The Contribution of Terpene Oxidation by O3, OH and NO3 to Aerosol Formation and the Influence of Water Vapour: Particle Size Distribution Studies Contribution to subproject CMD, HEP-7

It is established that secondary organic aerosol is formed during the reaction of ozone with biogenic alkenes. However, particle formation initiated by the reaction of terpenes with OH or NO3 is still uncertain, but might also contribute in a significant way to SOA (Secondary Organic Aerosol) formation. This laboratory study on the SOA size distribution tries to compare the ability to form new aerosol particles (number and volume concentration) via the three different terpene oxidation processes (ozone, OH, NO3) using a- and b-pinene as surrogates for endo- and exocyclic alkenes. The results indicate that the reaction with ozone is by far the most efficient reaction to initiate nucleation, thus producing new aerosol particles. The observations show that oxidation by OH as well as by NO3 cause much less intensive nucleation than by ozone. Moreover, the particle formation can rather be suppressed by lowering the initial monoterpene concentrations to 50 ppbv (e.g. threshold for a-pinene> 80 ppbv), at which nucleation during ozonolysis still occurs. On the contrary, the new aerosol volume produced in OH or NO3 reactions contributes in a similar amount to the atmospheric aerosol volume as found in the ozonolysis. This indicates that products of higher volatility are formed in both OH and NO3 reactions