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.     

Contribution of carbonyl photochemistry to aging of atmospheric secondary organic aerosol

The photodegradation of secondary organic aerosol (SOA) material by actinic UV radiation was investigated. SOA was generated via the dark reaction of ozone and d-limonene, collected onto quartz-fiber filters, and exposed to wavelength-tunable radiation. Photochemical production of CO was monitored in situ by infrared cavity ring-down spectroscopy. A number of additional gas-phase products of SOA photodegradation were observed by gas chromatography, including methane, ethene, acetaldehyde, acetone, methanol, and 1-butene. The absorption spectrum of SOA material collected onto CaF2 windows was measured and compared with the photolysis action spectrum for the release of CO, a marker for Norrish type-I photocleavage of carbonyls. Both spectra had a band at approximately 300 nm corresponding to the overlapping n --> pi* transitions in nonconjugated carbonyls. The effective extinction coefficient of freshly prepared SOA was estimated to be on the order of 15 L mol(-1) cm(-1) at 300 nm, implying one carbonyl group in every SOA constituent. The absorption by the SOA material slowly increased in the visible and near-UV during storage of SOA in open air in the dark, presumably as a result of condensation reactions that increased the degree of conjugation in the SOA constituents. These observations suggest that photolysis of carbonyl functional groups represents a significant sink for monoterpene SOA compounds in the troposphere, with an estimated lifetime of several hours over the continental United States.     

On molecular chirality within naturally occurring secondary organic aerosol particles from the central Amazon Basin

In this perspectives article, we reflect upon the existence of chirality in atmospheric aerosol particles. We then show that organic particles collected at a field site in the central Amazon Basin under pristine background conditions during the wet and dry seasons consist of chiral secondary organic material. We show how the chiral response from the aerosol particles can be imaged directly without the need for sample dissolution, solvent extraction, or sample preconcentration. By comparing the chiral-response images with optical images, we show that chiral responses always originate from particles on the filter, but not all aerosol particles produce chiral signals. The intensity of the chiral signal produced by the size resolved particles strongly indicates the presence of chiral secondary organic material in the particle. Finally, we discuss the implications of our findings on chiral atmospheric aerosol particles in terms of climate-related properties and source apportionment.     

Heterogeneous glyoxal oxidation: a potential source of secondary organic aerosol

Laboratory studies are described that suggest reactive uptake of glyoxal on particulate containing HNO(3) could contribute to the formation of secondary organic aerosol (SOA) in the upper troposphere (UT). Using a Knudsen cell flow reactor, glyoxal is observed to react on supercooled H(2)O/HNO(3) surfaces to form condensed-phase glyoxylic acid. This product was verified by derivatization and GC-MS analysis. The reactive uptake coefficient, γ, of glyoxal varies only slightly with the pressure of nitric acid, from γ = 0.5 to 3.0 × 10(-3) for nitric acid pressures between 10(-8) and 10(-6) Torr. The data do not show any dependence on temperature (181-201 K) or pressure of glyoxal (10(-7) to 10(-5) Torr). Using the determined reactive uptake kinetics in a simple model shows that glyoxal uptake to supercooled H(2)O/HNO(3) may account for 4-53% of the total organic mass fraction of aerosol in the UT.     

Real-time analysis of secondary organic aerosol particles formed from cyclohexene ozonolysis using a laser-ionization single-particle aerosol mass spectrometer.

A real-time analysis of secondary organic aerosol (SOA) particles formed from cyclohexene ozonolysis in a smog chamber was performed using a laser-ionization single-particle aerosol mass spectrometer (LISPA-MS). The instrument obtains both size and chemical compositions of individual aerosol particles with a high time-resolution (approximately 2 s at the maximum). Both positive and negative-ion mass spectra are obtained. Standard particles generated from dicarboxylic acid solutions using an atomizer were also analyzed. For both standard and SOA particles, the negative-ion mass spectra provided information about the molecular weights of the organic compounds in the particles, since the intense ions in the negative-ion mass spectra are mainly attributable to the molecular-related ions [M-H]-. It was demonstrated that the real-time single-particle analysis of SOA particles by the LISPA-MS technique can reveal the formation and transformation processes of SOA particle in smog chambers.     

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.     

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.     

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.     

OH-initiated heterogeneous aging of highly oxidized organic aerosol

The oxidative evolution ("aging") of organic species in the atmosphere is thought to have a major influence on the composition and properties of organic particulate matter but remains poorly understood, particularly for the most oxidized fraction of the aerosol. Here we measure the kinetics and products of the heterogeneous oxidation of highly oxidized organic aerosol, with an aim of better constraining such atmospheric aging processes. Submicrometer particles composed of model oxidized organics-1,2,3,4-butanetetracarboxylic acid (C(8)H(10)O(8)), citric acid (C(6)H(8)O(7)), tartaric acid (C(4)H(6)O(6)), and Suwannee River fulvic acid-were oxidized by gas-phase OH in a flow reactor, and the masses and elemental composition of the particles were monitored as a function of OH exposure. In contrast to our previous studies of less-oxidized model systems (squalane, erythritol, and levoglucosan), particle mass did not decrease significantly with heterogeneous oxidation. Carbon content of the aerosol always decreased somewhat, but this mass loss was approximately balanced by an increase in oxygen content. The estimated reactive uptake coefficients of the reactions range from 0.37 to 0.51 and indicate that such transformations occur at rates corresponding to 1-2 weeks in the atmosphere, suggesting their importance in the atmospheric lifecycle of organic particulate matter.     

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.     

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%.     

Evaluation of aerosol lead measurement techniques using laboratory generated super-micrometer particles

High levels of lead in some occupational environments still exist. These include lead paint abatement sites, smelting operations, small arms firing ranges, and other construction scenarios. New emerging technologies provide the capability to provide an on-site alternative to conventional laboratory methods for airborne lead. In this paper we describe the evaluation of two such technologies using laboratory prepared lead-laden super-micron aerosol particles. Size measurements by TSI Aerodynamic Particle Sizer™ indicated the fluctuation of the peak particle sizes varying less than 1% among different runs for a given solution concentration. The content of lead embedded in the particles varied from 14 to 18% between runs of three different lead solution concentrations. A commercially available instrument for airborne lead measurement, AeroLead™, showed promise of becoming fully validated with the addition of design enhancements, although not fully validated by the end of the research program. Some of these areas being reworked by the manufacturer include working electrode issues, such as a more uniform surface area. Once these have been addressed, the manufacturer plans to complete the field and laboratory validation procedures. In a subsequent study, the ABF-LIPS results shown in this paper indicated that the technology could be used to quantify lead in aerosol form with a signal-to-noise ratio of three or larger of approximately 100 μg m⁻³ or higher quantity in a few minutes of measurement interval. The estimated detection limit for Pb using the ABF-LIPS prototype was approximately 60 μg m⁻³. In comparison the Resource Conservation and Recovery Act (RCRA) limit for Pb emission is 250 μg m⁻³.     

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.     

Ozonolysis and photolysis of alkene-terminated self-assembled monolayers on quartz nanoparticles: implications for photochemical aging of organic aerosol particles

Photolysis of alkene-terminated self assembled monolayers (SAM) deposited on Degussa SiO(2) nanoparticles is studied following oxidation of SAM with a gaseous ozone/oxygen mixture. Infrared cavity ring-down spectroscopy is used to observe gas-phase products generated during ozonolysis and subsequent photolysis of SAM in real time. Reactions taking place during ozonolysis transform alkene-terminated SAM into a photochemically active state capable of photolysis in the tropospheric actinic window (lambda > 295 nm). Formaldehyde and formic acid are the observed photolysis products. Photodissociation action spectra of oxidized SAM and the observed pattern of gas-phase products are consistent with the well-established Criegee mechanism of ozonolysis of terminal alkenes. There is strong evidence for the presence of secondary ozonides (1,3,4-trioxalones) and other peroxides on the oxidized SAM surface. The data imply that photolysis plays a role in atmospheric aging of primary and secondary organic aerosol particles.     

Methods for characterization of organic compounds in atmospheric aerosol particles

Atmospheric aerosol particles of primary or secondary, biogenic or anthropogenic origin are highly complex samples of changing composition in time and space. To assess their effects on climate or human health, the size-dependent chemical composition of these ubiquitous atmospheric constituents must be known. The development of novel analytical methods has enabled more detailed characterization of the organic composition of aerosols. This review gives an overview of the methods used in the chemical characterization of atmospheric aerosol particles, with a focus on mass-spectrometry techniques for organic compounds, either alone or in combination with chromatographic separation. Off-line, on-site, and on-line methods are covered, and the advantages and limitations of the different methods are discussed. The main emphasis is on methods used for detailed characterization of the composition of the organic compounds in aerosol particles. We address and summarize the current state of analytical methods used in aerosol research and discuss the importance of developing novel sampling strategies and analytical instrumentation.

Open image in new window

Graphical Abstract Challenges in the atmospheric aerosol analytics

     

Real-time detection of individual secondary organic aerosol particle from photooxidation of toluene using aerosol time of flight mass spectrometer

Aromatic hydrocarbons are air pollutants in urban atmosphere and mainly from anthropogenic sources, i.e., emission from automotive exhaust and productive process of industry, and they constitute an important fraction of total volatile organic compounds (VOCs). Photochemical ozone and secondary organic aerosols (SOA) could be formed from the photochemical proc- esses of aromatic hydrocarbons. SOA may impact visibility of air, formation of clouds, change of the climate, and human health serio…     

Experimental study on synchrotron radiation photoionization of secondary organic aerosol derived from styrene ozonolysis

Secondary organic aerosol (SOA) produced by ozonolysis of styrene and other alkene compounds is a major part of fine particles in urban atmospheres. The atmospheric ozonolysis process of styrene is simulated in a smog chamber, and the formed SOA particles are detected on-line by a synchronous radiation vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS) in this study. Through molecular ion peaks in the photonionization mass spectra of SOA and the corresponding photoionization efficiency curve, combined with off-line measurement verification of ultraviolet visible and infrared absorption spectra, it is determined that formaldehyde, formic acid, benzene, phenol, benzaldehyde, and benzoic acid are the main constituents of styrene SOA. These provide new information for studying the atmospheric ozonolysis oxidation mechanism of styrene. VUV-PIMS can get over complicated sample preparation procedures, secondary pollution, and other shortcomings of the off-line method and is a useful instrument to measure constituents and unveil the formation process of SOA particles.     

Identification of secondary organic aerosols based on aerosol mass spectrometry

Secondary organic aerosol (SOA) is one of the major components of aerosols in the atmosphere and has not been well understood so far. Due to the complex chemical composition of organic aerosols, the identification of SOA has been a hotspot and difficult issue in the field of aerosol study. This study attempts to quantitatively identify SOA in winter of Shenzhen based on positive matrix factorization (PMF) analysis. Major sources were resolved and SOA was identified subsequently according to the characteristic ion fragments measured by highly time-resolved aerosol mass spectrometer measurement. It showed that in the winter of Shenzhen the average SOA concentration was 9.41 ± 6.33 μg/m³, accounting for 39.9 ± 21.8% of the total organic mass. Compared with primary organic aerosol (POA), the SOA concentrations had no large variation, suggestive of characteristics of regional secondary pollutants. The ratio of SOA/BC had pronounced diurnal variation, similar to that of O _x (O₃+NO₂), indicating SOA formation was significantly controlled by activity of photochemistry in the atmosphere. The most effective period for SOA formation was from 9 am ～3 pm since the SOA/BC ratio increased by 122% during this period. This study provides a new technical method and a new idea for SOA investigation.     

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.