Black carbon-containing particles have the ability to alter our climate by the direct and indirect effect. These particles have direct effects on the climate by absorption and scattering of solar radiation. They can have indirect effects by acting as cloud condensation nuclei (CCN), which in turn form cloud droplets and reflect solar radiation. To characterize these radiative effects, knowledge of condensational growth, activation and per-particle chemical species composition (mixing state) are important. In determining the particle evolution, it is necessary to consider entrainment of dry air into a cloud in order to achieve the cloud droplet spectra.
In this work, we used the particle-resolved model PartMC-MOSAIC together with a particle-resolved cloud parcel model, which has the ability to track the individual size and composition of thousands of particles through the aging processes of condensation, coagulation, and chemistry. An idealized urban plume scenario was simulated and an aged particle population was determined 1, 12, and 24 hours after the start of the simulation. Then, the particle populations were used to initialize the cloud parcel model. Previously, the modeled cloud was formed using a constant cooling rate with no entrainment. Here we revisit the study, but now include entrainment of non-cloudy air into the cloud.
We investigated the impact of entrainment on the cloud droplet number concentration (CDNC), liquid water content (LWC), number concentration size distribution, and the mass concentration of black carbon that is incorporated in the cloud. We found that the entrainment caused a decrease in LWC and CDNC in the idealized constant cooling rate scenarios, while the CDNC profile was more complex in less idealized scenarios. In addition, a broadening in the cloud droplet size distribution occurred due to entrainment. We also found that the mass of black carbon in the cloud was dependent on the maximum supersaturation and was largest for aged particles.