The influence of particle composition upon the evolution of urban ultrafine diesel particles on the neighbourhood scale
2018
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 ( C 16 −C 32 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 C 22 H 46 and
C 24 H 50 . The vapour pressure range, derived from published data,
is between 9.23 × 10 −3 and 8.94 × 10 −6 Pa for
C 22 H 46 and between 2.26 × 10 −3 and
2.46 × 10 −7 Pa for C 24 H 50 . 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).
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