Reaction of oleic acid particles with NO3 radicals: Products, mechanism, and implications for radical-initiated organic aerosol oxidation

The heterogeneous reaction of liquid oleic acid aerosol particles with NO3 radicals in the presence of NO2, N2O5, and O2 was investigated in an environmental chamber using a combination of on-line and off-line mass spectrometric techniques. The results indicate that the major reaction products, which are all carboxylic acids, consist of hydroxy nitrates, carbonyl nitrates, dinitrates, hydroxydinitrates, and possibly more highly nitrated products. The key intermediate in the reaction is the nitrooxyalkylperoxy radical, which is formed by the addition of NO3 to the carbon-carbon double bond and subsequent addition of O2. The nitrooxyalkylperoxy radicals undergo self-reactions to form hydroxy nitrates and carbonyl nitrates, and may also react with NO2 to form nitrooxy peroxynitrates. The latter compounds are unstable and decompose to carbonyl nitrates and dinitrates. It is noteworthy that in this reaction nitrooxyalkoxy radicals appear not to be formed, as indicated by the absence of the expected products of decomposition or isomerization of these species. This is different from gas-phase alkene-NO3 reactions, in which a large fraction of the products are formed through these pathways. The results may indicate that, for liquid organic aerosol particles in low NOx environments, the major products of the radical-initiated oxidation (including by OH radicals) of unsaturated and saturated organic compounds will be substituted forms of the parent compound rather than smaller decomposition products. These compounds will remain in the particle and can potentially enhance particle hygroscopicity and the ability of particles to act as cloud condensation nuclei.     

Organosulfate formation in biogenic secondary organic aerosol

Organosulfates of isoprene, alpha-pinene, and beta-pinene have recently been identified in both laboratory-generated and ambient secondary organic aerosol (SOA). In this study, the mechanism and ubiquity of organosulfate formation in biogenic SOA is investigated by a comprehensive series of laboratory photooxidation (i.e., OH-initiated oxidation) and nighttime oxidation (i.e., NO3-initiated oxidation under dark conditions) experiments using nine monoterpenes (alpha-pinene, beta-pinene, d-limonene, l-limonene, alpha-terpinene, gamma-terpinene, terpinolene, Delta(3)-carene, and beta-phellandrene) and three monoterpenes (alpha-pinene, d-limonene, and l-limonene), respectively. Organosulfates were characterized using liquid chromatographic techniques coupled to electrospray ionization combined with both linear ion trap and high-resolution time-of-flight mass spectrometry. Organosulfates are formed only when monoterpenes are oxidized in the presence of acidified sulfate seed aerosol, a result consistent with prior work. Archived laboratory-generated isoprene SOA and ambient filter samples collected from the southeastern U.S. were reexamined for organosulfates. By comparing the tandem mass spectrometric and accurate mass measurements collected for both the laboratory-generated and ambient aerosol, previously uncharacterized ambient organic aerosol components are found to be organosulfates of isoprene, alpha-pinene, beta-pinene, and limonene-like monoterpenes (e.g., myrcene), demonstrating the ubiquity of organosulfate formation in ambient SOA. Several of the organosulfates of isoprene and of the monoterpenes characterized in this study are ambient tracer compounds for the occurrence of biogenic SOA formation under acidic conditions. Furthermore, the nighttime oxidation experiments conducted under highly acidic conditions reveal a viable mechanism for the formation of previously identified nitrooxy organosulfates found in ambient nighttime aerosol samples. We estimate that the organosulfate contribution to the total organic mass fraction of ambient aerosol collected from K-puszta, Hungary, a field site with a similar organosulfate composition as that found in the present study for the southeastern U.S., can be as high as 30%.     

Photochemical aerosol formation from organic gases

It is found that many kinds of organic gases, which belong to aromatics, terpenes, and cycloalkanes, produce aerosols when irradiated by a deuterium lamp with spectrum ranging from 180 to 400 nm in wavelength, and by a krypton-fluoride excimer laser operated at 249 nm. Two initial processes are important, direct excitation of organic molecule by the ultraviolet light, and reaction of organic molecule with ozone produced photochemically by the ultraviolet light. It depends on compounds which process is dominant. For some compounds, both are possible.     

Kinetics, products, and mechanisms of secondary organic aerosol formation

Secondary organic aerosol (SOA) is formed in the atmosphere when volatile organic compounds (VOCs) emitted from anthropogenic and biogenic sources are oxidized by reactions with OH radicals, O(3), NO(3) radicals, or Cl atoms to form less volatile products that subsequently partition into aerosol particles. Once in particles, these organic compounds can undergo heterogenous/multiphase reactions to form more highly oxidized or oligomeric products. SOA comprises a large fraction of atmospheric aerosol mass and can have significant effects on atmospheric chemistry, visibility, human health, and climate. Previous articles have reviewed the kinetics, products, and mechanisms of atmospheric VOC reactions and the general chemistry and physics involved in SOA formation. In this article we present a detailed review of VOC and heterogeneous/multiphase chemistry as they apply to SOA formation, with a focus on the effects of VOC molecular structure on the kinetics of initial reactions with the major atmospheric oxidants, the subsequent reactions of alkyl, alkyl peroxy, and alkoxy radical intermediates, and the composition of the resulting products. Structural features of reactants and products discussed include compound carbon number; linear, branched, and cyclic configurations; the presence of C[double bond, length as m-dash]C bonds and aromatic rings; and functional groups such as carbonyl, hydroxyl, ester, hydroxperoxy, carboxyl, peroxycarboxyl, nitrate, and peroxynitrate. The intention of this review is to provide atmospheric chemists with sufficient information to understand the dominant pathways by which the major classes of atmospheric VOCs react to form SOA products, and the further reactions of these products in particles. This will allow reasonable predictions to be made, based on molecular structure, about the kinetics, products, and mechanisms of VOC and heterogeneous/multiphase reactions, including the effects of important variables such as VOC, oxidant, and NO(x) concentrations as well as temperature, humidity, and particle acidity. Such knowledge should be useful for interpreting the results of laboratory and field studies and for developing atmospheric chemistry models. A number of recommendations for future research are also presented.     

Kinetics and mechanism of the reaction of CF3 radicals with NO2

The reaction of CF₃ with NO₂ was studied at 296 ± 2K using two different absolute techniques. Absolute rate constants of (1.6 ± 0.3) × 10⁻¹¹ and (2.1 _−0.3⁺⁰⁷) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ were derived by IR fluorescence and UV absorption spectroscopy, respectively. The reaction proceeds via two reaction channels: CF₃ + NO₂ → CF₂O + FNO, (70 ± 12)% and CF₃ + NO₂ → CF₃O + NO, (30 ± 12)%. An upper limit of 11% for formation of other reaction products was determined. The overall rate constant was within the uncertainty independent of total pressure between 0.4 to 760 torr. © 1996 John Wiley & Sons, Inc.     

Mechanisms for the formation of secondary organic aerosol components from the gas-phase ozonolysis of alpha-pinene

Gas-phase ozonolysis of alpha-pinene was studied in static chamber experiments under 'OH-free' conditions. A range of multifunctional products-in particular low-volatility carboxylic acids-were identified in the condensed phase using gas chromatography coupled to mass spectrometry after derivatisation. The dependence of product yields on reaction conditions (humidity, choice of OH radical scavengers, added Criegee intermediate scavengers, NO(2)etc.) was investigated to probe the mechanisms of formation of these products; additional information was obtained by studying the ozonolysis of an enal and an enone derived from alpha-pinene. On the basis of experimental findings, previously suggested mechanisms were evaluated and detailed gas-phase mechanisms were developed to explain the observed product formation. Atmospheric implications of this work are discussed.     

Gas-phase reactions of organic radicals and diradicals with ions

Reactions of polyatomic organic radicals with gas phase ions have been studied at thermal energy using a flowing afterglow-selected ion flow tube (FA-SIFT) instrument. A supersonic pyrolysis nozzle produces allyl radical (CH2CHCH2) and ortho-benzyne diradical (o-C6H4) for reaction with ions. We have observed: [CH2CHCH2 + H3O+ --> C3H6+ + H2O], [CH2CHCH2 + HO- --> no ion products], [o-C6H4 + H3O+ --> C6H5+ + H2O], and [o-C6H4 + HO- --> C6H3- + H2O]. The proton transfer reactions with H3O+ occur at nearly every collision (kII approximately with 10(-9) cm3 s(-1)). The exothermic proton abstraction for o-C6H4 + HO- is unexpectedly slow (kII approximately with 10(-10) cm3 s(-1)). This has been rationalized by competing associative detachment: o-C6H4 + HO- --> C6H5O + e-. The allyl + HO- reaction proceeds presumably via similar detachment pathways.     

Alkenoxy radicals in gas-phase reactions of alkenes with oxygen atoms or ozone

Observations in the O₃ + trans-2-butene reaction system and in the O + trans-2-butene + O₂ reaction system suggest the intermediacy of alkenoxy radicals. A mechanism is proposed for the production of C_n and C_m (m <n) alkenoxy radicals by the reaction of C_nH_2n alkenes with oxygen atoms or with ozone.     

Heterogeneous chemistry of butanol and decanol with sulfuric acid: implications for secondary organic aerosol formation

Recent environmental chamber studies suggest that acid-catalyzed reactions between alcohols and aldehydes in the condensed phase lead to the formation of hemiacetals and acetals, enhancing secondary organic aerosol (SOA) growth. We report measurements of heterogeneous uptake of butanol and decanol on liquid H2SO4 in the range of 62-84 wt % and between 273 and 296 K. Both alcohols exhibit two distinct types of uptake behaviors (partially irreversible vs totally irreversible uptake), depending on the acid concentration and temperature. For the partially irreversible uptake, a fraction of the alcohol was physically absorbed while the other fraction underwent irreversible reaction. For the totally irreversible uptake, the alcohols were completely lost onto the sulfuric acid. The Henry's law solubility constant (H*) was determined from the time-dependent uptake, while the reactive uptake coefficients were calculated from the time-independent irreversible loss. Coexistence of butanol or decanol with octanal or decanal did not show enhanced uptake of the aldehydes in the sulfuric acid. Protonation and dissolution likely account for the reversible uptake, while formation of alkyl sulfate or dialkyl sulfate explains irreversible uptake of the alcohols. The results suggest that heterogeneous uptake of larger alcohols is unlikely of significant importance in the lower atmosphere except in the case of freshly nucleated aerosols that may have high acid concentrations.     

Pressure dependent mechanistic branching in the formation pathways of secondary organic aerosol from cyclic-alkene gas-phase ozonolysis

The gas-phase ozonolysis of cyclic-alkenes (1-methyl-cyclohexene, methylene-cyclohexane, α-pinene, β-pinene) is studied with respect to the pressure dependent formation of secondary organic aerosol (SOA). We find that SOA formation is substantially suppressed at lower pressures for all alkenes under study. The suppression coincides with the formation of ketene (α-pinene, 1-methyl-cyclohexene), ethene (1-methyl-cyclohexene) and the increased formation of CO (all alkenes) at lower reaction pressures. The formation of these products is independent of the presence of an OH scavenger and explained by an increased chemical activation of intermediate species in the hydroperoxide channel after the OH elimination. These findings underline the central role of the hydroperoxide pathway for SOA formation and give insight into the gas-phase ozonolysis mechanism after the stage of the Criegee intermediate chemistry.     

Heterogeneous reactions of HONO formation from NO2 and HNO3: a review

The photolysis of nitrous acid (HONO) is an important reaction of atmospheric chemistry due to the fact that it can be the source of OH radical in the troposphere. Despite its role as a radical precursor, the chemical mechanisms leading to HONO formation are not well understood. It is commonly assumed that HONO formation is due to both homogeneous and heterogeneous processes involving NO_x (mixture of NO and NO₂) in which the kinetic and mechanistic details are still under investigation. In this discussion, we would like to highlight the formation of HONO from NO₂ and nitric acid (HNO₃) in the presence of organic particulate. We understood that in the real case, many parameters can influence the reaction mechanism; however, this is just an effort to have a better understanding of the study of HONO formation in the atmospheric process.     

Kinetics of the gas-phase reactions of NO3 radicals and O3 with 3-methylfuran and the OH radical yield from the O3 reaction

Using relative rate methods, rate constants have been measured for the gas-phase reactions of 3-methylfuran with NO₃ radicals and O₃ at 296 ± 2 K and atmospheric pressure of air. The rate constants determined were (1.31 ± 0.461) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ for the NO₃ radical reaction and (2.05 ± 0.52) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for the O₃ reaction, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference reactions. Based on the cyclohexanone plus cyclohexanol yield in the presence of sufficient cyclohexane to scavenge > 95% of OH radicals formed, it is estimated that the O₃ reaction leads to the formation of OH radicals with a yield of 0.59, uncertain to a factor of ca. 1.5. In the troposphere, 3-methylfuran will react dominantly with the OH radical during daylight hours, and with the NO₃ radical during nighttime hours for nighttime NO₃ radical concentrations > 10⁷ molecule cm ⁻³. © 1996 John Wiley & Sons, Inc.     

Kinetics and products of the gas-phase reactions of divinyl sulfoxide with OH and NO3 radicals and O3

Using relative rate methods, rate constants for the gas-phase reactions of divinyl sulfoxide [CH 2CHS(O)CHCH 2; DVSO] with NO 3 radicals and O 3 have been measured at 296 +/- 2 K, and rate constants for the reaction with OH radicals have been measured over the temperature range of 277-349 K. Rate constants obtained for the NO 3 radical and O 3 reactions at 296 +/- 2 K were (6.1 +/- 1.4) x 10 (-16) and (4.3 +/- 1.0) x 10 (-19) cm (3) molecule (-1) s (-1), respectively. For the OH radical reaction, the temperature-dependent rate expression obtained was k = 4.17 x 10 (-12)e ((858 +/- 141)/ T ) cm (3) molecule (-1) s (-1) with a 298 K rate constant of (7.43 +/- 0.71) x 10 (-11) cm (3) molecule (-1) s (-1), where, in all cases, the errors are two standard deviations and do not include the uncertainties in the rate constants for the reference compounds. Divinyl sulfone was observed as a minor product of both the OH radical and NO 3 radical reactions at 296 +/- 2 K. Using in situ Fourier transform infrared spectroscopy, CO, CO 2, SO 2, HCHO, and divinyl sulfone were observed as products of the OH radical reaction, with molar formation yields of 35 +/- 11, 2.2 +/- 0.8, 33 +/- 4, 54 +/- 6, and 5.4 +/- 0.8%, respectively, in air. For the experimental conditions employed, aerosol formation from the OH radical-initiated reaction of DVSO in the presence of NO was minor, being approximately 1.5%. The data obtained here for DVSO are compared with literature data for the corresponding reactions of dimethyl sulfoxide.     

Kinetic study of the gas-phase reactions of OH and NO3 radicals and O3 with selected vinyl ethers

Kinetic studies on the gas-phase reactions of OH and NO3 radicals and ozone with ethyl vinyl ether (EVE), propyl vinyl ether (PVE) and butyl vinyl ether (BVE) have been performed in a 405 L borosilicate glass chamber at 298 +/- 3 K in synthetic air using in situ FTIR spectroscopy to monitor the reactants. Using a relative kinetic method rate coefficients (in units of cm3 molecule(-1) s(-1)) of (7.79 +/- 1.71) x 10(-11), (9.73 +/- 1.94) x 10(-11) and (1.13 +/- 0.31) x 10(-10) have been obtained for the reaction of OH with EVE, PVE and BVE, respectively, (1.40 +/- 0.35) x 10(-12), (1.85 +/- 0.53) x 10(-12) and (2.10 +/- 0.54) x 10(-12) for the reaction of NO3 with EVE, PVE and BVE, respectively, and (2.06 +/- 0.42) x 10(-16), (2.34 +/- 0.48) x 10(-16) and (2.59 +/- 0.52) x 10(-16) for the ozonolysis of EVE, PVE and BVE, respectively. Tropospheric lifetimes of EVE, PVE and BVE with respect to the reactions with reactive tropospheric species (OH, NO3 and O3) have been estimated for typical OH and NO3 radical and ozone concentrations.     

基于气溶胶质谱的二次有机气溶胶识别

二次有机气溶胶(SOA)是大气气溶胶十分重要的组成部分,也是目前人们认识最为薄弱的气溶胶组分.由于有机气溶胶化学组成的复杂性,对SOA进行有效的识别和估算一直是国际气溶胶研究领域的热点和难点问题.本研究尝试使用一种新方法来定量识别深圳冬季大气中的SOA:利用气溶胶质谱仪在线观测的高时间分辨率优势和质谱中的特征碎片离子,应用正定矩阵因子解析(PMF)模型对细粒子组分的主要来源进行解析,识别出其中的二次有机物.结果表明:深圳冬季大气细粒子中SOA浓度平均为9.41±6.33μg/m3,占总有机物质量的39.9±21.8%;相比于一次有机气溶胶(POA),SOA浓度水平变化较为平缓,体现了区域性二次污染物的特征.SOA/BC比值具有鲜明的日变化规律,且与Ox(O3+NO2)的日变化规律相似,说明SOA的生成过程显著地受控于大气光化学活性.深圳冬季大气SOA生成最活跃的时段约为9～15时,期间SOA/BC比值增长了122%.本文为研究我国大气二次有机气溶胶提供了一种新的技术方法和思路.     

Oxidation mechanism of aromatic peroxy and bicyclic radicals from OH-toluene reactions

Theoretical calculations have been performed to investigate mechanistic features of OH-initiated oxidation reactions of toluene. Aromatic peroxy radicals arising from initial OH and subsequent O(2) additions to the toluene ring are shown to cyclize to form bicyclic radicals rather than undergoing reaction with NO under atmospheric conditions. Isomerization of bicyclic radicals to more stable epoxide radicals possesses significantly higher barriers and, hence, has slower rates than O(2) addition to form bicyclic peroxy radicals. At each OH attachment site, only one isomeric pathway via the bicyclic peroxy radical is accessible to lead to ring cleavage. The study provides thermochemical and kinetic data for quantitative assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from toluene oxidation.     

Secondary organic aerosol formation from limonene ozonolysis: homogeneous and heterogeneous influences as a function of NO(x)

Limonene has a high emission rate both from biogenic sources and from household solvents. Here we examine the limonene + ozone reaction as a source for secondary organic aerosol (SOA). Our data show that limonene has very high potential to form SOA and that NO(x) levels, O(3) levels, and UV radiation all influence SOA formation. High SOA formation is observed under conditions where both double bonds in limonene are oxidized, but those conditions depend strongly on NO(x). At low NO(x), heterogeneous oxidation of the terminal double bond follows the initial limonene ozonolysis (at the endocyclic double bond) almost immediately, making the initial reaction rate limiting. This requires a high uptake coefficient between ozone and the first-generation, unsaturated organic particles. However, at high NO(x), this heterogeneous processing is inhibited and gas-phase oxidation of the terminal double bond dominates. Although this chemistry is slower, it also yields products with low volatility. UV light suppresses production of the lowest volatility products, as we have shown in earlier studies of the alpha-pinene + ozone reaction.     

Free-radical abstraction reactions with concerted fragmentation and NO formation

The enthalpy and activation energy of reactions involving attack by MeO₂^? and MeO₂^? on CH₂ groups of 2-butyl nitrite and 2-nitrosobutane have been calculated by quantum chemical methods. The abstraction of a hydrogen atom is accompanied, in the former case, by concerted N–O bond breaking and, in the latter case, by concerted C–N bond breaking, resulting in NO? formation. On the basis of the results obtained, an algorithm has been developed within the intersecting parabolas model for calculating the enthalpies, activation energies, and rate constants of these types of reactions involving alkyl, alkoxyl, aminyl, peroxyl, phenoxyl, thiyl, and hydroxyl radicals.     

异戊二烯与OH自由基光化学反应的二次有机气溶胶的生成

利用烟雾箱模拟装置,研究了异戊二烯与OH自由基反应的二次有机气溶胶(SOA)的生成.反应中生成的气相产物通过质子转移反应质谱仪(PTR-MS)测定,SOA的浓度及粒径谱分布通过高分辨率粒径谱仪(EEPS3090)测定.研究表明:甲基丙烯醛(MAC)/甲基乙烯基酮(MVK)、乙醛、甲醛、甲醇、甲酸/乙醇、乙醇醛、甲基乙二醛、丙酮/丙醛等为主要气相产物,各组实验中MAC/MVK和乙醛浓度达到最大时,其产率分别介于13.78%～37.72%和5.38%～9.34%(以C计)范围内;SOA生成量及其中值粒径随异戊二烯反应量的增加而增加,气相物质稳定后,SOA产率在5.6%～11.7%范围内,粒径在22～165nm范围内.