The impact of radiative
forcing from aerosols
University of Helsinki
The Intergovernmental Panel for Climate Change, in 2007, emphasised the need for understanding the complexity of the combined direct and indirect radiative forcing from aerosols and greenhouse gases (GHG). Although GHG budgets are relatively well known, there is uncertainty in the current estimates of radiative forcing from aerosols. Better understanding of the effects of aerosols in the atmosphere requires detailed information on how different sources (including those of biosphere) and transformation processes modify properties of aerosol particles and trace gases.
Trace gases and atmospheric aerosols are tightly connected via physical, chemical, meteorological and biological processes occurring in the atmosphere and at the atmosphere-biosphere interface. Aerosols serve as cloud condensation nuclei with a substantial effect on cloud properties and initiation of precipitation, but the mechanisms depend on processes occurring in the very early phases of aerosol formation.
Project description
The main focuses of our research unit have been:
- 1) formation and growth mechanisms of atmospheric aerosols, aerosol dynamics;
- 2) the effect of secondary biogenic aerosols on global aerosol load;
- 3) aerosol-cloud-climate interaction; and
- 4) relationships between the atmosphere and different ecosystems, particularly boreal forest. Our scientists have published over 700 peer-reviewed papers in the last five years, including 12 articles in Science and Nature.
Atmospheric aerosol formation is a result of photochemical reactions in the gas phase, particularly the formation of sulphuric acid and other vapours of very low volatility, such as multifunctional organic compounds and iodine oxides. One of our most significant recent results is the development of cluster spectrometers which measure particles below 3 nm size in field conditions (Figure 1).
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Fig 1. Total particle size distribution measured with the Neutral cluster and Air Ion Spectrometer negative charging mode in the range 0.8–40 nm. Negative and positive air-ion distributions were measured (also 0.8–40 nm) on 7 consecutive particle formation event days between 11 and 17 April 2007 in Hyytiälä. The observed formation and growth of particles and ions seemed to be almost simultaneous during particle formation. After formation, condensation of various vapours grows the particles in several hours. When particles reach 50-100 nm they can act as cloud condensation nuclei. In the last 13 years, ca. 1100 new formation events have been observed at
SMEAR II. |
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We have shown that long-term field measurements of atmospheric aerosol formation and growth processes are feasible and reveal important patterns on how atmosphere and biosphere interact. The work from our field stations provides a solid background and fertile base for interdisciplinary collaboration in theoretical, experimental and field work. The consortium operates five field stations in Finland, one of which can be accessed with a dynamic web-based tool, providing means for easy hands-on and insights to a complicated dataset (Figure 2).
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| Fig 2. A SmartSMEAR (www.atm.helsinki.fi /smart-SMEAR/) screenshot presenting air trajectories and data on a new-particle formation day, 16 May, 2006. Note that the figure shows only a part of the screen |
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Molecular simulations (Monte Carlo and Molecular Dynamics) were used to better comprehend the processes in nucleation and aerosol thermodynamics. We developed a cost-effective multi-level computational strategy combining linear-scaling density functional theory geometry optimisations and frequency calculations with perturbation theory or coupled-cluster energy corrections. This yielded reliable formation-free energies for medium-sized (on the order of 50 atoms) hydrogen-bonded clusters at affordable computational cost. The methodology has been applied to H2SO4•HSO4-•NH3 clusters containing up to four sulfuric acid molecules, demonstrating ammonia does not contribute to the ion-induced nucleation of sulfuric acid in atmospheric conditions. Our results indicate that the nucleation-enhancing role of amines in atmospheric conditions is likely to be greater than that of ammonia, despite the 2-3 orders of magnitude difference in concentrations. Further, it seems that amines, unlike ammonia, might play a role in ion-induced nucleation.
We have modelled the terrestrial photosynthesis, autotrophic respiration and volatile organic compound (VOC) biosynthesis at cellular, stomatal and leaf scales. This is accompanied by chamber measurements in laboratory and under field conditions where stand-level measurements can be performed.
Implications for global research
Fundamental processes need to be clarified to quantify aerosol radiative properties and the influence of aerosols on cloud microphysics and dynamics at the individual cloud level, and to understand changes and feedbacks in ecosystem carbon uptake dynamics. Advances in understanding the boundary layer meteorology are necessary to appreciate 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 link to regional-scale processes and further to global-scale phenomena.
To simulate global climate and air quality, the most recent progress on the above must be compiled, integrated and implemented in Earth System Models via novel parameterisations and process descriptions in different environments, including boreal forests and wetlands.
 University of Helsinki
Finnish Centre of Excellence in Physics, Chemistry, Biology and Meteorology of Atmospheric Composition and Climate Change
W: www.atm.helsinki.fi
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