Nucleation Particles in Diesel Exhaust: Composition Inferred from In Situ Mass Spectrometric Analysis J. SCHNEIDER,* N. HOCK, S. WEIMER, † AND S. BORRMANN Cloud Physics and Chemistry Department, Max Planck Institute for Chemistry, Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany U. KIRCHNER, R. VOGT, AND V. SCHEER Ford Forschungszentrum Aachen GmbH, Germany 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. 1. Introduction Understanding the processes of particle formation in diesel exhaust is of importance because of the discussion on adverse effects that diesel particles may have on human health. Diesel exhaust typically contains fine (Dp < 2.5 µm) and ultrafine (Dp < 0.1 µm) soot particles and, under certain conditions, nucleation particles (Dp < 0.05 µm) (1). Ultrafine particles reach deep into the lung and are therefore of special interest, although epidemiological studies with a focus on ultrafine particles are very sparse (2). The potential health effects of ultrafine solid and ultrafine soluble particles may be different, an aspect that has been neglected in epidemiological studies (3). Furthermore, particles from combustion processes may also influence the radiation budget of the atmosphere. It is presently under discussion whether particulate black carbon emissions contribute to the radiative forcing of the atmo- sphere on local scales or possibly also on a global scale (4) or whether the total short- and long-wave relaxed forcing by these aerosols remains near zero, as calculated by Penner et al. (5). The primary emissions of diesel engines include aerosol precursor gases (SO2, SO3,H2SO4,H2O, low-volatile organic species, and semivolatile organic species) as well as soot particles, which are fractallike agglomerates of approximately solid spheres with diameters of about 20 nm (1, 6). During the dilution and cooling process, a competition between nucleation of the low-volatile species and condensation on the surface of the existing particles occurs. A schematic of these processes is displayed in Figure 1. The fractallike soot particles have been observed to undergo compaction (7) when low- and semivolatile species condensate on their surface. This will result in higher density but lower diameter (mobility equivalent or aerodynamic) of the particles and less irregular shape. This accumulation mode (with soot particles as cores and various species as condensates) can be observed regularly during chassis dynamometer tests. The diameter of these particles ranges from about 50 to 500 nm (1, 8, 9). Under certain dilution conditions, nucleation particles with diam- eters in the range of 10-50 nm can be observed. This nucleation mode can be due to either nucleation of sulfuric acid water clusters or possibly also nucleation of low-volatile organics vapors, which might originate from lubricating oil. The latter pathway is indicated with the dotted line in Figure 1. It is thought that the sulfuric acid/sulfate fraction in total mass emission is dependent on the fuel sulfur content, while the soluble organics fraction (SOF), consisting mainly of unburned fuel and lube oil, is strongly dependent on engine operating conditions and is highest from heavy-duty engines at light loads when exhaust temperatures are low (1). For light-duty diesel cars, Maricq et al. (10) and Vogt et al. (11) could show that both high sulfur and the oxidation catalyst are mandatory conditions for sulfate formation, which resulted in nucleation particle formation. Shi and Harrison (12) found that binary nucleation of sulfuric acid/water with subsequent condensation of organic substances explained qualitatively their observations in diesel exhaust, but the calculated nucleation rate was too low. To explain this discrepancy, they suggested that other species (e.g., am- monia) might be involved in the nucleation, while Yu (13) suggested that chemiions could play a role in diesel exhaust nucleation. Kleemann et al. (14) found significant amounts of ammonium in diesel exhaust particles and observed * Corresponding author phone: +49 6131 305-586/596; fax: +49 6131 305-597; e-mail: schneider@mpch-mainz.mpg.de. † Present addresses: EMPA, CH-8600 Du ¨ bendorf, Switzerland, and Paul Scherrer Institute, CH-5232 Villigen, Switzerland. FIGURE 1. Schematic of the emissions and particle formation mechanisms in diesel exhaust. Solid lines indicate well-established processes, dotted lines indicate processes under discussion. Environ. Sci. Technol. 2005, 39, 6153-6161 10.1021/es049427m CCC: $30.25  2005 American Chemical Society VOL. 39, NO. 16, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6153 Published on Web 07/12/2005