The aerosol in a boreal forest in spring

Particles >30 nm diameter collected in a forest in Finland from 22 April–4 May 1998 and from 23 March–16 April 1999 were examined by transmission electron microscopy. The 1st period had no nucleation events, the second had many and differences in particle properties are described. Particles predominantly 100–200 nm in diameter and having a large organic content were found to accumulate in an enclosure above a snow surface in the forest. The explanation given is that on sunny days water from melting snow penetrated the soil and leaf litter beneath the snow, displacing gases produced by decomposition of organic material. It is assumed that the bursting of bubbles produced at the soil–snow interface by the escaping gases led to the production of the particles in the enclosure. Soil pit data at the site showed that on sunny days water commenced to penetrate to at least 29 cm soon after sunrise and reached a maximum in early afternoon. Particles similar to those found in the enclosure were present in the free atmosphere on days with nucleation events. Data from other observers showed that compounds that might be expected to arise from soil emissions were all present in greater concentrations on days with nucleation events than on days without. It is suggested that the material for production of condensable vapours leading to the nucleation and growth of particles was contained either in the escaping gases or in the aqueous material initially surrounding the particles produced by bubble bursting. The hypothesis has the advantage of explaining the very sharp maximum found in spring as the winter snow melts.