Modelling the microphysics of cloud aerosol interactions

• Ari Laaksonen, Ph.D., Professor
• Joni-Pekka Pietikäinen, M.Sc, Researcher
• Riikka Sorjamaa, M.Sc, Researcher

The indirect effect of aerosols is the most uncertain factor in the modelling of climate change. Some of atmospheric aerosol particles are able to work as cloud condensation nuclei (CCN) and activate to cloud droplets when cloud is formed. Properties of clouds, like albedo and drizzle formation capability, are dependent on the number of cloud droplets.

We are using a cloud microphysics model to simulate the activation of CCN to cloud droplets in liquid and mixed-phase clouds. With the model, we are able to study condensation/evaporation dynamics of different gases between gas/liquid/solid phase, coagulation between particles and chemistry both in gas and liquid phase with explicit liquid phase thermodynamics, which includes all relevant inorganic and also some organic species.

Our study can be divided to three main parts, effect of condensing volatile gases on cloud droplets formation in warm clouds, effect of organic compounds on cloud droplet formation in warm clouds and the condensation of trace gases in mixed-phase cloud.

1. Effect of condensing volatile gases on cloud/smog droplet formation

It has been shown that semivolatile gases, like nitric acid (HNO₃), hydrochloric acid (HCL) and ammonia (NH₃), can affect atmospheric cloud formation processes. The most well known of these is the HNO₃ effect. Due to HNO₃ the cloud droplet concentration become higher and cloud drop sizes smaller than in the absence of HNO₃. The increased droplet concentrations lead to increased cloud albedos, and may thereby affect the global radiation budget considerably. However, it is extremely difficult to estimate the magnitude of the HNO₃ effect on the radiation budget without climate model simulations. We are producing new parameterizations of the HNO₃ and HCl effect on cloud formation. Simple parameterization accounts for the increased droplet concentration relative to nitric and hydrochloric acid -free circumstances. These parameterizations are intended to be used in the large scale models such as GCM's to get estimation for the possible effect on the global radiation budget. (SR, J-PP, AL)

2. Effect of organic compounds on cloud/smog droplet formation

Organic compounds constitute the significant part of all aerosol particles. Depending on the characteristics of organic compound, there are several mechanisms affecting cloud droplet formation. Some organics are completely volatile and affect cloud droplet formation in the similar way as inorganic volatile gases. Some other organics are surface active, lowering the surface tension and thus the barrier for activation decreases. To make situation even more complicated, some organics are totally insoluble or only partly soluble, some are forming films on droplets slowing the water condensation and some are behaving in the very complex way in water mixtures. In our group, we are developing a thermodynamical module to take these effects into account in the cloud droplet formation. (RS, AP, TR, J-PP, AL)

3. Condensation of trace gases on mixed-phase clouds

In the atmospheric conditions, cloud droplets are able to freeze heterogeneously as soon as the water saturation ratio with respect to ice exceeds unity. However, usually aerosol drops become supercooled and freeze homogeneously at RH clearly above ice saturation point. After freezing, ice nuclei grow fast, and depending on conditions, rain formation occurs. We are developing model to study how trace gases are distributed between gas, liquid and solid phases prior to freezing, during the freezing and after the freezing. (J-PP, AL)