J.-F. Müller, K. Ceulemans, S. Compernolle Belgian Institute for Space Aeronomy, Brussels, Belgium

1. Factors influencing SOA yields in the simulation of α-pinene photooxidation experiments J.-F. Müller, K. Ceulemans, S. Compernolle Belgian Institute for Space Aeronomy, Brussels, Belgium L. Vereecken, J. Peeters Katholieke Universiteit Leuven, Belgium AGU Fall Symposium, Dec. 2007

2. semi-volatile compounds etc. O3, OH, NO3 Secondary Organic Aerosol (SOA) etc. volatile compounds Kp,i(partitioning coefficients) Experiments indicate that heterogeneous or particle-phase chemistry generates high-molecular weight compounds, enhancing SOA formation Previous model studies found necesary to increase the partitioning coefficients by several orders of magnitude in order to match SOA yields from laboratory experiments etc.

3. Oxidation by OH • mechanism based on advanced theoretical calculations • Peeters et al. 2001; Vereecken and Peeters, 2004; Fantechi et al., 2002 • important updates from Vereecken et al., PCCP, 2007

4. Ozonolysis • mechanism still preliminary, but theoretical work is in progress • pathways proposed so far to explain some key observed products (pinic acid, hydroxy pinonic acid) have been demonstrated to be negligible • As a consequence, the yield of organic acids is too low in this mechanism • Capouet et al., JGR, in press

5. Secondary chemistry • Explicit part of mechanism : degradation down to primary products + degradation of pinonaldehyde • But the degradation down to CO2 would require maybe billions of reactions (Aumont et al.) • Combinationof semi-generic chemistry (for high-yield compounds) and generic chemistry (for the rest) • Semi-generic compounds are lumped compounds for which the carbon number and all functionalities are defined, not their precise structure • Generic compounds are lumped compounds for which one functionality (RO2, ROOH, RONO2 etc.) is defined, and further subdivided into 4 volatility classes

6. Gas- and particulate phase concentrations at equilibrium Adsorption and desorption rates Partioning coefficient: Saturation vapour pressure Organic aerosol concentration obtained from a group contribution method (Capouet and Müller, 2006) Aerosol radius Adsorption rate: Accomodation coefficient (assumed > 0.1) Aerosol formation: Partitioning theory (Pankow, 1994)

7. Overall alpha-pinene mechanism • 5000 reactions, 1300 compounds (including the gas-aerosol partitioning reactions) • Capouet et al., J. Geophys. Res., in press • complete mechanism can be explored at http://www.oma.be/TROPO/boream/boreammodel • KPP/Rosenbrock as chemical solver

8. http://www.oma.be/TROPO/boream

9. Ozone formation: simulation of experiments Ozone (ppm) SAPRC, Carter et al. 2000 D(O3 -NO) (ppm) Kamens and Jaoui 2001 O3 (ppm)

10. SOA formation: Photooxidation experiments

11. Results: SOA yields • Simulations with additional acid formation channels in ozonolysis mechanism lead to better agreement in some (not all) low-VOC experiments • Simulations with additional particle-phase association reactions (ROOH+R’CHO) has little impact except in high-VOC ozonolysis experiments Stark contrast with previous modeling studies which required orders of magnitude enhancements of the partitioning coefficients in order to match observed SOA yields

12. Time series • Alpha-pinene decay is well reproduced in all cases • SOA formation occurs too late in the simulation of a few Nozière et al. experiments

13. SOA composition • Particulate compounds are multifunctional • Generic compounds are significant but not dominant in modeled SOA at sampling time • Hydroperoxides make up 25% of compounds in a high-NOx Nozière et al. experiment • Acids dominant (>50%) in ozonolysis experiments (Presto et al.)

14. Conclusions • Low-volatility hydroperoxides from OH-initiated oxidation contribute significantly to SOA even in presence of NO • Theoretical and laboratory work needed to elucidate origin of acids in ozonolysis • Particle-phase association reactions (ROOH+R’CHO) have only a small impact, except in some conditions (SOA yields enhanced by about 1/3 in high-VOC Presto et al. experiments); more laboratory work needed to investigate the elementary steps • Huge uncertainties in vapor pressures and activity coefficients, chemistry • Near future: calculate activity coefficients, fine-tune the mechanism and the vapor pressures in order to improve agreement • In case of satisfactory results, develop a reduced mechanism and SOA parameterization for implementation in a global model