Building an oxidation reactor in Taiwan: From volatile organic compounds to secondary organic aerosols

Large amounts of volatile organic compounds (VOCs) are emitted into the atmosphere from both human and natural sources. A significant portion of VOCs would be oxidized via their reactions with atmospheric oxidants like OH, NO₃, ozone, etc. The products of the oxidation reactions are often of low volatility and may condense to form secondary organic aerosols (SOA). To study the effect of VOC oxidation in aerosol formation, we are building an oxidation flow reactor system, which consists of (1) a 22-l aluminum chamber, (2) an ozone source with an ozone detector, (3) a UV-C (254 nm) lamp, (4) a photoionization detector to measure the effective VOC concentration, (5) various flow/concentration controlling apparatuses, and (6) a scanning mobility particle sizer to monitor the generated particles. Under the conditions of high UV and ozone levels, the oxidation process can be speeded up by orders of magnitude in this reactor. We hope to use this reactor: (i) to learn the “potential” mass of SOA that can be formed from a given VOC source like a traffic or industry site; (ii) to trace back the SOA source by utilizing the shortened reaction times; (iii) to learn the trends from VOC to SOA.     

Aqueous-phase OH oxidation of glyoxal: application of a novel analytical approach employing aerosol mass spectrometry and complementary off-line techniques

Aqueous-phase chemistry of glyoxal may play an important role in the formation of highly oxidized secondary organic aerosol (SOA) in the atmosphere. In this work, we use a novel design of photochemical reactor that allows for simultaneous photo-oxidation and atomization of a bulk solution to study the aqueous-phase OH oxidation of glyoxal. By employing both online aerosol mass spectrometry (AMS) and offline ion chromatography (IC) measurements, glyoxal and some major products including formic acid, glyoxylic acid, and oxalic acid in the reacting solution were simultaneously quantified. This is the first attempt to use AMS in kinetics studies of this type. The results illustrate the formation of highly oxidized products that likely coexist with traditional SOA materials, thus, potentially improving model predictions of organic aerosol mass loading and degree of oxidation. Formic acid is the major volatile species identified, but the atmospheric relevance of its formation chemistry needs to be further investigated. While successfully quantifying low molecular weight organic oxygenates and tentatively identifying a reaction product formed directly from glyoxal and hydrogen peroxide, comparison of the results to the offline total organic carbon (TOC) analysis clearly shows that the AMS is not able to quantitatively monitor all dissolved organics in the bulk solution. This is likely due to their high volatility or low stability in the evaporated solution droplets. This experimental approach simulates atmospheric aqueous phase processing by conducting oxidation in the bulk phase, followed by evaporation of water and volatile organics to form SOA.     

Nighttime radical observations and chemistry

The nitrate radical, NO(3), is photochemically unstable but is one of the most chemically important species in the nocturnal atmosphere. It is accompanied by the presence of dinitrogen pentoxide, N(2)O(5), with which it is in rapid thermal equilibrium at lower tropospheric temperatures. These two nitrogen oxides participate in numerous atmospheric chemical systems. NO(3) reactions with VOCs and organic sulphur species are important, or in some cases even dominant, oxidation pathways, impacting the budgets of these species and their degradation products. These oxidative reactions, together with the ozonolysis of alkenes, are also responsible for the nighttime production and cycling of OH and peroxy (HO(2) + RO(2)) radicals. In addition, reactions of NO(3) with biogenic hydrocarbons are particularly efficient and are responsible for the production of organic nitrates and secondary organic aerosol. Heterogeneous chemistry of N(2)O(5) is one of the major processes responsible for the atmospheric removal of nitrogen oxides as well as the cycling of halogen species though the production of nitryl chloride, ClNO(2). The chemistry of NO(3) and N(2)O(5) is also important to the regulation of both tropospheric and stratospheric ozone. Here we review the essential features of this atmospheric chemistry, along with field observations of NO(3), N(2)O(5), nighttime peroxy and OH radicals, and related compounds. This review builds on existing reviews of this chemistry, and encompasses field, laboratory and modelling work spanning more than three decades.     

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.     

Atmospheric organic aerosol production by heterogeneous acid-catalyzed reactions.

Exploratory evidence from our laboratories shows that acidic surfaces on atmospheric aerosols lead to very real and potentially multifold increases in secondary organic aerosol (SOA) mass and build-up of stabilized nonvolatile organic matter as particles age. One possible explanation for these heterogeneous processes are the acid-catalyzed (e.g., H2SO4 and HNO3) reactions of atmospheric multifunctional organic species (e.g., multifunctional carbonyl compounds) that are accommodated onto the particle phase from the gas phase. Volatile organic hydrocarbons (VOCs) from biogenic sources (e.g., terpenoids) and anthropogenic sources (aromatics) are significant precursors for multifunctional organic species. The sulfur content of fossil fuels, which is released into the atmosphere as SO2, results in the formation of secondary inorganic acidic aerosols or indigenous acidic soot particles (e.g., diesel soot). The predominance of SOAs contributing to PM2.5 (particulate matter, that is, 2.5 microm or smaller than 2.5 microm), and the prevalence of sulfur in fossil fuels suggests that interactions between these sources could be considerable. This study outlines a systematic approach for exploring the fundamental chemistry of these particle-phase heterogeneous reactions. If acid-catalyzed heterogeneous reactions of SOA products are included in next-generation models, the predicted SOA formation will be much greater and have a much larger impact on climate-forcing effects than we now predict. The combined study of both organic and inorganic acids will also enable greater understanding of the adverse health effects in biological pulmonary organs exposed to particles.     

Products and mechanism of secondary organic aerosol formation from reactions of linear alkenes with NO3 radicals

Secondary organic aerosol (SOA) formation from reactions of linear alkenes with NO(3) radicals was investigated in an environmental chamber using a thermal desorption particle beam mass spectrometer for particle analysis. A general chemical mechanism was developed to explain the formation of the observed SOA products. The major first-generation SOA products were hydroxynitrates, carbonylnitrates, nitrooxy peroxynitrates, dihydroxynitrates, and dihydroxy peroxynitrates. The major second-generation SOA products were hydroxy and oxo dinitrooxytetrahydrofurans, which have not been observed previously. The latter compounds were formed by a series of reactions in which delta-hydroxycarbonyls isomerize to cyclic hemiacetals, which then dehydrate to form substituted dihydrofurans (unsaturated compounds) that rapidly react with NO(3) radicals to form very low volatility products. For the approximately 1 ppmv alkene concentrations used here, aerosol formed only for alkenes C(7) or larger. SOA formed from C(7)-C(9) alkenes consisted only of second-generation products, whereas for larger alkenes first-generation products were also present and contributions increased with increasing carbon number apparently due to the formation of lower volatility products. The estimated mass fractions of first- and second-generation products were approximately 50:50, 30:70, 10:90, and 0:100, for 1-tetradecene, 1-dodecene, 1-decene, and 1-octene SOA, respectively. This study shows that delta-hydroxycarbonyls play a key role in the formation of SOA in alkene-NO(3) reactions and are likely to be important in other systems because delta-hydroxycarbonyls can also be formed from reactions of OH radicals and O(3) with hydrocarbons.     

对流层夜间化学研究

NO3自由基与N2O5是对流层夜间化学的关键物种。一方面NO3与O3等组分是夜间大气中的重要氧化剂,与它们的反应是生物排放挥发性有机物(VOCs)的主要汇;另一方面NO3与N2O5和雨滴或气溶胶颗粒物发生的异相反应则是大气中氮氧化合物NOx(NO,NO2)的主要清除过程,从而可以减轻对流层臭氧污染。研究它们的化学反应性质及对其进行实地测量,对深入理解大气氧化过程和全面了解区域乃至全球大气自净能力有重要意义。本文总结了近年来有关夜间化学的研究成果,介绍了以NO3和N2O5为中心的基本夜间化学过程、对流层中NO3与N2O5的源与汇以及外场测量技术的最新研究进展,并提出了尚待解决的一些问题。     

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.     

The statistical evolution of multiple generations of oxidation products in the photochemical aging of chemically reduced organic aerosol

The heterogeneous reactions of hydroxyl radicals (OH) with squalane and bis(2-ethylhexyl) sebacate (BES) particles are used as model systems to examine how distributions of reaction products evolve during the oxidation of chemically reduced organic aerosol. A kinetic model of multigenerational chemistry, which is compared to previously measured (squalane) and new (BES) experimental data, reveals that it is the statistical mixtures of different generations of oxidation products that control the average particle mass and elemental composition during the reaction. The model suggests that more highly oxidized reaction products, although initially formed with low probability, play a large role in the production of gas phase reaction products. In general, these results highlight the importance of considering atmospheric oxidation as a statistical process, further suggesting that the underlying distribution of molecules could play important roles in aerosol formation as well as in the evolution of key physicochemical properties such as volatility and hygroscopicity.     

Photo-oxidation of Isoprene with Organic Seed: Estimates of Aerosol Size Distributions Evolution and Formation Rates

ondary organic aerosol (SOA) formation from OH-initiated photo-oxidation of isoprene in the presence of organic seed aerosol. The dependence of the size distributions of SOA on both the level of pre-existing particles generated in situ from the photo-oxidation of trace hydrocarbons of indoor atmosphere and the concentration of precursor, has been investi-gated. It was shown that in the presence of high-level seed aerosol and low-level isoprene (typical urban atmospheric conditions), particle growth due to condensation of secondary organic products on pre-existing particles dominated; while in the presence of low-level seed aerosol and comparatively high-level isoprene (typical atmospheric conditions in rural re-gion), bimodal structures appeared in the size distributions of SOA, which corresponded to new particle formation resulting from homogeneous nucleation and particle growth due to condensation of secondary organic products on the per-existing particles respectively. The effects of concentrations of organic seed particles on SOA were also investigated. The particle size distributions evolutions as well as the corresponding formation rates of new particles indifferent conditions were also estimated.     

Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene

Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C(5)H(8)) leads to the formation of secondary organic aerosol (SOA). In the current study, the chemical composition of SOA from the photooxidation of isoprene over the full range of NO(x) conditions is investigated through a series of controlled laboratory chamber experiments. SOA composition is studied using a wide range of experimental techniques: electrospray ionization-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry, high-resolution mass spectrometry, online aerosol mass spectrometry, gas chromatography/mass spectrometry, and an iodometric-spectroscopic method. Oligomerization was observed to be an important SOA formation pathway in all cases; however, the nature of the oligomers depends strongly on the NO(x) level, with acidic products formed under high-NO(x) conditions only. We present, to our knowledge, the first evidence of particle-phase esterification reactions in SOA, where the further oxidation of the isoprene oxidation product methacrolein under high-NO(x) conditions produces polyesters involving 2-methylglyceric acid as a key monomeric unit. These oligomers comprise approximately 22-34% of the high-NO(x) SOA mass. Under low-NO(x) conditions, organic peroxides contribute significantly to the low-NO(x) SOA mass (approximately 61% when SOA forms by nucleation and approximately 25-30% in the presence of seed particles). The contribution of organic peroxides in the SOA decreases with time, indicating photochemical aging. Hemiacetal dimers are found to form from C(5) alkene triols and 2-methyltetrols under low-NO(x) conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NO(x) chamber experiments.     

Uptake of methacrolein into aqueous solutions of sulfuric acid and hydrogen peroxide

Multiphase acid-catalyzed oxidation by hydrogen peroxide has been suggested to be a potential route to secondary organic aerosol (SOA) formation from isoprene and its gas-phase oxidation products, but the kinetics and chemical mechanism remain largely uncertain. Here we report the first measurement of uptake of methacrolein into aqueous solutions of sulfuric acid and hydrogen peroxide in the temperature range of 253-293 K. The steady-state uptake coefficients were acquired and increased quickly with increasing sulfuric acid concentration and decreasing temperature. Propyne, acetone, and 2,3-dihydroxymethacrylic acid were suggested as the products. The chemical mechanism is proposed to be the oxidation of carbonyl group and C═C double bonds by peroxide hydrogen in acidic environment, which could explain the large content of polyhydroxyl compounds in atmospheric fine particles. These results indicate that multiphase acid-catalyzed oxidation of methacrolein by hydrogen peroxide can contribute to SOA mass in the atmosphere, especially in the upper troposphere.     

Highly Oxygenated Molecules from Atmospheric Autoxidation of Hydrocarbons: A Prominent Challenge for Chemical Kinetics Studies

Recent advances in chemical ionization mass spectrometry have allowed the detection of a new group of compounds termed highly oxygenated molecules (HOM). These are atmospheric oxidation products of volatile organic compounds (VOC) retaining most of their carbon backbone, and with O/C ratios around unity. Owing to their surprisingly high yields and low vapor pressures, the importance of HOM for aerosol formation has been easy to verify. However, the opposite can be said concerning the exact formation pathways of HOM from major aerosol precursor VOC. While the role of peroxy radical autoxidation, i.e., consecutive intramolecular H‐shifts followed by O₂ addition, has been recognized, the detailed formation mechanisms remain highly uncertain. A primary reason is that the autoxidation process occurs on sub‐second timescales and is extremely sensitive to environmental conditions like gas composition, temperature, and pressure. This, in turn, poses a great challenge for chemical kinetics studies to be able to mimic the relevant atmospheric reaction pathways, while simultaneously using conditions suitable for studying the short‐lived radical intermediates. In this perspective, we define six specific challenges for this community to directly observe the initial steps of atmospherically relevant autoxidation reactions and thereby facilitate vital improvements in the understanding of VOC degradation and organic aerosol formation.     

Kinetics of N(2)O(5) hydrolysis on secondary organic aerosol and mixed ammonium bisulfate-secondary organic aerosol particles

The kinetics of the hydrolysis reaction of N(2)O(5) on secondary organic aerosol (SOA) produced through the ozonolysis of α-pinene and on mixed ammonium bisulfate-SOA particles was investigated using an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. We report room temperature uptake coefficients, γ, on ammonium bisulfate and SOA particles at 50% relative humidity of 1.5 × 10(-2) ± 1.5 × 10(-3) and 1.5 × 10(-4) ± 2 × 10(-5), respectively. For the mixed ammonium bisulfate-SOA particles, γ decreased from 2.6 × 10(-3) ± 4 × 10(-4) to 3.0 × 10(-4) ± 3 × 10(-5) as the SOA mass fraction increased from 9 to 79, indicating a strong suppression in γ with the addition of organic material. There is an order-of-magnitude reduction in the uptake coefficient with the smallest amount of SOA material present and smaller additional reductions with increasing aerosol organic content. This newly coated organic layer may either decrease the mass accommodation coefficient of N(2)O(5) onto the particle or hinder the dissolution and diffusion of N(2)O(5) into the remainder of the aerosol after it has been accommodated onto the surface. The former corresponds to a surface effect and the latter to bulk processes. The low value of the uptake coefficient on pure SOA particles will likely make N(2)O(5) hydrolysis insignificant on such an aerosol, but atmospheric chemistry models need to account for the role that organics may play in suppressing the kinetics of this reaction on mixed organic-inorganic particles.     

α-蒎烯大气化学反应的研究进展

概述了国内外有关α-蒎烯的大气化学反应的研究进展．主要介绍了α-蒎烯与OH自由基、O3分子、NO3自由基的气相反应的机理及其产物以及形成二次有机气溶胶（SOA）的研究现状,还对α-蒎烯未来的研究动向等进行了阐述．     

Organic constituents on the surfaces of aerosol particles from southern Finland, amazonia, and california studied by vibrational sum frequency generation

This article summarizes and compares the analysis of the surfaces of natural aerosol particles from three different forest environments by vibrational sum frequency generation. The experiments were carried out directly on filter and impactor substrates, without the need for sample preconcentration, manipulation, or destruction. We discuss the important first steps leading to secondary organic aerosol (SOA) particle nucleation and growth from terpene oxidation by showing that, as viewed by coherent vibrational spectroscopy, the chemical composition of the surface region of aerosol particles having sizes of 1 μm and lower appears to be close to size-invariant. We also discuss the concept of molecular chirality as a chemical marker that could be useful for quantifying how chemical constituents in the SOA gas phase and the SOA particle phase are related in time. Finally, we describe how the combination of multiple disciplines, such as aerosol science, advanced vibrational spectroscopy, meteorology, and chemistry can be highly informative when studying particles collected during atmospheric chemistry field campaigns, such as those carried out during HUMPPA-COPEC-2010, AMAZE-08, or BEARPEX-2009, and when they are compared to results from synthetic model systems such as particles from the Harvard Environmental Chamber (HEC). Discussions regarding the future of SOA chemical analysis approaches are given in the context of providing a path toward detailed spectroscopic assignments of SOA particle precursors and constituents and to fast-forward, in terms of mechanistic studies, through the SOA particle formation process.     

UV photodissociation spectroscopy of oxidized undecylenic acid films

Oxidation of thin multilayered films of undecylenic (10-undecenoic) acid by gaseous ozone was investigated using a combination of spectroscopic and mass spectrometric techniques. The UV absorption spectrum of the oxidized undecylenic acid film is significantly red-shifted compared to that of the initial film. Photolysis of the oxidized film in the tropospheric actinic region (lambda > 295 nm) readily produces formaldehyde and formic acid as gas-phase products. Photodissociation action spectra of the oxidized film suggest that organic peroxides are responsible for the observed photochemical activity. The presence of peroxides is confirmed by mass-spectrometric analysis of the oxidized sample and an iodometric test. Significant polymerization resulting from secondary reactions of Criegee radicals during ozonolysis of the film is observed. The data strongly imply the importance of photochemistry in aging of atmospheric organic aerosol particles.     

Computation on IR of Radical Intermediates of Isoprene Reacting with OH Radical

The radical reactions of isoprene, the most abundant natural volatile organic compounds(VOC), is important to understand the atmospheric activities of isoprene and to evaluate the role of VOC in atmospheric pollution. Isoprene reaction with OH radical is such important radical reaction as its contribution to the isoprene decomposing in natural atmosphere. Meantime its radical products also make big contribution to other atmospheric reactions.