Model studies of volatile diesel exhaust particle formation: are organic vapours involved in nucleation and growth?

L Pirjola,M Karl,T Rönkkö,F Arnold

doi:10.5194/acp-15-10435-2015

Abstract

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.

Highlights

Regardless of many improvements in vehicle technology exhaust particles emitted from traffic constitute major air pollutants in urban environments (e.g. Pey et al, 2009)
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 semivolatile organic vapour combined with the homogeneous nucleation of GSA alone
The pre-existing core and soot mode concentrations had an opposite trend on the nucleation mode particles (NUP) formation, and the maximum NUP formation was predicted if a diesel particle filter (DPF) was used

Summary

Introduction

Regardless of many improvements in vehicle technology exhaust particles emitted from traffic constitute major air pollutants in urban environments (e.g. Pey et al, 2009). The mass emissions of diesel particles have been reduced due to the tightened emission regulations, the number emission of exhaust nanoparticles has been reported to be significant (Rönkkö et al, 2013; Lähde et al, 2010). These nonregulated particles can penetrate deepest into the human pulmonary and blood-vascular systems having hazardous health effects (Pope III and Dockery, 2006; Sioutas et al, 2005; Kettunen et al, 2007; Su et al, 2008; Alföldy et al, 2009).

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