Model studies of volatile diesel exhaust particle formation: are organic vapours involved in nucleation and growth?
- 1Department of Technology, Metropolia University of Applied Sciences, P.O. Box 4021, 00180 Helsinki, Finland
- 2Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
- 3Norwegian Institute for Air Research, P.O. Box 100, 2027 Kjeller, Norway
- 4Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, P.O. Box 692, 33101 Tampere, Finland
- 5Max-Planck-Institut für Kernphysik, Heidelberg, Germany
- 6Deutsches Zentrum für Luft and Raumfahrt (DLR), Obenpfaffenhofen, Germany
Abstract. A high concentration of volatile nucleation mode particles (NUP) formed in the atmosphere when the exhaust cools and dilutes has hazardous health effects and it impairs the visibility in urban areas. Nucleation mechanisms in diesel exhaust are only poorly understood. We performed model studies using two sectional aerosol dynamics process models AEROFOR and MAFOR on the formation of particles in the exhaust of a diesel engine, equipped with an oxidative after-treatment system and running with low fuel sulfur content (FSC) fuel, under laboratory sampling conditions where the dilution system mimics real-world conditions. Different nucleation mechanisms were tested. Based on the measured gaseous sulfuric acid (GSA) and non-volatile core and soot particle number concentrations of the raw exhaust, the model simulations showed that the best agreement between model predictions and measurements in terms of particle number size distribution was obtained by barrier-free heteromolecular homogeneous nucleation between the GSA and a semi-volatile organic vapour combined with the homogeneous nucleation of GSA alone. Major growth of the particles was predicted to occur due to the similar organic vapour at concentrations of (1−2) × 1012 cm−3. The pre-existing core and soot mode concentrations had an opposite trend on the NUP formation, and the maximum NUP formation was predicted if a diesel particle filter (DPF) was used. On the other hand, the model predicted that the NUP formation ceased if the GSA concentration in the raw exhaust was less than 1010 cm−3, which was the case when biofuel was used.