HOx radical regeneration in the oxidation of isoprene

Science advances, 2018

Nitrogen oxides (NO ) emitted from human activities are believed to regulate the atmospheric oxidation capacity of the troposphere. However, observational evidence is limited for the low-to-median NO concentrations prevalent outside of polluted regions. Directly measuring oxidation capacity, represented primarily by hydroxyl radicals (OH), is challenging, and the span in NO concentrations at a single observation site is often not wide. Concentrations of isoprene and its photo-oxidation products were used to infer the equivalent noontime OH concentrations. The fetch at an observation site in central Amazonia experienced varied contributions from background regional air, urban pollution, and biomass burning. The afternoon concentrations of reactive nitrogen oxides (NO ), indicative of NO exposure during the preceding few hours, spanned from 0.3 to 3.5 parts per billion. Accompanying the increase of NO concentration, the inferred equivalent noontime OH concentrations increased by at le...

Atmospheric Environment, 2012

The hydroxyl radical (OH) is the principal oxidizing agent in Earth's troposphere. Measurements of the total OH reactivity, defined as the reciprocal of the lifetime of OH, have been employed in field studies to estimate the entire amount of reactive trace gases in the atmosphere. Here, the OH reactivity technique has been employed in laboratory studies of the OH initiated atmospheric oxidation of the important biogenic volatile organic compound, isoprene. A significant discrepancy was observed between measured and calculated OH reactivity, indicating that OH was reacting with some oxidation products not measured in the experiment. The "missing" OH reactivity gradually increased as the oxidation progressed and contributed up to 40% of the total OH reactivity. In the presence of NO, the "missing" OH reactivity is likely due to a number of first-generation products produced in low yields, while in the absence of NO, alternative pathways that produce less reactive products are also occurring.

Proceedings of the National Academy of Sciences, 2016

Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO2) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the conf...

Atmospheric Chemistry and Physics, 2016

The emission, dispersion, and photochemistry of isoprene (C 5 H 8) and related chemical species in the convective boundary layer (CBL) during sunlit daytime were studied over a mixed forest in the southeastern United States by combining ground-based and aircraft observations. Fluxes of isoprene and monoterpenes were quantified at the top of the forest canopy using a high-resolution proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS). Snapshot (∼ 2 min sampling duration) vertical profiles of isoprene, methyl vinyl ketone (MVK) + methacrolein (MACR), and monoterpenes were collected from aircraft every hour in the CBL (100-1000 m). Both ground-based and airborne collected volatile organic compound (VOC) data are used to constrain the initial conditions of a mixed-layer chemistry model (MXLCH), which is applied to examine the chemical evolution of the O 3-NO x-HO x-VOC system and how it is affected by boundary layer dynamics in the CBL. The chemical loss rate of isoprene (∼ 1 h) is similar to the turbulent mixing timescale (0.1-0.5 h), which indicates that isoprene concentrations are equally dependent on both photooxidation and boundary layer dynamics. Analysis of a modelderived concentration budget suggests that diurnal evolution of isoprene inside the CBL is mainly controlled by surface emissions and chemical loss; the diurnal evolution Published by Copernicus Publications on behalf of the European Geosciences Union. 7726 L. Su et al.: Isoprene photooxidation in the southeastern US of O 3 is dominated by entrainment. The NO to HO 2 ratio (NO : HO 2) is used as an indicator of anthropogenic impact on the CBL chemical composition and spans a wide range (1-163). The fate of hydroxyl-substituted isoprene peroxyl radical (HOC 5 H 8 OO q ; ISOPOO) is strongly affected by NO : HO 2 , shifting from NO-dominant to NO-HO 2-balanced conditions from early morning to noontime. This chemical regime change is reflected in the diurnal evolution of isoprene hydroxynitrates (ISOPN) and isoprene hydroxy hydroperoxides (ISOPOOH).

Journal of Geophysical Research, 2000

We report laboratory kinetic studies of isoprene reactions initiated by the hydroxyl radical OH, using a turbulent flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. The rate constams for the reaction of isoprene with OH have been measured in the pressure range of 70 to 120 torr at 298 +_ 2 K and are found to be independent of pressure with an averaged value of (10.1 + 0.8) x 10 '• cm 3 molecule 4 s 4. The error limit given is within 1 standard deviation; a systematic error is estimated to be +_15%. We also describe direct observation of the OH-isoprene adduct based on ion-molecule reactions by using the CIMS method. The formation of the OH-isoprene adduct was used to extract the rate constam between OH and isoprene; within the uncertainty of the experiments the results were consistent with those obtained from the observed disappearance of OH. By monitoring the formation of the OH-isoprene adduct in the presence of oxygen molecules, an overall rate constam between OH-isoprene adduct and 02 has been first determined, with an averaged value of (2.8 +_ 0.7) x 10 '15 cm 3 molecule 'l s 'l at 76 torr and an estimated systematic error of +_50%. Atmospheric implications of the present results to the photochemical oxidation of isoprene are discussed. 1. Introduction Hydrocarbons play an important role in atmospheric chemistry [e.g., Atkinson, 1994; Seinfeld and Pandis, 1998]. Photochemical oxidation of hydrocarbons leads to ozone (O3), acid, and carbon dioxide (CO2) production, with major implications for local and regional air quality, acid deposition, and the greenhouse effect [IPCC Climate Change, 1994]. Isoprene (2-methyl-l,3-butadiene, CH2=C(CH3)CH=CH2) is one of the most abundant hydrocarbons emitted by the terrestrial biosphere, with a global averaged production rate of about 450 Tg yr -• [Rasmussen and Khalil, 1988] and is sufficiently reactive to influence oxidation levels over large portions of the continental troposphere [Trainer et al., 1987]. Many deciduous trees emit about 1% of the CO2 that they fix as isoprene. The biogenic isoprene comprises a hydrocarbon source that exceeds the emission of the nonmethane hydrocarbons of anthropogenic origin [Placet et al., 1990].

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.

Atmospheric Chemistry and Physics

Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant impact on air quality and climate. Compared with the oxidation of volatile organic compounds by ozone (O 3) and hydroxyl radical (OH), HOM formation in the oxidation by nitrate radical (NO 3), an important oxidant at nighttime and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO 3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM, including monomers (C 5), dimers (C 10), and trimers (C 15), both closed-shell compounds and open-shell peroxy radicals (RO 2), were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate firstand second-generation products. HOM monomers containing one to three N atoms (1-3N-monomers) were formed, starting with NO 3 addition to carbon double bond, forming peroxy radicals, followed by autoxidation. 1N-monomers were formed by both the direct reaction of NO 3 with isoprene and of NO 3 with first-generation products. 2N-monomers (e.g., C 5 H 8 N 2 O n(n=7-13) , C 5 H 10 N 2 O n(n=8-14)) were likely the termination products of C 5 H 9 N 2 O n q , which was formed by the addition of NO 3 to C5-hydroxynitrate (C 5 H 9 NO 4), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for ∼ 34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in the isoprene + NO 3 reaction under our experimental conditions. H shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation ("alkoxy-peroxy" pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO 2 and via the termination reactions of dimer RO 2 formed by further reaction of closed-shell dimers with NO 3 and possibly by the reaction of C5-RO 2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO 2 with monomer RO 2. The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 % +1.3 % −0.7 % , which corresponded to a SOA yield of ∼ 3.6 % Published by Copernicus Publications on behalf of the European Geosciences Union. 9682 D. Zhao et al.: Highly oxygenated organic molecule (HOM) formation assuming the molecular weight of C 5 H 9 NO 6 as the lower limit. This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO 3 .

Atmospheric Environment, 2001

This paper explores several aspects of the chemistry of a forested region in northwestern Greece, from data collected during the AEROBIC97 campaign. An observationally constrained box model has been constructed to enable comparisons between modelled concentrations of OH and HO and those determined by the #uorescence assay by gas expansion (FAGE) technique. These results represent the "rst comparison of measured and modelled OH concentrations in such an environment. The modelled OH concentrations are, on average,&50% of those measured (range of 16}61%) over 4 days of model and measurement comparison. Possible reasons for the model-measurement discrepancy are discussed. A rate of production analysis illustrates the dominance of isoprene and the monoterpenes on OH loss, as well as the signi"cance of the ozonolysis of biogenic species as an OH source. The measured and modelled [HO ]/[OH] ratio averaged between 11:00 and 15:00 h is much higher than has been found previously for similar NO V concentrations,&75 and 340, respectively, cf. 10}20. The high ratio re#ects the rapid recycling through the OH}HO oxidation chain, involving biogenic species. The high biogenic concentrations result in a midday OH lifetime of&0.15 s. Finally, for the conditions encountered during the campaign, there is high net photochemical ozone production, peaking at&20 ppbv h\ around 09:00 h.

Global Change Biology, 2012

Isoprene is emitted from many terrestrial plants at high rates, accounting for an estimated 1/3 of annual global volatile organic compound emissions from all anthropogenic and biogenic sources combined. Through rapid photooxidation reactions in the atmosphere, isoprene is converted to a variety of oxidized hydrocarbons, providing higher order reactants for the production of organic nitrates and tropospheric ozone, reducing the availability of oxidants for the breakdown of radiatively active trace gases such as methane, and potentially producing hygroscopic particles that act as effective cloud condensation nuclei. However, the functional basis for plant production of isoprene remains elusive. It has been hypothesized that in the cell isoprene mitigates oxidative damage during the stress-induced accumulation of reactive oxygen species (ROS), but the products of isoprene-ROS reactions in plants have not been detected. Using pyruvate-2-13 C leaf and branch feeding and individual branch and whole mesocosm flux studies, we present evidence that isoprene (i) is oxidized to methyl vinyl ketone and methacrolein (i ox ) in leaves and that i ox /i emission ratios increase with temperature, possibly due to an increase in ROS production under high temperature and light stress. In a primary rainforest in Amazonia, we inferred significant in plant isoprene oxidation (despite the strong masking effect of simultaneous atmospheric oxidation), from its influence on the vertical distribution of i ox uptake fluxes, which were shifted to low isoprene emitting regions of the canopy. These observations suggest that carbon investment in isoprene production is larger than that inferred from emissions alone and that models of tropospheric chemistry and biota-chemistry-climate interactions should incorporate isoprene oxidation within both the biosphere and the atmosphere with potential implications for better understanding both the oxidizing power of the troposphere and forest response to climate change.

Topics in Current Chemistry, 2012

In this chapter we discuss some of the recent work directed at further understanding the chemistry of our atmosphere in regions of low NO x , such as forests, where there are considerable emissions of biogenic volatile organic compounds, for example reactive hydrocarbons such as isoprene. Recent field measurements have revealed some surprising results, for example that OH concentrations are measured to be considerably higher than can be understood using current chemical mechanisms. It has also not proven possible to reconcile field measurements of other species, such as oxygenated VOCs, or emission fluxes of isoprene, using current mechanisms. Several complementary approaches have been brought to bear on formulating a solution to this problem, namely field studies using state-of-the-art instrumentation, chamber studies to isolate sub-sections of the chemistry, laboratory studies to measure rate coefficients, product branching ratios and photochemical yields, the development of ever more detailed chemical mechanisms, and high quality ab initio quantum theory to calculate the energy landscape for relevant reactions and to enable the rates of formation of products and intermediates for previously unknown and unstudied reactions to be predicted. The last few years have seen significant activity in this area, with several contrasting postulates put forward to explain the experimental findings, and here we attempt to synthesise the evidence and ideas.

Journal of Geophysical Research, 1998

Measurements of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and peroxymethacryloyl nitrate (MPAN) were made during the Southern Oxidants Study 1995 Nashville/Middle Tennessee Ozone Study at the Youth Inc. Ranch southeast of Nashville from June 29 to July 26. These measurements were made along with those of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK), other carbonyl compounds, and supporting measurements. This data set represents the first high-frequency, simultaneous measurements of MPAN and its precursor, MACR as well as PPN and its precursor propanal. The NOx sensitivity of isoprene chemistry can be studied with data from this site because large and widely fluctuating levels of NOx were experienced as a result of the proximity to the Nashville urban center. Mean mixing ratios of PAN, PPN, MPAN, and MACR were 485, 50, 30, and 290 parts per trillion by volume respectively. The mean diurnal cycle of MPAN closely tracks that of MACR and was found to be considerably sharper than the mean diurnal cycles of PAN, PPN, and 03, showing that MPAN is closely dependent on the availability of MACR. Considerable levels of MPAN and MACR appear to develop at night above a nocturnal boundary layer and are partly responsible for a commonly observed morning increase. Early morning OH reaction with MACR also produces MPAN in the morning. With the high summer temperatures at Youth Inc., the MPAN lifetime is largely determined by thermal decomposition, although OH and 03 chemistry could substantially reduce the ambient lifetime. Additional loss mechanisms for MPAN and dependence on a single source make MPAN more sensitive to photochemical activity and NO• than is PAN. Unless quickly advected to colder, less photochemically active regions of the atmosphere, the contribution of MPAN to 03 formation through long-range transport of NO• is likely to be less significant in comparison with PAN and PPN. Its influence is more limited to local 03 production, so MPAN can be a useful indicator of local-scale, active biogenic photochemistry.

Atmospheric Chemistry and Physics Discussions, 2016

Measurements of OH reactivity, the inverse lifetime of the OH-radical, can provide a top-down estimate of the total amount of reactive carbon in an airmass. Because OH reactivity is tied to the…

Atmospheric Chemistry and Physics, 2007

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Atmospheric Chemistry and Physics, 2013

Hydroxyl (OH) and hydroperoxyl (HO 2 ) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NO x conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NO x (NO + NO 2 ) and other inorganic species were used to constrain a zerodimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70 ± 0.31) for isoprene mixing ratios between 1-2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO 2 and isoprene-based peroxy radicals,

Atmospheric Chemistry and Physics, 2013

Hydroxyl (OH) and hydroperoxyl (HO 2 ) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NO x conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NO x (NO + NO 2 ) and other inorganic species were used to constrain a zerodimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70 ± 0.31) for isoprene mixing ratios between 1-2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO 2 and isoprene-based peroxy radicals,