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Kinetics of the Reactions of Isoprene-Derived Epoxides in Model Tropospheric Aerosol Solutions [1] Emily C. Minerath, Madeline P. Schultz and Matthew J. Elrod Environ. Sci. Technol., Article ASAP DOI: 10.1021/es902304p Polyols and organic sulfates have recently been identified in the secondary organic aerosol (SOA) formed in the photooxidation of isoprene in both the laboratory and under ambient atmospheric conditions. Nuclear magnetic resonance methods were used to monitor the bulk reaction kinetics of acid-catalyzed hydrolysis reactions for isoprene- and 1,3-butadiene-derived epoxides in order to determine the rates for such reactions in aerosols under the previously studied laboratory conditions and under ambient atmospheric conditions. The measured rate constants were found to vary over 7 orders of magnitude. For the fast case of the hydrolysis of 1,2-epoxyisoprene, the lifetime at neutral pH was found to be only 3 min. On the other hand, for the relatively slow reaction of 1,2-epoxy-3,4-hydroxybutane, the lifetime at the most acidic conditions commonly observed in tropospheric aerosols (pH 1.5) was found to be 7.7 h, a value that is still less than the several day lifetime of tropospheric aerosols. Therefore, the present results suggest that, despite a wide range in reactivities, several possible reactions of isoprene-derived epoxides should be kinetically efficient on atmospheric SOA. The reactions were also studied with the elevated sulfate concentrations that are often characteristic of tropospheric aerosols, and sulfate products were identified for all species except 1,2-epoxyisoprene. Other nucleophiles that may be present in aerosols (nitrate, chloride, bromide, and iodide) were also investigated, and it was found that nitrate and sulfate have similar nucleophilic strength, while the halides are much stronger nucleophiles in their reactions with epoxides. Therefore, aerosols which contain significant concentrations of these species may be expected to readily form species similar to those already identified for the reactions of epoxides with sulfate.

Atmospheric oxidation capacity sustained by a tropical forest Lelieveld J, Butler TM, Crowley JN, et al. NATURE Volume: 452 Issue: 7188 Pages: 737-740 Published: APR 10 2008 Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere(1-3), which are removed by oxidation reactions and deposition of reaction products(4-6). The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone(4). Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity(5-10). Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides(5-10) (NOx = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NOx air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40-80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.

Isoprene photooxidation: new insights into the production of acids and organic nitrates Paulot F, Crounse JD, Kjaergaard HG, et al. ATMOSPHERIC CHEMISTRY AND PHYSICS Volume: 9 Issue: 4 Pages: 1479-1501 Published: 2009 We describe a nearly explicit chemical mechanism for isoprene photooxidation guided by chamber studies that include time-resolved observation of an extensive suite of volatile compounds. We provide new constraints on the chemistry of the poorly-understood isoprene delta-hydroxy channels, which account for more than one third of the total isoprene carbon flux and a larger fraction of the nitrate yields. We show that the cis branch dominates the chemistry of the delta-hydroxy channel with less than 5% of the carbon following the trans branch. The modelled yield of isoprene nitrates is 12 +/- 3% with a large difference between the delta and beta branches. The oxidation of these nitrates releases about 50% of the NOx. Methacrolein nitrates (modelled yield similar or equal to 15 +/- 3% from methacrolein) and methylvinylketone nitrates (modelled yield similar or equal to 11 +/- 3% yield from methylvinylketone) are also observed. Propanone nitrate, produced with a yield of 1% from isoprene, appears to be the longest-lived nitrate formed in the total oxidation of isoprene. We find a large molar yield of formic acid and suggest a novel mechanism leading to its formation from the organic nitrates. Finally, the most important features of this mechanism are summarized in a condensed scheme appropriate for use in global chemical transport models.

Unexpected Epoxide Formation in the Gas-Phase Photooxidation of Isoprene Paulot F, Crounse JD, Kjaergaard HG, et al. SCIENCE Volume: 325 Issue: 5941 Pages: 730-733 Published: AUG 7 2009 Emissions of nonmethane hydrocarbon compounds to the atmosphere from the biosphere exceed those from anthropogenic activity. Isoprene, a five-carbon diene, contributes more than 40% of these emissions. Once emitted to the atmosphere, isoprene is rapidly oxidized by the hydroxyl radical OH. We report here that under pristine conditions isoprene is oxidized primarily to hydroxyhydroperoxides. Further oxidation of these hydroxyhydroperoxides by OH leads efficiently to the formation of dihydroxyepoxides and OH reformation. Global simulations show an enormous flux-nearly 100 teragrams of carbon per year-of these epoxides to the atmosphere. The discovery of these highly soluble epoxides provides a missing link tying the gas-phase degradation of isoprene to the observed formation of organic aerosols.

HOx radical regeneration in the oxidation of isoprene Peeters J, Nguyen TL, Vereecken L PHYSICAL CHEMISTRY CHEMICAL PHYSICS Volume: 11 Issue: 28 Pages: 5935-5939 Published: 2009 We propose, and quantify from first principles, two novel HOx-regenerating unimolecular reactions in isoprene oxidation, which are estimated to yield in pristine tropical forest conditions about 0.7 HO2 and 0.03 OH radicals per isoprene oxidized; it is further argued that the photolabile coproduct of HO2 can be a major source of OH, with a yield of the order of 1. The newly proposed chemistry could provide a rationalization for the unexpectedly high OH concentrations often observed in forested environments, such as over the Amazon forest in the recent Gabriel campaign.

Thermodynamics of the hydroxyl radical addition to isoprene Allodi MA, Kirschner KN, Shields GC Source: JOURNAL OF PHYSICAL CHEMISTRY A Volume: 112 Issue: 30 Pages: 7064-7071 Published: JUL 31 2008 Oxidation of isoprene by the hydroxyl radical leads to tropospheric ozone formation. Consequently, a more complete understanding of this reaction could lead to better models of regional air quality, a better understanding of aerosol formation, and a better understanding of reaction kinetics and dynamics. The most common first step in the oxidation of isoprene is the formation of an adduct, with the hydroxyl radical adding to one of four unsaturated carbon atoms in isoprene. In this paper, we discuss how the initial conformations of isoprene, s-trans and s-gauche, influences the pathways to adduct formation. We explore the formation of pre-reactive complexes at low and high temperatures, which are often invoked to explain the negative temperature dependence of this reaction's kinetics. We show that at higher temperatures the free energy surface indicates that a pre-reactive complex is unlikely, while at low temperatures the complex exists on two reaction pathways. The theoretical results show that at low temperatures all eight pathways possess negative reaction barriers, and reaction energies that range from -36.7 to -23.0 kcal.mol(-1). At temperatures in the lower atmosphere, all eight pathways possess positive reaction barriers that range from 3.8 to 6.0 kcal.mol(-1) and reaction energies that range from -28.8 to -14.4 kcal.mol(-1).

An isoprene mechanism intercomparison A.T. Archibald, M.E. Jenkin, D.E. Shallcross Atmospheric Environment (2009), doi: 10.1016/j.atmosenv.2009.09.016 The chemical mechanisms describing the photo-oxidation of isoprene in current Chemistry Transport Models CTMs) have been intercompared in a series of box model experiments. The mechanisms ranged in size and complexity from ~ 600 reactions to ~ 25 reactions. The box model experiments covered two isoprene emission strengths over a broad range of NO emissions to assess the performances of the mechanisms over the spectrum of atmospherically relevant conditions. There was some variability in the simulated oxidation rates of isoprene and formation rates of ozone. The variability in performance is a consequence of the details of the 18 underlying chemistry as represented in the mechanisms, and of the different assumptions and approximations made in mechanism reduction. These differences are illustrated and discussed for a series of species involved in the degradation of isoprene and the ozone formation mechanism, namely: HOx radicals; organic peroxy radicals (RO2); hydroperoxides; oxidized organic nitrogen compounds; and major carbonyl products. The results also confirm that all the considered isoprene mechanisms are unable to generate/recycle HOx at the rates needed to match recently reported observations at locations characterized by low levels of NOx.

Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol Brown SS, Degouw JA, Warneke C, et al. ATMOSPHERIC CHEMISTRY AND PHYSICS Volume: 9 Issue: 9 Pages: 3027-3042 Published: 2009 Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemically during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO3 in regionally polluted areas with large NOx levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a region with large emissions of both isoprene and NOx. Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO3 and were the most important factor determining the lifetime of this radical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO3. A determination of the mass of isoprene oxidized in darkness showed it to be a large fraction (>20%) of emitted isoprene. Organic nitrates produced from the NO3+isoprene reaction, though not directly measured, were estimated to account for 2-9% of total reactive nitrogen. The mass of isoprene oxidized by NO3 was comparable to and correlated with the organic aerosol loading for flights with relatively low organic aerosol background. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1 17%, and isoprene SOA mass derived from NO3 was calculated to exceed that due to OH by approximately 50%.

Improved simulation of isoprene oxidation chemistry with the ECHAM5/MESSy chemistry-climate model: lessons from the GABRIEL airborne field campaign Butler TM, Taraborrelli D, Fischer CBH, et al. ATMOSPHERIC CHEMISTRY AND PHYSICS Volume: 8 Issue: 16 Pages: 4529-4546 Published: 2008 The GABRIEL airborne field measurement campaign, conducted over the Guyanas in October 2005, produced measurements of hydroxyl radical (OH) concentration which are significantly higher than can be simulated using current generation models of atmospheric chemistry. Based on the hypothesis that this 'missing OH' is due to an as-yet undiscovered mechanism for recycling OH during the oxidation chain of isoprene, we determine that an OH recycling of about 40-50% (compared with 5-10% in current generation isoprene oxidation mechanisms) is necessary in order for our modelled OH to approach the lower error bounds of the OH observed during GABRIEL. Such a large amount of OH in our model leads to unrealistically low mixing ratios of isoprene. In order for our modelled isoprene mixing ratios to match those observed during the campaign, we also require that the effective rate constant for the reaction of isoprene with OH be reduced by about 50% compared with the lower bound of the range recommended by IUPAC. We show that a reasonable explanation for this lower effective rate constant could be the segregation of isoprene and OH in the mixed layer. Our modelling results are consistent with a global, annual isoprene source of about 500 Tg(C) yr(-1), allowing experimentally derived and established isoprene flux rates to be reconciled with global models.

Direct detection of OH formation in the reactions of HO2 with CH3C(O)O-2 and other substituted peroxy radicals Dillon TJ, Crowley JN ATMOSPHERIC CHEMISTRY AND PHYSICS Volume: 8 Issue: 16 Pages: 4877-4889 Published: 2008 This work details the first direct observation of OH as a product from (R1): HO2+ CH3C(O)O-2 -> (products), which has generally been considered an atmospheric radical termination process. The technique of pulsed laser photolysis radical generation, coupled to calibrated laser induced fluorescence detection was used to measure an OH product yield for (R1) of alpha(1) (298 K)=(0.5 +/- 0.2). This study of (R1) included the measurement of a rate coefficient k(1)(298 K) = ( 1.4 +/- 0.5) x 10(-11) cm(3) molecule(-1) s(-1), substantially reducing the uncertainties in modelling this important atmospheric reaction. OH was also detected as a product from the reactions of HO2 with three other carbonyl-containing peroxy radicals, albeit at smaller yield, e. g. (R2): HO2+ CH3C(O)CH2O2 -> (products), alpha(2) approximate to 0.15. By contrast, OH was not observed (alpha < 0.06) as a major product from reactions where carbonyl functionality was absent, e. g. HO2+ HOCH2CH2O2 (R8), and HO2+ CH3CH(OH)CH2O2 (R9).

Comparison of tropospheric gas-phase chemistry schemes for use within global models Emmerson KM, Evans MJ Source: ATMOSPHERIC CHEMISTRY AND PHYSICS Volume: 9 Issue: 5 Pages: 1831-1845 Published: 2009 Methane and ozone are two important climate gases with significant tropospheric chemistry. Within chemistry-climate and transport models this chemistry is simplified for computational expediency. We compare the state of the art Master Chemical Mechanism (MCM) with six tropospheric chemistry schemes (CRI-reduced, GEOS-CHEM and a GEOS-CHEM adduct, MOZART-2, TOMCAT and CBM-IV) that could be used within composition transport models. We test the schemes within a box model framework under conditions derived from a composition transport model and from field observations from a regional scale pollution event. We find that CRI-reduced provides much skill in simulating the full chemistry, yet with greatly reduced complexity. We find significant variations between the other chemical schemes, and reach the following conclusions. 1) The inclusion of a gas phase N2O5+H2O reaction in one scheme and not others is a large source of uncertainty in the inorganic chemistry. 2) There are significant variations in the calculated concentration of PAN between the schemes, which will affect the long range transport of reactive nitrogen in global models. 3) The representation of isoprene chemistry differs hugely between the schemes, leading to significant uncertainties on the impact of isoprene on composition. 4) Differences are found in NO3 concentrations in the night-time chemistry. Resolving these four issues through further investigative laboratory studies will reduce the uncertainties within the chemical schemes of global tropospheric models.

A two transition state model for radical-molecule reactions: Applications to isomeric branching in the OH-Isoprene reaction Greenwald EE, North SW, Georgievskii Y, et al. Source: JOURNAL OF PHYSICAL CHEMISTRY A Volume: 111 Issue: 25 Pages: 5582-5592 Published: JUN 28 2007 A two transition state model is applied to the prediction of the isomeric branching in the addition of hydroxyl radical to isoprene. The outer transition state is treated with phase space theory fitted to long-range transition state theory calculations on an electrostatic potential energy surface. High-level quantum chemical estimates are applied to the treatment of the inner transition state. A one-dimensional master equation based on an analytic reduction from two-dimensions for a particular statistical assumption about the rotational part of the energy transfer kernel is employed in the calculation of the pressure dependence of the addition process. We find that an accurate treatment of the two separate transition state regions, at the energy and angular momentum resolved level, is essential to the prediction of the temperature dependence of the addition rate. The transition from a dominant outer transition state to a dominant inner transition state is shown to occur at about 275 K, with significant effects from both transition states over the 30-500 K temperature range. Modest adjustments in the ab initio predicted inner saddle point energies yield predictions that are in quantitative agreement with the available high-pressure limit experimental observations and qualitative agreement with those in the falloff regime. The theoretically predicted capture rate is reproduced to within 10% by the expression [1.71 x 10(-10)(T/298)(-2.58) exp(-608.6/RT) + 5.47 x 10(-11)(T/298)(-1.78) exp(-97.3/RT); with R = 1.987 and T in K] cm(3) molecule(-1) s(-1) over the 30-500 K range. A 300 K branching ratio of 0.67:0.02:0.02:0.29 was determined for formation of the four possible OH-isoprene adduct isomers 1, 2, 3, and < BO > 4 </BO >, respectively, and was found to be relatively insensitive to temperature. An Arrhenius activation energy of -0.77 kcal/mol was determined for the high-pressure addition rate constants around 300 K. Rate Constants for the Gas-Phase beta-Myrcene plus OH and Isoprene plus OH Reactions as a Function of Temperature Hites RA, Turner AM INTERNATIONAL JOURNAL OF CHEMICAL KINETICS Volume: 41 Issue: 6 Pages: 407-413 Published: JUN 2009 Rate constants for the gas-phase reactions of the hydroxyl radical with the biogenic hydrocarbons, beta-myrcene and isoprene, were measured using the relative rate technique over the temperature range 313-413 K and at similar to 1 atm total pressure. OH was produced by the photolysis of H2O2, and helium was the diluent gas. The reactants were detected by online mass spectrometry, which resulted in high time resolution allowing for large amounts of data to be collected and used in the determination of the Arrhenius parameters. Many experiments were performed over the temperature range of interest, leading to more accurate parameters than previous investigations. The following Arrhenius expression has been determined for these reactions (in units of cm(3) molecule(-1) s(-1)): for isoprene k = (3.14(-0.19)(+0.29)) x 10(-11) exp[(338 +/- 19)/T] and for beta-myrcene k = (9.19(-1.89)(+2.38)) exp[(1071 +/- 82)/T]. The Arrhenius plot for the isoprene + OH reaction indicates curvature in this relationship and is given by k = (3.47 +/- 0.14) x 10(-17) T-2 exp[(1036 +/- 14)/T]. Our measured rate constant for the beta-myrcene + OH reaction at 298 K is higher, but not significantly, than current literature values. This is the first report of beta-myrcene's rate constant with OH as a function of temperature.

Amplified Trace Gas Removal in the Troposphere Hofzumahaus A, Rohrer F, Lu KD, et al. SCIENCE Volume: 324 Issue: 5935 Pages: 1702-1704 Published: JUN 26 2009 The degradation of trace gases and pollutants in the troposphere is dominated by their reaction with hydroxyl radicals (OH). The importance of OH rests on its high reactivity, its ubiquitous photochemical production in the sunlit atmosphere, and most importantly on its regeneration in the oxidation chain of the trace gases. In the current understanding, the recycling of OH proceeds through HO2 reacting with NO, thereby forming ozone. A recent field campaign in the Pearl River Delta, China, quantified tropospheric OH and HO2 concentrations and turnover rates by direct measurements. We report that concentrations of OH were three to five times greater than expected, and we propose the existence of a pathway for the regeneration of OH independent of NO, which amplifies the degradation of pollutants without producing ozone.

C-12/C-13 kinetic isotope effects of the gas-phase reactions of isoprene, methacrolein, and methyl vinyl ketone with OH radicals Iannone R, Koppmann R, Rudolph J Source: ATMOSPHERIC ENVIRONMENT Volume: 43 Issue: 19 Pages: 3103-3110 Published: JUN 2009 The stable-carbon kinetic isotope effects (KIEs) for the gas-phase reactions of isoprene, methacrolein (MACR), and methyl vinyl ketone (MVK) with OH radicals were studied in a 25 L reaction chamber at (298 +/- 2) K and ambient pressure. The time dependence of both the stable-carbon isotope ratios and the concentrations was determined using a gas-chromatography combustion isotope ratio mass spectrometry (GCC-IRMS) system. The volatile organic compounds (VOCs) used in the KIE experiments had natural-abundance isotopic composition thus KIE data obtained from these experiments can be directly applied to atmospheric studies of isoprene chemistry. All C-12/(13)/C KIE values are reported as epsilon values, where epsilon = (KIE - 1) x 1000 parts per thousand, and KIE = k(12)/k(13). The following average stable-carbon KIEs were obtained: (6.56 +/- 0.12)parts per thousand (isoprene), (6.47 +/- 0.27)parts per thousand (MACR), and (7.58 +/- 0.47)parts per thousand (MVK). The measured KIEs all agree within uncertainty to an inverse molecular mass (MM) dependence of (OH)epsilon(parts per thousand) = (487 +/- 18)MM-1, which was derived from two previous studies [J. Geophys. Res. 2000, 105, 29329-29346: J. Phys. Chem. A 2004, 108, 11537-11544]. Upon adding the isoprene, MACR, and MVK (OH)epsilon values from this study, the inverse MM dependence changes only marginally to (OH)epsilon(parts per thousand) = (485 +/- 14)MM-1. The addition of these isoprene (OH)epsilon values to a recently measured set of (O3)epsilon values in an analogous study [Atmos. Environ. 2008, 42, 8728-8737] allows for estimates of the average change in the C-12/C-13 ratio due to processing in the troposphere.

Global chemical transport model study of ozone response to changes in chemical kinetics and biogenic volatile organic compounds emissions due to increasing temperatures: Sensitivities to isoprene nitrate chemistry and grid resolution Ito A, Sillman S, Penner JE Source: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Volume: 114 Article Number: D09301 Published: MAY 1 2009 Global modeling studies show a wide variability in the response of the O-3 budget to climate change as projected by applying Intergovernmental Panel on Climate Change scenarios in climate models. We employ sensitivity studies to elucidate the major uncertainties in the response of tropospheric O-3 to perturbations in biogenic volatile organic compounds (BVOC) emissions and reaction rate coefficients due to changes in temperature. The change in global O-3 burden due to an increase in BVOC emissions associated with a +5 K depends critically on the assumed treatment for the fraction of NOx recycled (0-100%) from isoprene nitrate (+9 to +34 Tg), in contrast to the chemical reaction rate coefficients response (-8 to -9 Tg). The model O-3 burden shows sensitivity (40 Tg) to the NOx recycling efficiencies (0-100%) similar to the burden's sensitivity to the grid resolution (4 degrees x 5 degrees -1 degrees x 1 degrees). The correlation of O-3 with total alkyl nitrates (Sigma ANs) in the surface air at a California forest site shows sensitivity to the NOx recycling (40-100%) similar to the correlation's sensitivity to the horizontal resolution (4 degrees x 5 degrees -1 degrees x 1 degrees). The results of the sensitivity simulations imply that the slope of O-3 to Sigma ANs might be used to constrain the yield of isoprene nitrate and NOx recycling fraction, but better agreement could be achieved by using a higher-resolution model with even higher NOx recycling from isoprene nitrate. Our results suggest that the reduction of NOx recycling from isoprene nitrate be set apart from that due to the effect of the grid resolution in the chemical transport model.

New particle formation in forests inhibited by isoprene emissions Astrid Kiendler-Scharr, Ju¨rgen Wildt, Miikka Dal Maso, Thorsten Hohaus, Einhard Kleist, Thomas F. Mentel, Ralf Tillmann, Ricarda Uerlings, Uli Schurr & Andreas Wahner NATURE Vol: 461 Pages: 381-384 Published: September 2009 It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate. The most abundant VOCs emitted by terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes. New particle formation in boreal regions is related to monoterpene emissions and causes an estimated negative radiative forcing of about 20.2 to 20.9Wm22. The annual variation in aerosol growth rates during particle nucleation events correlates with the seasonality of monoterpene emissions of the local vegetation, with a maximum during summer. The frequency of nucleation events peaks, however, in spring and autumn. Here we present evidence from simulation experiments conducted in a plant chamber that isoprene can significantly inhibit new particle formation. The process leading to the observed decrease in particle number concentration is linked to the high reactivity of isoprene with the hydroxyl radical (OH). The suppression is stronger with higher concentrations of isoprene, but with little dependence on the specific VOC mixture emitted by trees. A parameterization of the observed suppression factor as a function of isoprene concentration suggests that the number of new particles produced depends on the OH concentration and VOCs involved in the production of new particles undergo three to four steps of oxidation by OH. Our measurements simulate conditions that are typical for forested regions and may explain the observed seasonality in the frequency of aerosol nucleation events, with a lower number of nucleation events during summer compared to autumn and spring. Biogenic emissions of isoprene are controlled by temperature and light, and if the relative isoprene abundance of biogenic VOC emissions increases in response to climate change or land use change, the new particle formation potential may decrease, thus damping the aerosol negative radiative forcing effect.

Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: comparison of measurements with the box model MECCA 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. Atmos. Chem. Phys. Discuss., 8, 15239-15289, 2008. As a major source region of the hydroxyl radical OH, the Tropics largely control the oxidation capacity of the atmosphere on a global scale. However, emissions of hydrocarbons from the tropical rainforest that react rapidly with OH can potentially deplete the amount of OH and thereby reduce the oxidation capacity. The airborne GABRIEL field campaign in equatorial South America (Suriname) in October 2005 investigated the influence of the tropical rainforest on the HOx budget (HOx=OH+HO2). The first observations of OH and HO2 over a tropical rainforest are compared to steady state concentrations calculated with the atmospheric chemistry box model MECCA. The important precursors and sinks for HOx chemistry, measured during the campaign, are used as constraining parameters for the simulation of OH and HO2. Significant underestimations of HOx are found by the model over land during the afternoon, with mean ratios of observation to model of 12.2±3.5 and 4.1±1.4 for OH and HO2, respectively. The discrepancy between measurements and simulation results is correlated to the abundance of isoprene. While for low isoprene mixing ratios (above ocean or at altitudes >3 km), observation and simulation agree fairly well, for mixing ratios >200 pptV (<3 km over the rainforest) the model tends to underestimate the HOx observations as a function of isoprene. Box model simulations have been performed with the condensed chemical mechanism of MECCA and with the detailed isoprene reaction scheme of MCM, resulting in similar results for HOx concentrations. Simulations with constrained HO2 concentrations show that the conversion from HO2 to OH in the model is too low. However, by neglecting the isoprene chemistry in the model, observations and simulations agree much better. An OH source similar to the strength of the OH sink via isoprene chemistry is needed in the model to resolve the discrepancy. A possible explanation is that the oxidation of isoprene by OH not only dominates the removal of OH but also produces it in a similar amount. Several additional reactions which directly produce OH have been implemented into the box model, suggesting that upper limits in producing OH are still not able to reproduce the observations (improvement by factors of ≈2.4 and ≈2 for OH and HO2, respectively). We determine that OH has to be recycled to 94% instead of the simulated 38% to match the observations, which is most likely to happen in the isoprene degradation process, otherwise additional sources are required. Measurements of OH and HO2 yields from the gas phase ozonolysis of isoprene T. L. Malkin, A. Goddard, D. E. Heard, and P. W. Seakins Atmos. Chem. Phys. Discuss., 9, 17579-17631, 2009 The reactions of ozone with alkenes are an important source of hydroxyl (OH) radicals; however, quantification of their importance is hindered by uncertainties in the absolute OH yield. Hydroxyl radical yields for the gas-phase ozonolysis of isoprene are determined in this paper by four different methods: (1) The use of cyclohexane as an OH scavenger, and the production of cyclohexanone, (2) The use of 1,3,5-trimethylbenzene as an OH tracer, and the diminution in its concentration, (3) A kinetic method in which the OH yield was obtained by performing a series of pseudo-first-order experiments in the presence or absence of an OH scavenger (cyclohexane), (4) The OH and HO2 yields were determined by fitting the temporal OH and HO2 profiles following direct detection of absolute OH and HO2 concentrations by laser induced fluorescence at low pressure (Fluorescence Assay by Gas Expansion-FAGE). The following OH yields for the ozonolysis of isoprene were obtained, relative to alkene consumed, for each method: (1) Scavenger (0.25 ± 0.04), (2) Tracer (0.25 ± 0.03), (3) Kinetic study (0.27 ± 0.02), and (4) Direct observation (0.26 ± 0.02), the error being one standard deviation. An averaged OH yield of 0.26 ± 0.02 is recommended at room temperature and atmospheric pressure and this result is compared with recent literature determinations. The HO2 yield was directly determined for the first time using FAGE to be 0.26 ± 0.03.

A product study of the isoprene+NO3 reaction Perring AE, Wisthaler A, Graus M, et al. Source: ATMOSPHERIC CHEMISTRY AND PHYSICS Volume: 9 Issue: 14 Pages: 4945-4956 Published: 2009 Oxidation of isoprene through reaction with NO3 radicals is a significant sink for isoprene that persists after dark. The main products of the reaction are multifunctional nitrates. These nitrates constitute a significant NOx sink in the nocturnal boundary layer and they likely play an important role in formation of secondary organic aerosol. Products of the isoprene+NO3 reaction will, in many locations, be abundant enough to affect nighttime radical chemistry and to persist into daytime where they may represent a source of NOx. Product formation in the isoprene + NO3 reaction was studied in a smog chamber at Purdue University. Isoprene nitrates and other hydrocarbon products were observed using Proton Transfer Reaction-Mass Spectrometry (PTR-MS) and reactive nitrogen products were observed using Thermal Dissociation-Laser Induced Fluorescence (TD-LIF). The organic nitrate yield is found to be 65 +/- 12% of which the majority was nitrooxy carbonyls and the combined yield of methacrolein and methyl vinyl ketone (MACR+MVK) is found to be similar to 10%. PTR-MS measurements of nitrooxy carbonyls and TD-LIF measurements of total organic nitrates agreed well. The PTR-MS also observed a series of minor oxidation products which were tentatively identified and their yields quantified These other oxidation products are used as additional constraints on the reaction mechanism.

OH-initiated degradation of several hydrocarbons in the atmosphere simulation chamber SAPHIR Poppe D, Brauers T, Dorn HP, et al. Source: JOURNAL OF ATMOSPHERIC CHEMISTRY Volume: 57 Issue: 3 Pages: 203-214 Published: JUL 2007 Degradation of isoprene, m-xylene, n-octane, propene, and methacrolein by hydroxyl radicals has been studied in the simulation chamber SAPHIR under burden of trace gases as they are typical for the moderately polluted planetary boundary layer. Measured time series of the hydrocarbon mixing ratios and the OH concentrations were used to determine the rate constants. The hydrocarbons were measured with gas chromatography and proton transfer reaction mass spectrometry. OH was measured with the Julich DOAS (differential optical absorption spectroscopy) instrument. In all cases except methacrolein good agreement was found with the reference rate constants taken from theMaster Chemical Mechanism ( MCM3.1). The data for methacrolein are consistent with the results of Karl et al. ( J. Atmos. Chem 55, 2006, doi:10.1007/s10874-006-9034-x) who reported a 12% smaller value. The degradation of hydrocarbons provides an independent method to analyse precision and accuracy of the OH measurements. A precision of better than 4% over a period of nearly 4 months was found. The accuracy is within the limitations given by the light absorption cross section of OH. Both results are consistent with earlier results by Hausmann et al.

Mainz Isoprene Mechanism 2 (MIM2): an isoprene oxidation mechanism for regional and global atmospheric modelling Taraborrelli D, Lawrence MG, Butler TM, et al. Source: ATMOSPHERIC CHEMISTRY AND PHYSICS Volume: 9 Issue: 8 Pages: 2751-2777 Published: 2009 We present an oxidation mechanism of intermediate size for isoprene (2-methyl-1,3-butadiene) suitable for simulations in regional and global atmospheric chemistry models, which we call MIM2. It is a reduction of the corresponding detailed mechanism in the Master Chemical Mechanism (MCM v3.1) and intended as the second version of the well-established Mainz Isoprene Mechanism (MIM). Our aim is to improve the representation of tropospheric chemistry in regional and global models under all NOx regimes. We evaluate MIM2 and re-evaluate MIM through comparisons with MCM v3.1. We find that MIM and MIM2 compute similar O-3, OH and isoprene mixing ratios. Unlike MIM, MIM2 produces small relative biases for NOx and organic nitrogen-containing species due to a good representation of the alkyl and peroxy acyl nitrates (RONO2 and RC(O)OONO2). Moreover, MIM2 computes only small relative biases with respect to hydrogen peroxide (H2O2), methyl peroxide (CH3OOH), methanol (CH3OH), formaldehyde (HCHO), peroxy acetyl nitrate (PAN), and formic and acetic acids (HCOOH and CH3C(O)OH), being always below approximate to 6% in all NOx scenarios studied. Most of the isoprene oxidation products are represented explicitly, including methyl vinyl ketone (MVK), methacrolein (MACR), hydroxyacetone and methyl glyoxal. MIM2 is mass-conserving with respect to carbon, including CO2 as well. Therefore, it is suitable for studies assessing carbon monoxide (CO) from biogenic sources, as well as for studies focused on the carbon cycle. Compared to MIM, MIM2 considers new species like acetaldehyde (CH3CHO), propene (CH2=CHCH3) and glyoxal (CHOCHO) with global chemical production rates for the year 2005 of 7.3, 9.5 and 33.8 Tg/yr, respectively. Our new mechanism is expected to substantially improve the results of atmospheric chemistry models by representing many more intermediates, that are transported and deposited, which allows us to test model results with many more new measurements. MIM2 allows regional and global models to easily incorporate new experimental results on the chemistry of organic species.

Thwarting the seeds of clouds Ziemann, PJ NATURE Vol: 461 Pages: 353-354 Published: September 2009 Atmospheric oxidation of hydrocarbons emitted from plants leads to the formation of aerosol particles that affect cloud properties. Contrary to what was thought, this process might add to global warming.

Unimolecular β-Hydroxyperoxy Radical Decomposition with OH Recycling in the Photochemical Oxidation of Isoprene Gabriel da Silva*, Claire Graham and Zhe-Fei Wang Environmental Science & Technology Publication Date (Web): November 30, 2009 A novel process in the photochemical oxidation of isoprene that recycles hydroxyl (OH) radicals has been identified using first-principles computational chemistry. Isoprene is the dominant biogenic volatile organic compound (VOC), and its oxidation controls chemistry in the forest boundary layer and is also thought to contribute to cloud formation in marine environments. The mechanism described here involves rapid unimolecular decomposition of the two major peroxy radicals (β-hydroxyperoxy radicals) produced by OH-initiated isoprene oxidation. Peroxy radicals are well-known as key intermediates in VOC oxidation, but up to now were only thought to be destroyed in bimolecular reactions. The process described here leads to OH recycling with up to around 60% efficiency in environments with low levels of peroxy radicals and NOx. In forested environments reaction of the β-hydroxyperoxy radicals with HO2 is expected to dominate, with a small contribution from the mechanism described here. Peroxy radical decomposition will be more important in the unpolluted marine boundary layer, where lower levels of NO and HO2 are encountered.

Developments in Laboratory Studies of Gas-Phase Reactions for Atmospheric Chemistry with Applications to Isoprene Oxidation and Carbonyl Chemistry Seakins, PW; Blitz, MA ANNUAL REVIEW OF PHYSICAL CHEMISTRY, VOL 62, page 351-373 DOI: 10.1146/annurev-physchem-032210-102538 Publication year: 2011 Laboratory studies of gas-phase chemical processes are a key tool in understanding the chemistry of our atmosphere and hence tackling issues such as climate change and air quality. Laboratory techniques have improved considerably with greater emphasis on product detection, allowing the measurement of site-specific rate coefficients. Radical chemistry lies at the heart of atmospheric chemistry. In this review we consider issues around radical generation and recycling from the oxidation of isoprene and from the chemical reactions and photolysis of carbonyl species. Isoprene is the most globally significant hydrocarbon, but uncertainties exist about its oxidation in unpolluted environments. Recent experiments and calculations that cast light on radical generation are reviewed. Carbonyl compounds are the dominant first-generation products from hydrocarbon oxidation. Chemical oxidation can recycle radicals, or photolysis can be a net radical source. Studies have demonstrated that high-resolution and temperature-dependent studies are important for some significant species.

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