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

Irina Nikolova, Xiaoming Cai, Mohammed Salim Alam, Soheil Zeraati-Rezaei, Jian Zhong, A. Rob MacKenzie, Roy M. Harrison
2018 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> 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<span class="thinspace"></span><span class="inline-formula">&amp;lt;</span><span class="thinspace"></span>30<span class="thinspace"></span>nm) to changes in particle composition, saturation vapour
more » ... tion vapour pressure, and the mass accommodation coefficient. We use a multicomponent aerosol microphysics model based on surrogate molecule (<span class="inline-formula">C<sub>16</sub>−C<sub>32</sub></span> <span class="inline-formula"><i>n</i></span>-alkane) volatilities. For standard atmospheric conditions (298<span class="thinspace"></span>K, 1013.25<span class="thinspace"></span>hPa), and over timescales (ca. 100<span class="thinspace"></span>s) relevant for dispersion on the neighbourhood scale (up to 1<span class="thinspace"></span>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<span class="thinspace"></span>s of model runtime, is sensitive to the vapour pressure parameterisations for particles with compositions centred on surrogate molecules between <span class="inline-formula">C<sub>22</sub>H<sub>46</sub></span> and <span class="inline-formula">C<sub>24</sub>H<sub>50</sub></span>. The vapour pressure range, derived from published data, is between 9.23<span class="thinspace"></span><span class="inline-formula">×</span><span class="thinspace"></span>10<span class="inline-formula"><sup>−3</sup></span> and 8.94<span class="thinspace"></span><span class="inline-formula">×</span><span class="thinspace"></span>10<span class="inline-formula"><sup>−6</sup></span> Pa for <span class="inline-formula">C<sub>22</sub>H<sub>46</sub></span> and between 2.26<span class="thinspace"></span><span class="inline-formula">×</span><span class="thinspace"></span>10<span class="inline-formula"><sup>−3</sup></span> and 2.46<span class="thinspace"></span><span class="inline-formula">×</span><span class="thinspace"></span>10<span class="inline-formula"><sup>−7</sup></span> Pa for <span class="inline-formula">C<sub>24</sub>H<sub>50</sub></span>. 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<span class="thinspace"></span>% 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).</p>
doi:10.5194/acp-18-17143-2018 fatcat:bn2nuk5ydzcoxdfucitq5h65sa