ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-7499-2012 Peroxy radical chemistry and OH radical production during the NO<sub>3</sub>-initiated oxidation of isoprene Kwan A. J. 1 5 8 Chan A. W. H. 2 6 Ng N. L. 2 7 Kjaergaard H. G. 3 Seinfeld J. H. 1 2 Wennberg P. O. 1 4 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA Copenhagen Center for Atmospheric Chemistry, Department of Chemistry, University of Copenhagen, 2100 Copenhagen Ø, Denmark Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA now at: Energy Storage Division, NEXT ENERGY EWE-Forschungszentrum für Energietechnologie e.V., 26129 Oldenburg, Germany now at: Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA 94720, USA now at: School of Chemical and Biomolecular Engineering and School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA soon at (September 2012): United States Agency for International Development, Washington, DC, 20523 USA 17 08 2012 12 16 7499 7515 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/7499/2012/acp-12-7499-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/7499/2012/acp-12-7499-2012.pdf

Peroxy radical reactions (RO<sub>2</sub> + RO<sub>2</sub>) from the NO<sub>3</sub>-initiated oxidation of isoprene are studied with both gas chromatography and a chemical ionization mass spectrometry technique that allows for more specific speciation of products than in previous studies of this system. We find high nitrate yields (~ 80%), consistent with other studies. We further see evidence of significant hydroxyl radical (OH) formation in this system, which we propose comes from RO<sub>2</sub> + HO<sub>2</sub> reactions with a yield of ~38–58%. An additional OH source is the second generation oxidation of the nitrooxyhydroperoxide, which produces OH and a dinitrooxyepoxide with a yield of ~35%. The branching ratio of the radical propagating, carbonyl- and alcohol-forming, and organic peroxide-forming channels of the RO<sub>2</sub> + RO<sub>2</sub> reaction are found to be ~18–38%, ~59–77%, and ~3–4%, respectively. HO<sub>2</sub> formation in this system is lower than has been previously assumed. Addition of RO<sub>2</sub> to isoprene is suggested as a possible route to the formation of several isoprene C<sub>10</sub>-organic peroxide compounds (ROOR). The nitrooxy, allylic, and C<sub>5</sub> peroxy radicals present in this system exhibit different behavior than the limited suite of peroxy radicals that have been studied to date.

 References Apel, E C., Riemer, D D., Hills, A., Baugh, W., Orlando, J., Faloona, I., Tan, D., Brune, W., Lamb, B., Westberg, H., Carroll, M A., Thornberry, T., and Geron, C D.: Measurement and interpretation of isoprene fluxes and isoprene, methacrolein, and methyl vinyl ketone mixing ratios at the PROPHET site during the 1998 Intensive, J. Geophys. Res., 107, 4034–4048, 2002. Arey, J., Aschmann, S M., Kwok, E. S C., and Atkinson, R.: Alkyl nitrate, hydroxyalkyl nitrate, and hydroxycarbonyl formation from the NO<sub>x</sub>-air photooxidations of \chemC_5-\chemC_8 n-alkanes, J. Phys. Chem. A, 105, 1020–1027, 2001. Atkinson, R.: Gas-phase tropospheric chemistry of volatile organic compounds: 1. Alkanes and alkenes, J. Phys. Chem. Ref. Data, 26, 215–290, 1997. Atkinson, R. and Arey, J.: Gas-phase tropospheric chemistry of biogenic volatile organic compounds: a review, Atmos. Environ., 37, Supplement 2, 197–219, 2003. Atkinson, R. and Arey, J.: Mechanisms of the gas-phase reactions of aromatic hydrocarbons and PAHs with OH and \chemNO_3 radicals, Polycyclic Aromat. Compd., 27, 15–40, 2007. Atkinson, R., Aschmann, S., and Winer, A.: Alkyl nitrate formation from the reaction of a series of branched \chemRO_2 radicals with NO as a function of temperature and pressure, J. Atmos. Chem., 5, 91–102, 1987. Atkinson, R., Aschmann, S M., and Pitts, J N.: Rate constants for the gas-phase reactions of the \chemNO_3 radical with a series of organic compounds at 296 +/− 2 K, J. Phys. Chem., 92, 3454–3457, 1988. Atkinson, R., Baulch, D. L., Cox, R. A., Crowley, J. N., Hampson, R. F., Hynes, R. G., Jenkin, M. E., Rossi, M. J., Troe, J., and IUPAC Subcommittee: Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II – gas phase reactions of organic species, Atmos. Chem. Phys., 6, 3625–4055, http://dx.doi.org/10.5194/acp-6-3625-2006doi:10.5194/acp-6-3625-2006, 2006. Barnes, I., Bastian, V., Becker, K H., and Tong, Z.: Kinetics and products of the reactions of \chemNO_3 with monoalkenes, dialkenes, and monoterpenes, J. Phys. Chem., 94, 2413–2419, 1990. Beaver, M. R., Clair, J. M. St., Paulot, F., Spencer, K. M., Crounse, J. D., LaFranchi, B. W., Min, K. E., Pusede, S. E., Wooldridge, P. J., Schade, G. W., Park, C., Cohen, R. C., and Wennberg, P. O.: Importance of biogenic precursors to the budget of organic nitrates: observations of multifunctional organic nitrates by CIMS and TD-LIF during BEARPEX 2009, Atmos. Chem. Phys., 12, 5773–5785, http://dx.doi.org/10.5194/acp-12-5773-2012doi:10.5194/acp-12-5773-2012, 2012. Bench, G., Fallon, S., Schichtel, B., Malm, W., and McDade, C.: Relative contributions of fossil and contemporary carbon sources to PM 2.5 aerosols at nine Interagency Monitoring for Protection of Visual Environments (IMPROVE) network sites, J. Geophys. Res.-Atmos., 112, D10205, http://dx.doi.org/10.1029/2006JD007708doi:10.1029/2006JD007708, 2007. Berndt, T. and Boge, O.: Gas-phase reaction of \chemNO_3 radicals with isoprene: A kinetic and mechanistic study, Int. J. Chem. Kinet., 29, 755–765, 1997. Bey, I., Aumont, B., and Toupance, G.: The nighttime production of OH radicals in the continental troposphere, Geophys. Res. Lett., 24, 1067–1070, 1997. Bey, I., Aumont, B., and Toupance, G.: A modeling study of the nighttime radical chemistry in the lower continental troposphere 2. Origin and evolution of HO<sub>x</sub>, J. Geophys. Res.-Atmos., 106, 9991–10001, 2001a. Bey, I., Jacob, D., Yantosca, R., Logan, J., Field, B., Fiore, A., Li, Q., Liu, H., Mickley, L., and Schultz, M.: Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation, J. Geophys. Res., 106, 23073–23096, 2001b. Biggs, P., Canosa-Mas, C E., Fracheboud, J., Shallcross, D E., and Wayne, R P.: Investigation into the kinetics and mechanism of the reaction of NO<sub>3</sub> with CH<sub>3</sub> and CH<sub>3</sub>O at 298 K between 0.6 and 8.5 Torr: is there a chain decomposition mechanism in operation?, J. Chem. Soc. Faraday Trans., 90, 1197–1204, 1994. Birdsall, A W., Andreoni, J F., and Elrod, M J.: Investigation of the role of bicyclic peroxy radicals in the oxidation mechanism of toluene, J. Phys. Chem. A, 114, 39, 10655–10633, 2010. Birdsall, A W. and Elrod, M J.: Comprehensive NO-dependent study of the products of the oxidation of atmospherically relevant aromatic compounds, J. Phys. Chem. A, 115, 21, 5397–5407, 2011. Brown, S S., Osthoff, H D., Stark, H., Dube, W P., Ryerson, T B., Warneke, C., de~Gouw, J A., Wollny, A G., Parrish, D D., Fehsenfeld, F C., and Ravishankara, A R.: Aircraft observations of daytime \chemNO_3 and \chemN_2O_5 and their implications for tropospheric chemistry, J. Photoch. Photobio. A, 176, 270–278, 2005. Brown, S S., Ryerson, T B., Wollny, A G., Brock, C A., Peltier, R., Sullivan, A P., Weber, R J., Dube, W P., Trainer, M., Meagher, J F., Fehsenfeld, F C., and Ravishankara, A R.: Variability in nocturnal nitrogen oxide processing and its role in regional air quality, Science, 311, 67–70, 2006. Brown, S. S., deGouw, J. A., Warneke, C., Ryerson, T. B., Dubé, W. P., Atlas, E., Weber, R. J., Peltier, R. E., Neuman, J. A., Roberts, J. M., Swanson, A., Flocke, F., McKeen, S. A., Brioude, J., Sommariva, R., Trainer, M., Fehsenfeld, F. C., and Ravishankara, A. R.: Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol, Atmos. Chem. Phys., 9, 3027–3042, http://dx.doi.org/10.5194/acp-9-3027-2009doi:10.5194/acp-9-3027-2009, 2009. Canosa-Mas, C E., King, M D., Lopez, R., Percival, C J., Wayne, R P., Shallcross, D E., Pyle, J A., and Daele, V.: Is the reaction between CH<sub>3</sub>C(O)O<sub>2</sub> and NO<sub>3</sub> important in the night-time troposphere?, J. Chem. Soc. Faraday Trans., 92, 2211–2222, 1996. Carlton, A. G., Wiedinmyer, C., and Kroll, J. H.: A review of Secondary Organic Aerosol (SOA) formation from isoprene, Atmos. Chem. Phys., 9, 4987–5005, http://dx.doi.org/10.5194/acp-9-4987-2009doi:10.5194/acp-9-4987-2009, 2009. Carslaw, N., Carpenter, L J., Plane, J. M C., Allan, B J., Burgess, R A., Clemitshaw, K C., Coe, H., and Penkett, S A.: Simultaneous observations of nitrate and peroxy radicals in the marine boundary layer, J. Geophys. Res.-Atmos., 102, 18917–18933, 1997. Carter, W. and Atkinson, R.: Atmospheric chemistry of alkanes, J. Atmos. Chem., 3, 377–405, 1985. Chameides, W L., Lindsay, R W., Richardson, J., and Kiang, C S.: The role of biogenic hydrocarbons in urban photochemical smog – Atlanta as a case study, Science, 241, 1473–1475, 1988. Claeys, M., Graham, B., Vas, G., Wang, W., Vermeylen, R., Pashynska, V., Cafmeyer, J., Guyon, P., Andreae, M O., Artaxo, P., and Maenhaut, W.: Formation of secondary organic aerosols through photooxidation of isoprene, Science, 303, 1173–1176, 2004. Cocker, D R., Flagan, R C., and Seinfeld, J H.: State-of-the-art chamber facility for studying atmospheric aerosol chemistry, Environ. Sci. Technol., 35, 2594–2601, 2001. Crounse, J D., McKinney, K A., Kwan, A J., and Wennberg, P O.: Measurement of gas-phase hydroperoxides by chemical ionization mass spectrometry, Anal. Chem., 78, 6726–6732, 2006. Crounse, J D., Paulot, F., Kjaergaard, H G., and Wennberg, P O.: Peroxy radical isomerization in the oxidation of isoprene, Phys. Chem. Chem. Phys., 13, 13607–13613, 2011. Daele, V., Laverdet, G., Le Bras, G., and Poulet, G.: Kinetics of the reactions CH<sub>3</sub>O + NO, CH<sub>3</sub>O + NO<sub>3</sub>, and CH<sub>3</sub>O<sub>2</sub> + NO<sub>3</sub>, J. Phys. Chem., 99, 1470–1477, 1995. Davidson, J A., Viggiano, A A., Howard, C J., Dotan, I., Fehsenfeld, F C., Albritton, D L., and Ferguson, E E.: Rate constants for reactions of \chemO_2^+, \chemNO_2^+, \chemNO^+, \chemH_3O^+, \chemCO_3^-, \chemNO_2^-, and halide ions with \chemN_2O_5 at 300 K, J. Chem. Phys., 68, 2085–2087, 1978. de~Gouw, J A., Middlebrook, A M., Warneke, C., Goldan, P D., Kuster, W C., Roberts, J M., Fehsenfeld, F C., Worsnop, D R., Canagaratna, M R., Pszenny, A. A P., Keene, W C., Marchewka, M., Bertman, S B., and Bates, T S.: Budget of organic carbon in a polluted atmosphere: Results from the New England Air Quality Study in 2002, J. Geophys. Res.-Atmos., 110, D16305, http://dx.doi.org/10.1029/2004JD005623doi:10.1029/2004JD005623, 2005. de~Gouw, J A., Brock, C A., Atlas, E L., Bates, T S., Fehsenfeld, F C., Goldan, P D., Holloway, J S., Kuster, W C., Lerner, B M., Matthew, B M., Middlebrook, A M., Onasch, T B., Peltier, R E., Quinn, P K., Senff, C J., Stohl, A., Sullivan, A P., Trainer, M., Warneke, C., Weber, R J., and Williams, E J.: Sources of particulate matter in the northeastern United States in summer: 1. Direct emissions and secondary formation of organic matter in urban plumes, J. Geophys. Res.-Atmos., 113, D08301, http://dx.doi.org/10.1029/2007JD009243doi:10.1029/2007JD009243, 2008. Dibble, T S.: Failures and limitations of quantum chemistry for two key problems in the atmospheric chemistry of peroxy radicals, Atmos. Environ., 42, 5837–5848, 2008. Dillon, T. J. and Crowley, J. N.: Direct detection of OH formation in the reactions of HO<sub>2</sub> with CH<sub>3</sub>C(O)O<sub>2</sub> and other substituted peroxy radicals, Atmos. Chem. Phys., 8, 4877–4889, http://dx.doi.org/10.5194/acp-8-4877-2008doi:10.5194/acp-8-4877-2008, 2008. Donahue, N M., Robinson, A L., and Pandis, S N.: Atmospheric organic particulate matter: From smoke to secondary organic aerosol, Atmos. Environ., 43, 94–106, 2009. Eddingsaas, N C., VanderVelde, D G., and Wennberg, P O.: Kinetics and products of the acid-catalyzed ring-opening of atmospherically relevant butyl epoxy alcohols, J. Phys. Chem. A, 114, 8106–8113, 2010. Faloona, I., Tan, D., Brune, W., Hurst, J., Barket, D., Couch, T L., Shepson, P., Apel, E., Riemer, D., Thornberry, T., Carroll, M A., Sillman, S., Keeler, G J., Sagady, J., Hooper, D., and Paterson, K.: Nighttime observations of anomalously high levels of hydroxyl radicals above a deciduous forest canopy, J. Geophys. Res.-Atmos., 106, 24315–24333, 2001. Fan, J W. and Zhang, R Y.: Atmospheric oxidation mechanism of isoprene, Environ. Chem., 1, 140–149, 2004. Fiore, A M., Horowitz, L W., Purves, D W., Levy, H., Evans, M J., Wang, Y X., Li, Q B., and Yantosca, R M.: Evaluating the contribution of changes in isoprene emissions to surface ozone trends over the eastern United States, J. Geophys. Res.-Atmos., 110, D12303, http://dx.doi.org/10.1029/2004JD005485doi:10.1029/2004JD005485, 2005. Forkel, R., Klemm, O., Graus, M., Rappengluck, B., Stockwell, W R., Grabmer, W., Held, A., Hansel, A., and Steinbrecher, R.: Trace gas exchange and gas phase chemistry in a Norway spruce forest: A study with a coupled 1-dimensional canopy atmospheric chemistry emission model, Atmos. Environ., 40, S28–S42, 2006. Francisco-Marquez, M., Alvarez-Idaboy, J R., Galano, A., and Vivier-Bunge, A.: A possible mechanism for furan formation in the tropospheric oxidation of dienes, Environ. Sci. Technol., 39, 8797–8802, 2005. Fry, J. L., Kiendler-Scharr, A., Rollins, A. W., Wooldridge, P. J., Brown, S. S., Fuchs, H., Dubé, W., Mensah, A., dal Maso, M., Tillmann, R., Dorn, H.-P., Brauers, T., and Cohen, R. C.: Organic nitrate and secondary organic aerosol yield from NO<sub>3</sub> oxidation of β-pinene evaluated using a gas-phase kinetics/aerosol partitioning model, Atmos. Chem. Phys., 9, 1431–1449, http://dx.doi.org/10.5194/acp-9-1431-2009doi:10.5194/acp-9-1431-2009, 2009. Fuentes, J D., Wang, D., Bowling, D R., Potosnak, M., Monson, R K., Goliff, W S., and Stockwell, W R.: Biogenic hydrocarbon chemistry within and above a mixed deciduous forest, J. Atmos. Chem., 56, 165–185, 2007. Garden, A L., Paulot, F., Crounse, J D., Maxwell-Cameron, I J., Wennberg, P O., and Kjaergaard, H G.: Calculation of conformationally weighted dipole moments useful in ion-molecule collision rate estimates, Chem. Phys. Lett., 474, 45–50, 2009. Geyer, A. and Stutz, J.: Vertical profiles of \chemNO_3, \chemN_2O_5, \chemO_3, and NO<sub>x</sub> in the nocturnal boundary layer: 2. Model studies on the altitude dependence of composition and chemistry, J. Geophys. Res.-Atmos., 109, D12307, http://dx.doi.org/10.1029/2003JD004211doi:10.1029/2003JD004211, 2004. Geyer, A., Alicke, B., Ackermann, R., Martinez, M., Harder, H., Brune, W., di~Carlo, P., Williams, E., Jobson, T., Hall, S., Shetter, R., and Stutz, J.: Direct observations of daytime \chemNO_3: Implications for urban boundary layer chemistry, J. Geophys. Res.-Atmos., 108, 4368, http://dx.doi.org/10.1029/2002JD002967doi:10.1029/2002JD002967, 2003a. Geyer, A., Bachmann, K., Hofzumahaus, A., Holland, F., Konrad, S., Klupfel, T., Patz, H W., Perner, D., Mihelcic, D., Schafer, H J., Volz-Thomas, A., and Platt, U.: Nighttime formation of peroxy and hydroxyl radicals during the BERLIOZ campaign: Observations and modeling studies, J. Geophys. Res.-Atmos., 108, 8249, http://dx.doi.org/10.1029/2001JD000656doi:10.1029/2001JD000656, 2003b. Golz, C., Senzig, J., and Platt, U.: \chemNO_3-initiated oxidation of biogenic hydrocarbons, Chemosphere, 3, 339–352, 2001. Griffin, R J., Cocker, D R., Flagan, R C., and Seinfeld, J H.: Organic aerosol formation from the oxidation of biogenic hydrocarbons, J. Geophys. Res.-Atmos., 104, 3555–3567, 1999. Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P. I., and Geron, C.: Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature), Atmos. Chem. Phys., 6, 3181–3210, http://dx.doi.org/10.5194/acp-6-3181-2006doi:10.5194/acp-6-3181-2006, 2006. Hallquist, M., Wangberg, I., Ljungstrom, E., Barnes, I., and Becker, K H.: Aerosol and product yields from NO<sub>3</sub> radical-initiated oxidation of selected monoterpenes, Environ. Sci. Technol., 33, 553–559, 1999. Harley, P., Vasconcellos, P., Vierling, L., Pinheiro, C. C D., Greenberg, J., Guenther, A., Klinger, L., De~Almeida, S S., Neill, D., Baker, T., Phillips, O., and Malhi, Y.: Variation in potential for isoprene emissions among Neotropical forest sites, Global Change Biol., 10, 630–650, 2004. Harrison, R M., Shi, J P., and Grenfell, J L.: Novel nighttime free radical chemistry in severe nitrogen dioxide pollution episodes, Atmos. Environ., 32, 2769–2774, 1998. Hasson, A S., Ho, A W., Kuwata, K T., and Paulson, S E.: Production of stabilized Criegee intermediates and peroxides in the gas phase ozonolysis of alkenes 2. Asymmetric and biogenic alkenes, J. Geophys. Res.-Atmos., 106, 34143–34153, 2001. Hasson, A S., Tyndall, G S., and Orlando, J J.: A product yield study of the reaction of \chemHO_2 radicals with ethyl peroxy (\chemC_2H_5O_2), acetyl peroxy (\chemCH_3C(O)O_2), and acetonyl peroxy (\chemCH_3C(O)CH_2O_2) radicals, J. Phys. Chem. A, 108, 5979–5989, 2004. Hasson, A S., Kuwata, K T., Arroyo, M C., and Petersen, E B.: Theoretical studies of the reaction of hydroperoxy radicals (\chemHO_2) with ethyl peroxy (\chemCH_3CH_2O_2), acetyl peroxy (\chemCH_3C(O)O_2) and acetonyl peroxy (\chemCH_3C(O)CH_2O_2) radicals, J. Photoch. Photobio. A, 176, 218–230, 2005. Hasson, A S., Tyndall, G S., Orlando, J J., Singh, S., Hernandez, S Q., Campbell, S., and Ibarra, Y: Branching ratios for the reaction of selected carbonyl-containing peroxy radicals with hydroperoxy radicals, J. Phys. Chem. A, 116, 6264–6281, 2012. Heald, C L., Jacob, D J., Park, R J., Russell, L M., Huebert, B J., Seinfeld, J H., Liao, H., and Weber, R J.: A large organic aerosol source in the free troposphere missing from current models, Geophys. Res. Lett., 32, L18809, http://dx.doi.org/10.1029/2005GL023831doi:10.1029/2005GL023831, 2005. Heintz, F., Platt, U., Flentje, H., and Dubois, R.: Long-term observation of nitrate radicals at the tor station, Kap Arkona (Rugen), J. Geophys. Res.-Atmos., 101, 22891–22910, 1996. Horowitz, L W., Liang, J Y., Gardner, G M., and Jacob, D J.: Export of reactive nitrogen from North America during summertime: Sensitivity to hydrocarbon chemistry, J. Geophys. Res.-Atmos., 103, 13451–13476, 1998. Horowitz, L W., Fiore, A M., Milly, G P., Cohen, R C., Perring, A., Wooldridge, P J., Hess, P G., Emmons, L K., and Lamarque, J F.: Observational constraints on the chemistry of isoprene nitrates over the eastern United States, J. Geophys. Res.-Atmos., 112, D12S08, http://dx.doi.org/10.1029/2006JD007747doi:10.1029/2006JD007747, 2007. Jay, K. and Stieglitz, L.: The gas-phase addition of NO<sub>x</sub> to olefins, Chemosphere, 19, 1939–1950, 1989. Jenkin, M E., Hurley, M D., and Wallington, T J.: Investigation of the radical product channel of the \chemCH_3C(O)O_2+\chemHO_2 reaction in the gas phase, Phys. Chem. Chem. Phys., 9, 3149–3162, 2007. Jenkin, M E., Hurley, M D., and Wallington, T J.: Investigation of the radical product channel of the \chemCH_3C(O)CH_2O_2+\chemHO_2 reaction in the gas phase, Phys. Chem. Chem. Phys., 10, 4274–4280, 2008. Jenkin, M E., Hurley, M A., and Wallington, T J.: Investigation of the radical product channel of the \chemCH_3OCH_2O_2 + \chemHO_2 reaction in the gas phase, J. Phys. Chem. A, 114, 408–416, 2010. Johnson, D., Jenkin, M E., Wirtz, K., and Martin-Reviejo, M.: Simulating the formation of secondary organic aerosol from the photooxidation of aromatic hydrocarbons, Environ. Chem., 2, 35–48, 2005. Keywood, M D., Varutbangkul, V., Bahreini, R., Flagan, R C., and Seinfeld, J H.: Secondary organic aerosol formation from the ozonolysis of cycloalkenes and related compounds, Environ. Sci. Technol., 38, 4157–4164, 2004. Kroll, J H., Ng, N L., Murphy, S M., Flagan, R C., and Seinfeld, J H.: Secondary organic aerosol formation from isoprene photooxidation under high-NO<sub>x</sub> conditions, Geophys. Res. Lett., 32, L18808, http://dx.doi.org/10.1029/2005GL023637doi:10.1029/2005GL023637, 2005. Kroll, J H., Ng, N L., Murphy, S M., Flagan, R C., and Seinfeld, J H.: Secondary organic aerosol formation from isoprene photooxidation, Environ. Sci. Technol., 40, 1869–1877, 2006. Kwok, E. S C. and Atkinson, R.: Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure-activity relationship – an update, Atmos. Environ., 29, 1685–1695, 1995. Kwok, E. S C., Aschmann, S M., Arey, J., and Atkinson, R.: Product formation from the reaction of the \chemNO_3 radical with isoprene and rate constants for the reactions of methacrolein and methyl vinyl ketone with the \chemNO_3 radical, Int. J. Chem. Kinet., 28, 925–934, 1996. 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, 737–740, 2008. Lesclaux, R.: Combination of peroxyl radicals in the gas phase, in: Peroxyl Radicals, edited by: Alfassi, Z., 81–112, Wiley, 1997. 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. A-Gen., 26, 1805–1961, 1992. Matsunaga, A. and Ziemann, P J.: Yields of beta-hydroxynitrates and dihydroxynitrates in aerosol formed from OH radical-initiated reactions of linear alkenes in the presence of NO<sub>x</sub>, J. Phys. Chem. A, 113, 599–606, 2009. Mayo, F R.: Free radical autoxidations of hydrocarbons, Acc. Chem. Res., 1, 193–201, 1968. Mellouki, A., Talukdar, R K., Bopegedera, A., and Howard, C J.: Study of the kinetics of the reactions of \chemNO_3 with \chemHO_2 and OH, Int. J. Chem. Kinet., 25, 25–39, 1993. Neeb, P. and Moortgat, G K.: Formation of OH radicals in the gas-phase reaction of propene, isobutene, and isoprene with \chemO_3: Yields and mechanistic implications, J. Phys. Chem. A, 103, 9003–9012, 1999. Ng, N. L., Chhabra, P. S., Chan, A. W. H., Surratt, J. D., Kroll, J. H., Kwan, A. J., McCabe, D. C., Wennberg, P. O., Sorooshian, A., Murphy, S. M., Dalleska, N. F., Flagan, R. C., and Seinfeld, J. H.: Effect of NO$_\textx$ level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes, Atmos. Chem. Phys., 7, 5159–5174, http://dx.doi.org/10.5194/acp-7-5159-2007doi:10.5194/acp-7-5159-2007, 2007. Ng, N. L., Kwan, A. J., Surratt, J. D., Chan, A. W. H., Chhabra, P. S., Sorooshian, A., Pye, H. O. T., Crounse, J. D., Wennberg, P. O., Flagan, R. C., and Seinfeld, J. H.: Secondary organic aerosol (SOA) formation from reaction of isoprene with nitrate radicals (NO<sub>3</sub>), Atmos. Chem. Phys., 8, 4117–4140, http://dx.doi.org/10.5194/acp-8-4117-2008doi:10.5194/acp-8-4117-2008, 2008. O'Brien, J M., Czuba, E., Hastie, D R., Francisco, J S., and Shepson, P B.: Determination of the hydroxy nitrate yields from the reaction of \chemC_2-\chemC_6 alkenes with OH in the presence of NO, J. Phys. Chem. A, 102, 8903–8908, 1998. Orzechowska, G E. and Paulson, S E.: Photochemical sources of organic acids. 1. Reaction of ozone with isoprene, propene, and 2-butenes under dry and humid conditions using SPME, J. Phys. Chem. A, 109, 5358–5365, 2005. Osborne, D A. and Waddington, D J.: Reactions of oxygenated radicals in the gas phase: 7. Reactions of methylperoxyl radicals and alkenes, J. Chem. Soc. Perk. T., 2, 925–930, 1980. Paulot, F., Crounse, J D., Kjaergaard, H G., Kurten, 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, H. G., 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, 1479–1501, http://dx.doi.org/10.5194/acp-9-1479-2009doi:10.5194/acp-9-1479-2009, 2009b. Peeters, J., Nguyen, T L., and Vereecken, L.: HO<sub>x</sub> radical regeneration in the oxidation of isoprene, Phys. Chem. Chem. Phys., 11, 5935–5939, 2009. Penkett, S A., Burgess, R A., Coe, H., Coll, I., Hov, O., Lindskog, A., Schmidbauer, N., Solberg, S., Roemer, M., Thijsse, T., Beck, J., and Reeves, C E.: Evidence for large average concentrations of the nitrate radical (\chemNO_3) in Western Europe from the HANSA hydrocarbon database, Atmos. Environ., 41, 3465–3478, 2007. Perring, A. E., Wisthaler, A., Graus, M., Wooldridge, P. J., Lockwood, A. L., Mielke, L. H., Shepson, P. B., Hansel, A., and Cohen, R. C.: A product study of the isoprene+NO<sub>3</sub> reaction, Atmos. Chem. Phys., 9, 4945–4956, http://dx.doi.org/10.5194/acp-9-4945-2009doi:10.5194/acp-9-4945-2009, 2009. Platt, U. and Janssen, C.: Observation and role of the free radicals \chemNO_3, ClO, BrO and IO in the troposphere, Faraday Discuss., 100, 175–198, 1995. Platt, U., Perner, D., Schroder, J., Kessler, C., and Toennissen, A.: The diurnal variation of \chemNO_3, J. Geophys. Res.-Oc. Atm., 86, 1965–1970, 1981. Quinn, P K., Bates, T S., Coffman, D., Onasch, T B., Worsnop, D., Baynard, T., de~Gouw, J A., Goldan, P D., Kuster, W C., Williams, E., Roberts, J M., Lerner, B., Stohl, A., Pettersson, A., and Lovejoy, E R.: Impacts of sources and aging on submicrometer aerosol properties in the marine boundary layer across the Gulf of Maine, J. Geophys. Res., 111, D23S36, http://dx.doi.org/10.1029/2006JD007582doi:10.1029/2006JD007582, 2006. Ren, X R., Harder, H., Martinez, M., Lesher, R L., Oliger, A., Shirley, T., Adams, J., Simpas, J B., and Brune, W H.: HO<sub>x</sub> concentrations and OH reactivity observations in New York City during PMTACS-NY2001, Atmos. Environ., 37, 3627–3637, 2003. Roberts, J M., Williams, J., Baumann, K., Buhr, M P., Goldan, P D., Holloway, J., Hubler, G., Kuster, W C., McKeen, S A., Ryerson, T B., Trainer, M., Williams, E J., Fehsenfeld, F C., Bertman, S B., Nouaime, G., Seaver, C., Grodzinsky, G., Rodgers, M., and Young, V L.: Measurements of PAN, PPN, and MPAN made during the 1994 and 1995 Nashville Intensives of the Southern Oxidant Study: Implications for regional ozone production from biogenic hydrocarbons, J. Geophys. Res.-Atmos., 103, 22473–22490, 1998. Rollins, A. W., Kiendler-Scharr, A., Fry, J. L., Brauers, T., Brown, S. S., Dorn, H.-P., Dubé, W. P., Fuchs, H., Mensah, A., Mentel, T. F., Rohrer, F., Tillmann, R., Wegener, R., Wooldridge, P. J., and Cohen, R. C.: Isoprene oxidation by nitrate radical: alkyl nitrate and secondary organic aerosol yields, Atmos. Chem. Phys., 9, 6685–6703, http://dx.doi.org/10.5194/acp-9-6685-2009doi:10.5194/acp-9-6685-2009, 2009. Ruppert, L. and Becker, K. H.: A product study of the OH radical-initiated oidation of isoprene: formation of \chemC_5-unsaturated diols, Atmos. Environ., 34, 1529–1542, 2000. Sakulyanontvittaya, T., Duhl, T., Wiedinmyer, C., Helmig, D., Matsunaga, S., Potosnak, M., Milford, J., and Guenther, A.: Monoterpene and sesquiterpene emission estimates for the United States, Environ. Sci. Technol., 42, 1623–1629, 2008. Sander, S P., Abbatt, J., Barker, J R., Burkholder, J B., Friedl, R R., Golden, D M., Huie, R E., Kolb, C E., Kurylo, M J., Moortgat, G K., Orkin, V L., and Wine, P H.: Chemical kinetics and photochemical data for use in atmospheric studies: Evaluation Number 17, Jet Propulsion Laboratory, 2011. Schichtel, B., Malm, W., Bench, G., Fallon, S., McDade, C., Chow, J., and Watson, J.: Fossil and contemporary fine particulate carbon fractions at 12 rural and urban sites in the United States, J. Geophys. Res., 113, D02311, http://dx.doi.org/10.1029/2007JD008605doi:10.1029/2007JD008605, 2008. Seeley, J V., Meads, R F., Elrod, M J., and Molina, M J.: Temperature and pressure dependence of the rate constant for the HO<sub>2</sub> + NO reaction, J. Phys. Chem., 100, 4026–4031, 1996. Sharkey, T D., Singsaas, E L., Vanderveer, P J., and Geron, C.: Field measurements of isoprene emission from trees in response to temperature and light, Tree Physiol., 16, 649–654, 1996. Simmons, K E. and van Sickle, D.: Addition abstraction competition of acylperoxy radicals reacting with alkenes – cooxidation of cyclohexene and valeraldehyde, J. Am. Chem. Soc., 95, 7759–7763, 1973. Simpson, D., Yttri, K E., Klimont, Z., Kupiainen, K., Caseiro, A., Gelencser, A., Pio, C., Puxbaum, H., and Legrand, M.: Modeling carbonaceous aerosol over Europe: Analysis of the CARBOSOL and EMEP EC/OC campaigns, J. Geophys. Res.-Atmos., 112, D23S14, http://dx.doi.org/10.1029/2006JD008158doi:10.1029/2006JD008158, 2007. Sinha, A., Lovejoy, E R., and Howard, C J.: Kinetic study of the reaction of \chemHO_2 with ozone, J. Chem. Phys., 87, 2122–2128, 1987. Skov, H., Hjorth, J., Lohse, C., Jensen, N R., and Restelli, G.: Products and mechanisms of the reactions of the nitrate radical (\chemNO_3) with isoprene, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene in air, Atmos. Environ. A-Gen., 26, 2771–2783, 1992. Smith, N., Plane, J. M C., Nien, C F., and Solomon, P A.: Nighttime radical chemistry in the San Joaquin Valley, Atmos. Environ., 29, 2887–2897, 1995. Spittler, M., Barnes, I., Bejan, I., Brockmann, K J., Benter, T., and Wirtz, K.: Reactions of \chemNO_3 radicals with limonene and alpha-pinene: Product and SOA formation, Atmos. Environ., 40, S116–S127, 2006. Stark, M S. and Waddington, D J.: Oxidation of propene in the gas-phase, Int. J. Chem. Kinet., 27, 123–151, 1995. Starn, T K., Shepson, P B., Bertman, S B., Riemer, D D., Zika, R G., and Olszyna, K.: Nighttime isoprene chemistry at an urban-impacted forest site, J. Geophys. Res.-Atmos., 103, 22437–22447, 1998. St Clair, J M., McCabe, D C., Crounse, J D., Steiner, U., and Wennberg, P O.: Chemical ionization tandem mass spectrometer for the in situ measurement of methyl hydrogen peroxide, Rev. Sci. Instrum., 81, 094102, http://dx.doi.org/10.1063/1.3480552doi:10.1063/1.3480552, 2010. Steinbacher, M., Dommen, J., Ordonez, C., Reimann, S., Gruebler, F., Staehelin, J., Andreani-Aksoyoglu, S., and Prevot, A. S H.: Volatile organic compounds in the Po Basin. part B: Biogenic VOCs, J. Atmos. Chem., 51, 293–315, 2005. Stroud, C A., Roberts, J M., Williams, E J., Hereid, D., Angevine, W M., Fehsenfeld, F C., Wisthaler, A., Hansel, A., Martinez-Harder, M., Harder, H., Brune, W H., Hoenninger, G., Stutz, J., and White, A B.: Nighttime isoprene trends at an urban forested site during the 1999 Southern Oxidant Study, J. Geophys. Res.-Atmos., 107, 4291, http://dx.doi.org/10.1029/2001JD000959doi:10.1029/2001JD000959, 2002. Su, T. and Chesnavich, W J.: Parameterization of the ion-polar molecule collision rate constant by trajectory calculations, J. Chem. Phys., 76, 5183–5185, 1982. Suh, I., Lei, W F., and Zhang, R Y.: Experimental and theoretical studies of isoprene reaction with \chemNO_3, J. Phys. Chem. A, 105, 6471–6478, 2001. Sullivan, A P., Peltier, R E., Brock, C A., de~Gouw, J A., Holloway, J S., Warneke, C., Wollny, A G., and Weber, R J.: Airborne measurements of carbonaceous aerosol soluble in water over northeastern United States: Method development and an investigation into water-soluble organic carbon sources, J. Geophys. Res.-Atmos., 111, D23S46, http://dx.doi.org/10.1029/2006JD007072doi:10.1029/2006JD007072, 2006. Surratt, J D., Murphy, S M., Kroll, J H., Ng, N L., Hildebrandt, L., Sorooshian, A., Szmigielski, R., Vermeylen, R., Maenhaut, W., Claeys, M., Flagan, R C., and Seinfeld, J H.: Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene, J. Phys. Chem. A, 110, 9665–9690, 2006. Surratt, J D., Chan, A. W H., Eddingsaas, N C., Chan, M N., Loza, C L., Kwan, A J., Hersey, S P., Flagan, R C., Wennberg, P O., and Seinfeld, J H.: Reactive intermediates revealed in secondary organic aerosol formation from isoprene, P. Natl. Acad. Sci. USA, 107, 6640–6645, 2010. 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<sub>x</sub> abundances and HO<sub>x</sub> production rates in the Nashville urban plume, J. Geophys. Res.-Atmos., 107, 4146, http://dx.doi.org/10.1029/2001JD000932doi:10.1029/2001JD000932, 2002. van Sickle, D., Mayo, F R., and Arluck, R M.: Liquid-phase oxidation of cyclopentene, J. Amer. Chem. Soc., 87, 4832–4837, 1965a. van Sickle, D., Mayo, F R., and Arluck, R M.: Liquid-phase oxidations of cyclic alkenes, J. Amer. Chem. Soc., 87, 4824–4832, 1965b. Vaughan, S., Canosa-Mas, C E., Pfrang, C., Shallcross, D E., Watson, L., and Wayne, R P.: Kinetic studies of reactions of the nitrate radical (\chemNO_3) with peroxy radicals (\chemRO_2): an indirect source of OH at night?, Phys. Chem. Chem. Phys., 8, 3749–3760, 2006. Vereecken, L. and Peeters, J.: Decomposition of substituted alkoxy radicals-part I: a generalized structure-activity relationship for reaction barrier heights, Phys. Chem. Chem. Phys., 11, 9062–9074, 2009. Volkamer, R., Jimenez, J L., San~Martini, F., Dzepina, K., Zhang, Q., Salcedo, D., Molina, L T., Worsnop, D R., and Molina, M J.: Secondary organic aerosol formation from anthropogenic air pollution: Rapid and higher than expected, Geophys. Res. Lett., 33, L17811, http://dx.doi.org/10.1029/2006GL026899doi:10.1029/2006GL026899, 2006. von Friedeburg, C., Wagner, T., Geyer, A., Kaiser, N., Vogel, B., Vogel, H., and Platt, U.: Derivation of tropospheric \chemNO_3 profiles using off-axis differential optical absorption spectroscopy measurements during sunrise and comparison with simulations, J. Geophys. Res.-Atmos., 107, 4168, http://dx.doi.org/10.1029/2001JD000481doi:10.1029/2001JD000481, 2002. von Kuhlmann, R., Lawrence, M. G., Pöschl, U., and Crutzen, P. J.: Sensitivities in global scale modeling of isoprene, Atmos. Chem. Phys., 4, 1–17, http://dx.doi.org/10.5194/acp-4-1-2004doi:10.5194/acp-4-1-2004, 2004. Wallington, T., Nielsen, O., and Sehested, J.: Reactions of organic peroxy radicals in the gas phase, in: Peroxyl Radicals, edited by: Alfassi, Z., 113–172, Wiley, 1997. Warneke, C., de~Gouw, J A., Goldan, P D., Kuster, W C., Williams, E J., Lerner, B M., Jakoubek, R., Brown, S S., Stark, H., Aldener, M., Ravishankara, A R., Roberts, J M., Marchewka, M., Bertman, S., Sueper, D T., McKeen, S A., Meagher, J F., and Fehsenfeld, F C.: Comparison of daytime and nighttime oxidation of biogenic and anthropogenic VOCs along the New England coast in summer during New England Air Quality Study 2002, J. Geophys. Res.-Atmos., 109, D10309, http://dx.doi.org/10.1029/2003JD004424doi:10.1029/2003JD004424, 2004. Wayne, R P., Barnes, I., Biggs, P., Burrows, J P., Canosamas, C E., Hjorth, J., Lebras, G., Moortgat, G K., Perner, D., Poulet, G., Restelli, G., and Sidebottom, H.: The nitrate radical – physics, chemistry, and the atmosphere, Atmos. Environ. A-Gen., 25, 1–203, 1991. Weber, R J., Sullivan, A P., Peltier, R E., Russell, A., Yan, B., Zheng, M., de~Gouw, J., Warneke, C., Brock, C., Holloway, J S., Atlas, E L., and Edgerton, E.: A study of secondary organic aerosol formation in the anthropogenic-influenced southeastern United States, J. Geophys. Res.-Atmos., 112, D13302, http://dx.doi.org/10.1029/2007JD008408doi:10.1029/2007JD008408, 2007. Williams, J., Roberts, J M., Fehsenfeld, F C., Bertman, S B., Buhr, M P., Goldan, P D., 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. Zhang, D. and Zhang, R Y.: Unimolecular decomposition of nitrooxyalkyl radicals from \chemNO_3-isoprene reaction, J. Chem. Phys., 116, 9721–9728, 2002. Ziemann, P.: Evidence for low-volatility diacyl peroxides as a nucleating agent and major component of aerosol formed from reactions of O<sub>3</sub> with cyclohexene and homologous compounds, J. Phys. Chem. A, 106, 4390–4402, 2002.