Computation of maximum rate of water-sulphuric acid nucleation in diesel exhaust

The origin of nucleation mode observed in the diesel engine exhaust is unclear. In this work, the mechanism of simple classical homogeneous water-sulphuric acid nucleation was studied using the parameterization of Vehkamäki, Kulmala, Lehtinen, and Noppel [2003. Modelling binary homogeneous nucleation of water-sulfuric acid vapours: Parameterisation for high temperature emissions. Environmental Science and Technology, 37, 3392-3398]. In the simple model used, condensation and coagulation were taken into account as sink terms in respective equations. The focus of the study was on the total amount of stable clusters formed, which provides an upper limit for nucleation mode number concentration. It was seen that the nucleation can be achieved even with relatively low sulphuric acid concentrations (of the order of 5×1017m-3). However, the efficiency depends strongly on the cooling and dilution experienced by the exhaust. According to the results obtained, the assumption of homogeneous sulphuric acid-water nucleation depicted by Vehkamäki parameterization gives physically meaningful results with low-sulphur content fuels if the sulphur-to-sulphuric acid conversion factor is close to 100%. Nonetheless, there are published results of nucleation mode in diesel exhaust which cannot be explained by the sulphuric acid-water mechanism.