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 souther n Finland as part of the BIOFOR (Biogenic aerosol formation in the boreal f orest) project in the summer of 1998 and spring of 1999. Scots pine branch emission measurements were made with the chamber technique for selected day s. Steady state 'OH and NO3' concentrations were calculated and source term s 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 i nvestigate the source of new particles (nucleation events) observed at the site. Forest emission rates of monoterpenes were not found to be exceptiona lly high prior to or during the occurrence of events. Neither the relative composition of the monoterpene emission nor that of the forest concentratio ns showed significant deviations prior to or during event periods. Source t erms for secondary organic compounds were only slightly higher (weakly sign ificant) on event days as compared to non-event days and did not correlate to maxima in ultra-fine particle concentrations on event days. Nucleation e vents were not observed during nighttime when the production of secondary o rganic compounds, and probably of secondary organic aerosol was greatest. T hus, we conclude that the oxidation products of the terpenes. were not the nucleating species observed at Hyytiala. Correlations between nocturnal inc reases in particulate organic carbon and the source term for secondary orga nic compounds indicate that the increase could have been due to condensatio n processes if the aerosol yield was at least 10-15%. Sulphur dioxide and N H3 concentrations, as well as the H2SO4 source term were significantly high er during event periods, and the H2SO4 source term together with the NH3 co ncentration correlated well with the daily maxima in ultra-fine particle nu mber concentrations, The results indicate that SO2 and NH3 were involved in the mechanism for nucleation events at Hyytiala. 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 terpen es. The data indicate that a secondary organic aerosol yield on the order o f 10% would suffice.