Modelling of diesel exhaust aerosol during laboratory sampling

A modelling approach which applies fundamental mechanisms of aerosol formation and dynamics to simulate the effect of sampling conditions on exhaust particle distributions is presented. The model examines the evolution of exhaust aerosol immediately after its mixing with dilution air, which is assumed to occur adiabatically and instantaneously. Particle number and mass balances are expressed separately for two aerosol modes: the nucleation mode (mean diameter <30 nm) and the accumulation mode (>30 nm). The number and mass balances include homogeneous nucleation, condensation on existing particles, intra- and inter-mode coagulation and diffusional losses to the walls. The model takes into account the sulphuric acid-water system and a heavy (C16) hydrocarbon in diesel exhaust. First, it is shown that the number concentration of the nucleation mode can be described if a barrierless nucleation is assumed for the sulphuric acid. At a second step, it is computationally demonstrated that the measured volume concentration in the nucleation mode can only be reached, if hydrocarbon condensation occurs on the sulphuric acid-water nuclei. Finally, it is shown that the nucleation mode is significantly suppressed by the presence of soot mode, due to the high surface area of the latter, which consumes volatile species. The model may be used to estimate the effect of sampling conditions on exhaust particle size distributions, with emphasis on the nucleation mode profiles.