Abstract
Abstract. 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 (diameter < 30 nm) to changes in particle composition, saturation vapour pressure, and the mass accommodation coefficient. We use a multicomponent aerosol microphysics model based on surrogate molecule (C16−C32 n-alkane) volatilities. For standard atmospheric conditions (298 K, 1013.25 hPa), and over timescales (ca. 100 s) relevant for dispersion on the neighbourhood scale (up to 1 km), the choice of a particular vapour pressure dataset changes the range of compounds that are appreciably volatile by two to six carbon numbers. The nucleation-mode peak diameter, after 100 s of model runtime, is sensitive to the vapour pressure parameterisations for particles with compositions centred on surrogate molecules between C22H46 and C24H50. The vapour pressure range, derived from published data, is between 9.23 × 10−3 and 8.94 × 10−6 Pa for C22H46 and between 2.26 × 10−3 and 2.46 × 10−7 Pa for C24H50. Therefore, the vapour pressures of components in this range are 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 non-volatile material is also considered; non-volatile core fractions of more than 5 % are inconsistent with the field measurements that we test the model against. We consider mass accommodation coefficient values less than unity and find that model runs with more volatile vapour pressure parameterisations and lower accommodation coefficients are similar to runs with less volatile vapour pressure parameterisations and higher accommodation coefficients. The new findings of this study may also be used to identify semi-volatile organic compound (SVOC) compositions that play dominating roles in the evaporative shrinkage of the nucleation mode observed in field measurements (Dall'Osto et al., 2011).
Highlights
Ultrafine particles (UFPs, with particle diameter Dp < 100 nm) have become an increasingly important focus of urban air research over the last 2 decades
The purpose of this study was to evaluate the importance of particle composition and saturation vapour pressure on the evolution of urban ultrafine diesel particles on the neighbourhood scale ( 1 km) by means of numerical simulations
We presented the effect of evaporation on the size-resolved ultrafine particles and looked at the evolution of the nucleationmode peak diameter (Dpg,nuc) depending on particle semi-volatile organic compound (SVOC) composition, vapour pressure, fraction of non-volatile core in the particles, and the value of the mass accommodation coefficient
Summary
Ultrafine particles (UFPs, with particle diameter Dp < 100 nm) have become an increasingly important focus of urban air research over the last 2 decades. The main source of UFPs in outdoor urban air is typically road traffic (Kumar et al, 2014). Harrison et al (2011) reported that on a busy highway in central London, UK, 71.9 % of particles by number were traffic-generated; of this 71.9 %, 27.4 % were found in the semi-volatile exhaust nucleation mode (size between 15 and 30 nm), 38 % were in the exhaust solid mode (size > 30 nm), and the remaining 6.5 % were from brake dust and resuspension (size > 2000 nm). Nucleation-mode particles are defined as particles with a diameter of less than.
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