Aerosol particles are ubiquitous in the Earth's atmosphere and contribute significantly to the biogeochemical cycling of trace gas species and the Earth's radiative flux budget. One of the largest uncertainties relating to anthropogenic climate forcing concerns atmospheric aerosols [Houghton et al., 1995]. Until their natural background source is quantified, the influence of anthropogenic aerosols cannot be determined. Additionally, until the mechanisms leading to background and anthropogenic aerosol formation are understood within a theoretical framework, the ability to predict climate change due to atmospheric aerosols will not be achieved.
The number concentration of atmospheric particles is determined primarily by the formation rate of new particles, typically of the order of 1-2 nm in size, formed through gas-to-particle conversion processes. Once stable particle clusters are formed, they can grow through coagulation and condensation processes to quasi-stable sizes of 50-100 nm where they can directly (and indirectly through cloud formation) influence the radiative budget.

Due to measurement limitations associated with particle sizes down to <1 nm, the source regions and production mechanisms of natural secondary (gas-to-particle conversion) aerosols have proven somewhat difficult to elucidate. Current instrumentation allows measurements down to about 3 nm, so in reality we are measuring only recently formed "ultrafine" particles rather than actual new particles. Similar difficulties limit measurements of gas phase precursors at very low mixing ratios. Additionally, in the natural background environment, intensive bursts with significant new particle formation appear to occur infrequently and are difficult to predict. In consequence, field experiments aimed at elucidating the mechanisms and species involved in nucleation have been difficult to design.

Similar obstacles have slowed development of theoretical frameworks to explain nucleation. A lack of thermodynamic and surface tension data on the multiplicity of possible nucleation species has limited the accuracy of nucleation models; however, significant advances in the development of more thermodynamically consistent binary and ternary nucleation models have been achieved [Kulmala et al., 1998; Korhonen et al., 1999].

Although natural particle formation in the boundary layer appears to be infrequent in most background environments (such as the marine boundary layer and remote continental sites), two environments have been identified as regions of significant new particle production and form the focus of this newsletter: The coastal zone, which can produce extended nucleation events lasting several hours, with peak particle concentrations exceeding 1,000,000 cm-3 [O'Dowd et al., 1998]; and forested regions, where nucleation is observed to occur over spatial scales greater than 1000 km [Mäkelä et al., 1997].

The European Commission has funded three dedicated pan-European aerosol nucleation research programs under its "4th Framework Environment & Climate Research Programme (1998-2000)." Two of these focus on in situ atmospheric field measurements at locations where natural new particle formation is known to occur on a regular basis: (1) New Particle Formation and Fate in the Coastal Environment (PARFORCE), which examines nucleation in the coastal environment (at the Mace Head Atmospheric Monitoring Station), and (2) Biogenic Aerosol Formation in the Boreal Forest (BIOFOR), which examines nucleation in the forested environment (at the Hyytiälä SMEAR II research station). The third, Nucleation Processes from Oxidation of Biogenic Volatile Organic Compounds (NUCVOC), examines through laboratory studies the nucleation potential of various common atmospheric VOCs.

The following articles summarize the preliminary progress achieved on all three projects. Two additional articles summarize the facilities and characteristics of the research stations: Mace Head, Ireland and Hyytiälä, Finland. To avoid repetition of similar experimental and modeling techniques used on both BIOFOR and PARFORCE projects, these are discussed only in the PARFORCE article.

References

1. Houghton, J.T., L.G. Meira Filho, B.A. Callendar, N. Harris, A. Katenberg, and K. Maskell, [eds], IPCC Climate Changes: The Science of Climate Change (Cambridge University Press), 1995.
2. Korhonen, P., M. Kulmala, A. Laaksonen, Y. Viisanen, R. McGraw, and J.H. Seinfeld., Ternary nucleation of H2SO4, NH3, and H2O in the atmosphere, J. Geophys. Res. (submitted).
3. Kulmala M., A. Laaksonen, and L. Pirjola, Parameterizat-ions for sulfuric acid/water nucleation rates, J. Geophys. Res., 103, 8301-8308, 1998.
4. Mäkelä, J.M., P. Aalto, V. Jokinen, T. Pohja, A. Nissinen, S. Palmroth, T. Markkanen, K. Seitsonen, H. Lihavainen, and M. Kulmala, Observations of ultrafine aerosol particle formation and growth in boreal forest, Geophys. Res. Letts., 24, 1219-1222, 1997.
   O'Dowd, C.D., M. Geever, M.K. Hill, S.G. Jennings,
5. M.H. Smith, New particle formation: Spatial scales and nucleation rates in the coastal environment, Geophys. Res. Letts., 25, 1661-1664,1998.