二次有机气溶胶的特征和形成机制

二次有机气溶胶(SOA)的形成是当今大气化学过程的研究热点之一. 城市大气中二次有机碳占颗粒物总有机碳的17%~65%. 单萜烯和芳香族化合物分别是SOA最重要的天然和人为源前体物,在大气中与×OH, NO3-和O3等氧化剂发生多途径反应形成有机酸、多官能团羰基化合物、硝基化合物等半挥发性有机物,通过吸附、吸收等过程进入颗粒相,改变了气溶胶的特性及其环境效应. 有机化合物表现出较强的源特征性,可以作为示踪分子解析颗粒物来源,二元羧酸是SOA的潜在示踪物. 论述了SOA研究的最新进展并指出了未来的研究方向.     

二次有机气溶胶中二羰基化合物的研究进展

二次有机气溶胶的形成是大气环境化学的研究热点。二羰基化合物对二次有机气溶胶形成具有重要的研究意义。由于二羰基化合物在液相中发生聚合反应,在液相中的分配大于Henry定律的预测值,影响二次有机气溶胶的形成。通过二羰基化合物乙二醛和甲基乙二醛在大气中光氧化和异相反应的研究,评述了二羰基化合物对二次有机气溶胶形成的研究发展。     

The Thermal-Stability of Oligomers in Alpha-Pinene Secondary Organic Aerosol

Recent studies characterizing laboratory generated secondary organic aerosol (SOA) have shown that 50% or more of the particle mass is made up of oligomeric species, which based on molecular weight and structure should be nonvolatile. On the other hand, about 75% the particle mass is lost when SOA is passed through a denuder heated to 100°C, suggesting that it is semi-volatile. To study the reason for this discrepancy, the chemical effects of a thermal denuder (TD) on oligomers in laboratory SOA formed by ozonolysis of α-pinene were examined with high-resolution molecular mass spectrometry. Both monomers and oligomers in the particle phase were lost to the gas phase upon heating to 100°C. Additionally, when the species released to the gas phase during heating were allowed to readsorb onto the remaining particle mass, almost all of the readsorbed material was monomeric. Both of these observations suggest that oligomers decompose when heated in a TD. Chemical analysis of the remaining particle mass after passing through the TD showed formation of many new compounds that corresponded to oligomer decomposition products. While similar experiments should be performed with other aerosol types, these results suggest that care must be taken when attempting to use a TD at an elevated temperature to infer the volatility of SOA at ambient temperature.

Copyright 2012 American Association for Aerosol Research     

种子气溶胶对甲苯光氧化生成二次有机气溶胶的生长影响

在烟雾腔内进行了羟基自由基启动的甲苯光氧化生成二次有机气溶胶的实验.通过向烟雾腔内添加硫酸铵、硝酸铵、硅酸钠和氯化钙4种不同的种子气溶胶,考察它们对二次有机气溶胶形成的影响.结果表明:种子气溶胶的浓度越低,越有利于二次有机气溶胶的生成;在反应开始阶段,种子气溶胶对二次有机气溶胶的形成起促进作用,随着反应时间的增长,反而起阻止作用:在相同的种子气溶胶浓度下,硅酸钠和硝酸铵对二次有机气溶胶的形成的影响几乎相同;4种种子气溶胶对甲苯氧化生成的二次有机气溶胶的生长影响顺序为:氯化钙》硝酸铵、硅酸钠》硫酸铵.     

Collection Efficiency of the Aerosol Mass Spectrometer for Chamber-Generated Secondary Organic Aerosols

The collection efficiency (CE) of the aerosol mass spectrometer (AMS) for chamber-generated secondary organic aerosol (SOA) at elevated mass concentrations (range: 19–207 μg m^?3; average: 64 μg m^?3) and under dry conditions was investigated by comparing AMS measurements to scanning mobility particle sizer (SMPS), Sunset semi-continuous carbon monitor (Sunset), and gravimetric filter measurements. While SMPS and Sunset measurements are consistent with gravimetric filter measurements throughout a series of reactions with varying parent hydrocarbon/oxidant combinations, AMS CE values were highly variable ranging from unity to <15%. The majority of mass discrepancy reflected by low CE values does not appear to be due to particle losses either in the aerodynamic lens system or in the vacuum chamber as the contributions of these mechanisms to CE are low and negligible, respectively. As a result, the largest contribution to CE in the case of chamber-generated SOA appears to be due to particle bounce at the vaporizer surface before volatilization, which is consistent with earlier studies that have investigated the CE of ambient and select laboratory-generated particles. CE values obtained throughout the series of reactions conducted here are also well correlated with the f ₄₄/f ₅₇ ratio, thereby indicating both that the composition of the organic fraction has an important impact on the CE of chamber-generated SOA and that this effect may be linked to the extent to which the organic fraction is oxidized.

Copyright 2013 American Association for Aerosol Research     

甲苯与氯原子光氧化产生二次有机气溶胶粒子实时探测研究

在烟雾腔内,用紫外光照射甲苯、氯气、一氧化氮和空气的混合物,启动甲苯和氯原子的光氧化反应和一系列的后续反应,产生非挥发性和半挥发性有机化合物.半挥发性有机化合物可以在气态和粒子态之间进行分配,产生二次有机气溶胶粒子.用实验室自制的气溶胶飞行时间质谱仪实时测量这些粒子产物的分子成分和粒径分布,并初步探讨了这些组分的可能反应机理.     

Secondary Organic Aerosol Coating Formation and Evaporation: Chamber Studies Using Black Carbon Seed Aerosol and the Single-Particle Soot Photometer

We report a protocol for using black carbon (BC) aerosol as the seed for secondary organic aerosol (SOA) formation in an environmental chamber. We employ a single-particle soot photometer (SP2) to probe single-particle SOA coating growth dynamics and find that SOA growth on nonspherical BC aerosol is diffusion-limited. Aerosol composition measurements with an Aerodyne high resolution time-of-flight aerosol mass spectrometer (AMS) confirm that the presence of BC seed does not alter the composition of SOA as compared to self-nucleated SOA or condensed SOA on ammonium sulfate seed. We employ a 3-wavelength photoacoustic soot spectrometer (PASS-3) to measure optical properties of the systems studied, including fullerene soot as the surrogate BC seed, nucleated naphthalene SOA from high-NO_x photooxidation, and nucleated α-pinene SOA from low-NO_x photooxidation. A core-and-shell Mie scattering model of the light absorption enhancement is in good agreement with measured enhancements for both the low- and high-NO_x α-pinene photooxidation systems, reinforcing the assumption of a core-shell morphology for coated BC particles. A discrepancy between measured and modeled absorption enhancement factors in the naphthalene photooxidation system is attributed to the wavelength-dependence of refractive index of the naphthalene SOA. The coating of high-NO_x α-pinene SOA decreases after reaching a peak thickness during irradiation, reflecting a volatility change in the aerosol, as confirmed by the relative magnitudes of f₄₃ and f₄₄ in the AMS spectra. The protocol described here provides a framework by which future studies of SOA optical properties and single-particle growth dynamics may be explored in environmental chambers.

Copyright 2013 American Association for Aerosol Research     

Interpretation of Secondary Organic Aerosol Formation from Diesel Exhaust Photooxidation in an Environmental Chamber

Secondary organic aerosol (SOA) formation from diesel exhaust was investigated using an environmental chamber. Particle volume measurement based solely on mobility diameter underestimated the SOA formation from diesel exhaust due to the external void space of agglomerate particles. Therefore, particle mass concentration and fractal-like dimension was determined from the particle effective density as a function of particle mass using an aerosol particle mass analyzer and scanning mobility particle sizer (APM–SMPS). Continuous aging of aerosol measured by an increase of atomic ratio (O/C) underscored the importance of multigenerational oxidation of low-volatile organic vapors emitted from diesel engine as a possible significant source of ambient oxygenated SOA. Higher particle effective densities were observed when raw exhaust was injected into a full bag as opposed to filling a bag with diluted exhaust using an ejector diluter. This suggests that the dilution method, in addition to dilution ratio, may impact the evaporation of semivolatile species. This study demonstrates the critical need to evaluate particle mass when evaluating SOA formation onto fractal particles such as diesel exhaust.     

Water Uptake by NaCl Particles Prior to Deliquescence and the Phase Rule

Using an environmental transmission electron microscope (ETEM), we show that a significant amount of water, far exceeding the multilayers caused by surface adsorption, is reversibly associated prior to deliquescence with substrate-supported NaCl particles (dry diameters of ～ 40 nm to 1.5 μ m; ～ 18°C). We hypothesize that the water is present as an aqueous solution containing dissolved Na and Cl ions. Water uptake occurs at relative humidities (RH) as low as 70%, and the resulting liquid layer coating the particles is stable over extended times if the RH is held constant. We exposed CaSO ₄ and CaSO ₄ · 2H ₂ O particles to elevated RH values in the ETEM to show that chemically nonspecific condensation of gas-phase water on the TEM substrate does not explain our observations. Furthermore, damage to the NaCl surface induced by the electron beam and small fluctuations in RH do not seem to contribute to or otherwise affect water uptake. We have similar observations of water association for other alkali halide particles, including NaBr and CsCl, prior to deliquescence. To explain the observations, we derive the phase rule for this geometry and show that it allows for the coexistence of liquid, solid, and vapor for the binary NaCl/H ₂ O system across a range of RH values. The derivation includes the effects of heterogeneous pressure because of the Laplace-Young relations for the subsystems. Furthermore, in view of the lever rule and the absence of similar observations for free-floating pure NaCl aerosol particles, we hypothesize that the surface energy necessary to support these effects is provided by sample-substrate interactions. Thus, the results of this study may be relevant to atmospheric systems in which soluble compounds are associated with insoluble materials.     

Effects of Relative Humidity on Ozone and Secondary Organic Aerosol Formation from the Photooxidation of Benzene and Ethylbenzene

The formation of ozone and secondary organic aerosol (SOA) from benzene–NO _x and ethylbenzene–NO_x irradiations was investigated under different levels of relative humidity (RH) in a smog chamber. In benzene and ethylbenzene irradiations, the intensity of the bands of O?H, C?O, C?O, and C?OH from SOA samples all greatly increased with increasing RH. The major substances in SOA were determined to be carboxylic acids and glyoxal hydrates. It was also found that SOA contained aromatic products, and NO₂- and ONO₂-containing products. The results show that the increase in RH can greatly reduce the maximum O₃ by the transfer of NO₂- and ONO₂-containing products into the particle phase. During the process of evaporation, the lost substances from the collected SOA have similar structures for both benzene and ethylbenzene. This demonstrates that ethyl-containing substances are very stable and difficult to evaporate. For benzene, some of glyoxal hydrates were left to form C?O?C- and C?O-containing species like hemiacetal and acetal after evaporation, whereas for ethylbenzene, glyoxal favored cross reactions with ethylglyoxal during evaporation. Only a few species in SOA were released into the gas phase during evaporation while a large part of SOA remained, which is mainly composed of carboxylic acid. It is concluded that the aqueous radical reactions and the hydration from glyoxal can be enhanced under high RH conditions, which can irreversibly enhance the formation of SOA from both benzene and ethylbenzene.

Copyright 2014 American Association for Aerosol Research     

Local and Regional Secondary Organic Aerosol: Insights from a Year of Semi-Continuous Carbon Measurements at Pittsburgh

We present Scanning Mobility CCN Analysis (SMCA) as a novel method for obtaining rapid measurements of size-resolved cloud condensation nuclei (CCN) distributions and activation kinetics. SMCA involves sampling the monodisperse outlet stream of a Differential Mobility Analyzer (DMA) operated in scanning voltage mode concurrently with CCN and condensation particle counters. By applying the same inversion algorithm as used for obtaining size distributions with a scanning mobility particle sizer (SMPS), CCN concentration and activated droplet size are obtained as a function of mobility size over the timescale of an SMPS scan (typically 60–120 s). Methods to account for multiple charging, particle non-sphericity, and limited counting statistics are presented. SMCA is demonstrated using commercial SMPS and CFSTGC instruments with the manufacturer-provided control software. The method is evaluated for activation of both laboratory aerosol and ambient aerosol obtained during the 2004 NEAQS-ITCT2k4 field campaign. It is shown that SMCA reproduces the results obtained with a DMA operating in voltage “stepping” mode.     

On the Importance of Organic Oxygen for Understanding Organic Aerosol Particles

This study shows how aerosol organic oxygen data could provide new information about organic aerosol mass, aqueous solubility of organic aerosols, formation of secondary organic aerosol (SOA) and the relative contributions of anthropogenic and biogenic sources. For more than two decades, atmospheric aerosol organic mass (OM) concentration has been estimated by multiplying the measured carbon content by an assumed (OM)-to-organic carbon (OC) factor, usually 1.4. However, this factor can vary from 1.0 to 2.5 depending on location. This large uncertainty about aerosol organic mass limits our understanding of the influence of organic aerosol on climate, visibility and health. New examination of organic aerosol speciation data shows that the oxygen content is responsible for the observed range in the OM-to-OC factor. When organic oxygen content is excluded, the ratio of non-oxygen organic mass to carbon mass varies very little across different environments (1.12 to 1.14). The non-oxygen-OM-to-OC factor for all studied sites (urban and non-urban) averaged 1.13. The uncertainty becomes an order of magnitude smaller than the uncertainty in the best current estimates of organic mass to organic carbon ratios (1.6 ± 0.2 for urban and 2.1 ± 0.2 for non-urban areas). This analysis suggests that, when aerosol organic oxygen data become available, organic aerosol mass can be quite accurately estimated using just OC and organic oxygen (OO) without the need to know whether the aerosol is fresh or aged. In addition, aerosol organic oxygen data will aid prediction of water solubility since compounds with OO-to-OC higher than 0.4 have water solubilities higher than 1 g per 100 g water.     

VO2薄膜相变及其温度滞后

以攀钢产V2O5为原料，采用无机胶体法制备电阻突变VO2薄膜，研究了VO2薄膜的电阻突变温度、突变数量级及其突变温度滞后. 结果表明，VO2薄膜相变温度为35℃，制备方法和衬底材质对VO2薄膜的电阻突变数量级有较大影响. 以普通玻璃和石英玻璃作衬底、用H2还原法，可使VO2薄膜的电阻突变达到2~3个数量级，用N2热分解法仅能达到1.5~2个数量级；普通玻璃衬底上VO2薄膜的电阻突变数量级小于石英玻璃衬底上的VO2薄膜的电阻突变数量级. VO2薄膜的电阻突变温度滞后为1~6℃，电阻突变数量级、衬底材质和制备方法对其有较大影响.     

滞后低温相变储能微胶囊的制备和性能研究

本文采用三聚氰胺-尿素-甲醛树脂(MUF)为壳层,PCM为芯材,制备了一种新型的有机低温微胶囊化相变材料(MPCMs),并利用红外光谱、光学显微镜和粒度仪对微胶囊进行表征,同时采用差示扫描量热仪(DSC)和热失重仪(TGA)分别研究了相变材料(PCM)和PCM微胶囊的相变特性和热稳定性。结果表明:制备的PCM微胶囊大小均一、平均粒径为1μm左右、光滑的球形颗粒。随着芯壳比的增加,微胶囊的相变潜热逐渐增大,结晶温度逐渐降低,微胶囊化PCM的熔点与纯PCM的结晶温度最大差值达85.8℃,微胶囊化PCM呈现出优异的相变滞后性能和良好的热稳定性。     

Organic Aerosol Speciation: Intercomparison of Thermal Desorption Aerosol GC/MS (TAG) and Filter-Based Techniques

We compared measurements of organic molecular markers made using a novel Thermal Desorption Aerosol Gas Chromatograph/Mass Spectrometer (TAG) with two offline filter-based methods: solvent extraction GC/MS (SE-GC/MS) and thermal desorption GC/MS (TD-GC/MS). TAG is designed for automated, high time-resolved measurements of organic aerosol speciation. Laboratory and field measurements were performed to compare TAG and filter performance for n-alkanes, n-alkanoic acids, cholesterol, PAHs, and hopanes. Laboratory measurements of model organic aerosol mixtures of known composition were made in the Carnegie Mellon University smog chamber, and field measurements were made in downtown Pittsburgh. There was excellent agreement between techniques for hopanes and several PAHs, which are important markers for motor vehicle emissions. Agreement was also strong for moderately polar and nonpolar species in the high-concentration smog chamber experiments. Poorer agreement between filter and TAG observations was obtained for n-alkanes in ambient measurements. To further investigate the differences in n-alkane performance between these methods, potential matrix effects and internal consistency within the TAG and filter ambient air data sets were examined. We spiked a subset of ambient TAG samples with deuterated internal standards to examine potential matrix effects. Under typical conditions, there was little bias in recovery of deuterated standards. At extreme ambient aerosol levels, however, there were large biases in relative recovery, indicating matrix effects may be important under those conditions. Similar results were observed in laboratory experiments with engine lubricating oil. Applying internal standards in the TAG system would help track and correct for matrix effects influencing compound recovery.     

Chemistry of Secondary Organic Aerosol Formation from OH Radical-Initiated Reactions of Linear,Branched, and Cyclic Alkanes in the Presence of NO x

The effects of molecular structure on the products and mechanisms of SOA formation from OH radical-initiated reactions of linear, branched, and cyclic alkanes in the presence of NO _x were investigated in a series of environmental chamber experiments. SOA mass spectra were obtained in real time and off line using a thermal desorption particle beam mass spectrometer and used to identify reaction products. Real-time mass spectra were used to classify products according to their temporal behavior, and off-line temperature-programmed thermal desorption analysis of collected SOA was used to separate products by volatility prior to mass spectral analysis and to gain information on compound vapor pressures. A reaction mechanism that includes gas- and particle-phase reactions was developed that explains the formation of SOA products and is consistent with the various lines of mass spectral information. Results indicate that the SOA products formed from the reactions of linear, branched, and cyclic alkanes are similar, but differ in a few important ways. Proposed first-generation SOA products include alkyl nitrates, 1,4-hydroxynitrates, 1,4-hydroxycarbonyls, and dihydroxycarbonyls. The 1,4-hydroxycarbonyls and dihydroxycarbonyls rapidly isomerize in the particle phase to cyclic hemiacetals that then dehydrate to volatile dihydrofurans. This conversion process is catalyzed by HNO ₃ formed in the chamber and is slowed by the presence of NH ₃ . Volatile products can react further with OH radicals, forming multi-generation products containing various combinations of the same functional groups present in first-generation products. For linear and branched alkanes, the products are acyclic or monocyclic, whereas for cyclic alkanes they are acyclic, monocyclic, or bicyclic. Some of the products, especially those formed from ring-opening reactions of cyclic alkanes appear to be low volatility oligomers. The implications of the results for the formation of atmospheric SOA are discussed.     

Aerosol Growth in a Steady-State,Continuous Flow Chamber: Application to Studies of Secondary Aerosol Formation

An analytical solution for the steady-state aerosol size distribution achieved in a steady-state, continuous flow chamber is derived, where particle growth is occurring by gas-to-particle conversion and particle loss is occurring by deposition to the walls of the chamber. The solution is presented in the case of two condensing species. By fitting the predicted steady-state aerosol size distribution to that measured, one may infer information about the nature of the condensing species from the calculated values of the species's molecular weights. The analytical solution is applied to three sets of experiments on secondary organic aerosol formation carried out in the U.S. Environmental Protection Agency irradiated continuous flow reactor, with parent hydrocarbons: toluene, f -pinene, and a mixture of toluene and f -pinene. Fits to the observed size distributions are illustrated by assuming two condensing products for each parent hydrocarbon; this is a highly simplified picture of secondary organic aerosol formation, which is known to involve considerably more than two condensing products. While not based on a molecular-level model of the gas-to-particle conversion process, the model does allow one to evaluate the extent to which the observed size distribution agrees with that based on a simple, two-component picture of condensation, and to study the sensitivity of those size distributions to variation of the essential properties of the condensing compounds, such as molecular weight. An inherent limitation of the steady-state experiment is that it is not possible to calculate the vapor pressures of the condensing species.     

Evolved Gas Analysis of Secondary Organic Aerosols

Secondary organic aerosols have been characterized by evolved gas analysis (EGA). Hydrocarbons selected as aerosol precursors were representative of anthropogenic emissions (cyclohexene, cyclopentene, 1-decene and 1-dodecene, n-dodecane, o-xylene, and 1,3,5-tri-methylbenzene) and of biogenic emissions (the terpenes α-pinene, β-pinene and d-limonene and the sesquiterpene trans-caryophyllene). Also analyzed by EGA were samples of secondary, primary (highway tunnel), and ambient (urban) aerosols before and after exposure to ozone and other photochemical oxidants. The major features of the EGA thermograms (amount of CO₂ evolved as a function of temperature) are described. The usefulness and limitations of EGA data for source apportionment of atmospheric particulate carbon are briefly discussed.     

Bias in Filter-Based Aerosol Light Absorption Measurements Due to Organic Aerosol Loading: Evidence from Ambient Measurements

During the 2006 Texas Air Quality Study/Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS 2006) a filter-based (Particle Soot Absorption Photometer, or PSAP) and a photoacoustic-based aerosol light absorption technique were deployed and here the data are compared. The level of agreement between the two techniques with ambient aerosol depended on the abundance of organic aerosol (OA), with the ratio of OA to light absorbing carbon (LAC) mass (R _OA?LAC ) of particular importance. When OA mass concentration was low the agreement between the methods was within instrumental uncertainties (PSAP measuring 12% higher), however at high (R _OA?LAC ) (～15–20) the difference in agreement was between 50 and 80%. This difference is similar to the bias observed in the laboratory studies of a companion paper using non-absorbing OA and LAC (Cappa et al. 2008a Cappa, C., Lack, D., Burkholder, J. and Ravishankara, A. 2008a. Bias in Filter-Based Aerosol Light Absorption Measurements Due to Organic Aerosol Loading: Evidence from Laboratory Measurements. Aerosol Sci. Technol, 42: 1022–1033. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]). It was found that most of the OA was oxidized and non-absorbing in nature. We postulate that the observed differences results from a bias in the filter-based measurements due to (a) the redistribution of liquid-like organic particulate matter (PM) around the fiber filters thereby modifying the filter surface and subsequent light scattering, and (b) the possible coating and absorption enhancement of pre-existing absorbing PM (i.e., soot) as OA deposition and redistribution occurs. We stop short or recommending a universal correction using these findings due to the magnitude of the bias showing some dependence on air mass type. Any use of this data for a correction must consider the uncertainties in measuring OA mass concentrations, LAC mass concentrations and type of OA present. The observed PSAP bias may have a significant impact on the accuracy of aerosol single scatter albedo (SSA) and LAC mass concentrations derived from filter-based aerosol absorption methods from regions impacted by large amounts of OA. Although this study was performed using the PSAP we caution users of other filter-based instrumentation to the possibility of a similar bias in those instruments.     

常用炸药潮解的原因及一般规律

潮解性并不是在任何一个相对湿度下都存在的,而是存在某一特定的相对湿度数值,当小于这一值时就没有潮解性,只有相对湿度超过这一值时,才表现出显著的吸水能力。通过计算部分炸药在20℃时的吸湿点,以及对常见炸药不同温度时水及盐的饱和蒸汽压试验,分析吸湿点与温度的关系,探索其潮解的原因及一般规律。