Biogenic emissions and gaseous precursors to forest aerosols

Measurements of ambient monoterpenes, sulphur dioxide, nitric acid, ammonia and particulate organic carbon were made in a Scots pine forest in southern Finland as part of the BIOFOR (Biogenic aerosol formation in the boreal forest) project in the summer of 1998 and spring of 1999. Scots pine branch emission measurements were made with the chamber technique for selected days. Steady state −OH and NO·3concentrations were calculated and source terms for the production of secondary and condensable gases from the oxidation of terpenes and of SO₂ were determined. The purpose of the project was to investigate the source of new particles (nucleation events) observed at the site. Forest emission rates of monoterpenes were not found to be exceptionally high prior to or during the occurrence of events. Neither the relative composition of the monoterpene emission nor that of the forest concentrations showed significant deviations prior to or during event periods. Source terms for secondary organic compounds were only slightly higher (weakly significant) on event days as compared to non-event days and did not correlate to maxima in ultra-fine particle concentrations on event days. Nucleation events were not observed during nighttime when the production of secondary organic compounds, and probably of secondary organic aerosol was greatest. Thus, we conclude that the oxidation products of the terpenes were not the nucleating species observed at Hyytiälä. Correlations between nocturnal increases in particulate organic carbon and the source term for secondary organic compounds indicate that the increase could have been due to condensation processes if the aerosol yield was at least 10–15%. Sulphur dioxide and NH₃ concentrations, as well as the H₂SO₄ source term were significantly higher during event periods, and the H₂SO₄ source term together with the NH₃ concentration correlated well with the daily maxima in ultra-fine particle number concentrations. The results indicate that SO₂ and NH₃ were involved in the mechanism for nucleation events at Hyytiälä. The H₂SO₄ source term was not high enough to account for the entirety of the observed growth rate of the new particles. A substantial part of the growth ought to have been due to condensation of secondary organic material from the oxidation of terpenes. The data indicate that a secondary organic aerosol yield on the order of 10% would suffice.