Abstract
Abstract. Combustion and other high-temperature processes frequently result in the emission of aerosols in the form of polydisperse fractal-like aggregates made of condensed-phase nanoparticles (soot for instance). If certain conditions are met, the emitted aerosol particles are known to evolve into important cloud condensation nuclei (CCN) in the atmosphere. In this work, the hygroscopic parameter κ of complex morphology aggregates is calculated from the supersaturation-dependent activated fraction Fa=Fa(SS) in the frame of κ-Köhler theory. The particle size distribution is approximated with the morphology-corrected volume equivalent diameter calculated from the electrical mobility diameter by taking into account the diameter of the primary particle and the fractal dimension of the aggregate experimentally obtained from transmission electron microscopy measurements. Activation experiments are performed in water supersaturation conditions using a commercial CCN-100 condensation nuclei counter. The model is tested in close-to-ideal conditions of size-selected, isolated spherical particles (ammonium sulfate nanoparticles dispersed in nitrogen), then with complex polydisperse fractal-like aggregates (soot particles activated by exposure to ozone with κ as low as 5×10-5) that represent realistic anthropogenic emissions in the atmosphere.
Highlights
Soot particles formed during the incomplete combustion of hydrocarbons and emitted in exhaust are potentially important contributors to the radiative forcing of the atmosphere as they adsorb and scatter the solar radiation, but they can act as cloud condensation nuclei (CCN) or ice nuclei (IN) and trigger the formation of persistent clouds (Bond et al, 2013)
Ammonium sulfate is well known for the isolated, quasispherical particles that can be generated by atomization of aqueous solution, and for this reason it is often used as a reference material for activation experiments (Petters and Kreidenweis, 2007; Rose et al, 2008) and in this work to test the validity of Eq (3) before moving to complex morphology aggregates
An example transmission electron microscopy (TEM) image of the ammonium sulfate particles obtained after size selection is shown in Fig. 3a, and the corresponding particle projection dp (TEM) and electrical mobility dm (SMPS) distributions are shown in Fig. 3b and c, respectively
Summary
Soot particles formed during the incomplete combustion of hydrocarbons and emitted in exhaust are potentially important contributors to the radiative forcing of the atmosphere as they adsorb and scatter the solar radiation (direct effect), but they can act as cloud condensation nuclei (CCN) or ice nuclei (IN) and trigger the formation of persistent clouds (indirect effect) (Bond et al, 2013). Estimations of the magnitude of the direct and indirect effects are subject to large uncertainty, and commonly accepted values span the range [+0.25, +1.09] W m−2 for the direct effect and [−1.20, 0.00] W m−2 for the indirect effect comprehensive of all aerosol–cloud interactions (Stocker et al, 2014) Such large uncertainties result from the combination of several difficult-to-predict behaviors of the soot particles in the atmosphere when compared to mineral and biogenic aerosols. Their small size and low density enable a long lifetime in the atmosphere, which can reach several weeks (Govardhan et al, 2017) Their complex morphology and large specific surface allow many possible surface interactions that can deeply affect their reactivity (Monge et al, 2010; Browne et al, 2015). To better understand the effect of soot particles on the radiative balance of the atmosphere, it is important to understand how their
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