The Influence of Aerosols on Cloud Formation

The goal of this project is to elucidate one of the least understood
aspects of atmospheric chemistry and global change, namely the ability
of aerosols to act as Cloud Condensation Nuclei (CCNs) and therefore
its indirect effect on the Earth's radiation balance.

It is widely accepted that aerosol particles, water vapor and clouds
are involved in maintaining the radiation balance of the atmosphere.
They are equally important for the chemical reactivity and cycles of
atmospheric trace compounds that either have low volatility or display
a particular affinity towards water. Microphysical processes involving
aerosols have been extensively studied with the result that they are
known in relatively great detail for inorganic compounds. On the other
hand, the knowledge of the interaction of trace gases and organic
vapors with aerosol particles, and aerosols' role as CCNs (i.e. aerosols'
indirect effect on the Earth's radiation balance) is relatively unknown.

Project Description
The focus of this Ph.D. project is on aerosols' indirect effect on the
Earth's radiation balance, since when aerosols acts as CCNs they can
change cloud properties.

Topics which are currently not understood and therefore interesting to
investigate are the reactions of NOx and acids with seasalt particles
and the intrusion of organic molecules into aerosols. The focus should
be to understand whether these aerosols are able to uptake water and
thereby contribute to cloud formation. For the understanding of the
formation of cloud droplets it is important to know and understand the
chemical composition of the aerosols and their capacity as CCNs.

The Ph.D. student can for example study:
1: Uptake processes and their impact on aerosol modeling. The
   investigations will focus on the dependence of the uptake coefficients
   with respect to temperature, size and composition of the aerosol.

2: The possibilities for aerosols to participate in cloud formation (its
   CCN capacity) can be studied by considering how likely it is for the
   aerosols to participate in uptake processes involving water vapor.

3: Aerosols' indirect influence on the Earth's radiation balance by
   studying the effects aerosols have on cloud properties for different
   kinds of clouds.

The Ph.D. student will have the opportunity to combine experiments with
quantum and classical microscopic models along with atmospheric modeling.
However, all three research disciplines do not have to be included in the
proposal, though atmospheric modeling of aerosols must be a part of the
research proposal.

   Molecular dynamical simulations and quantum chemical calculations in
   cooperation with Kurt V. Mikkelsen, University of Copenhagen, Department
   of Chemistry.

   Laboratory experiments in cooperation with Merete Bilde, University of
   Copenhagen, Department of Chemistry.

   Atmospheric modeling: microphysical aerosol modeling and investigations
   of aerosols indirect influence on the radiation balance in cooperation
   with Allan Gross and Anette Guldberg, Danish Meteorological Institute.

The experiments will focus on the hygroscopic properties of aerosols of
different chemical compositions and sizes and their ability to act as
CCN. Experiments will be performed using a Tandem Differential Mobility
Analyzer (TDMA) system combined with a CCN chamber. The existing system
will be modified and extended with a humidifier, an organic vapor
production system and a CCN chamber. The addition of a CCN chamber to the
TDMA system will provide unique possibilities for studying the ability of
aerosol particles to act as CCN in a controlled environment.

Microscopic Modeling
An aerosol prototype will be established using a dielectric function that
depends on molecular composition and concentration. The verification of
this model will be determined by its ability of reproducing the
experimental knowledge about aerosols containing sulphate. For determining
the probability of aerosols to act as CCNs we calculate uptake coefficients
as a function of composition and size of the aerosols. We determine these
rate constants by combining quantum mechanical methods of the reaction
partner with classical mechanical descriptions of the aerosol. We have
denoted this approach AERODYN.

Atmospheric Modeling
Two topics can be combined here: aerosols' ability to act as CCNs and
aerosols' indirect effect on the Earth radiation balance.

Aerosols can influence the cloud droplet concentration and droplet size
distribution i.e. its ability to act as a CCN. A model combining the
atmospheric transport-chemistry model MOON and the micro physics processes of
aerosols must be developed. Information and parameters estimated during the
laboratory experiments and microscopic simulations will be implemented in this
model. Typical atmospheric cases (stratospheric, tropospheric and inside the
atmospheric boundary layer) should be set up aiming at an improved understanding
of the influence natural and anthropogenic sources have on aerosol formation and
distribution in the atmosphere.

Aerosols' indirect effect on the Earth's radiation balance is not well
understood. The idea in this part of the project is to use knowledge
obtained in the micro physical aerosol modeling part to study the effect
of aerosols on cloud formation and cloud properties for different kinds of
clouds (water clouds, ice clouds). The aim of this study is to improve the
parameterization of these effects that is needed for implementation in a
full 3-dimensional model of the atmosphere used for studying the role
aerosols play in climate change.

We offer a PhD scholarship which provides a unique opportunity to connect
three research disciplines. Connecting molecular dynamical simulations with
laboratory experiments and micro physical and macroscopic aerosol modeling
on the level described above has not been done before.

Contact Persons
Kurt V. Mikkelsen, University of Copenhagen, [email protected].
Merete Bilde, University of Copenhagen, [email protected]
Allan Gross, Danish Meteorological Institute, [email protected]
Anette Guldberg, Danish Meteorological Institute, [email protected]