Gas emissions from soil and leaf litter as a source of new particle formation

The hypothesis is tested that the nucleating material responsible for the production of new particle formation over boreal forests, particularly in spring, is a result of the release of gases of decomposition of organic material in soil and leaf litter generated by micro-organisms. It is shown that the supply of moisture to the soil by melting snow in spring or by rain in summer greatly enhances the frequency of nucleation events. While low air temperatures and low pre-existing particle surface areas also increase the probability of new particle formation, freezing of the soil and leaf litter reduce it. The observations are consistent with the hypothesis and the seasonal and interannual variability are difficult to explain without it.