The Reaction of CH3O2 Radicals with OH Radicals: A Neglected Sink for CH3O2 in the Remote Atmosphere Christa Fittschen,*,† Lisa K. Whalley,‡,§ and Dwayne E. Heard‡,§ †
Université Lille 1, PhysicoChimie des Processus de Combustion et de l’Atmosphère PC2A, Cité Scientifique, Bat. C11, 59655 Villeneuve d’Ascq, France ‡ School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, U.K. § National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, U.K. shows that R1 can be competitive with one of the major recognized sinks for CH3O2. With the goal of undertaking a more detailed examination of the role of R1 in the chemistry of remote atmospheres, we have included reaction R1 into a model constrained using data from a field campaign1 at the Cape Verde Atmospheric Observatory, which took place in May 2007, chosen as typical example. Figure 1 shows the sinks for CH3O2 in the form of a pie-chart, more details on the model employed can be found in Whalley et al.1 The left pie-chart shows the distribution of the rate of removal of CH3O2 through reaction with its sinks in the model without R1, with roughly a third of all CH3O2 radicals reacting through different pathways with HO2 (black pie-slice); a third reacts with NOx (blue pie-slice, [NO] = 2.1 ppt, [NO2] = 12.3 ppt) and a third reacts via self-and cross reactions (green pieslice); for simplicity, the distribution of the three pie-slices through different pathways is not shown (see Whalley et al.1 for more details). The right pie-chart shows output from the same model, but now including R1: the impact of this reaction is depicted by the red pie-slice and represents around 25% of the overall sink for CH3O2, making this reaction as important as the reaction with HO2 radicals. Remote low NOx environments that lead to long RO2 he methyl peroxy radical, CH3O2, is a critical intermediate lifetimes and smaller blue pie-slices (this pie-slice is dominant in the atmospheric oxidation of hydrocarbons. Other than in polluted environments) are widely represented in the Earth’s its reaction with NO, the major reaction pathway currently atmosphere, and hence neglecting a major reaction path for an described in atmospheric chemistry models (e.g., the Master important reactive intermediate can lead to significant errors in Chemical Mechanism MCM)1 is reaction with HO2 and self- or the interpretation of observations. However, in order to cross reaction with other peroxy radicals. We argue in this understand the ultimate impact of this reaction, a detailed viewpoint that the reaction of CH3O2 radicals with OH radicals, knowledge of the reaction products is required. Currently so far disregarded in atmospheric chemistry models, can nothing is known about potential products, but three different become a major sink for CH3O2 radicals under remote exothermic reaction paths can be envisaged: conditions. Therefore, this reaction should routinely be included into models describing the chemistry in low NOx OH + CH3O2 → CH 2O2 + H 2O (R1a) environments. Indeed, very little is known about this class of reactions, and OH + CH3O2 → CH3O + HO2 (R1b) only very recently has the rate constant of the reaction between OH + CH3O2 → CH3OH + O2 the simplest peroxy radical, CH3O2, and OH radicals: (R1c)
T
CH3O2 + OH → products
Using a model Archibald et al.3 have investigated the impact of the three analogous possible pathways of R1 for 6 peroxy radicals up to C4 using rate constants of 0.5, 1.0, and 1.5 × 10−10 cm3 molecule−1 s−1 for the reaction of each peroxy radical with OH. Those model studies focused on the change in the concentration of stable species following the inclusion of R1
(R1)
been measured.2 With a large rate constant of k1 = (2.8 ± 1.4) × 10−10 cm3 molecule−1 s−1, it can be expected that R1 will be a non-negligible sink for CH3O2 radicals. HO2 radicals are major reaction partners of CH3O2 radicals in the remote environment, thus comparing the ratio of the concentrations [HO2]/[OH] (≈ 50 in remote environments) with the inverse ratio of the corresponding rate constants (kOH+CH3O2/kHO2+CH3O2 ≈ 50) © 2014 American Chemical Society
Received: May 21, 2014 Published: June 20, 2014 7700
dx.doi.org/10.1021/es502481q | Environ. Sci. Technol. 2014, 48, 7700−7701
Environmental Science & Technology
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Figure 1. Relative rates of removal of CH3O2 simulated for noon conditions at the Cape Verde Atmospheric Observatory. Left: R1 is omitted; right: R1 is included. Black pie-slice: loss via the reaction of CH3O2 with HO2. Blue pie-slice: reaction of CH3O2 with NOx ([NO] = 2.1 ppt, [NO2] = 12.3 ppt). Green pie-slice: self- and cross reactions. Red pie-slice: removal via R1
product yields are needed in order to estimate the detailed impact of this reaction class on the composition of the atmosphere. A combined experimental and theoretical approach will be needed, especially for the study of larger peroxy radicals.
into the mechanism. For all scenarios Archibald et al. found only a small effect on the mixing ratios of O3, NOx, as well as OH and HO2 radicals. However, an increase of up to a factor of 160 in the mixing ratios of HCOOH was observed if the reaction proceeded through R1a, that is, formation of a Criegee intermediate. An increase of up to a factor of 8 in the mixing ratio of CH3OH was observed if the major pathway was R1c. Such changes are non-negligible, and yet the conditions used by Archibald et al. underestimated the impact of R1 on radical levels for large areas of the Earth’s atmosphere: • The NOx concentration used by Archibald (≈50 ppt at noon) is considerably higher than used to generate Figure 1 (noontime [NO] = 2.1 ppt and [NO2] = 12.3 ppt for Cape Verde). From Figure 1 it can be seen, that even with NOx concentrations of ∼14 ppt roughly 25% of the CH3O2 reacts with NOx (blue pie-slice) with this fraction increasing with NOx, and the fraction via R1 decreasing. • Even the largest rate constant used for R1 is smaller by nearly a factor of 2 than the recently determined rate constant of Bossolasco et al.2 • A ratio of [HO2]/[OH] = 110 was assumed, while measurements in remote marine boundary layers have shown considerably lower ratios, for example [HO2]/ [OH] = 66 at Cape Verde1. A lower ratio directly increases the fraction of CH3O2 radicals reacting through R1. R1 may also provide the source of a missing oxidant for SO24 if proceeding through R1a. Including R1 could also explain the unexpected deviation from photostationary state as recently observed in the Southern Atlantic,5 where HO2/OH ratios of ≈30 together with [NOx] concentrations well below 20 ppt make R1 the predominant reaction path for CH3O2. From the points made above, it is clear that the reaction of CH3O2, and probably also other peroxy radicals, with OH radicals should not be omitted from atmospheric chemistry models used to describe remote conditions. We recommend that strong efforts should be made to improve the understanding of this class of reactions. The rate constants for reaction of OH with peroxy radicals other than CH3O2 need to be determined, but more importantly the determination of
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS CF thanks the French ANR for funding under contract No. ANR-11-LabEx-0005-01 CaPPA (Chemical and Physical Properties of the Atmosphere). LKW and DEH wish to thank the Natural Environment Research Council for funding.
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REFERENCES
(1) Whalley, L. K.; Furneaux, K. L.; Goddard, A.; Lee, J. D.; Mahajan, A.; Oetjen, H.; Read, K. A.; Kaaden, N.; Carpenter, L. J.; Lewis, A. C.; Plane, J. M. C.; Saltzman, E. S.; Wiedensohler, A.; Heard, D. E. The chemistry of OH and HO2 radicals in the boundary layer over the tropical Atlantic Ocean. Atmos. Chem. Phys. 2010, 10 (4), 1555−1576. (2) Bossolasco, A.; Faragó, E. P.; Schoemaecker, C.; Fittschen, C. Rate constant of the reaction between CH3O2 and OH radicals. Chem. Phys. Lett. 2014, 593, 7−13. (3) Archibald, A. T.; Petit, A. S.; Percival, C. J.; Harvey, J. N.; Shallcross, D. E. On the importance of the reaction between OH and RO2 radicals. Atmos. Sci. Lett. 2009, 10 (2), 102−108. (4) Berresheim, H.; Adam, M.; Monahan, C.; O’Dowd, C.; Plane, J. M. C.; Bohn, B.; Rohrer, F. Missing SO2 oxidant in the coastal atmosphere? - Evidence from high resolution measurements of OH and atmospheric sulfur compounds. Atmos. Chem. Phys. Discuss. 2014, 14 (1), 1159−1190. (5) Hosaynali Beygi, Z.; Fischer, H.; Harder, H. D.; Martinez, M.; Sander, R.; Williams, J.; Brookes, D. M.; Monks, P. S.; Lelieveld, J. Oxidation photochemistry in the Southern Atlantic boundary layer: unexpected deviations of photochemical steady state. Atmos. Chem. Phys. 2011, 11 (16), 8497−8513.
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dx.doi.org/10.1021/es502481q | Environ. Sci. Technol. 2014, 48, 7700−7701
