Finnish Centre of Excellence in Physics, Chemistry, Biology and Meteorology of Atmospheric Composition and Climate Change

The IPCC has emphasised the complexity of the combined direct and indirect radiative forcing from both aerosols and greenhouse gases. Atmospheric aerosol particles influence the Earth’s radiation balance directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei (CCN). The uncertainty in current estimates of radiative forcing is still large, and one of the most significant issues to resolve is how the different components may interact with each other.

An important phenomenon associated with the atmospheric aerosol system is the formation of new atmospheric aerosol particles. Atmospheric aerosol formation consists of a complicated set of processes that include the production of nanometer-size clusters from gaseous vapours, the growth of these clusters to detectable sizes, and their simultaneous removal by coagulation with the preexisting aerosol particle population. Once formed, aerosol particles need to grow further to sizes >50-100 nm in diameter until they are able to influence climate, even though smaller particles may have influences on human health and atmospheric chemistry. While aerosol formation has been observed to take place almost everywhere in the atmosphere, serious gaps in our knowledge regarding this phenomenon still exist. These gaps range from the basic process-level understanding of atmospheric aerosol formation to its various impacts on atmospheric chemistry, climate, human health and environment.

Fig 1: The SMEAR II -station is built to study material and energy flows in atmosphere - vegetation - soil - continuum at different temporal and spatial scales.The station is a versatile and automatic unit operating in continuous and long-term manner since 1996

Tackling uncertainties

Our main objective is to contribute to the reduction of scientific uncertainties concerning global climate change issues, particularly those related to aerosols and clouds. The main focus of our research unit is in the following topics:

• The formation and growth mechanisms of atmospheric aerosols, aerosol dynamics;

• The effect of secondary biogenic aerosols on global aerosol load;

• Aerosol-cloud-climate interaction; and,

• The relationships between the atmosphere and different ecosystems, particularly boreal forest.

The scientists in our group have published over 660 papers in peerreviewed journals during the last five years, including ten articles in Science or Nature.

The core of our activities is in continuous measurements and database of atmospheric and ecological mass fluxes and aerosol precursors and CO2/aerosol/trace gas interactions in SMEAR field stations (Fig 1); and in focused experiments and modelling to understand the observed patterns. Our research plan has been designed specifically to focus on those aspects of the research chain (from molecular scale to global scale) where the uncertainties are largest. One of the most significant recent results obtained is the observation that the formation of new particles and their subsequent growth to CCN sizes (Fig 2).

Fig 2: Aerosol size distribution on a new particle formation day at Hyytiälä SMEAR II station. Shortly before noon newly formed particles start to appear at 3 nm size, and this lasts for a few hours. After their formation, condensation of various vapours grows these particles during several hours.When the particles reach sizes of 50-100 nm they can act as cloud condensation nuclei. During the last 11 years, ca. 1000 new particle formation events have been observed at SMEAR II

Processes and simulations

To better comprehend the processes involved in nucleation and aerosol thermodynamics, we start from molecular simulations (Monte Carlo and Molecular Dynamics). Knowledge on these microscopic processes of nucleation, in conjunction with condensation/evaporation and coagulation is required to understand aerosol dynamics, particle concentrations and composition. Significant advances in laboratory data and modelling techniques are being developed for a number of important aerosol systems. Similarly, photosynthesis, autotrophic respiration and VOC synthesis are modelled at cellular, stomatal and leaf (or shoot for conifers) scales, and the model approach is accompanied by chamber measurements in laboratory and under field conditions.

Fundamental aerosol and carbon cycle processes need to be recognised in order to quantify aerosol radiative properties and the influence of aerosols on cloud microphysics and dynamics at the scale of individual clouds, and to understand changes in carbon uptake dynamics. At larger scales, advances in our insight of the boundary layer meteorology are needed to understand atmospheric aerosol transport, trace gas (e.g. CO2, CH4, N2O, O3, SO2, NOX, and VOC) and water vapour exchange and the consequent deposition processes. Boundary layer studies form a link to regional-scale processes and further to global-scale phenomena. In order to be able to simulate global climate and air quality, the most recent progress on this chain of processes must be compiled, integrated and implemented in Climate Change (CC) and Air Quality (AQ) numerical models via novel parameterisations in different environments.

T: +358 (0)9 191 50756
E: markku.kulmala@helsinki.fi
W: www.atm.helsinki.fi/indexeng.php