Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 10 17 2010 10.5194/acp-10-8097-2010 http://www.atmos-chem-phys.net/10/8097/2010/ http://www.atmos-chem-phys.net/10/8097/2010/acp-10-8097-2010.html http://www.atmos-chem-phys.net/10/8097/2010/acp-10-8097-2010.pdf 8097 8118 2010-09-01 Impacts of mechanistic changes on HO_x formation and recycling in the oxidation of isoprene A. T. Archibald M. C. Cooke S. R. Utembe D. E. Shallcross R. G. Derwent M. E. Jenkin atmos.chem@btinternet.com School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK rdscientific, Newbury, Berkshire, RG14 6LH, UK Atmospheric Chemistry Services, Okehampton, Devon, EX20 1FB, UK now at: NCAS-Climate, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK Recently reported model-measurement discrepancies for the concentrations of the HO_x radical species (OH and HO₂) in locations characterized by high emission rates of isoprene have indicated possible deficiencies in the representation of OH recycling and formation in isoprene mechanisms currently employed in numerical models; particularly at low levels of NO_x. Using version 3.1 of the Master Chemical Mechanism (MCM v3.1) as a base mechanism, the sensitivity of the system to a number of detailed mechanistic changes is examined for a wide range of NO_x levels, using a simple box model. The studies consider sensitivity tests in relation to three general areas for which experimental and/or theoretical evidence has been reported in the peer-reviewed literature, as follows: (1) implementation of propagating channels for the reactions of HO₂ with acyl and β-oxo peroxy radicals with HO₂, with support from a number of studies; (2) implementation of the OH-catalysed conversion of isoprene-derived hydroperoxides to isomeric epoxydiols, as characterised by Paulot et al.~(2009a); and (3) implementation of a mechanism involving respective 1,5 and 1,6 H atom shift isomerisation reactions of the β-hydroxyalkenyl and cis-δ-hydroxyalkenyl peroxy radical isomers, formed from the sequential addition of OH and O₂ to isoprene, based on the theoretical study of Peeters et al. (2009). All the considered mechanistic changes lead to simulated increases in the concentrations of OH, with (1) and (2) resulting in respective increases of up to about 7% and 16%, depending on the level of NO_x. (3) is found to have potentially much greater impacts, with enhancements in OH concentrations of up to a factor of about 3.3, depending on the level of NO_x, provided the (crucial) rapid photolysis of the hydroperoxy-methyl-butenal products of the cis-δ-hydroxyalkenyl peroxy radical isomerisation reactions is represented, as also postulated by Peeters et al.~(2009). Additional tests suggest that the mechanism with the reported parameters cannot be fully reconciled with atmospheric observations and existing laboratory data without some degree of parameter refinement and optimisation which would probably include a reduction in the peroxy radical isomerisation rates and a consequent reduction in the OH enhancement propensity. However, an order of magntitude reduction in the isomerisation rates is still found to yield notable enhancements in OH concentrations of up to a factor of about 2, with the maximum impact at the low end of the considered NO_x range. <br><br> A parameterized representation of the mechanistic changes is optimized and implemented into a reduced variant of the Common Representative Intermediates mechanism (CRI v2-R5), for use in the STOCHEM global chemistry-transport model. The impacts of the modified chemistry in the global model are shown to be consistent with those observed in the box model sensitivity studies, and the results are illustrated and discussed with a particular focus on the tropical forested regions of the Amazon and Borneo where unexpectedly elevated concentrations of OH have recently been reported. Archibald, A. T., Jenkin, M. E., and Shallcross, D. E.: An isoprene mechanism intercomparison, Atmos. Environ., doi:10.1016/j.atmosenv.2009.09.016, accepted, 2009. Atkinson, R., Baulch, D. L., Cox, R. A., Crowley, J. N., Hampson, R. F., Hynes, R. G., Jenkin, M. E., Rossi, M. J. and Troe J.: Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I – gas phase reactions of O_x, HO_x, NO_x and SO_x species, Atmos. Chem. Phys., 4, 1461–1738, doi:10.5194/acp-4-1461-2004, 2004. Atkinson, R., Baulch, D. L., Cox, R. A., Crowley, J. N., Hampson, R. F., Hynes, R. G., Jenkin, M. E., Rossi, M. J., and Troe J.: Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II – reactions of organic species. Atmos. Chem. Phys., 6, 3625–4055, doi:10.5194/acp-6-3625-2006, 2006. Baker, J., Arey, J., and Atkinson, R.: Formation and reaction of hydroxycarbonyls from the reaction of OH radicals with 1,3-butadiene and isoprene, Environ. Sci. Technol., 39, 4091–4099, 2005. Benkelberg, H.-J., Böge, O., Seuwen, R., and Warneck, P.: Product distributions from the OH radical-induced oxidation of but-1-ene, methyl-substituted but-1-enes and isoprene in NO_x-free air, Phys. Chem. Chem. Phys., 2, 4029–4039, 2000. Butler, T. M., Taraborrelli, D., Brühl, C., Fischer, H., Harder, H., Martinez, M., Williams, J., Lawrence, M. G., and Lelieveld, J.: Improved simulation of isoprene oxidation chemistry with the ECHAM 5/MESSy chemistry-climate model: lessons from the GABRIEL airborne field campaign, Atmos. Chem. Phys., 8(16), 4529–4546, 2008. Calvert, J. G., Atkinson, R., Kerr, J. A., Madronich, S., Moortgat, G. K., Wallington, T. J., and Yarwood, G.: The Mechanisms of Atmospheric Oxidation of Alkenes, Oxford University Press, New York, USA, ISBN: 0195131770, 2000. Chen, S., Ren, X., Mao, J., Chen, Z., Brune, W. H., Lefer, B. Rappenglück, B., Flynn, J., Olson, J., and Crawford, J. H.: A comparison of chemical mechanisms based on TRAMP-2006 field data, Atmos. Environ., doi:10.1016/j.atmosenv.2009.05.027, 2009. Collins, W., Stevenson, D., Johnson, C., and Derwent, R.: Tropospheric Ozone in a Global-Scale Three-Dimensional Lagrangian Model and Its Response to NO_x Emission Controls, J. Atmos. Chem., 26(3), 223–274, 1997. Derwent, R. G., Stevenson, D. S., Doherty, R. M., Collins, W. J., Sanderson, M. G., and Johnson, C. E.: Radiative forcing from surface NO_x emissions: spatial and seasonal variations, Clim. Change, 88(4), 385–401, 2008. Da Silva, G., Graham, C., and Wang, Z.-F.: Unimolecular β-hydroxyperoxy radical decomposition with OH recycling in the photochemical oxidation of isoprene, Environ. Sci. Technol., 44, 250–256, 2010. Dibble, T. S.: Intramolecular hydrogen bonding and double H-atom transfer in peroxy and alkoxy radicals from isoprene, J. Phys.Chem. A., 108, 2199–2207, 2004a. Dibble, T. S.: Prompt chemistry of alkenoxy radical products of the double H-atom transfer of alkoxy radicals from isoprene, J. Phys. Chem. A., 108, 2208–2215, 2004b. Dillon, T. J. and Crowley, J. N.: Direct detection of OH formation in the reactions of HO₂ with CH₃C(O)O₂ and other substituted peroxy radicals, Atmos. Chem. Phys., 8(16), 4877–4889, doi:10.5194/acp-8-4877-2009, 2008. Dlugi, R., Berger, M., Zelger, M., Hofzumahaus, A., Siese, M., Holland, F., Wisthaler, A., Grabmer, W., Hansel, A., Koppmann, R., Kramm, G., Möllmann-Coers, M. and Knaps, A.: Turbulent exchange and segregation of HOx radicals and volatile organic compounds above a deciduous forest, Atmos. Chem. Phys., 10, 6215–6235, doi:10.5194/acp-10-6215-2010, 2010. Eerdekens, G., Ganzeveld, L., Vilà-Guerau de Arellano, J., Klüpfel, T., Sinha, V., Yassaa, N., Williams, J., Harder, H., Kubistin, D., Martinez, M., and Lelieveld, J.: Flux estimates of isoprene, methanol and acetone from airborne PTR-MS measurements over the tropical rainforest during the GABRIEL 2005 campaign, Atmos. Chem. Phys., 9, 4207–4227, doi:10.5194/acp-9-4207-2009, 2009. Gierczak, T., Burkholder, J. B., Talukdar, R. K., Mellouki, A., Barone, S. B., and Ravishankara, A. R.: Atmospheric fate of methyl vinyl ketone and methacrolein, J. Photochem. Photobiol. A: Chemistry, 110, 1–10, 1997. Greenwald, E., North, S., Georgievskii, Y., and Klippenstein, S.: A two transition state model for radical-molecule reactions: applications to isomeric branching in the OH-isoprene reaction, J. Phys. Chem. A., 111, 5582–5592, 2007. Guenther, A., Hewitt, C. N., Erickson, D., Fall, R., Geron, C., Graedel, T., Harley, P., Klinger, L., Lerdau, M., McKay, W.A., Pierce, T., Scholes, B., Steinbrecher, R., Tallamraju, R., Taylor, J., and Zimmerman, P.: A global model of natural volatile organic compound emissions, J. Geophys. Res., 100, 8873–8892, 1995. Hasson, A. S., Tyndall, G. S. and Orlando, J. J.: A product yield study of the reaction of HO₂ Radicals with ethyl peroxy (C₂H$_5$O₂), acetyl peroxy (CH₃C(O)O₂), and acetonyl peroxy (CH₃C(O)CH₂O₂) radicals, J. Phys. Chem. A, 108(28), 5979–5989, 2004. Heard, D. E. and Pilling, M. J.: Measurement of OH and HO₂ in the troposphere, Chem. Rev, 103(12), 5163–5198, 2003. Horowitz, L. W., Fiore, G. P., Milly, A. M., Cohen, R. C., Perring, A., Wooldridge, P. J., Hess, P. G., Emmons, L. K., and Lamarque, J.: Observational constraints on the chemistry of isoprene nitrates over the eastern United States, J. Geophys. Res., 112(D12), D12S08, doi:10.1029/2006JD007747, 2007. IUPAC: Current recommendation of the IUPAC Subcommittee for Gas Kinetic Data Evaluation, available at: http://www.iupac-kinetic.ch.cam.ac.uk/, 2010. Jenkin, M. E. and Clemitshaw K. C.: Ozone and other secondary photochemical pollutants: chemical processes governing their formation in the planetary boundary layer, Atmos. Environ., 34, 2499–2527, 2000. Jenkin, M. E., Saunders, S. M., and Pilling, M. J.: The tropospheric degradation of volatile organic compounds: a protocol for mechanism development, Atmos. Environ., 31(1), 81–104, 1997. Jenkin, M. E., Boyd, A. A., and Lesclaux, R.: Peroxy radical kinetics resulting from the OH-initiated oxidation of 1, 3-butadiene, 2, 3-dimethyl-1, 3-butadiene and isoprene, J. Atmos. Chem., 29(3), 267–298, 1998. Jenkin, M. E., Hurley, M. D., and Wallington, T. J.: Investigation of the radical product channel of the CH₃C(O)O₂ + HO₂ reaction in the gas phase, Phys. Chem. Chem. Phys., 9(24), 3149–3162, 2007. Jenkin, M. E., Hurley, M. D., and Wallington, T. J.: Investigation of the radical product channel of the CH₃C(O)CH₂O₂ + HO₂ reaction in the gas phase, Phys. Chem. Chem. Phys, 10(29), 4274–4280, 2008a. Jenkin, M. E., Watson, L. A., Utembe, S. R., and Shallcross, D. E.: A Common Representative Intermediates (CRI) mechanism for VOC degradation. Part 1: Gas phase mechanism development, Atmos. Environ., 42(31), 7185–7195, 2008b. Jenkin, M. E., Hurley, M. D., and Wallington, T. J.: Investigation of the radical product channel of the CH₃OCH₂O₂ + HO₂ reaction in the gas phase, J. Phys. Chem. A, 114, 408–416, 2010. Jorand, F., Heiss, A., Perrin, O., Sahetchian, K., Kerhoas, L., and Einhorn, J.: Isomeric hexyl-ketohydroperoxides formed by reactions of hexoxy and hexylperoxy radicals in oxygen, Int. J. Chem. Kinet., 35(8), 354–366, 2003. Karl, T., Guenther, A., Turnipseed, A., Tyndall, G., Artaxo, P. and Martin, S.: Rapid formation of isoprene photo-oxidation products observed in Amazonia, Atmos. Chem. Phys., 9, 7753–7767, doi:10.5194/acp-9-7753-2009, 2009. Kubistin, D., Harder, H., Martinez, M., Rudolf, M., Sander, R., Bozem, H., Eerdekens, G., Fischer, H., Gurk, C., Klüpfel, T., Königstedt, R., Parchatka, U., Schiller, C. L., Stickler, A., Taraborrelli, D., Williams, J. and Lelieveld, J.: Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: comparison of measurements with the box model MECCA, Atmos. Chem. Phys. Discuss., 8(4), 15239–15289, doi:10.5194/acpd-8-15239-2008, 2008. Kuhn, U., Andreae, M. O., Ammann, C., Aráujo, A. C., Brancaleoni, E., Ciccioli, P., Dindorf, T., Frattoni, M., Gatti, L. V., Ganzeveld, L., Kruijt, B., Lelieveld, J., Lloyd, J., Meixner, F. X., Nobre, A. D., P\" oschl, U., Spirig, C., Stefani, P., Thielmann, A., Valentini, R., and Kesselmeier, J.: Isoprene and monoterpene fluxes from Central Amazonian rainforest inferred from tower-based and airborne measurements, and implications on the atmospheric chemistry and the local carbon budget, Atmos. Chem. Phys., 7, 2855–2879, doi:10.5194/acp-7-2855-2007, 2007. Kwok, E. S. C. and Atkinson, R.: Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure-reactivity relationship: an update, Atmos. Environ., 29, 1685–1695, 1995. Lee, Y. N., Zhou, X., and Hallock, K.: Atmospheric carbonyl compounds at a rural southeastern U.S. site, J. Geophys. Res., 100, 25933–25944, 1995. Lee, Y. N., Zhou, X., Kleinman, L. I., Nunnermacker, L. J., Springston, S. R., Daum, P. H., Newman, L., Keigley, W. G., Holdren, M. W., Spicer, C. W., Parrish, D. D., Holloway, J., Williams, J., Roberts, J. M., Ryerson, T. B., Fehsenfeld, F. C., Young, V., and Fu, B.: Atmospheric chemistry and distribution of formaldehyde and several multi-oxygenated carbonyl compounds during the 1995 Nashville/Middle Tennessee ozone study, J. Geophys. Res., 103, 22449–22462, 1998. Lee, W., Baasandorj, M., Stevens, P. S., and Hites, R. A.: Monitoring OH-initiated oxidation kinetics of isoprene and its products using online mass spectrometry, Environ. Sci. Technol., 39, 1030–1036, 2005. Lelieveld, J., Butler, T. M., Crowley, J. N., Dillon, T. J., Fischer, H., Ganzeveld, L., Harder, H., Lawrence, M. G., Martinez, M., Taraborrelli, D., and Williams, J.: Atmospheric oxidation capacity sustained by a tropical forest, Nature, 452(7188), 737–740, 2008. Lightfoot, P. D., Cox, R. A., Crowley, J. N., Destriau, M., Hayman, G. D., Jenkin, M. E., Moortgat, G. K., and Zabel, F.: Organic peroxy radicals: kinetics, spectroscopy and tropospheric chemistry. Atmos. Environ., 26A, 1805–1964, 1992. Lockwood, A. L., Shepson, P. B., Fiddler, M. N. and Alaghmand, M.: Isoprene nitrates: preparation, separation, identification, yields, and atmospheric chemistry, Atmos. Chem. Phys., 10, 6169–6178, doi:10.5194/acp-10-6169-2010, 2010. Martinez, M., Harder, H., Kovacs, T. A., Simpas, J. B., Bassis, J., Lesher, R., Brune, W. H., Frost, G. J., Williams, E. J., and Stroud, C. A.: OH and HO₂ concentrations, sources, and loss rates during the Southern Oxidants Study in Nashville, Tennessee, summer 1999, J. Geophys. Res., 108, 4617, doi:10.1029/2003JD003551, 2003. Miller, A. M., Yeung, L. Y., Kiep, A. C., and Elrod, M. J.: Overall rate constant measurements of the reactions of alkene-derived hydroxyalkylperoxy radicals with nitric oxide, Phys. Chem. Chem. Phys., 6, 3402–3407, 2004. Park, J, Jongsma, C. G., Zhang, R. Y., and North, S. W.: OH/OD initiated oxidation of isoprene in the presence of O₂ and NO, J. Phys. Chem. A, 108(48), 10688–10697, 2004. Patchen, A. K., Pennino, M. J., Kiep, A. C., and Elrod, M. J.: Direct kinetics study of the product-forming channels of the reaction of isoprene-derived hydroxyperoxy radicals with NO, Int. J. Chem. Kinet., 39, 353–361, 2007. Paulot, F., Crounse, J. D., Kjaergaard, H. G., Kürten, A., St. Clair, J. M., Seinfeld, J. H., and Wennberg, P. O.: Unexpected epoxide formation in the gas-phase photooxidation of isoprene, Science, 325, 730–733, 2009a. Paulot, F., Crounse, J. D., Kjaergaard, Kroll, J. H., Seinfeld, J. H. and Wennberg, P. O.: Isoprene photooxidation: new insights into the production of acids and organic nitrates. Atmos. Chem. Phys., 9(4), 1479–1501, doi:10.5194/acp-9-1479-2009, 2009b. Peeters, J., Boullart, W., and Van Hoeymissen, J.: Site specific partial rate constants for OH addition to alkenes and dienes. In Proc. EUROTRAC Symp. `94, Garmisch-Partenkirchen, FRG. April 1994, 110–114, 1994. Peeters, J., Nguyen, T. L., and Vereecken, L.: HO_x radical regeneration in the oxidation of isoprene, Phys. Chem. Chem. Phys., 28, 5935–5939, 2009. Perrin, O., Heiss, A., Doumenc, F., and Sahetchian, K.: Determination of the isomerization rate constant HOCH₂CH₂CH₂CH(OO.)CH$_3 \quad \to $ HOC.HCH₂CH₂CH(OOH)CH₃. Importance of intramolecular hydroperoxy isomerization in tropospheric chemistry. J. Chem. Soc. Faraday Trans., 94, 2323–2335, 1998. Perring, A. E., Bertram, T. H., Wooldridge, P. J., Fried, A., Heikes, B. G., Dibb, J., Crounse, J. D., Wennberg, P. O., Blake, N. J., Blake, D. R., Brune, W. H., Singh, H. B., and Cohen, R. C.: Airborne observations of total RONO₂: new constraints on the yield and lifetime of isoprene nitrates, Atmos. Chem. Phys., 9, 1451–1463, doi:10.5194/acp-9-1451-2009, 2009. Pinho, P. G., Pio, C. A., and Jenkin, M. E.: Evaluation of isoprene degradation in the detailed tropospheric chemical mechanism, MCM v3, using environmental chamber data, Atmos. Environ., 39, 1303–1322, 2005. Pugh, T. A. M., MacKenzie, A. R., Hewitt, C. N., Langford, B., Edwards, P. M., Furneaux, K. L., Heard, D. E., Hopkins, J. R., Jones, C. E., Karunaharan, A., Lee, J., Mills, G., Misztal, P., Moller, S., Monks, P. S. and Whalley, L. K.: Simulating atmospheric composition over a South-East Asian tropical rainforest: Performance of a chemistry box model, Atmos. Chem. Phys.., 10, 279–298, 2010a. Pugh,~T. A. M., MacKenzie,~A. R., Langford,~B., Nemitz,~E., Misztal,~P. K., and Hewitt,~C. N.: The influence of small-scale variations in isoprene concentrations on atmospheric chemistry over a tropical rainforest, Atmos. Chem. Phys. Discuss., 10, 18197–18234, doi:10.5194/acpd-10-18197-2010, 2010b. Raber, W. H. and Moortgat, G. K.: Progress and Problems in Atmospheric Chemistry, edited by: Barker, J. World Scientific Publishing Company, Singapore, 318–373, 1996. Ren, X., Olson, J. R., Crawford, J. H., Brune, W. H., Mao, J., Long, R. B., Chen, Z., Chen, G., Avery, M. A., and Sachse, G. W.: HO_x chemistry during INTEX-A 2004: Observation, model calculation, and comparison with previous studies, J. Geophys. Res., 113, D05310, doi:10.1029/2007JD009166, 2008. Roberts, J. M., Flocke, F., Stroud, C. A., Hereid, D., Williams, E. J., Fehsenfeld, F. C., Brune, W., Martinez, M., and Harder, H.: Ground-based measurements of PANs during the 1999 Southern Oxidants Study Nashville intensive, J. Geophys. Res., 107(D21), 4554, doi:10.1029/2001JD000947, 2002. Roberts, J. M., Marchewka, M., Bertman, S. B., Sommariva, R., Warneke, C., de Gouw, J., Kuster, W., Goldan, P., Williams, E., Lerner, B. M., Murphy, P., and Fehsenfeld, F. C.: Measurements of PANs during the New England Air Quality Study 2002, J. Geophys. Res., 112, D20306, doi:10.1029/2007JD008667, 2007. Ruppert, L. and Becker, K.-H.: A product study of the OH radical-initiated oxidation of isoprene: formation of C$_5$-unsaturated diols, Atmos. Environ., 34, 1529–1542, 2000. Saunders, S. M., Jenkin, M. E., Derwent, R. G. and Pilling, M. J.: Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part A): tropospheric degradation of non-aromatic volatile organic compounds, Atmos. Chem. Phys., 3, 161–180, doi:10.5194/acp-3-161-2003, 2003. Spaulding, R. S., Schade, G. W., Goldstein, A. H., and Charles, M. J.: Characterization of secondary atmospheric photooxidation products: Evidence for biogenic and anthropogenic sources, J. Geophys. Res., 108(D8), 4247, doi:10.1029/2002JD002478, 2003. Sprengnether, M., Demerjian, K. L., Donahue, N. M., and Anderson, J. G.: Product analysis of the OH oxidation of isoprene and 1, 3-butadiene in the presence of NO. J. Geophys. Res., 107(D15), doi:10.1029/2001JD000716, 2002. Stavrakou, T., Peeters, J. and Müller, J.-F.: Improved global modelling of HO_x recycling in isoprene oxidation: evaluation against the GABRIEL and INTEX-A aircraft campaign measurements. Atmos. Chem. Phys. Discuss., 10, 16551–16588, doi:10.5194/acpd-10-16551-2010, 2010. Tan, D., Faloona, I., Simpas, J. B., Brune, W., Shepson, P. B., Couch, T. L., Sumner, A. L., Carroll, M. A., Thornberry, T., Apel, E., Reimer, D., and Stockwell, W.: HO_x budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign, J. Geophys. Res., 106(D20), 24407–24427, doi:10.1029/2001JD900016, 2001. Taraborrelli, D., Lawrence, M. G., Butler, T. M., Sander, R., and Lelieveld, J.: Mainz Isoprene Mechanism 2 (MIM2): an isoprene oxidation mechanism for regional and global atmospheric modelling, Atmos. Chem. Phys., 9(8), 2751–2777, doi:10.5194/acp-9-2751-2009, 2009. Thornton, J. A., Wooldridge, P. J., Cohen, R. C., Martinez, M., Harder, H., Brune,W. H., Williams, E. J., Roberts, J. M., Fehsenfeld, F. C., Hall, S. R., Shetter, R. E., Wert, B. P., and Fried, A.: Ozone production rates as a function of NO_x abundances and HO_x production rates in the Nashville urban plumes. J. Geophys. Res., 107(D12), 4146, doi:10.1029/2001JD000932, 2002. Utembe, S. R., Cooke, M. C., Archibald, A. T., Jenkin, M. E., Derwent, R. G., and Shallcross, D. E.: Using a reduced Common Representative Intermediates (CRIv2-R5) Mechanism to Simulate Tropospheric Ozone in a 3-D Lagrangian Chemistry Transport Model, Atmos. Environ., 44, 1609–1622, 2010. Watson, L. A., Shallcross, D. E., Utembe, S. R., and Jenkin, M. E.: A Common Representative Intermediates (CRI) mechanism for VOC degradation. Part 2: Gas phase mechanism reduction, Atmos. Environ., 42(31), 7196–7204, 2008. Wiedinmyer, C., Greenberg, J., Guenther, A., Hopkins, B., Baker, K., Geron, C., Palmer, P. I., Long, B. P., Turner, J. R., and Pétron, G.: Ozarks Isoprene Experiment (OZIE): measurements and modeling of the "isoprene volcano", J. Geophys. Res., 110, D18307, doi:10.1029/2005JD005800, 2005. Williams, J., Roberts, J. M., Fehsenfeld, F. C., Bertman, S. B., Buhr, M. P., Goldan, P. D., G. Hubler, G., Kuster, W. C., Ryerson, T. B., Trainer, M., and Young, V.: Regional ozone from biogenic hydrocarbons deduced from airborne measurements of PAN, PPN and MPAN, Geophys. Res. Lett., 24, 1099–1102, 1997. Yu, J., Jeffries, H. E., and Le Lacheur, R. M.: Identifying airborne carbonyl compounds in isoprene reaction products by their PFBHA oximes using gas chromatography/ion trap mass spectrometry, Environ. Sci. Technol., 29, 1923–1932, 1995. Zhao, J, Zhang, R. Y., and North, S. W.: Oxidation mechanism of delta-hydroxyisoprene alkoxy radicals: hydrogen abstraction versus 1,5 H-shift, Chem. Phys. Lett., 369(1–2), 204–213, 2003. Zhao, J, Zhang, R. Y., Fortner, E. C., and North, S. W.: Quantification of hydroxycarbonyls from OH-isoprene reactions, J. Am. Chem. Soc., 126, 2686–2687, 2004.