High Sensitivity of Diesel Soot Morphological and Optical Properties to

Article pubs.acs.org/est

High Sensitivity of Diesel Soot Morphological and Optical Properties to Combustion Temperature in a Shock Tube Chong Qiu,*,†,‡ Alexei F. Khalizov,§,∥ Brian Hogan,⊥ Eric L. Petersen,⊥ and Renyi Zhang*,‡,§ †

Department of Chemistry & Industrial Hygiene, University of North Alabama, Florence, Alabama 35632-5049, United States Department of Chemistry, §Department of Atmospheric Sciences, and ⊥Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States ∥ Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States ‡

S Supporting Information *

ABSTRACT: Carbonaceous particles produced from combustion of fossil fuels have strong impacts on air quality and climate, yet quantitative relationships between particle characteristics and combustion conditions remain inadequately understood. We have used a shock tube to study the formation and properties of diesel combustion soot, including particle size distributions, effective density, elemental carbon (EC) mass fraction, massmobility scaling exponent, hygroscopicity, and light absorption and scattering. These properties are found to be strongly dependent on the combustion temperature and fuel equivalence ratio. Whereas combustion at higher temperatures (∼2000 K) yields fractal particles of a larger size and high EC content (90 wt %), at lower temperatures (∼1400 K) smaller particles of a higher organic content (up to 65 wt %) are produced. Single scattering albedo of soot particles depends largely on their organic content, increasing drastically from 0.3 to 0.8 when the particle EC mass fraction decreases from 0.9 to 0.3. The mass absorption cross-section of diesel soot increases with combustion temperature, being the highest for particles with a higher EC content. Our results reveal that combustion conditions, especially the temperature, may have significant impacts on the direct and indirect climate forcing of atmospheric soot aerosols.

■

INTRODUCTION Diesel engines are widely used in on-road and off-road vehicles, marine vessels, and electrical generators. A characteristic feature of the diesel engine is that the fuel is sprayed into the combustion chamber in liquid form where part of the fuel evaporates and mixes with the air and the rest of the fuel remains as droplets. Although the global fuel equivalence ratio (the actual fuel/ oxygen ratio divided by stoichiometric fuel/oxygen ratio) in diesel engines corresponds to lean combustion conditions, local gas pockets rich in fuel vapor and liquid fuel droplets are present during the combustion process. Diffusion-limited combustion of rich vapor-air mixtures and surface combustion of the fuel droplets1 produce significant amounts of particulate elemental 2 carbon (EC), with an emission factor of 0.84+0.44 −0.29 g C per kg fuel and a total estimated annual global emission rate of 1150−1320 Gg.3 In addition to EC, the combustion process in diesel engines introduces unburned fuel, lubricating oil, and ash.4 The interaction between different exhaust constituents upon leaving the hot combustion chamber forms diesel soot particles that contain variable amounts of organic carbon (OC), inorganic sulfates, and metal oxides, depending on the fuel composition and engine operating conditions.5,6 The formation of internally mixed particles may change the properties of diesel soot, altering its environmental impacts. For example, polycyclic aromatic hydrocarbons (PAHs) comprising a significant fraction of the nascent soot OC are well-known carcinogens and mutagens.7 The presence of PAH on diesel soot particles, which have the © 2014 American Chemical Society

ability to penetrate deep into human lungs, exacerbates the adverse health effects of soot exposure.8 Upon entering the atmosphere, soot particles are subjected to further transformations, which involve the phase partitioning of organic and inorganic gaseous species,9−17 heterogeneous reactions with oxidants,18 and coagulation with other aerosols. In particular, photochemical oxidation of volatile organic compounds (VOCs) leads to various products,19−21 some of which can contribute to formation of organic coating on soot particles.14,15,17 The rate at which the internal mixing occurs may depend on the composition and properties of nascent soot particles. The resulting internally mixed soot has significantly altered properties, including effective density, morphology, hygroscopicity, single scattering albedo (SSA), and chemical reactivity.14,15,22−26 As a result of internal mixing, the contribution of soot aerosol to climate forcing through the direct effect (absorption and scattering of the solar radiation) and indirect effect (serving as cloud condensation nuclei) is significantly modified.27 Currently, large uncertainties in the understanding of the climate forcing by aerosols considerably hinder accurate predictions of climate change.27 In addition, internally mixed soot particles may exert a profound impact on photochemistry and local weather.28−32 Received: Revised: Accepted: Published: 6444

December 16, 2013 March 31, 2014 May 6, 2014 May 6, 2014 dx.doi.org/10.1021/es405589d | Environ. Sci. Technol. 2014, 48, 6444−6452

Environmental Science & Technology

Article

Figure 1. Schematic of the experimental setup (a), typical laser signal attenuation traces for combustion of diesel and propane at 1950 K behind the reflected shock wave (b), and diesel soot yield as a function of the combustion temperature (c).

spectrometer (CRDS).42 This approach has been used to obtain comprehensive data on the formation and properties of soot from combustion of propane/oxygen mixtures under a broad range of conditions.42 In this study, we report the application of the combined shocktube generation and postshock characterization approach to the investigation of soot from combustion of diesel fuel premixed with oxygen. The major goal was to elucidate the dependence of soot particle concentration, composition, morphology, effective density, hygroscopicity, and optical properties on the combustion temperature and pressure for diesel/oxygen mixtures with different fuel equivalence ratios.

The formation and atmospheric transformations of soot emitted by different sources are currently subjects of intensive research. For example, Kittelson et al. evaluated emissions from real diesel engines, via both on-road and dynamometer measurements, providing emission levels and properties of diesel soot.4 Surrogate combustion sources, including diffusion flame burners,33,34 premixed flame burners,35−37 and spark discharge generators,38 are also widely applied for characterization of soot particle properties. Recently, shock tubes have received increasing attention as a tool for studying the production of nanoparticles from fossil fuel combustion.39−41 The shock tube can reproduce the high temperature and pressure inside the internal combustion engine, with the advantages of a wide choice of fuels and fuel-to-oxygen ratios, along with independently tunable fuel equivalence ratio and temperature.39 For instance, the shock-tube approach has allowed exploring the soot formation under high equivalence ratio−high temperature conditions, which cannot be achieved using flame soot sources.42 Combined with in situ light emission and laser extinction measurements, shock-tube studies can provide time-resolved data on the soot formation upon combustion of aliphatic and aromatic hydrocarbons, including the nucleation and growth of primary soot spherules and the effects of various additives.43 However, owing to the transient nature of the combustion process in the shock tube, limited information can be obtained regarding the composition and morphology of soot particles from direct in situ measurements.42 Also, in situ laser extinction is not sufficiently sensitive to detect small soot particles produced at a low mass yield in experiments with leaner fuel−air mixtures.42 These limitations have been circumvented using a novel approach that integrates shock-tube combustion experiments with postshock measurements of soot aerosol that is transferred from the shock tube following the combustion event.42 Postshock measurements are performed using a suite of aerosol instruments, including a tandem differential mobility analyzer (TDMA), an aerosol particle mass analyzer (APM), a nephelometer, and a cavity ring-down

■

EXPERIMENTAL SECTION Shock-Tube Combustion Experiments. A schematic of the shock tube is shown in Figure 1a. The details of the shocktube facility, standard operation procedures, and soot yield calculations based on in situ laser extinction measurements have been reported previously42 and are described in detail in Supporting Information. No tailoring of the driver gas was required to extend the postshock test times for the present diesel combustion experiments because soot formed promptly behind the reflected shockwave. The fuel used in all experiments was Diesel No. 2 Neat Standard (Mineral oil type A, CAS# 68476-346, Sigma-Aldrich). Measurements of Combustion Soot Properties. After the tube reached pressure equilibration, a known volume (typically 20 L) of the gas mixture was transferred from the shock tube to a ∼1 m3 environmental chamber prefilled with particle-free zero air (Figure 1) where it was stirred thoroughly with a fan, resulting in a dilution factor of about 50. Aerosol was sampled continuously from the chamber and was analyzed using a suite of aerosol instruments. Details of the soot aerosol sampling and characterization techniques have been reported elsewhere,42 and only a brief description is provided below. The mobility diameter, mass, nonvolatile mass fraction, and hygroscopicity of soot particles were measured using an 6445

dx.doi.org/10.1021/es405589d | Environ. Sci. Technol. 2014, 48, 6444−6452


Article

Figure 2. Mobility size distributions of soot generated from the combustion of diesel/O2 mixtures at different temperatures with the fuel equivalence ratio ϕ of (a) 2.8, (b) 8.1 and (c) 15. In panel d, the averaged mobility diameter Davg of soot is plotted as a function of combustion temperature for three different diesel/O2 ratios.

integrated aerosol analytical system consisting of two DMAs, an APM, a condensation particle counter (CPC), a thermal denuder, and an aerosol humidifier.24 The system was operated as a scanning mobility particle sizer (SMPS), a hygroscopicity- or thermo-TDMA, and a DMA-APM by means of automated threeway valves. The mobility size distribution of soot aerosol in the chamber was measured in the SMPS mode. At each combustion condition, the size distribution of the particles in the chamber was measured twice, and the average of the two scans was used as the final result. The size distributions are repeatable within experimental uncertainties (Supplementary Figure S5). The averaged mobility diameter was calculated as Davg =

∫ n(D)·D·dD ∫ n(D)·dD

The hygroscopicity-TDMA (HTDMA) was used to determine the uptake of water by nascent soot by comparing the particle mobility size before (Dp,0) and after (Dp,1) the exposure to 90% relative humidity (RH). The hygroscopic diameter growth factor (hGfd) was calculated as hGfd = ((Dp,1)/(Dp,0)). Similarly, thermo-TDMA (TTDMA, also known as volatilityTDMA) was used to determine the volatile particle fraction by comparing the particle mobility size before (Dp,0) and after (Dp,1) processing in the thermal denuder maintained at 300 °C with an estimated residence time of 10 s. The thermal diameter growth factor (tGfd) was also calculated as tGfd = ((Dp,1)/(Dp,0)). The mass growth factor, corresponding to the particle EC fraction f EC, was calculated from the particle masses before (m0) and after (m1) thermal denuding, as determined by DMA-APM, using the equation ⎛ (m ) ⎞ fEC = ⎜ 1 ⎟ ⎝ (m 0 ) ⎠

(1)

where n(D) is the particle number size distribution. The mass of particles with known mobility sizes was determined in the DMA-APM mode, and the effective density ρeff was derived as ρeff =

6m πDp3

The particle shape was characterized from the dynamic shape factor, χ,

χ= (2)

(4)

where Dve is the volume equivalent diameter, and Cp and Cve are the Cunningham slip correction factors calculated for Dp and Dve, respectively.45 More details on the calculation of Dve can be found in Supporting Information. Light scattering (bsca) and extinction (bext) at 532 nm were determined by a commercial integrating nephelometer (TSI 3563) and a home-built CRDS, respectively.23 The absorption coefficient, babs, was calculated as bext − bsca and the aerosol SSA was calculated as bsca/bext. The mass absorption cross-section (MAC) was determined as babs/CEC, where CEC is the mass concentration of EC and is calculated by integrating the particle

where Dp is the mobility diameter of the particle determined by DMA and m is the particle mass determined by APM. Mass measurements were calibrated using commercial polystyrene latex (PSL) spheres as size standards following the procedure reported previously with high accuracy (better than 5%).34 The mass-mobility measurements were used to derive the massmobility scaling exponent Dm,44 which is similar but not identical to the mass fractal dimension, Dfm.34

ρeff ∝ Dp3 − Dm

Dp Cve · Dve Cp

(3) 6446



Article

Figure 3. Properties of soot particles produced under different combustion conditions: (a) size-resolved EC mass faction; (b) averaged EC mass faction; (c) effective density of nascent soot; (d) effective density of thermally denuded soot; (e) dynamic shape factor χ; and (f) fractal dimension. All soot particles were produced from the combustion of diesel/O2/Ar mixtures with a fuel equivalence ratio ϕ of 15. The dash lines in panels a, b, and e show the trend of the changes in size-resolved EC mass fraction, averaged EC mass fraction, and dynamic shape factor as the combustion temperature changes, respectively. Numbers above lines in panels c and d correspond to Dm. Details of the regression analysis in panels c and d can be found in Supporting Information.

number size distribution, n(D), accounting for the size dependencies of the effective density ρeff(D) and EC mass fraction f EC(D): 780 nm

C EC =

∫10 nm

π 3 D n(D)ρeff (D)fEC (D) dD 6

instantaneously after the passage of the reflected shockwave. Under comparable combustion conditions, the soot inception delay is shorter and the soot yield is higher for diesel fuel than for propane. The soot yield is defined as the ratio of mass of soot over mass of carbon in the original fuel. Figure 1c shows the yields of EC calculated from the in situ light extinction measurements for combustion experiments conducted with a fuel-to-oxygen ratio ϕ = 15 at different combustion temperatures. There is a clear increasing trend in soot production from 1400 to 2050 K followed by a decrease at combustion temperatures above 2200 K. Similar trends were also observed for the other two fuel equivalent ratios (2.8 and 8.1). Reduced EC yield at lower temperatures occurs because of the slow dehydrogenation

(5) 46

The contribution from forward scattering was minor, and details on the uncertainties in light scattering and extinction measurements are described in Supporting Information.

■

RESULTS AND DISCUSSION In Situ Soot Measurements. As shown in Figure 1b, the formation of soot from combustion of diesel fuel occurs almost 6447



Article

Figure 3b illustrates the temperature dependence of f EC_avg, which starts at 25−45% in the temperature range of 1400−1600 K, increases sharply around 1600−1800 K, and reaches a plateau at ∼90% for combustion temperatures above 1900 K. The temperature dependence of f EC_avg is not very sensitive to the fuel equivalence ratio, as indicated by similar trends for the three data sets obtained using distinctively different values of ϕ = 2.8, 8.1, and 15 (Figure 3b). This observation suggests that for ϕ above the sooting limit, the combustion temperature is the key factor that determines the EC/OC ratio in soot produced from homogeneous combustion of vaporized diesel fuel. Particle Effective Density and Mass-Mobility Scaling Exponent. Effective densities of soot particles measured in our study are significantly lower than the material density of EC (1.77 g cm−3),51 in agreement with previous measurements of soot sampled from diesel engines (0.5−0.9 g cm−3 for soot particles of 50−150 nm).50,52 The relationship between the particle effective density and mobility diameter is highly sensitive to the combustion temperature (Figure 3c). Whereas for soot produced at lower combustion temperatures (e.g., ∼1650 K) the effective density is virtually independent of the particle mobility size (blue diamonds), at higher temperatures (T > 1900 K), there is a significant decrease in the particle effective density with increasing mobility diameter (black squares and red triangles). For instance, the effective density decreases from 0.9 to 0.09 g cm−3 on going from 81 to 707 nm particle diameter for soot produced at ∼2164 K. Such trends in the effective density indicate that the particles formed at low and high temperatures have different morphologies. The mass-mobility scaling exponents Dm obtained by fitting eq 3 to size-dependent effective density data shown in Figure 3c indicate that combustion of diesel fuel at high temperature (>1900 K) produces fractal particles (Dm = 1.7−2.0), while particles from low-temperature combustion are compact (Dm = 2.9−3.0). For the intermediate combustion temperature (∼1800 K), two distinct morphologies can be distinguished in a single soot sample, depending on the particle mobility size (Figure 3c, green circles). Smaller 50−350 nm particles are compact (Dm = 2.93), showing nearly constant effective density. Larger particles exhibit effective density decreasing with increasing size, corresponding to fractal aggregates with Dm = 1.84. Note that the Dm at intermediate combustion temperatures may have larger uncertainties due to the limited data in the mass-mobility measurements. The dependence of Dm on combustion temperature is summarized in Figure 3f. Thermal denuding reduces the particle mobility diameter and mass by removing the OC coating from the EC backbone. However, the effective density and fractal dimension remain practically unchanged (Figure 3d), even for particles with high initial OC content. Since the process of thermal denuding induces no appreciable restructuring of soot aggregates,53 it is safe to conclude that the structure of the soot backbone is the same in nascent and thermally denuded diesel soot. These results are in agreement with previous reports of negligible restructuring for soot with variable fraction of OC from shock-tube combustion of propane42 and premixed flame combustion of ethylene.54 On the contrary, aqueous sulfuric acid coatings have been shown to cause almost complete rearrangement of the soot backbone, resulting in compact aggregates with closely packed primary spherules.34 The different behaviors of the coatings composed of nonpolar OC and sulfuric acid are caused by the difference in the physical properties of these two coating

and growth of soot nuclei. Hindered soot production at higher temperatures is caused by the oxidation of soot primary spherules and their precursor molecules (acetylene, benzene, PAHs, etc.).47 Soot yield can be derived more accurately from the postshock measurements, and the discrepancies between these two approaches have been discussed in details previously.42 Postshock Measurements. Particle Size Distribution and Number Concentration. Figure 2a−c shows mobility size distributions of soot aerosol that was produced in the shock tube under different conditions and then transferred to the environmental chamber. The total concentration of soot particles in the chamber ranged from 1.1 × 104 to 5.2 × 104 cm−3, comparable to that of soot produced from propane.42 For all fuel equivalence ratios, two major size modes can be identified. The first mode is at 20−60 nm with a position almost independent of the fuel equivalence ratio; the mode peak intensity decreases when fuel equivalence ratio increases from 2.8 to 15. The peak position of the second mode is sensitive to fuel equivalence ratio, varying from 100 to 700 nm when the ratio increases from 2.8 to 15 (Figure 2a−c). Furthermore, both modes vary significantly with the combustion temperature. As shown in Figure 2a−c, when combustion temperature increases from 1400 to 1600 K, the position of the first mode shifts to larger size, sometimes by as much as 60 nm (Figure 2a). When the temperature increases beyond 1600 K, the size distribution shifts back to smaller sizes. The size distribution of particles produced at ∼2100 K is more similar to the distribution at ∼1400 K than that at ∼1600 K, with the amplitude of the second mode increased substantially. Generally, larger particles are produced from richer fuel/oxygen mixtures and at higher temperatures. For the fuel equivalence ratio in the range from 2.8 to 15, average mobility diameter Davg increases linearly when the combustion temperature increases from 1400 to 2200 K (Figure 2d). Size distributions of soot particles produced from mixtures with fuel equivalence ratios of 2.8 and 8.1 at temperatures below 1600 K are in agreement with the size distributions of diesel soot obtained in on-road and dynamometer measurements of emissions from real diesel engines.4 Particle EC Mass Fraction. To evaluate the particle EC content, soot aerosol from the chamber was size-selected and passed through the thermal denuder operating at 300 °C to remove volatile particle constituents. Evaporation time scale is estimated to be within 1 s based on the results of Riipinen et al.48 It was assumed that all the remaining particle mass is EC.49 The mass fraction of EC ( f EC) was calculated from the mass ratio of heated and nascent particles. The value of f EC increases with increasing combustion temperature, and above 1700 K the EC becomes the major particle constituent (Figure 3a). Depending on the particle mobility size, f EC levels off at 78−92% when the combustion temperature reaches ∼2000 K. For soot produced below 1700 K, the f EC is low (∼35%) and shows little variation with the mobility size (Supplementary Figure S1). On the contrary, for soot produced above 1700 K the f EC is sizedependent, increasing with particle diameter. For example, at ∼1900 K the EC fraction is 60% for 50 nm particles and 90% for 700 nm particles. The decrease in the fraction of EC in shocktube experiments at lower temperatures is in agreement with observations of larger amounts of condensed organic material in diesel soot emitted at lower engine loads.50 For each individual diesel combustion experiment, there is a linear dependence of the heated versus nascent particle mass (Supplementary Figure S2). The slope of this dependence can be interpreted as the size-averaged EC mass fraction f EC_avg.42 6448



Article

materials, including the higher material density and surface tension of sulfuric acid. To gain more insight into the shape of compact and fractal soot particles, their dynamic shape factors χ were calculated using eq 4. For compact particles produced at the lowest combustion temperature (∼1677 K), χ = 1.02−1.05, indicating their nearly spherical shape (Figure 3e). For comparison, sodium chloride particles of similar sizes typically have χ = 1.08.55,56 For fractal particles, the shape factor generally shows an increasing trend with increasing combustion temperature and particle mobility size, reaching a value of χ = 4.61 at 1904 K for 707 nm particles, in agreement with previous studies.34,35,42 Thermal denuding of soot results in a uniform increase of the shape factor by Δχ = 0.2−0.4, as shown in Supplementary Figure S3. The dynamic shape factor increases because the removal of the coating material exposes the irregular soot backbone, resulting in a higher drag on the particle. The trends shown in Figure 3c−e are also applicable to soot produced using other fuel equivalence ratios. Particle Optical Properties. Light extinction and scattering by soot aerosol that was transferred from the shock tube to the chamber were quantified as a function of fuel equivalence ratio and combustion temperature. Light extinction coefficient bext is sensitive to the equivalence ratio, varying between 2−200 Mm−1, 30−2700 Mm−1, and 500−3000 Mm−1 for ϕ of 2.8, 8.1, and 15, respectively. Also, bext shifts to higher values when combustion temperature increases. Scattering coefficient bsca shows a temperature dependence that is similar to the temperature dependence of bext. Direct climate forcing by soot aerosol depends on the ratio of scattering and extinction, i.e., SSA. Figure 4a illustrates the strong dependence of soot SSA on the combustion temperature. For all of the fuel equivalence ratios tested in our study, SSA is relatively high (0.9) for soot produced at lower combustion temperature (1400 K) but decreases to values as low as 0.2 when the combustion temperature exceeds 2100 K. Values of SSA near 1 indicate that the aerosol is mainly scattering, and only for values below about 0.8 the particles could exert a net warming effect in the atmosphere for clear-sky conditions.57 For all tested fuel equivalence ratios, SSA decreases linearly with increasing f EC_avg (Figure 4b). This trend is somewhat different from the trend that has been observed in our previous study for the soot produced from the combustion of propane,42 where SSA decreased linearly with the increase in f EC_avg only for soot produced from richer mixtures (ϕ = 8.0) but showed no clear relation of f EC_avg for soot from leaner mixtures (ϕ = 2.5). Using light absorption coefficients derived from the difference between light extinction and scattering together with the data on particle mobility size distributions, effective densities, and EC fractions we calculated the mass absorption cross-section (MAC) for soot produced under different combustion conditions, assuming that EC is the only component that absorbs light at 532 nm.49 Along with SSA, MAC is the property most relevant to the balance between negative and positive climate forcing by aerosol. As shown in Figure 4c, for each fuel equivalent ratio (2.8, 8.1, and 15), MAC values increase in the temperature range from 1400 to 2000 K but decrease somewhat above 2000 K. The lowest values of MAC observed in our study are ∼2.0 m2 g−1, being practically independent of ϕ. The lowest MAC values were generally observed at lower combustion temperatures, in agreement with the higher particle-phase OC content produced at lower temperatures. The maximum values of MAC are 5.5, 13.0, and 13.0 m2 g−1 for ϕ of 2.8, 8.1, and 15, respectively, suggesting that the richness of fuel affects the light absorption of

Figure 4. Optical properties of soot particles generated from the combustion of diesel/O2/Ar mixtures with different fuel equivalence ratios. The single scattering albedo (SSA) is plotted as the function of combustion temperature (a) and averaged elemental carbon (EC) fraction (b) for three different diesel/O2 ratios. The mass absorption cross-section (MAC, m2 g−1) of soot is plotted as a function of combustion temperature in panel c. Details of the regression analysis in panels a and b can be found in Supporting Information.

the aerosol products. Although for pure graphitic EC particles a MAC of 7.5 ± 1.2 m2 g−1 has been suggested,2 significant positive and negative deviations from this value are well documented.2,58 The variation in MAC may be caused by several factors, including the size, mixing state, and morphology of soot aggregates that act to either increase or decrease the magnitude of light absorption.59,60 For instance, the higher maximum values of MAC obtained in experiments with ϕ = 8.1 and 15 than in experiments with ϕ = 2.8 may be due to the presence of the second larger mode in the size distributions of soot produced using richer mixtures (Figure 2a−c). For fractal soot, MAC has been shown to increase with the increasing mobility size of 6449



Article

aggregates.14 Very low values of MAC observed in our measurements at lower temperatures may be a result of the overestimated mass fraction of EC, especially for the aerosol produced at low combustion temperature (∼1400 K) with fuel equivalence ratio ϕ of 2.8. Whereas we assume that all of the material remaining after thermal denuding is EC, some of this material in fact could be low-volatility OC, such as high molecular weight PAH, which are significantly weaker light absorbers than EC at 532 nm. With increasing temperature, a larger fraction of fuel is converted to EC, leading to smaller negative deviations and a higher MAC. Also, for particles produced below 2000 K, light absorption may be additionally enhanced from the lensing effect by the OC coating.59 Particle Hygroscopicity. To evaluate the cloud-forming ability of freshly produced diesel soot, hygroscopic growth factors were measured as a function of particle mobility size at 90% RH using HTDMA. In all measurements, including soot from combustion at lower (∼1400 K) and higher (∼2000 K) temperatures, no change in the particle mobility diameter was observed upon humidification, corresponding to hGfd = (1.000 ± 0.005). As discussed above, diesel soot particles from high-temperature combustion are fractal aggregates composed primarily of EC, along with a small fraction of OC coatings. It is well-known that soot with a high EC content is hydrophobic24 and shows hGfd = 1. However, the presence of thin coatings of water-soluble materials may result in water uptake at an elevated relative humidity, causing partial aggregate restructuring, which manifests in hGfd < 1.14,15,24,61,62 The absence of restructuring in our HTDMA measurements for diesel soot from higher-temperature combustion experiments confirms the hydrophobic nature of the OC coating. This hydrophobic material, largely consisting of PAH and long-chain aliphatic hydrocarbons, is also the main component of compact, nearly spherical soot particles produced at lower combustion temperatures, explaining their hGfd = 1. If these compact particles contained even partially hydrophilic OC, then hGfd > 1 would have been observed. Our hygroscopicity measurements thus indicate that OC coatings on nascent diesel soot particles are hydrophobic, and hence freshly produced diesel particles have negligible contribution to atmospheric cloud condensation nuclei (CCN). Implications. In diesel engines, soot is formed from homogeneous and heterogeneous combustion of vaporized and liquid fuel, respectively, in a temperature range of 1000− 2600 K.1 Our study focuses on the formation and properties of soot aerosols produced from homogeneous combustion of diesel fuel in locally rich pockets. Our results are also applicable to recently developed engines realizing the homogeneous combustion of highly diluted diesel fuel to reduce NOx and soot emissions.1 Using shock-tube experiments, we demonstrate that combustion temperature is the most crucial factor in determining the composition and physical properties of soot particles from homogeneous combustion of diesel fuel in the temperature range of 1400−2200 K. Lower-temperature combustion (1900 K) yields particles consisting mainly of EC and having fractal morphology. At intermediate combustion temperatures (1700− 1900 K), both OC-like and EC-like particles are formed in comparable amounts. Under similar conditions (i.e., fuel loading, fuel equivalence ratio, and combustion temperature), homogeneous combustion of diesel fuel is more efficient in soot production than that from propane combustion,42 including an

earlier inception time, a higher soot yield, and a larger particle size. The temperature-dependent composition, size, and morphology of soot particles have significant effects on their resulting atmospheric impacts and aging transformations. For instance, soot formed at higher temperatures (∼2000 K) is EC-like, with a lower effective density, more fractal morphology, and lower SSA. On the other hand, soot produced at lower temperatures (∼1400 K) shows OC-like properties, with a higher effective density, nearly spherical shape, and larger SSA. Thus, the direct climate forcing of freshly emitted diesel soot varies considerably, depending on the combustion conditions. Diesel soot with a higher EC content strongly absorbs solar radiation and warms the atmosphere, while OC-like soot scatters light more significantly and reduces the amount of solar radiation reaching the ground. Furthermore, the chemical composition and physical properties of nascent soot have an impact on the rate of its atmospheric aging. For example, recent studies have shown that for nascent soot with a higher EC content and fractal morphology, the coating formation initially does not lead to growth in particle mobility size but instead results in particle compaction.14,15 A higher OC content in nascent soot produced at lower combustion temperatures facilitates the coating formation by enhancing partitioning or heterogeneous reactions of semivolatile organics.12,63−66 Further studies should focus on soot formation from heterogeneous combustion of liquid diesel fuel droplets, including the effects from common additives such as lubricating oils and biofuels.

■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Experimental details and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*Phone: (256) 765-4339 . Fax: (256) 765-4958. E-mail: cqiu@ una.edu. *Phone: (979) 845-7656. Fax: (979) 862-4466. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CBET-0932705) and the Robert A. Welch Foundation (A1417).

■

REFERENCES

(1) Dec, J. E. Advanced compression-ignition engines-understanding the in-cylinder processes. Proc. Combust. Inst. 2009, 32, 2727. (2) Bond, T. C.; Bergstrom, R. W. Light absorption by carbonaceous particles: An investigative review. Aerosol Sci. Technol. 2006, 40, 27. (3) Bond, T. C.; Doherty, S. J.; Fahey, D. W.; Forster, P. M.; Berntsen, T.; DeAngelo, B. J.; Flanner, M. G.; Ghan, S.; Kärcher, B.; Koch, D.; Kinne, S.; Kondo, Y.; Quinn, P. K.; Sarofim, M. C.; Schultz, M. G.; Schulz, M.; Venkataraman, C.; Zhang, H.; Zhang, S.; Bellouin, N.; Guttikunda, S. K.; Hopke, P. K.; Jacobson, M. Z.; Kaiser, J. W.; Klimont, Z.; Lohmann, U.; Schwarz, J. P.; Shindell, D.; Storelvmo, T.; Warren, S. G.; Zender, C. S. Bounding the role of black carbon in the climate system: A scientific assessment. J. Geophys. Res. Atmos. 2013, 118, 5380. (4) Kittelson, D. B.; Watts, W. F.; Johnson, J. P. On-road and laboratory evaluation of combustion aerosols - Part 1: Summary of diesel engine results. J. Aerosol Sci. 2006, 37, 913.

6450



Article

(5) Burtscher, H. Physical characterization of particulate emissions from diesel engines: a review. J. Aerosol Sci. 2005, 36, 896. (6) Kittelson, D. B. Engines and nanoparticles: a review. J. Aerosol Sci. 1998, 29, 575. (7) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. Combustion aerosols: Factors governing their size and composition and implications to human health. J. Air Waste Manage. Assoc. 2000, 50, 1565. (8) Scheepers, P. T. J.; Bos, R. P. Combustion of diesel fuel from a toxicological perspective. 2. Toxicity. Int. Arch. Occup. Environ. Health 1992, 64, 163. (9) Zhang, R.; Khalizov, A. F.; Pagels, J.; Zhang, D.; Xue, H.; McMurry, P. H. Variability in morphology, hygroscopicity, and optical properties of soot aerosols during atmospheric processing. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 10291. (10) Xue, H.; Khalizov, A. F.; Wang, L.; Zheng, J.; Zhang, R. Effects of coating of dicarboxylic acids on the mass−mobility relationship of soot particles. Environ. Sci. Technol. 2009, 43, 2787. (11) Donahue, N. M.; Robinson, A. L.; Pandis, S. N. Atmospheric organic particulate matter: From smoke to secondary organic aerosol. Atmos. Environ. 2009, 43, 94. (12) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; Goldstein, A. H.; Hamilton, J. F.; Herrmann, H.; Hoffmann, T.; Iinuma, Y.; Jang, M.; Jenkin, M. E.; Jimenez, J. L.; Kiendler-Scharr, A.; Maenhaut, W.; McFiggans, G.; Mentel, T. F.; Monod, A.; Prévôt, A. S. H.; Seinfeld, J. H.; Surratt, J. D.; Szmigielski, R.; Wildt, J. The formation, properties and impact of secondary organic aerosol: current and emerging issues. Atmos. Chem. Phys. 2009, 9, 5155. (13) Ranjan, M.; Presto, A. A.; May, A. A.; Robinson, A. L. Temperature dependence of gas−particle partitioning of primary organic aerosol emissions from a small diesel engine. Aerosol Sci. Technol. 2011, 46, 13. (14) Qiu, C.; Khalizov, A. F.; Zhang, R. Soot aging from OH-initiated oxidation of toluene. Environ. Sci. Technol. 2012, 46, 9464. (15) Khalizov, A. F.; Lin, Y.; Qiu, C.; Guo, S.; Collins, D.; Zhang, R. Role of OH-initiated oxidation of isoprene in aging of combustion soot. Environ. Sci. Technol. 2013, 47, 2254. (16) May, A. A.; Presto, A. A.; Hennigan, C. J.; Nguyen, N. T.; Gordon, T. D.; Robinson, A. L. Gas-particle partitioning of primary organic aerosol emissions: (2) Diesel vehicles. Environ. Sci. Technol. 2013, 47, 8288. (17) Ma, Y.; Brooks, S. D.; Vidaurre, G.; Khalizov, A. F.; Wang, L.; Zhang, R. Rapid modification of cloud-nucleating ability of aerosols by biogenic emissions. Geophys. Res. Lett. 2013, 40, 6293. (18) Liu, Y.; Liu, C.; Ma, J.; Ma, Q.; He, H. Structural and hygroscopic changes of soot during heterogeneous reaction with O3. Phys. Chem. Chem. Phys. 2010, 12, 10896. (19) Lei, W.; Zhang, R.; Sean McGivern, W.; Derecskei-Kovacs, A.; North, S. W. Theoretical study of isomeric branching in the isoprene− OH reaction: implications to final product yields in isoprene oxidation. Chem. Phys. Lett. 2000, 326, 109. (20) Zhang, R.; Suh, I.; Lei, W.; Clinkenbeard, A. D.; North, S. W. Kinetic studies of OH-initiated reactions of isoprene. J. Geophys. Res. Atmos. 2000, 105, 24627. (21) Lei, W.; Zhang, R. Theoretical study of hydroxyisoprene alkoxy radicals and their decomposition pathways. J. Phys. Chem. A 2001, 105, 3808. (22) Kamens, R. M.; Coe, D. L. A large gas-phase stripping device to investigate rates of PAH evaporation from airborne diesel soot particles. Environ. Sci. Technol. 1997, 31, 1830. (23) Khalizov, A. F.; Xue, H.; Wang, L.; Zheng, J.; Zhang, R. Enhanced light absorption and scattering by carbon soot aerosol internally mixed with sulfuric acid. J. Phys. Chem. A 2009, 113, 1066. (24) Khalizov, A. F.; Zhang, R.; Zhang, D.; Xue, H.; Pagels, J.; McMurry, P. H. Formation of highly hygroscopic soot aerosols upon internal mixing with sulfuric acid vapor. J. Geophys. Res. 2009, 114, D05208.

(25) Xue, H.; Khalizov, A. F.; Wang, L.; Zheng, J.; Zhang, R. Effects of dicarboxylic acid coating on the optical properties of soot. Phys. Chem. Chem. Phys. 2009, 11, 7869. (26) Adam, T. W.; Chirico, R.; Clairotte, M.; Elsasser, M.; Manfredi, U.; Martini, G.; Sklorz, M.; Streibel, T.; Heringa, M. F.; DeCarlo, P. F.; Baltensperger, U.; De Santi, G.; Krasenbrink, A.; Zimmermann, R.; Prevot, A. S. H.; Astorga, C. Application of modern online instrumentation for chemical analysis of gas and particulate phases of exhaust at the European Commission Heavy-Duty Vehicle Emission Laboratory. Anal. Chem. 2010, 83, 67. (27) Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K. B.; Tignor, M.; Miller, H. L. IPCC Climate Change 2007: The Physical Science Basis; Cambridge University Press: New York, 2007. (28) Lei, W.; Zhang, R.; Tie, X.; Hess, P. Chemical characterization of ozone formation in the Houston-Galveston area: A chemical transport model study. J. Geophys. Res. Atmos. 2004, 109, D12301. (29) Li, G.; Zhang, R.; Fan, J.; Tie, X. Impacts of black carbon aerosol on photolysis and ozone. J. Geophys. Res. Atmos. 2005, 110, D23206. (30) Fan, J.; Zhang, R.; Tao, W.-K.; Mohr, K. I. Effects of aerosol optical properties on deep convective clouds and radiative forcing. J. Geophys. Res. Atmos. 2008, 113, D08209. (31) Zelenay, V.; Mooser, R.; Tritscher, T.; Křepelová, A.; Heringa, M. F.; Chirico, R.; Prévôt, A. S. H.; Weingartner, E.; Baltensperger, U.; Dommen, J.; Watts, B.; Raabe, J.; Huthwelker, T.; Ammann, M. Aging induced changes on NEXAFS fingerprints in individual combustion particles. Atmos. Chem. Phys. 2011, 11, 11777. (32) Healy, R. M.; Sciare, J.; Poulain, L.; Crippa, M.; Wiedensohler, A.; Prévôt, A. S. H.; Baltensperger, U.; Sarda-Estève, R.; McGuire, M. L.; Jeong, C. H.; McGillicuddy, E.; O’Connor, I. P.; Sodeau, J. R.; Evans, G. J.; Wenger, J. C. Quantitative determination of carbonaceous particle mixing state in Paris using single-particle mass spectrometer and aerosol mass spectrometer measurements. Atmos. Chem. Phys. 2013, 13, 9479. (33) Zhang, D.; Zhang, R. Y. Laboratory investigation of heterogeneous interaction of sulfuric acid with soot. Environ. Sci. Technol. 2005, 39, 5722. (34) Pagels, J.; Khalizov, A. F.; McMurry, P. H.; Zhang, R. Y. Processing of soot by controlled sulphuric acid and water condensationMass and mobility relationship. Aerosol Sci. Technol. 2009, 43, 629. (35) Slowik, J. G.; Stainken, K.; Davidovits, P.; Williams, L. R.; Jayne, J. T.; Kolb, C. E.; Worsnop, D. R.; Rudich, Y.; DeCarlo, P. F.; Jimenez, J. L. Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements. Part 2: Application to combustion-generated soot aerosols as a function of fuel equivalence ratio. Aerosol Sci. Technol. 2004, 38, 1206. (36) Northrop, W. F.; Madathil, P. V.; Bohac, S. V.; Assanis, D. N. Condensational growth of particulate matter from partially premixed low temperature combustion of biodiesel in a compression ignition engine. Aerosol Sci. Technol. 2011, 45, 26. (37) Ghazi, R.; Tjong, H.; Soewono, A.; Rogak, S. N.; Olfert, J. S. Mass, mobility, volatility, and morphology of soot particles generated by a McKenna and inverted burner. Aerosol Sci. Technol. 2012, 47, 395. (38) Horvath, H.; Gangl, M. A low-voltage spark generator for production of carbon particles. J. Aerosol Sci. 2003, 34, 1581. (39) Petersen, E. L.; Rickard, M. J. A.; Corfton, M. W.; Abbey, E. D.; Traum, M. J.; Kalitan, D. M. A facility for gas- and condensed-phase measurements behind shock waves. Meas. Sci. Technol. 2005, 16, 1716. (40) Mathieu, O.; Djebaili-Chaumeix, N.; Paillard, C. E.; Douce, F. Experimental study of soot formation from a diesel fuel surrogate in a shock tube. Combust. Flame 2009, 156, 1576. (41) Eremin, A. V. Formation of carbon nanoparticles from the gas phase in shock wave pyrolysis processes. Prog. Energy Combust. Sci. 2012, 38, 1. (42) Khalizov, A. F.; Hogan, B.; Qiu, C.; Petersen, E. L.; Zhang, R. Characterization of soot aerosol produced from combustion of propane in a shock tube. Aerosol Sci. Technol. 2012, 46, 925. (43) Kellerer, H.; Muller, A.; Bauer, H. J.; Wittig, S. Soot formation in a shock tube under elevated pressure conditions. Combust. Sci. Technol. 1996, 113, 67. 6451



Article

(44) Sorensen, C. M. The mobility of fractal aggregates: A Review. Aerosol Sci. Technol. 2011, 45, 765. (45) Hinds, W. C. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles; Wiley: New York, 1999. (46) Qian, F.; Ma, L.; Thompson, J. E. Modeling and measurements of angular truncation for an aerosol albedometer. J. Eur. Opt. Soc. Rapid Publ. 2012, 7, 12021. (47) Richter, H.; Howard, J. B. Formation of polycyclic aromatic hydrocarbons and their growth to soota review of chemical reaction pathways. Prog. Energy Combust. Sci. 2000, 26, 565. (48) Riipinen, I.; Pierce, J. R.; Donahue, N. M.; Pandis, S. N. Equilibration time scales of organic aerosol inside thermodenuders: Evaporation kinetics versus thermodynamics. Atmos. Environ. 2010, 44, 597. (49) Petzold, A.; Ogren, J. A.; Fiebig, M.; Laj, P.; Li, S. M.; Baltensperger, U.; Holzer-Popp, T.; Kinne, S.; Pappalardo, G.; Sugimoto, N.; Wehrli, C.; Wiedensohler, A.; Zhang, X. Y. Recommendations for reporting ″black carbon″ measurements. Atmos. Chem. Phys. 2013, 13, 8365. (50) Park, K.; Cao, F.; Kittelson, D. B.; McMurry, P. H. Relationship between particle mass and mobility for diesel exhaust particles. Environ. Sci. Technol. 2003, 37, 577. (51) Park, K.; Kittelson, D. B.; Zachariah, M. R.; McMurry, P. H. Measurement of inherent material density of nanoparticle agglomerates. J. Nanopart. Res. 2004, 6, 267. (52) Barone, T. L.; Lall, A. A.; Storey, J. M. E.; Mulholland, G. W.; Prikhodko, V. Y.; Frankland, J. H.; Parks, J. E.; Zachariah, M. R. Sizeresolved density measurements of particle emissions from an advanced combustion diesel engine: Effect of aggregate morphology. Energy Fuels 2011, 25, 1978. (53) Bambha, R. P.; Dansson, M. A.; Schrader, P. E.; Michelsen, H. A. Effects of volatile coatings and coating removal mechanisms on the morphology of graphitic soot. Carbon 2013, 61, 80. (54) Slowik, J. G.; Cross, E. S.; Han, J. H.; Kolucki, J.; Davidovits, P.; Williams, L. R.; Onasch, T. B.; Jayne, J. T.; Kolb, C. E.; Worsnop, D. R. Measurements of morphology changes of fractal soot particles using coating and denuding experiments: Implications for optical absorption and atmospheric lifetime. Aerosol Sci. Technol. 2007, 41, 734. (55) Kelly, W. P.; McMurry, P. H. Measurement of particle density by inertial classification of differential mobility analyzer−Generated monodisperse aerosols. Aerosol Sci. Technol. 1992, 17, 199. (56) Wang, Z.; King, S. M.; Freney, E.; Rosenoern, T.; Smith, M. L.; Chen, Q.; Kuwata, M.; Lewis, E. R.; Pöschl, U.; Wang, W.; Buseck, P. R.; Martin, S. T. The dynamic shape factor of sodium chloride nanoparticles as regulated by drying rate. Aerosol Sci. Technol. 2010, 44, 939. (57) Haywood, J. M.; Shine, K. P. The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary radiation budget. Geophys. Res. Lett. 1995, 22, 603. (58) Levy, M. E.; Zhang, R.; Khalizov, A. F.; Zheng, J.; Collins, D. R.; Glen, C. R.; Wang, Y.; Yu, X.-Y.; Luke, W.; Jayne, J. T.; Olaguer, E. Measurements of submicron aerosols in Houston, Texas during the 2009 SHARP field campaign. J. Geophys. Res. Atmos. 2013, 118, 10. (59) Bond, T. C.; Habib, G.; Bergstrom, R. W. Limitations in the enhancement of visible light absorption due to mixing state. J. Geophys. Res. Atmos. 2006, 111, D20211. (60) Wang, Y.; Khalizov, A.; Levy, M.; Zhang, R. New Directions: Light absorbing aerosols and their atmospheric impacts. Atmos. Environ. 2013, 81, 713. (61) Weingartner, E.; Burtscher, H.; Baltensperger, U. Hygroscopic properties of carbon and diesel soot particles. Atmos. Environ. 1997, 31, 2311. (62) Tritscher, T.; Juranyi, Z.; Martin, M.; Chirico, R.; Gysel, M.; Heringa, M. F.; DeCarlo, P. F.; Sierau, B.; Prevot, A. S. H.; Weingartner, E.; Baltensperger, U. Changes of hygroscopicity and morphology during ageing of diesel soot. Environ. Res. Lett. 2011, 6, No. 034026. (63) Wang, L.; Lal, V.; Khalizov, A. F.; Zhang, R. Heterogeneous chemistry of alkylamines with sulfuric acid: Implications for atmospheric formation of alkylaminium sulfates. Environ. Sci. Technol. 2010, 44, 2461.

(64) Wang, L.; Khalizov, A. F.; Zheng, J.; Xu, W.; Ma, Y.; Lal, V.; Zhang, R. Atmospheric nanoparticles formed from heterogeneous reactions of organics. Nat. Geosci. 2010, 3, 238. (65) Qiu, C.; Wang, L.; Lal, V.; Khalizov, A. F.; Zhang, R. Heterogeneous reactions of alkylamines with ammonium sulfate and ammonium bisulfate. Environ. Sci. Technol. 2011, 45, 4748. (66) Zhang, R.; Khalizov, A.; Wang, L.; Hu, M.; Xu, W. Nucleation and growth of nanoparticles in the atmosphere. Chem. Rev. 2012, 112, 1957.

6452


High Sensitivity of Diesel Soot Morphological and Optical Properties to

Recommend Documents