Gas-Phase Formation of Sulfurous Acid (H2SO3) in the Atmosphere.

Sulfurous acid (H2SO3) is known to be thermodynamically instable decomposing into SO2 and H2O. All attempts to detect this elusive acid in solution failed up to now. Reported H2SO3 formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H2SO3 is formed in the OH radical-initiated gas-phase oxidation of methanesulfinic acid (CH3S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of 53 - 17 + 7 ${{53}_{-17}^{+\ 7}}$ %. Further main products are SO2, SO3 and methanesulfonic acid. CH3S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H2SO3 production of ∼8 million metric tons from the OH+CH3S(O)OH reaction. The investigated H2SO3 depletion in the presence of water vapor results in k(H2O+H2SO3) <3×10-18 cm3 molecule-1 s-1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H2SO3, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions.

Highly Efficient Autoxidation of Triethylamine.

Autoxidation has been acknowledged as a major oxidation pathway in a broad range of atmospherically important compounds including isoprene and monoterpenes. More recently, autoxidation has also been identified as central and even dominant in the atmospheric oxidation of the rather small nonhydrocarbons dimethyl sulfide (DMS) and trimethylamine (TMA). Here, we find even faster autoxidation in the aliphatic amine triethylamine (TEA). The atmospherically dominating autoxidation leads to highly oxygenated and functionalized compounds. Products with as many as three hydroperoxy (OOH) groups and an O:C ratio larger than 1 are formed. We present theoretical multiconformer transition-state theory (MC-TST) calculations of the unimolecular reactions in the autoxidation following the OH + TEA reaction and calculate peroxy radical H-shift rate coefficients >20 s-1 for the first two generations of H-shifts. The efficient autoxidation in TEA is verified by the observation of the proposed highly oxidized products and radicals in flow-tube experiments. We find that the initial OH hydrogen abstraction at the α-carbon is strongly favored, with the ß-carbon abstraction yield being less than a few percent.

Direct sulfuric acid formation from the gas-phase oxidation of reduced-sulfur compounds.

Sulfuric acid represents a fundamental precursor for new nanometre-sized atmospheric aerosol particles. These particles, after subsequent growth, may influence Earth´s radiative forcing directly, or indirectly through affecting the microphysical and radiative properties of clouds. Currently considered formation routes yielding sulfuric acid in the atmosphere are the gas-phase oxidation of SO2 initiated by OH radicals and by Criegee intermediates, the latter being of little relevance. Here we report the observation of immediate sulfuric acid production from the OH reaction of emitted organic reduced-sulfur compounds, which was speculated about in the literature for decades. Key intermediates are the methylsulfonyl radical, CH3SO2, and, even more interestingly, its corresponding peroxy compound, CH3SO2OO. Results of modelling for pristine marine conditions show that oxidation of reduced-sulfur compounds could be responsible for up to ∼50% of formed gas-phase sulfuric acid in these areas. Our findings provide a more complete understanding of the atmospheric reduced-sulfur oxidation.

Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere.

Dimeric accretion products have been observed both in atmospheric aerosol particles and in the gas phase. With their low volatilities, they are key contributors to the formation of new aerosol particles, acting as seeds for more volatile organic vapors to partition onto. Many particle-phase accretion products have been identified as esters. Various gas- and particle-phase formation pathways have been suggested for them, yet evidence remains inconclusive. In contrast, peroxide accretion products have been shown to form via gas-phase peroxy radical (RO2) cross reactions. Here, we show that these reactions can also be a major source of esters and other types of accretion products. We studied α-pinene ozonolysis using state-of-the-art chemical ionization mass spectrometry together with different isotopic labeling approaches and quantum chemical calculations, finding strong evidence for fast radical isomerization before accretion. Specifically, this isomerization seems to happen within the intermediate complex of two alkoxy (RO) radicals, which generally determines the branching of all RO2-RO2 reactions. Accretion products are formed when the radicals in the complex recombine. We found that RO with suitable structures can undergo extremely rapid C-C ß scissions before recombination, often resulting in ester products. We also found evidence of this previously overlooked RO2-RO2 reaction pathway forming alkyl accretion products and speculate that some earlier peroxide identifications may in fact be hemiacetals or ethers. Our findings help answer several outstanding questions on the sources of accretion products in organic aerosol and bridge our knowledge of the gas phase formation and particle phase detection of accretion products. As esters are inherently more stable than peroxides, this also impacts their further reactivity in the aerosol.

Peroxy Radical and Product Formation in the Gas-Phase Ozonolysis of α-Pinene under Near-Atmospheric Conditions: Occurrence of an Additional Series of Peroxy Radicals O,O-C10H15O(O2)yO2 with y = 1-3.

Ozonolysis of α-pinene, C10H16, and other monoterpenes is considered to be one of the important chemical process in the atmosphere leading to condensable vapors, which are relevant to aerosol formation and, finally, for Earth's radiation budget. The formation of peroxy (RO2) radicals, O,O-C10H15(O2)xO2 with x = 0-3, and closed-shell products has been probed from the ozonolysis of α-pinene for close to atmospheric reaction conditions. (The "O,O" in the chemical formulas indicates the two carbonyl groups formed in the ozonolysis.) An additional series of RO2 radicals, O,O-C10H15O(O2)yO2 with y = 1-3, emerged in the presence of NO additions of (1.7-50) × 109 molecules cm-3, whose formation can be explained via different processes starting from alkoxy (RO) radicals, such as the RO-driven autoxidation. The main closed-shell product is a substance with the composition C10H16O3, probably pinonic acid, obtained with a molar yield (lower limit) of 0.26+0.27-0.14 independent of NO. Total molar product yields accounted for up to 0.71+0.72-0.38 indicating reasonable detection sensitivity of the analytical technique applied. For the isomeric O,O-C10H15O2 radicals, an average rate coefficient k(RO2 + NO) = (1.5 ± 0.3) × 10-11 cm3 molecule-1 s-1 at 295 ± 2 K was determined. Product analysis showed a lowering in the formation of highly oxygenated organic molecules (HOMs) by a factor of ∼2.2 when adding 5 × 1010 molecules cm-3 of NO. The comparison with former results revealed that total HOM suppression by NO in the α-pinene ozonolysis is slightly stronger than in the OH + α-pinene reaction.

Hydrotrioxide (ROOOH) formation in the atmosphere.

Organic hydrotrioxides (ROOOH) are known to be strong oxidants used in organic synthesis. Previously, it has been speculated that they are formed in the atmosphere through the gas-phase reaction of organic peroxy radicals (RO2) with hydroxyl radicals (OH). Here, we report direct observation of ROOOH formation from several atmospherically relevant RO2 radicals. Kinetic analysis confirmed rapid RO2 + OH reactions forming ROOOH, with rate coefficients close to the collision limit. For the OH-initiated degradation of isoprene, global modeling predicts molar hydrotrioxide formation yields of up to 1%, which represents an annual ROOOH formation of about 10 million metric tons. The atmospheric lifetime of ROOOH is estimated to be minutes to hours. Hydrotrioxides represent a previously omitted substance class in the atmosphere, the impact of which needs to be examined.

Peroxy Radical Processes and Product Formation in the OH Radical-Initiated Oxidation of α-Pinene for Near-Atmospheric Conditions.

α-Pinene, C10H16, represents one of the most important biogenic emissions into the atmosphere. The formation of RO2 radicals HO-C10H16Ox, x = 2-6, and their closed-shell products from the OH + α-pinene reaction has been measured for close to atmospheric reaction conditions in the presence of NO with concentrations of (1.7-490) × 109 molecules cm-3. Main closed-shell products are substances with the composition C10H16O2 and C10H16O4, most likely carbonyls, obtained with molar yields in the range 0.42-0.45 and 0.17-0.19, respectively, for NO concentrations >5 × 1010 molecules cm-3. The corresponding total product yields amount to 0.75-0.81, indicating efficient product detection by the mass spectrometric method applied. All stated molar yields represent lower limit values affected with an uncertainty of [Formula: see text]. Kinetic and product analysis consistently revealed the suppression of the formation of highly oxygenated organic molecules (HOMs) by a factor of 2-2.2 for the highest NO concentration used. The findings of this study provide insights into the RO2 radical processes of the OH + α-pinene reaction for atmospheric conditions and give an overview about the first-generation products.

Atmospheric Fate of the CH3SOO Radical from the CH3S + O2 Equilibrium.

The atmospheric oxidation mechanisms of reduced sulfur compounds are of great importance in the biogeochemical sulfur cycle. The CH3S radical represents an important intermediate in these oxidation processes. Under atmospheric conditions, CH3S will predominantly react with O2 to form the peroxy radical CH3SOO. The formed CH3SOO has two competing unimolecular reaction pathways: isomerization to CH3SO2, which further decomposes into CH3 and SO2, or a hydrogen shift followed by HO2 loss, leading to CH2S. Previous theoretical calculations have suggested that CH2S formation should be the dominant pathway, in disagreement with existing experimental results. Our large active space multireference configuration interaction calculations agree with the experimental results that the formation of CH3 and SO2 is the dominant route and the formation of CH2S and HO2 can, at most, be a minor pathway. We support the calculations with new experiments starting from the OH + CH3SH reaction for CH3S formation under low NOx conditions and find a SO2 yield of 0.86 ± 0.18 within our reaction time of 7.9 s. Model simulations of our experiments show that the SO2 yield converges to 0.98. This combined theoretical and experimental study thus furthers the understanding of the general oxidation mechanisms of sulfur compounds in the atmosphere.

Trimethylamine Outruns Terpenes and Aromatics in Atmospheric Autoxidation.

Autoxidation in the atmosphere has been realized in the last decade as an important process that forms highly oxidized products relevant for the formation of secondary organic aerosol and likely with detrimental human health effects. It is experimentally shown that the OH radical-initiated oxidation of trimethylamine, the most highly emitted amine in the atmosphere, proceeds via rapid autoxidation steps dominating its atmospheric oxidation process. All three methyl groups are functionalized within a timescale of 10 s following the reaction with OH radicals leading to highly oxidized products. The exceptionally large density of functional groups in the oxidized products is expected to define their chemical properties. A detailed reaction mechanism based on theoretical calculations is able to describe the experimental findings. The comparison with results of the reinvestigated OH radical- and ozone-initiated autoxidation of a series of terpenes and aromatics reveals the trimethylamine process as the most efficient one discovered up to now for atmospheric conditions.

Efficient alkane oxidation under combustion engine and atmospheric conditions.

Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O2. This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol. Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions. Even at high concentrations of NOX, which typically rapidly terminates autoxidation in urban areas, the studied C6-C10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality.

SO2 formation and peroxy radical isomerization in the atmospheric reaction of OH radicals with dimethyl disulfide.

The atmospheric reaction of OH radicals with dimethyl disulfide, CH3SSCH3, proceeds primarily via OH addition forming CH3S and CH3SOH as reactive intermediates, and to a lesser extent via H-abstraction resulting in the peroxy radical CH3SSCH2OO in the presence of O2. The latter undergoes a fast two-step isomerization process leading to HOOCH2SSCHO. CH3S and CH3SOH are both converted to SO2 and CH3O2 with near unity yields under atmospheric conditions.

Atmospheric Autoxidation of Amines.

Autoxidation has been acknowledged as a major oxidation pathway in a broad range of atmospherically important compounds including isoprene, monoterpenes, and very recently, dimethyl sulfide. Here, we present a high-level theoretical multiconformer transition-state theory study of the atmospheric autoxidation in amines exemplified by the atmospherically important trimethylamine (TMA) and dimethylamine and generalized by the study of the larger diethylamine. Overall, we find that the initial hydrogen shift reactions have rate coefficients greater than 0.1 s-1 and autoxidation is thus an important atmospheric pathway for amines. This autoxidation efficiently leads to the formation of hydroperoxy amides, a new type of atmospheric nitrogen-containing compounds, and for TMA, we experimentally confirm this. The conversion of amines to hydroperoxy amides may have important implications for nucleation and growth of atmospheric secondary organic aerosols and atmospheric OH recycling.

Highly Oxygenated Organic Molecules (HOM) from Gas-Phase Autoxidation Involving Peroxy Radicals: A Key Contributor to Atmospheric Aerosol.

Highly oxygenated organic molecules (HOM) are formed in the atmosphere via autoxidation involving peroxy radicals arising from volatile organic compounds (VOC). HOM condense on pre-existing particles and can be involved in new particle formation. HOM thus contribute to the formation of secondary organic aerosol (SOA), a significant and ubiquitous component of atmospheric aerosol known to affect the Earth's radiation balance. HOM were discovered only very recently, but the interest in these compounds has grown rapidly. In this Review, we define HOM and describe the currently available techniques for their identification/quantification, followed by a summary of the current knowledge on their formation mechanisms and physicochemical properties. A main aim is to provide a common frame for the currently quite fragmented literature on HOM studies. Finally, we highlight the existing gaps in our understanding and suggest directions for future HOM research.

Accretion Product Formation from Ozonolysis and OH Radical Reaction of α-Pinene: Mechanistic Insight and the Influence of Isoprene and Ethylene.

α-Pinene (C10H16) represents one of the most important biogenic emissions in the atmosphere. Its oxidation products can significantly contribute to the secondary organic aerosol (SOA) formation. Here, we report on the formation mechanism of C19 and C20 accretion products from α-pinene oxidation, which are believed to be efficient SOA precursors. Measurements have been performed in a free-jet flow system. Detection of RO2 radicals and accretion products was carried out by recent mass spectrometric techniques using different ionization schemes. Observed C10-RO2 radicals from α-pinene ozonolysis were O,O-C10H15(O2) xO2 with x = 0, 1, 2, 3 and from the OH radical reaction HO-C10H16(O2)αO2 with α = 0, 1, 2. All detected C20 accretion products can be explained via the accretion reaction RO2 + R'O2 → ROOR' + O2 starting from the measured C10-RO2 radicals. We speculate that C19 accretion products are formed in an analogous way assuming CH2O elimination. Addition of isoprene (C5H8), producing C5-RO2 radicals, leads to C15 accretion products formed via cross-reactions with C10-RO2 radicals. This process is competing with the formation of C19/C20 products from the pure α-pinene oxidation. A similar behavior has been observed for ethylene additives that form C12 accretion products. In the atmosphere, a complex accretion product spectrum from self- and cross-reactions of available RO2 radicals can be expected. Modeling atmospheric conditions revealed that C19/C20 product formation is only reduced by a factor of 1.2 or 3.6 in isoprene-dominated environments assuming a 2- or 15-fold isoprene concentration over α-pinene, respectively, as present in different forested areas.

Accretion Product Formation from Self- and Cross-Reactions of RO2 Radicals in the Atmosphere.

Hydrocarbons are emitted into the Earth's atmosphere in very large quantities by human and biogenic activities. Their atmospheric oxidation processes almost exclusively yield RO2 radicals as reactive intermediates whose atmospheric fate is not yet fully unraveled. Herein, we show that gas-phase reactions of two RO2 radicals produce accretion products composed of the carbon backbone of both reactants. The rates for accretion product formation are very high for RO2 radicals bearing functional groups, competing with those of the corresponding reactions with NO and HO2 . This pathway, which has not yet been considered in the modelling of atmospheric processes, can be important, or even dominant, for the fate of RO2 radicals in all areas of the atmosphere. Moreover, the vapor pressure of the formed accretion products can be remarkably low, characterizing them as an effective source for the secondary organic aerosol.

Direct Probing of Criegee Intermediates from Gas-Phase Ozonolysis Using Chemical Ionization Mass Spectrometry.

Criegee intermediates (CIs), mainly formed from gas-phase ozonolysis of alkenes, are considered as atmospheric oxidants besides OH and NO3 radicals as well as ozone. Direct CI measurement techniques are inevitably needed for reliable assessment of CIs' role in atmospheric processes. We found that CIs from ozonolysis reactions can be directly probed by means of chemical ionization mass spectrometry with a detection limit of about 104-105 molecules cm-3. Results from quantum chemical calculations support the experimental findings. The simplest CI, CH2OO, is detectable as an adduct with protonated ethers, preferably with protonated tetrahydrofuran. Kinetic measurements yielded k(CH2OO + SO2) = (3.3 ± 0.9) × 10-11 and k(CH2OO + acetic acid) = (1.25 ± 0.30) × 10-10 cm3 molecule-1 s-1 at 295 ± 2 K, in very good agreement with recent measurements using diiodomethane photolysis for CH2OO generation. CIs from the ozonolysis of cyclohexene, acting as surrogate for cyclic terpenes, are followed as protonated species (CI)H+ using protonated amines as reagent ions. Kinetic investigations indicate a different reactivity of cyclohexene-derived CIs compared with that of simple CIs, such as CH2OO. It is supposed that the aldehyde group significantly influences the CI reactivity of the cyclohexene-derived CIs. The direct CI detection method presented here should allow study of the formation and reactivity of a wide range of different CIs formed from atmospheric ozonolysis reactions.

Formation of Highly Oxidized Radicals and Multifunctional Products from the Atmospheric Oxidation of Alkylbenzenes.

Aromatic hydrocarbons contribute significantly to tropospheric ozone and secondary organic aerosols (SOA). Despite large efforts in elucidating the formation mechanism of aromatic-derived SOA, current models still substantially underestimate the SOA yields when comparing to field measurements. Here we present a new, up to now undiscovered pathway for the formation of highly oxidized products from the OH-initiated oxidation of alkyl benzenes based on theoretical and experimental investigations. We propose that unimolecular H-migration followed by O2-addition, a so-called autoxidation step, can take place in bicyclic peroxy radicals (BPRs), which are important intermediates of the OH-initiated oxidation of aromatic compounds. These autoxidation steps lead to the formation of highly oxidized multifunctional compounds (HOMs), which are able to form SOA. Our theoretical calculations suggest that the intramolecular H-migration in BPRs of substituted benzenes could be fast enough to compete with bimolecular reactions with HO2 radicals or NO under atmospheric conditions. The theoretical findings are experimentally supported by flow tube studies using chemical ionization mass spectrometry to detect the highly oxidized peroxy radical intermediates and closed-shell products. This new unimolecular BPR route to form HOMs in the gas phase enhances our understanding of the aromatic oxidation mechanism, and contributes significantly to a better understanding of aromatic-derived SOA in urban areas.

endo-Cyclization of unsaturated RO2 radicals from the gas-phase ozonolysis of cyclohexadienes.

Unsaturated RO2 radicals from the ozonolysis of cyclodienes can readily undergo an endo-cyclization step under atmospheric conditions forming a new ring-containing RO2 radical after further O2 addition. This path represents an extension of the atmospheric autoxidation scheme forming highly oxidized multifunctional organic compounds (HOMs). HOMs play an important role for Earth's organic aerosol burden.

Highly Oxidized Second-Generation Products from the Gas-Phase Reaction of OH Radicals with Isoprene.

The gas-phase reaction of OH radicals with isoprene has been investigated in an atmospheric pressure flow tube at 293 ± 0.5 K with special attention to the second-generation products. Reaction conditions were optimized to achieve a predominant reaction of RO2 radicals with HO2 radicals. Chemical ionization-atmospheric pressure interface-time-of-flight mass spectrometry served as the analytical technique in order to follow the formation of RO2 radicals and closed-shell products containing at least four O atoms in the molecule. The reaction products were detected as adducts with the reagent ions using acetate, lactate, or nitrate in the ionization process. Observed signals were attributed to a series of C5-products with multiple hydroxy, hydroperoxy, and probably carbonyl groups. H/D exchange experiments supported the product identification. The generation of the detected second-generation products can be mechanistically explained starting from the OH radical reaction of hydroxy hydroperoxide isomers, HO-C5H8-OOH. These isomers represent the dominant products of the initial OH radical attack on isoprene. Dihydroxy dihydroperoxides, (HO)2-C5H8-(OOH)2, were analyzed as the main second-generation products beside the dihydroxy epoxides. A simple kinetic analysis revealed that the observed second-generation products in total (other than dihydroxy epoxides) were formed with an estimated molar yield of 10.0-1.5+2.1 % with respect to converted hydroxy hydroperoxides. A formation yield of 5.8-0.9+1.3 % has been deduced for the main product (HO)2-C5H8-(OOH)2. The detected, highly oxidized isoprene products represent potential secondary organic aerosol precursors. An annual, global (HO)2-C5H8-(OOH)2 formation strength of (16-35) × 106 metric tons is estimated based on product measurements of this study and literature data regarding the formation of the dihydroxy epoxide isomers for an annual isoprene emission of 454 × 106 metric tons of carbon.