Size-dependent mixing characteristics of volatile and nonvolatile components in diesel exhaust aerosols

Mixing characteristics of particles of different volatilities from a diesel engine were studied with two tandem differential mobility analyzers (TDMAs) and an aerosol particle mass analyzer (APM). In both TDMA systems, a heater was located in the aerosol path between the first and second DMAs. Diesel exhaust particles that were size-selected in the first DMA were passed through the heater, and the change in particle size due to loss of volatile components was determined by the second DMA. On the basis of the volatility measurements, the particles could be separated into two overlapping modes that varied in peak diameter and magnitude depending on the engine operating conditions. Particles in the smaller size mode were almost completely volatile, while those in the larger size mode contained a nonvolatile core. The TDMA data inversion technique used here allowed accurate determination of the mixing ratios of the two types of particles. These data were in turn used to validate a simple fitting method that uses two log-normal curves to obtain the mixing ratios. In some experiments, the APM was used downstream of a TDMA to directly measure the particle mass loss due to evaporation. The loss determined bythe TDMA-APM system was significantly greater than that calculated from mobility size changes measured solely with the TDMA. The TDMA-APM results were used to calculate the size-dependent mass concentrations of volatile and nonvolatile components for particles in the size range from 70 to 200 nm.     

Hydrocarbon condensation in heavy-duty diesel exhaust

The semivolatile mass fraction of diesel exhaust particles was studied using size-resolved on-line techniques (DMA-ELPI; TDMA-ELPI). The average density of the semivolatile liquid on the particles was measured to be approximately 0.8 g/cm3. The measured size resolved values of mass transfer imply that condensation, or diffusion-limited mass transfer, plays a major role in driving the volatile matter to the diesel exhaust particles. The measured mass change values correspond to highly size dependent mass fractions for the semivolatile component, ranging from approximately 20-80%. Integrated over particle size distribution, the volatile mass fractions were 25 and 45% for the two load points studied. Calculation, based on the measured particle properties, indicates that only 10% volatile mass fraction could be explained by monolayer adsorption. The size resolved changes in particle effective density, fractal dimension, volatile mass fractions and mass are all in agreement with theoretical considerations of condensation.     

Toward distinguishing woodsmoke and diesel exhaust in ambient particulate matter

Particulate matter (PM) from biomass burning and diesel exhaust has distinct X-ray spectroscopic, carbon specific signatures, which can be employed for source apportionment. Characterization of the functional groups of a wide selection of PM samples (woodsmoke, diesel soot, urban air PM) was carried out using the soft X-ray spectroscopy capabilities at the synchrotron radiation sources in Berkeley (ALS) and Brookhaven (NSLS). The spectra reveal that diesel exhaust particulate (DEP) matter is made up from a semigraphitic solid core and soluble organic matter, predominantly with carboxylic functional groups. Woodsmoke PM has no or a less prevalent, graphitic signature, instead it contains carbon-hydroxyl groups. Using these features to apportion the carbonaceous PM in ambient samples we estimate that the relative contribution of DEP to ambient PM in an urban area such as Lexington, KY and St. Louis, MO is 7% and 13.5%, respectively. These values are comparable to dispersion modeling data from nonurban and urban areas in California, and with elemental carbon measurements in urban locations such as Boston, MA, Rochester, NY, and Washington, DC.     

Sampling atmospheric carbonaceous aerosols using an integrated organic gas and particle sampler

Measurement of particle-bound organic carbon (OC) may be complicated by sampling artifacts such as adsorption of gas-phase species onto particles or filters or evaporation of semivolatile compounds off the particles. A denuder-based integrated organic gas and particle sampler (IOGAPS), specifically designed to minimize sampling artifacts, has been developed to sample atmospheric carbonaceous aerosols. IOGAPS is designed to first remove gas-phase chemicals via sorption to the XAD-coated denuder, and subsequently particles are trapped on a quartz filter. A backup sorbent system consisting of sorbent- (XAD-4 resin) impregnated filters (SIFs) was used to capture the semivolatile OC that evaporates from the particles accumulated on the upstream quartz filter. A traditional filter pack (FP) air sampler, which uses a single quartz filter to collect the particles, was employed for comparison in this study. Elemental and organic carbon were determined from filter punches by a thermal optical transmittance aerosol carbon analyzer. Field measurements show that there was no significant difference between the elemental carbon concentrations determined by the FP and IOGAPS, indicating that particle loss during the transit through the denuder tube was negligible. Compared with the OC determined by FP (3.9-12.6 microg of C/m3), the lower OC observed on the quartz filter in the IOGAPS (2.2-6.0 microg of C/m3) was expected because of the removal of gas-phase organics by the denuder. Higher semivolatile organic carbon (SVOC) on the backup SIFs during the night (1.24-8.43 microg of C/m3) suggests that more SVOC, emitted from primary sources or formed as secondary organic compounds, partitions onto the particles during the night because of the decreased ambient temperature. These data illustrate the utility of an IOGAPS system to more accurately determine the particle-bound OC in comparison to FP-based systems.     

Relationship between particle mass and mobility for diesel exhaust particles

We used the aerosol particle mass analyzer (APM) to measure the mass of mobility-classified diesel exhaust particles. This information enabled us to determine the effective density and fractal dimension of diesel particles as a function of engine load. We found that the effective density decreases as particle size increases. TEM images showed that this occurs because particles become more highly agglomerated as size increases. Effective density and fractal dimension increased somewhat as engine load decreased. TEM images suggest that this occurs because these particles contain more condensed fuel and/or lubricating oil. Also, we observed higher effective densities when high-sulfur EPA fuel (approximately 360 ppm S) was used than for Fischer-Tropsch fuel (approximately 0 ppm S). In addition, the effective density provides the relationship between mobility and aerodynamic equivalent diameters. The relationship between these diameters enables us to intercompare, in terms of a common measure of size, mass distributions measured with the scanning mobility particle sizer (SMPS) and a MOUDI impactor without making any assumptions about particle shape or density. We show that mass distributions of diesel particles measured with the SMPS-APM are in good agreement with distributions measured with a MOUDI and a nano-MOUDI for particles larger than approximately 60 nm. However, significantly more mass and greater variation were observed by the nano-MOUDI for particles smaller than 40 nm than by the SMPS-APM.     

Characterizing and biological monitoring of polycyclic aromatic hydrocarbons in exposures to diesel exhaust

Polycyclic aromatic hydrocarbons (PAHs) are one of the most important and carcinogenic components in diesel exhaust (DE). Therefore, ambient PAHs concentrations were measured and characterized for work areas in a locomotive engine inspection plant. Pre- and post-shift urine samples and concurrent air samples were collected on 17 workers to measure the concentration of urinary 1-hydroxypyrene (1-OHP), a metabolite of pyrene. Increased urinary 1-OHP concentrations were observed over at least three consecutive sampling days. The biological kinetics of pyrene metabolism was studied with a one-compartment pharmokinetic model. The conversion rate and elimination rate of 1-OHP were estimated using nonlinear mixed-effects model, and validated with multiple nonlinear regression models by assessing the pattern of elimination rates of each worker separately. Urinary 1-OHP was confirmed to be a sensitive marker of PAHs exposure with mean half-life of 29 h in this population of Chinese workers. The study results would be beneficial to future occupational and environmental studies of PAH exposure.     

Significance of semivolatile diesel exhaust organics for secondary HONO formation

The atmospheric origin of nitrous acid (HONO) is largely unknown despite its estimated importance as an OH source during daytime due to its rapid photolysis. Recently, primary HONO contained in automobile exhaust as well as secondary HONO formation on soot particles have been invoked as possible HONO sources, but none of them is able to account for the observed HONO to NOx ratios of up to 0.04 in the atmosphere. In this paper, we show that semivolatile and/or water-soluble species contained in diesel exhaust are significantly involved in secondary HONO formation. These species are not associated with soot when the exhaust exits the tailpipe. To quantify these species and to assess the reaction kinetics leading to HONO, experiments were performed in which filtered but hot diesel exhaust gas interacted with a glass surface as well as a water film mimicking dry and wet surfaces to which exhaust might be exposed. A fraction of 0.023 of the NOx emitted was heterogeneously converted to HONO, which is at least three times more than the primary HONO emissions by diesel engines and a fraction of 50 larger than HONO formed on diesel soot particles that do not contain the semivolatile organics.     

Nucleation mode formation in heavy-duty diesel exhaust with and without a particulate filter

Particle size distribution measurement with direct tailpipe sampling is employed to study the effect of a continuously regenerating diesel particulate filter (CRDPF) on emissions of a heavy-duty diesel engine. The CRDPF consists of an oxidation catalyst and a filter. Tests are conducted using 2 and 40 ppm sulfur content fuels and two steady-state driving modes. The formation of nucleation mode with and without CRDPF is found to depend on different parameters. Without after-treatment, size distribution is observed to have a nucleation mode only at low load. Being independent of the fuel sulfur level (with these low sulfur level fuels), this nucleation mode is suggested to form mainly from hydrocarbons. With a CRDPF-equipped engine, nucleation mode, which was not observed without CRDPF, was found at high load mode only. This nucleation mode formation was found to correlate positively with fuel sulfur content. It is suggested that sulfuric acid is a main nucleating species in this situation, resulting from the effective conversion of SO2 to SO3 in the oxidation catalyst. Using a thermodenuder confirms that the nucleation mode particles are semivolatile in nature.     

Effect of lubricant on the formation of heavy-duty diesel exhaust nanoparticles

The effect of lubricants on nanoparticle formation in heavy-duty diesel exhaust with and without a continuously regenerating diesel particulate filter (CRDPF) is studied. A partial flow sampling system with a particle size distribution measurement starting from 3 nm, approximately, is used. Tests are conducted using four different lubricant formulations, a very low sulfur content fuel, and four steady-state driving modes. A well-documented test procedure was followed for each test. Two different kinds of nanoparticle formation were observed, and both were found to be affected bythe lubricant but in differentway. Without CRDPF, nanoparticles were observed at low loads. No correlation between lubricant sulfur and these nanoparticles was found. These nanoparticles are suggested to form mainly from hydrocarbons. With CRDPF, installed nanoparticles were formed only at high load. The formation correlated positively with the lubricant (and fuel) sulfur level, suggesting that sulfuric compounds are the main nucleating species in this situation. Storage effects of CRDPF had an effect on nanoparticle concentration as the emissions of nanoparticles decreased over time.     

Formation of methyl nitrite and methyl nitrate during plasma treatment of diesel exhaust

FTIR spectroscopy was used to identify CH3ONO and CH3ONO2 as products of the nonthermal plasma treatment of simulated diesel exhaust. This is the first observation of CH3ONO formation in such systems. The yield of CH3ONO relativeto CH3ONO2 scaled linearly with the average [NO]/ [NO2] ratio in the system. A plot of [CH3ONO]/[CH3ONO2] versus [NO]/[NO2] gives a slope of 1.81 +/- 0.30. This result is indistinguishable from the literature value of the rate constant ratio k(CH3O + NO)/k(CH3O + NO2) = (2.6 x 10(-11))/ (1.5 x 10(-11)) = 1.73 +/- 0.37. The experimental observations suggest that reactions of CH3O radicals with NO and NO2 are the sources of CH3ONO and CH3ONO2 in such systems. The linear relationship between the yields of CH3ONO and CH3ONO2 provides a means of estimating the yield of these compounds during nonthermal plasma treatment of diesel exhaust.     

Organic aerosol formation from photochemical oxidation of diesel exhaust in a smog chamber

Diluted exhaust from a diesel engine was photo-oxidized in a smog chamber to investigate secondary organic aerosol (SOA) production. Photochemical aging rapidly produces significant SOA, almost doubling the organic aerosol contribution of primary emissions after several hours of processing at atmospherically relevant hydroxyl radical concentrations. Less than 10% of the SOA mass can be explained using a SOA model and the measured oxidation of known precursors such as light aromatics. However, the ultimate yield of SOA is uncertain because it is sensitive to treatment of particle and vapor losses to the chamber walls. Mass spectra from an aerosol mass spectrometer (AMS) reveal that the organic aerosol becomes progressively more oxidized throughout the experiments, consistent with sustained, multi-generational production. The data provide strong evidence that the oxidation of a wide array of precursors that are currently not accounted for in existing models contributes to ambient SOA formation.     

Chemical speciation of vanadium in particulate matter emitted from diesel vehicles and urban atmospheric aerosols

We report on the development and application of an integrated set of analytical tools that enable accurate measurement of total, extractable, and, importantly, the oxidation state of vanadium in sub-milligram masses of environmental aerosols and solids. Through rigorous control of blanks, application of magnetic-sector-ICPMS, and miniaturization of the extraction/separation methods we have substantially improved upon published quantification limits. The study focused on the application of these methods to particulate matter (PM) emissions from diesel vehicles, both in baseline configuration without after-treatment and also equipped with advanced PM and NO(x) emission controls. Particle size-resolved vanadium speciation data were obtained from dynamometer samples containing total vanadium pools of only 0.2-2 ng and provide some of the first measurements of the oxidation state of vanadium in diesel vehicle PM emissions. The emission rates and the measured fraction of V(V) in PM from diesel engines running without exhaust after-treatment were both low (2-3 ng/mile and 13-16%, respectively). The V(IV) species was measured as the dominant vanadium species in diesel PM emissions. A significantly greater fraction of V(V) (76%) was measured in PM from the engine fitted with a prototype vanadium-based selective catalytic reductors (V-SCR) retrofit. The emission rate of V(V) determined for the V-SCR equipped vehicle (103 ng/mile) was 40-fold greater than that from the baseline vehicle. A clear contrast between the PM size-distributions of V(V) and V(IV) emissions was apparent, with the V(V) distribution characterized by a major single mode in the ultrafine (<0.25 μm) size range and the V(IV) size distribution either flat or with a small maxima in the accumulation mode (0.5-2 μm). The V(V) content of the V-SCR PM (6.6 μg/g) was 400-fold greater than that in PM from baseline (0.016 μg/g) vehicles, and among the highest of all environmental samples examined. Synchrotron based V 1s XANES spectroscopy of vanadium-containing fine-particle PM from the V-SCR identified V(2)O(5) as the dominant vanadium species.     

山西老陈醋晾晒前、后挥发性香味物质比较分析

山西老陈醋晾晒前、后样品采用同时蒸馏萃取处理,然后气-质谱联机、稀释法气相色谱-嗅闻分析。基于保留指数、质谱、标准品,气-质谱联机分析共鉴定出183种挥发性化合物,包括含硫化合物、含氮杂环、含氧杂环、醛类、酮类、醇类、酸类、酯类、酚类、醚类、其它类。两种醋中主要挥发性化合物均包括乙酸、乙偶姻、糠醛、2,3-丁二酮等,而晒后醋中吡嗪类化合物尤其四甲基吡嗪的含量显著增加。基于保留指数、气味特征、气-质谱联机鉴定结果、标准品,从两种醋中共鉴定出50种香气活性物质,其中晒前醋中稀释因子高的为乙酸、乙偶姻、2,3-丁二酮、乳酸乙酯、愈创木酚等10种化合物,晒后醋中稀释因子高的为3-甲硫基丙醇、糠醛、苯乙醛、愈创木酚等20种化合物。相比之下,醋晾晒前后主要变化为酯类物质减少,一些含氮杂环化合物尤其是吡嗪类化合物及含硫化合物的含量增加。研究结果可为山西老陈醋生产工艺的升级改造提供基础数据。     

On-road measurement of particle emission in the exhaust plume of a diesel passenger car

Particle size distributions were measured under real world dilution conditions in the exhaust plume of a diesel passenger car closely followed by a mobile laboratory on a high speed test track. Under carefully controlled conditions the exhaust plume was continuously sampled and analyzed inside the mobile laboratory. Exhaust particle size distribution data were recorded together with exhaust gas concentrations, i.e., CO, CO2, and NO(x), and compared to data obtained from the same vehicle tested on a chassis dynamometer. Good agreement was found for the soot mode particles which occurred at a geometric mean diameter of approximately 50 nm and a total particle emission rate of 10(14) particles km(-1). Using 350 ppm high sulfur fuel and the standard oxidation catalyst a bimodal size distribution with a nucleation mode at 10 nm was observed at car velocities of 100 km h(-1) and 120 km h(-1), respectively. Nucleation mode particles were only present if high sulfur fuel was used with the oxidation catalyst installed. This is in agreement with prior work that these particles are of semivolatile nature and originate from the nucleation of sulfates formed inside the catalyst. Temporal effects of the occurrence of nucleation mode particles during steady-state cruising and the dynamical behavior during acceleration and deceleration were investigated.     

Nucleation particles in diesel exhaust: composition inferred from in situ mass spectrometric analysis

Mass spectrometric measurements of size and composition of diesel exhaust particles have been performed under various conditions: chassis dynamometer tests, field measurements near a German motorway, and individual car chasing. Nucleation particles consisting of volatile sulfate and organic material could be detected both at the chassis dynamometer test facility and during individual car chasing. We found evidence that if nucleation occurs, sulfuric acid/water is the nucleating agent. Low-volatile organics species condense only on the preexisting sulfuric acid/water clusters. Nucleation was found to depend strongly on various parameters such as exhaust dilution conditions, fuel sulfur content, and engine load. The latter determines the fraction of the fuel sulfur that is converted to sulfuric acid. The organic compounds (volatile and low-volatile) condense only on preexisting particles, such as both sulfuric acid nucleation particles and larger accumulation mode soot particles. On the latter, sulfuric acid also condenses, if the conditions for nucleation are not given. The overall ratio of sulfate to organic (volatile and low-volatile) is also strongly dependent on the engine load. It was found that the production of nucleation particles even at high engine load can be suppressed by using low-sulfur fuel.     

Sorption of a diverse set of organic vapors to diesel soot and road tunnel aerosols

In a traffic-dominated environment sorption of organic pollutants to exhaust aerosols can strongly determine their further fate. The sorption properties of two aerosol samples representing different exhaust sources have been determined for a large set of diverse organic vapors. For pure diesel soot we could identify adsorption to elemental carbon (EC) as the dominant sorption process. We used our experimental equilibrium adsorption coefficients to derive a predictive model for adsorption on soot in line with adsorption models for other surfaces published earlier. On road tunnel aerosols, both adsorption to EC and absorption in organic matter (OM) governed the observed sorption and the data could not be further evaluated in terms of a specific sorption mechanism.