The influence of particle composition upon the evolution of urban ultrafine diesel particles on the neighbourhood scale

A recent study demonstrated that diesel particles in urban air undergo evaporative shrinkage when advected to a cleaner atmosphere (Harrison et al., 2016). We explore, in a structured and systematic way, the sensitivity of nucleation-mode diesel particles to changes in particle composition and saturation vapour pressure. We use a multi-component aerosol microphysics model based on surrogate molecule (C16-C32 n-alkane) volatilities. For standard atmospheric conditions (298K, 1013.25hPa), and over timescales (ca. 100s) relevant for dispersion on the neighbourhood scale (up to 1km), the choice of a particular vapour pressure dataset changes the range of compounds that are appreciably volatile by 2–6 carbon numbers. The nucleation-mode peak diameter, after 100s of model runtime, is sensitive to the vapour pressure parameterisations for particles with compositions centred on surrogate molecules between C22H46 and C24H50. The vapour pressures of components in this range are therefore critical for the modelling of nucleation-mode aerosol dynamics on the neighbourhood scale and need to be better constrained. Laboratory studies have shown this carbon number fraction to derive predominantly from engine lubricating oil. The accuracy of vapour pressure data for other (more and less volatile) components from laboratory experiments, is less critical. The influence of a core of involatile material is also considered.

The new findings of this study may also be used to identify the Semi-Volatile Organic Compound (SVOC) compositions that play dominating roles in the evaporative shrinkage of the nucleation mode observed in field measurements (e.g. Dall’Osto et al., 2011). As well as reconciling model and observations, identifying the most significant vapour pressure regime for nucleation-mode dynamics offers a way to improve the computing efficiency of urban aerosol models by adopting simplified schemes for those less important components: e.g., an equilibrium scheme for low-carbon-number components and a linear scheme for high-carbon-number components.