Effect of oxidation catalysts on diesel soot particles

Effect of catalyzed diesel particulate filter and its catalyst loading on emission characteristics of a non-road diesel engine

In this study, the effects of a diesel oxidation catalyst (DOC) coupled with a catalyzed diesel particulate filter (CDPF) with different catalyst loadings on the power, fuel consumption, gaseous and particulate emissions from a non-road diesel engine were investigated. Results showed that the after-treatment had a negligible effect on the power and fuel consumption. The reduction effect of the DOC on the CO and hydrocarbon (HC) increased with the engine load. Further reductions occurred coupling with the CDPF. Increasing the catalyst loading resulted in a more significant reduction in the HC emissions than CO emissions. The DOC could increase the NO₂ proportion to 37.9%, and more NO₂ was produced when coupled with the CDPF below 250°C; above 250°C, more NO₂ was consumed. The after-treatment could reduce more than 99% of the particle number (PN) and 98% of the particle mass (PM). Further reductions in the PN and PM occurred with a higher CDPF catalyst loading. The DOC had a better reduction effect on the nucleation particles than the accumulation ones, but the trend reversed with the CDPF. The DOC shifted the particle size distribution (PSD) to larger particles with an accumulation particle proportion increasing from 13% to 20%, and the geometric mean diameter (GMD) increased from 18.2 to 26.0 nm. The trend reversed with the CDPF and the accumulation particle proportion declined to less than 10%. A lower catalyst loading on the CDPF led to a higher proportion of nucleation particles and a smaller GMD.

Catalytic effect of diesel PM derived ash on PM oxidation activity

With diesel particulate filter and gasoline particulate filter periodical regeneration, more and more ash accumulates on the substrate of filter. Ash gathering on the substrate of filter leads to more contact area of particulate matter and ash. Specific ingredients in ash present catalytic effects on particulate matter oxidation. However, the catalytic effect of diesel particulate matter derived ash on its oxidation, mimicking the ash accumulating on filter substrate, is still uncovered using experiments. In this paper, diesel particulate matter derived ash was put at the bottom of particulate matter samples to imitating the soot loading on filter substrate which was covered by much ash. The results indicated that the burnout temperature of diesel particulate matter was in the range of 500-600 °C; while it was 600-700 °C for Printex (U). The burnout temperature drop by ash was lower than 10 °C for diesel particulate matter. The maximum mass loss rate corresponded to approximately 450 °C for diesel particulate matter, and it was changed minorly by ash and ramp rates. However, the temperature corresponding to the maximum mass loss rate was seriously retarded by high ramp rates for Printex (U), and ash presented limited effect on it. The maximum activation energy drop by ash was approximately 60 kJ/mol at the initial stage of oxidation for diesel particulate matter. The activation energy was approximately 132.19, 114.78, 157.26, and 144.67 kJ/mol for diesel PM, diesel PM-ash, Printex (U), and Printex (U)-ash, respectively. Organic compounds dropped gradually in the oxidation process of diesel particulate matter. Nanostructure evolutions of diesel particulate matter and Printex (U) were similar, experiencing smaller sizes and void cores at the end of oxidation process.

The health effects of traffic-related air pollution: A review focused the health effects of going green

Traffic-related air pollution (TRAP) is global concern due to both the ecological damage of TRAP and the adverse health effects in Humans. Several strategies to reduce TRAP have been implemented, including the use of sustainable fuels, after-treatment technologies, and new energy vehicles. Such approaches can reduce the exhaust of particulate matter, adsorbed chemicals and a range of gases, but from a health perspective these approaches are not always successful. This review aims to discuss the approaches taken, and to then describe the likely health effects of these changes.

Atmospheric Pressure Plasma for Diesel Particulate Matter Treatment: A Review

The purification of diesel exhaust gas is of great importance to prevent the atmospheric emission of major pollutants such as diesel particulate matter and nitrogen oxides and meet the environmental regulations. The atmospheric-pressure plasma is attracting increasing interest and is a promising after-treatment technology for purifying diesel emission at low temperatures. However, when compared with the numerous publications on nitrogen oxides reduction by non-thermal plasma, using non-thermal plasma to particulate matter treatment have relatively limited. This work provides a comprehensive review of the plasma applications for diesel particulate matter treatment, including self-regenerating diesel particulate filter, diesel particulate matter removal, and simultaneous removal of diesel particulate matter and nitrogen oxides. The treatment of particulate matter from both simulated particulate matter sources and actual diesel engines also discussed in this comprehensive review. The challenge to this technology is limited energy consumption for plasma, which should be less than 5% (~30 J/L) of the overall fuel consumption. Until now, the atmospheric-pressure plasma has been no commercial implementation in diesel exhaust gas treatment, so more research is needed to be done in this field.

A phenomenological model for particle number and size distributions of a diesel engine with a diesel oxidation catalyst

Particle number is a key index for evaluating particulate emissions, and diesel oxidation catalysts (DOCs) are one of the most important technologies for controlling the particulate emissions of a diesel engine. In this paper, a novel phenomenological one-dimensional model was established to predict particle number and size distributions at a DOC outlet with the aim of investigating the effects of DOC on particle number emissions. The phenomenological model consisted of two submodels: submodel-1, a global kinetic model for calculating particle size in particle number and size distributions after particles had passed through the DOC, and submodel-2, an original global parametric model for calculating the particle number at the DOC outlet. The effects of the sampling process, fuel properties, and the engine operating condition were considered in submodel-2. An 8.8 L, direct-injection, heavy-duty diesel engine was tested. The particle number and size distributions at the DOC inlet and outlet were determined using an engine exhaust particle sizer. The test data, coupled with literature results, were used to calibrate and validate the phenomenological model. This model was then applied to investigate the influence of various factors on particle number and size distributions at the DOC outlet. It was found that dilution temperature, fuel sulfur content, exhaust gas temperature, and gas hourly space velocity (GHSV) played a key role in the particle number after DOC oxidation. The particle number concentration at the DOC outlet increased as fuel sulfur content and exhaust gas temperature increased and decreased as GHSV and dilution temperature increased. In general, results proved that this phenomenological model was accurate enough to predict particle number and size distributions at a DOC outlet under most operating conditions. It may serve as a useful tool for research and development focusing on PM reduction of diesel engines and air pollution control.

Preparation and application of effective different catalysts for simultaneous control of diesel soot and NOX emissions: An overview

Soot particulates and nitrogen oxides (NO_X) from diesel engine exhaust have been causing serious problems to human health and the global environment.

Carbonaceous composition changes of heavy-duty diesel engine particles in relation to biodiesels, aftertreatments and engine loads

Three biodiesels and two aftertreatments were tested on a heavy-duty diesel engine under the US FTP transient cycle and additional four steady engine loads. The objective was to examine their effects on the gaseous and particulate emissions, with emphasis given to the organic and elemental carbon (OC and EC) in the total particulate matter. Negligible differences were observed between the low-sulfur (B1S50) and ultralow-sulfur (B1S10) biodiesels, whereas small reductions of OC were identified with the 10% biodiesel blend (B10). The use of diesel oxidation catalyst (DOC1) showed moderate reductions of EC and particularly OC, resulting in the OC/EC ratio well below unity. The use of DOC plus diesel particulate filter (DOC2+DPF) yielded substantial reductions of OC and particularly EC, resulting in the OC/EC ratio well above unity. The OC/EC ratios were substantially above unity at idle and low load, whereas below unity at medium and high load. The above changes in particulate OC and EC are discussed with respect to the fuel content, pollutant removal mechanisms and engine combustion conditions. Overall, the present study shows that the carbonaceous composition of PM could change drastically with engine load and aftertreatments, and to a lesser extent with the biodiesels under study.

The effects of emission control strategies on light-absorbing carbon emissions from a modern heavy-duty diesel engine

Control of atmospheric black carbon (BC) and brown carbon (BrC) has been proposed as an important pathway to climate change mitigation, but sources of BC and BrC are still not well understood. In order to better identify the role of modern heavy-duty diesel engines on the production of BC and BrC, emissions from a heavy-duty diesel engine operating with different emission control strategies were examined using a source dilution sampling system. The effect of a diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) on light-absorbing carbon (LAC) was evaluated at three steady-state engine operation modes: idle, 50% speed and load, and 100% speed and load. LAC was measured with four different engine configurations: engine out, DOC out, DPF out, and engine out with an altered combustion calibration. BC and BrC emission rates were measured with the Aethalometer (AE-31). EC and BC emission rates normalized to the mass of CO₂emitted increased with increasing engine speed and load. Emission rates normalized to brake-specific work did not exhibit similar trends with speed and load, but rather the highest emission rate was measured at idle. EC and OC emissions were reduced by 99% when the DOC and DPF architecture was applied. The application of a DPF was equally effective at removing 99% of the BC fraction of PM, proving to be an important control strategy for both LAC and PM. BC emissions were unexpectedly increased across the DOC, seemingly due to a change aerosol optical properties. Removal of exhaust gas recirculation (EGR) flow due to simulated EGR cooler failure caused a large increase in OC and BrC emission rates at idle, but had limited influence during high load operation. LAC emissions proved to be sensitive to the same control strategies effective at controlling the total mass of diesel PM.

IMPLICATIONS

In the context of black carbon emissions, very small emission rates of brown carbon were measured over a range of control technologies and engine operating conditions. During specific idle engine operation without EGR and adjusted fueling conditions, brown carbon can be formed in significant amounts, requiring careful management tactics. Control technologies for particulate matter are very effective for light-absorbing carbon, reducing black carbon emissions to near zero for modern engines equipped with a DPF. Efforts to control atmospheric brown carbon need to focus on other sources other than modern diesel engines, such as biomass burning.

Effects of particulate oxidation catalyst on unregulated pollutant emission and toxicity characteristics from heavy-duty diesel engine

To evaluate the effects of particulate oxidation catalyst (POC) on unregulated pollutant emission and toxicity characteristics, polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), soot, soluble organic fractions (SOF) and sulphate emissions emitted from a heavy-duty diesel engine retrofitted with a POC were investigated on a diesel bench. The particulate matter (PM) in the exhaust was collected by Teflon membrane, and the PAHs and VOCs were analysed by a gas chromatography/mass spectrometer (GC/MS). The results indicate that the POC exhibits good performance on the emission control of VOCs, PAHs and PM. The POC and the diesel particulate filters (DPF) both show a good performance on reducing the VOCs emission. Though the brake-specific emission (BSE) reductions of the total PAHs by the POC were lower than those by the DPF, the POC still removed almost more than 50% of the total PAHs emission. After the engine was retrofitted with the POC, the reductions of the PM mass, SOF and soot emissions were 45.2-89.0%, 7.8-97.7% and 41.7-93.3%, respectively. The sulphate emissions decreased at low and medium loads, whereas at high load, the results were contrary. The PAHs emissions were decreased by 32.4-69.1%, and the contributions of the PAH compounds were affected by the POC, as well as by load level. The benzo[a]pyrene equivalent (BaPeq) of PAHs emissions were reduced by 35.9-97.6% with the POC. The VOCs emissions were reduced by 21.8-94.1% with the POC, and the reduction was more evident under high load.

Experimental study on the nitrogen dioxide and particulate matter emissions from diesel engine retrofitted with particulate oxidation catalyst

A particulate oxidation catalyst (POC) was employed to perform experiments on the engine test bench to evaluate the effects on the nitrogen dioxide (NO2) and particulate matter (PM) emissions from diesel engine. The engine exhaust was sampled from both upstream and downstream of the POC. The results showed that the POC increased the ratios of NO2/NOx significantly in the middle and high loads, the ratio of NO2/nitrogen oxides (NOx) increased 4.5 times on average under all experiment modes with the POC. An engine exhaust particle sizer (EEPS) was used to study the particle number-weighted size distributions and the abnormal particle emissions with the POC. The results indicated that the average reduction rate of particle number (PN) was 61% in the operating range of the diesel engine. At the engine speed of 1,400 r/min, the reduction rates of PN tended to decrease with the larger particle size. In the long time run under the steady mode (520 Nm, 1,200 r/min), abnormal particle emissions after the POC happened seven times in the first hour, and the average PN concentration of these abnormal emission peaks was much higher than that in normal state. The particle emissions of peaks 1-5 equaled the particles emitted downstream of the POC in normal state for 1.9h in number concentration, and for 3.6h in mass concentration. The PN concentrations tended to increase over time in 5h under the steady engine mode and the increase of the PN in the size range of 6.04-14.3 nm was more evident.

Study of Miller timing on exhaust emissions of a hydrotreated vegetable oil (HVO)-fueled diesel engine

The effect of intake valve closure (IVC) timing by utilizing Miller cycle and start of injection (SOI) on particulate matter (PM), particle number and nitrogen oxide (NOx) emissions was studied with a hydrotreated vegetable oil (HVO)-fueled nonroad diesel engine. HVO-fueled engine emissions, including aldehyde and polyaromatic hydrocarbon (PAH) emissions, were also compared with those emitted with fossil EN590 diesel fuel. At the engine standard settings, particle number and NOx emissions decreased at all the studied load points (50%, 75%, and 100%) when the fuel was changed from EN590 to HVO. Adjusting IVC timing enabled a substantial decrease in NOx emission and combined with SOI timing adjustment somewhat smaller decrease in both NOx and particle emissions at IVC -50 and -70 degrees CA points. The HVO fuel decreased PAH emissions mainly due to the absence of aromatics. Aldehyde emissions were lower with the HVO fuel with medium (50%) load. At higher loads (75% and 100%), aldehyde emissions were slightly higher with the HVO fuel. However, the aldehyde emission levels were quite low, so no clear conclusions on the effect of fuel can be made. Overall, the study indicates that paraffinic HVO fuels are suitable for emission reduction with valve and injection timing adjustment and thus provide possibilities for engine manufacturers to meet the strictening emission limits.

Mutagenicity of diesel engine exhaust is eliminated in the gas phase by an oxidation catalyst but only slightly reduced in the particle phase

Concerns about adverse health effects of diesel engine emissions prompted strong efforts to minimize this hazard, including exhaust treatment by diesel oxidation catalysts (DOC). The effectiveness of such measures is usually assessed by the analysis of the legally regulated exhaust components. In recent years additional analytical and toxicological tests were included in the test panel with the aim to fill possible analytical gaps, for example, mutagenic potency of polycyclic aromatic hydrocarbons (PAH) and their nitrated derivatives (nPAH). This investigation focuses on the effect of a DOC on health hazards from combustion of four different fuels: rapeseed methyl ester (RME), common mineral diesel fuel (DF), SHELL V-Power Diesel (V-Power), and ARAL Ultimate Diesel containing 5% RME (B5ULT). We applied the European Stationary Cycle (ESC) to a 6.4 L turbo-charged heavy load engine fulfilling the EURO III standard. The engine was operated with and without DOC. Besides regulated emissions we measured particle size and number distributions, determined the soluble and solid fractions of the particles and characterized the bacterial mutagenicity in the gas phase and the particles of the exhaust. The effectiveness of the DOC differed strongly in regard to the different exhaust constituents: Total hydrocarbons were reduced up to 90% and carbon monoxide up to 98%, whereas nitrogen oxides (NO(X)) remained almost unaffected. Total particle mass (TPM) was reduced by 50% with DOC in common petrol diesel fuel and by 30% in the other fuels. This effect was mainly due to a reduction of the soluble organic particle fraction. The DOC caused an increase of the water-soluble fraction in the exhaust of RME, V-Power, and B5ULT, as well as a pronounced increase of nitrate in all exhausts. A high proportion of ultrafine particles (10-30 nm) in RME exhaust could be ascribed to vaporizable particles. Mutagenicity of the exhaust was low compared to previous investigations. The DOC reduced mutagenic effects most effectively in the gas phase. Mutagenicity of particle extracts was less efficiently diminished. No significant differences of mutagenic effects were observed among the tested fuels. In conclusion, the benefits of the DOC concern regulated emissions except NO(X) as well as nonregulated emissions such as the mutagenicity of the exhaust. The reduction of mutagenicity was particularly observed in the condensates of the gas phase. This is probably due to better accessibility of gaseous mutagenic compounds during the passage of the DOC in contrast to the particle-bound mutagens. Concerning the particulate emissions DOC especially decreased ultrafine particles.

Effects of biodiesel, engine load and diesel particulate filter on nonvolatile particle number size distributions in heavy-duty diesel engine exhaust

Diesel engine exhaust contains large numbers of submicrometer particles that degrade air quality and human health. This study examines the number emission characteristics of 10-1000 nm nonvolatile particles from a heavy-duty diesel engine, operating with various waste cooking oil biodiesel blends (B2, B10 and B20), engine loads (0%, 25%, 50% and 75%) and a diesel oxidation catalyst plus diesel particulate filter (DOC+DPF) under steady modes. For a given load, the total particle number concentrations (N(TOT)) decrease slightly, while the mode diameters show negligible changes with increasing biodiesel blends. For a given biodiesel blend, both the N(TOT) and mode diameters increase modestly with increasing load of above 25%. The N(TOT) at idle are highest and their size distributions are strongly affected by condensation and possible nucleation of semivolatile materials. Nonvolatile cores of diameters less than 16 nm are only observed at idle mode. The DOC+DPF shows remarkable filtration efficiency for both the core and soot particles, irrespective of the biodiesel blend and engine load under study. The N(TOT) post the DOC+DPF are comparable to typical ambient levels of ≈ 10(4)cm(-3). This implies that, without concurrent reductions of semivolatile materials, the formation of semivolatile nucleation mode particles post the after treatment is highly favored.

Effect of diesel oxidation catalysts on the diesel particulate filter regeneration process

A Diesel Particulate Filter (DPF) regeneration process was investigated during aftertreatment exhaust of a simulated diesel engine under the influence of a Diesel Oxidation Catalyst (DOC). Aerosol mass spectrometry analysis showed that the presence of the DOC decreases the Organic Carbon (OC) fraction adsorbed to soot particles. The activation energy values determined for soot nanoparticles oxidation were 97 ± 5 and 101 ± 8 kJ mol(-1) with and without the DOC, respectively; suggesting that the DOC does not facilitate elementary carbon oxidation. The minimum temperature necessary for DPF regeneration was strongly affected by the presence of the DOC in the aftertreatment. The conversion of NO to NO(2) inside the DOC induced the DPF regeneration process at a lower temperature than O(2) (ΔT = 30 K). Also, it was verified that the OC fraction, which decreases in the presence of the DOC, plays an important role to ignite soot combustion.

Dependence between nonvolatile nucleation mode particle and soot number concentrations in an EGR equipped heavy-duty Diesel engine exhaust

Heavy duty diesel engine exhaust characteristics were studied with direct tailpipe sampling on an engine dynamometer. The exhaust particle size distributions, total particle mass, and gaseous emissions were measured with different load conditions without after-treatment. The measured particle size distributions were bimodal; distinctive accumulation and nucleation modes were detected for both volatile and dry particle samples. The condensing volatile compounds changed the characteristics of the nonvolatile nucleation mode while the soot/accumulation mode characteristics (concentration and diameter) were unchanged. A clear dependence between the soot and the nonvolatile nucleation mode number concentrations was detected. While the concentration of the soot mode decreased, the nonvolatile nucleation mode concentration increased. The soot mode number concentration decrease was related to soot-NOx trade-off; the decrease of the exhaust gas recirculation rate decreased soot emission and increased NOx emission. Simultaneously detected increase of the nonvolatile nucleation mode concentration may be caused by the decrease of the soot mode sink or by changed combustion characteristics. However, the total particle number concentration increased with decreasing soot mode number concentration. The proportion of the particle number concentration between the nonvolatile nucleation and soot mode followed the NO2:NO ratio linearly. While ratio NO2:NO increased the proportion of soot mode number concentration in total number concentration increased. Regardless of the mechanism that causes the balance between the soot mode and the nonvolatile nucleation mode emissions, the changes in the particle number size distribution should be taken into account while the particle mass emissions are controlled with combustion optimization.

Experimental investigation on regulated and unregulated emissions of a diesel/methanol compound combustion engine with and without diesel oxidation catalyst

The use of methanol in combination with diesel fuel is an effective measure to reduce particulate matter (PM) and nitrogen oxides (NOx) emissions from in-use diesel vehicles. In this study, a diesel/methanol compound combustion (DMCC) scheme was proposed and a 4-cylinder naturally-aspirated direct-injection diesel engine modified to operate on the proposed combustion scheme. The effect of DMCC and diesel oxidation catalyst (DOC) on the regulated emissions of total hydrocarbons (THC), carbon monoxide (CO), NOx and PM was investigated based on the Japanese 13 Mode test cycle. Certain unregulated emissions, including methane, ethyne, ethene, 1,3-butadiene, BTX (benzene, toluene, xylene), unburned methanol and formaldehyde were also evaluated based on the same test cycle. In addition, the soluble organic fraction (SOF) in the particulate and the particulate number concentration and size distribution were investigated at certain selected modes of operation. The results show that the DMCC scheme can effectively reduce NOx, particulate mass and number concentrations, ethyne, ethene and 1,3-butadiene emissions but significantly increase the emissions of THC, CO, NO(2), BTX, unburned methanol, formaldehyde, and the proportion of SOF in the particles. After the DOC, the emission of THC, CO, NO(2), as well as the unregulated gaseous emissions, can be significantly reduced when the exhaust gas temperature is sufficiently high while the particulate mass concentration is further reduced due to oxidation of the SOF.

Nanoparticle emissions from a heavy-duty engine running on alternative diesel fuels

We have studied the effect of three different fuels (fossil diesel fuel (EN590); rapeseed methyl ester (RME); and synthetic gas-to-liquid (GTL)) on heavy-duty diesel engine emissions. Our main focus was on nanoparticle emissions of the engine. Our results show that the particle emissions from a modern diesel engine run with EN590, GTL, or RME consisted of two partly nonvolatile modes that were clearly separated in particle size. The concentration and geometric mean diameter of nonvolatile nucleation mode cores measured with RME were substantially greater than with the other fuels. The soot particle concentration and soot particle size were lowest with RME. With EN590 and GTL, a similar engine load dependence of the nonvolatile nucleation mode particle size and concentration imply a similar formation mechanism of the particles. For RME, the nonvolatile core particle size was larger and the concentration dependence on engine load was clearly different from that of EN590 and GTL. This indicates that the formation mechanism of the core particles is different for RME. This can be explained by differences in the fuel characteristics.

Diesel nucleation mode particles: semivolatile or solid?

Although the preponderance of current data points to semivolatile diesel nuclei particles composed of sulfuric acid and heavy hydrocarbons, the question remains as to what extent, if any, they contain solid cores. We present evidence here of a "solid" particle nucleation mode that accompanies normal soot emissions in the case of two modern light-duty diesel vehicles run with ultralow sulfur fuel. This mode is most prominent at idle, but also appears at speeds below approximately 30 mph, and is highly sensitive to the level of exhaust gas recirculation (EGR). The nuclei particles are examined for their volatility and electrical charge. In stark contrast to "conventional" nuclei particles, they remain nonvolatile to >400 degrees C and exhibit a bipolar charge with a Boltzmann temperature of 580 degrees C. Their nonvolatile nature rules out sulfate and heavy hydrocarbons as primary constituents, and their electrical charge requires formation in a high-temperature environment capable of generating bipolar ions. This suggests that "solid" nuclei particles form during combustion but remain distinct from soot particles, analogous to what has been found recently in flames. As concerns about potential emissions of nonvolatile nanoparticles have already surfaced, an important conclusion of this study is that diesel particulate filters remove the "solid" nucleation mode with an efficiency comparable to soot

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.

Nucleation mode particles with a nonvolatile core in the exhaust of a heavy duty diesel vehicle

The characteristics of the nucleation mode particles of a Euro IV heavy-duty diesel vehicle exhaust were studied. The NOx and PM emissions of the vehicle were controlled through the use of cooled EGR and high-pressure fuel injection techniques; no exhaust gas after-treatment was used. Particle measurements were performed in vehicle laboratory and on road. Nucleation mode dominated the particle number size distribution in all the tested driving conditions. According to the on-road measurements, the nucleation mode was already formed after 0.7 s residence time in the atmosphere and no significant changes were observed for longer residence times. The nucleation mode was insensitive to the fuel sulfur content, dilution air temperature, and relative humidity. An increase in the dilution ratio decreased the size of the nucleation mode particles. This behavior was observed to be linked to the total hydrocarbon concentration in the diluted sample. In volatility measurements, the nucleation mode particles were observed to have a nonvolatile core with volatile species condensed on it. The results indicate that the nucleation mode particles have a nonvolatile core formed before the dilution process. The core particles have grown because of the condensation of semivolatile material, mainly hydrocarbons, during the dilution.