10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Single Particle Measurements of Size and Mixing State of Black Carbon Particles Combined with Simplified κ-Köhler Theory Explains Their Droplet Activation Behaviour Observed in Fog and Clouds
MARTIN GYSEL, Ghislain Motos, Julia Schmale, Joel Corbin, Marco Zanatta, Robin Modini, Urs Baltensperger, Paul Scherrer Institute
Abstract Number: 548 Working Group: Clouds and Climate
Abstract Black carbon (BC) is an aerosol component, mainly emitted from anthropogenic sources, with peculiar properties. BC is the major light-absorbing component of atmospheric aerosols due to its high mass absorption coefficient (MAC), therefore causing substantial climate warming through aerosol-radiation interactions (ARI). BC in pure form is a poor cloud condensation nuclei (CCN) due to its insolubility, which makes wet deposition through the droplet nucleation pathway inefficient. Freshly emitted BC is often externally mixed, while it tends to acquire coatings during atmospheric aging processes. Such coatings can increase the MAC of BC. However, the corresponding effect on ARI may be partly compensated by reduced BC burdens as a consequence of better CCN activity and thus more efficient wet removal. In this study we address the droplet activation of BC in ambient fog and clouds, and assess whether BC activation can be predicted from measured BC properties.
A first field experiment was conducted in Zurich, Switzerland, an urban site where fog regularly occurs and where a mixture of fresh and aged BC is encountered. A second field experiment was conducted at the Jungfraujoch, Switzerland, a high-alpine site frequently exposed to clouds and with highly aged free tropospheric aerosol. Aerosol was sampled during fog/cloud periods using a total inlet to collect the aerosol as a whole (interstitial plus droplet residual particles) and an interstitial inlet to exclusively collect interstitial particles. Aerosol size distribution (mobility particle size spectrometer) as well as BC core size distribution and mixing state (single particle soot photometer, SP2) were measured behind both inlets along with complementary measurements.
Comparison of the measurements behind these two inlets makes it possible to compare the properties of particles that activated to cloud droplets with those remaining interstitial. The size distribution data provide the activation cut-off diameter representative of BC-free particles, from which the fog/cloud peak supersaturation is inferred. The SP2 provides, within its detection limits, the volume equivalent total particle and BC core diameters of BC-containing particles. This information combined with coating hygroscopicity was used as input for κ-Köhler theory to infer the critical supersaturation for CCN activation of BC on a single particle basis. Validity of these predictions was tested by comparing predicted critical supersaturations measured for BC particles behind the two inlets for fog/clouds with known peak supersaturation.
The fog in Zurich was characterized by large activation cut-off diameters of BC-free particles, which implies very low peak supersaturation. The BC particle activation agreed with predictions both qualitatively and quantitatively: activated fraction increased with increasing coating thickness for a fixed core size, the coating thickness at 50% activation decreased with increasing core size, and the observed dependence of the activation cut-off on size and mixing state of BC matched the theoretical line for the peak supersaturation in the fog events. Similar agreement between prediction and measurements was also found for the clouds at Jungfraujoch, though with more limited “data coverage” as clearly higher peak supersaturations shift the activation cut-off diameters towards the lower limits of detection of the SP2.
In conclusion, we achieved successful closure between observed droplet activation of BC and predictions based on measured size and mixing state for a range of peak supersaturations. This shows that assuming spherical core-shell morphology, which is implicitly done in both SP2 data analysis and κ-Köhler theory, describes activation behavior of atmospheric BC in good approximation. This validates application of κ-Köhler theory based on droplet activation in model simulations in which BC size and mixing state is available on a mass or volume basis.