What was done
The authors analyzed a five-year (1999-2004) April-September database of aerosol number-size distribution obtained from three different monitoring stations in the Finnish boreal zone of Europe - Varrio (67°46'N, 29°35'E), Pallas (68°01'N, 24°10'E) and Hyytiala (61°51'N, 24°17'E) - in an investigation of "the characteristic changes of the aerosol population in air masses undergoing marine to continental transition over forested areas in northern Norway, Sweden and Finland" that are "substantially lacking in anthropogenic aerosol sources."

What was learned
Tunved et al. say their results "clearly show that a substantial gas-to-particle [trans]formation of biogenic volatile organic carbon [BVOC] to secondary organic aerosol [SOA] takes place over the boreal forest in northern Europe," and that "these BVOCs are most likely emitted from the forest itself and, based on previous findings, are most likely constituted of terpenes." In addition, they say their analysis "suggests an apparent mass yield in the range of 5 to 10%," and that "the boreal forest typically sustains 1000 to 2000 particles cm^-3 in a climatic relevant size range (~40 to 100 nm)." Also, they report that because they "provided a similar mechanistic and quantitative behavior for two widely separate locations (more than 700 km apart), the derived relations suggest that similar mechanisms control the aerosol number and mass evolution over large areas in the boreal regions of the Northern Hemisphere."

What it means
The ten European researchers state that their findings have "important implications for radiation budget estimates and relevancy for the evaluation of feedback loops believed to determine our future climate," noting that their study establishes that boreal forest "is a major source of climate-relevant aerosol particles" that are "capable of competing with the anthropogenic CCN [cloud condensation nuclei] loadings transported over forested areas." Indeed, their findings support an important multi-stage connection between the metabolic activity of trees and the propensity for the atmosphere to produce clouds, which relationship is self-protecting of the biosphere, as per the following scenario. As the air's CO2 content rises, its aerial fertilization effect boosts boreal forest productivity, which leads to an enhanced evolution of biogenic gases, which leads to the production of more cloud condensation nuclei, which leads to the creation of (1) more clouds, (2) longer-lived clouds and (3) brighter clouds, all three of which phenomena lead to more solar radiation being reflected back to space, which acts to oppose the increase in the atmosphere's greenhouse effect produced by the initial rise in the atmosphere's CO2 concentration.