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

1. Evaluation of a Detailed Chemical Mechanism for Alpha-Pinene Degradation and Subsequent Secondary Aerosol Formation K. Ceulemans, J.-F. Müller, S. Compernolle Belgian Institute for Space Aeronomy, Brussels, Belgium • L. Vereecken, J. Peeters • Katholieke Universiteit Leuven, Belgium ACM Conference Davis December 2008

2. Outline • BOREAM: model for alpha-pinene oxidation and subsequent secondary aerosol formation • Possible impact of gas-phase oligomerization reactions • Evaluation against dark ozonolysis experiments

3. Alpha-Pinene Oxidation Model • Detailed explicit gas phase model with additional generic chemistry and aerosol formation module • 10000 reactions, 2500 compounds • Capouet et al., J. Geophys. Res., 2008 • complete mechanism can be explored at http://www.aeronomie.be/tropo/boream • KPP(Kinetic PreProcessor) /Rosenbrock as chemical solver

4. Explicit chemistry • mechanism based on advanced theoretical calculations and SARs • Oxidation by OH, O3 and NO3 • important updates from Vereecken et al., PCCP, 2007 • leads to many different stable primary products

5. Primary products: explicit or through lumped generic species APIN + OH APIN2OH APIN2OH JbCH3OHcHO2 JbCH3OHcHO2 + HO2 JbCH3OHcHOOH (primary product) JbCH3OHcHOOH + OH L10HPO2 (semi-generic: 10C, OH, OOH and O2-groups) JbCH3OHcHOOH + OH JbCH3OHcO Explicit chemistry Semi-generic chemistry • L10HPO2 + NO L10HPO + NO2 • L10HPO2 + HO2 L10HPP + O2 • … • L10HPP + OH L10KPP + HO2 • L10KPP + OH LXeO2 + HO2 (LXeO2: generic low volatility peroxy-radical) P: hydroperoxide, H : alcohol , K: ketone O: oxyradical O2: peroxyradical generic chemistry • LXeO2 + NO LXeO + NO2 • LXeO2 + HO2 LXeOOH • … • LXeOOH + OH LXeO • LXeO + O2 LXeCHO + HO2 LX indicates a generic species, e indicates the volatility class (11 classes provided) Products (explicit or generic) are allowed to partition to aerosol phase

6. Activity coefficient Aerosol formation and Partitioning • Molecules can partition between particulate and gas phase • Pankow partitioning coefficient: • Vapor pressure: calculated with group contribution method (see talk of Steven Compernolle) • Activity coefficient: takes into account mixture effects, calculated with UNIFAC-based method (Compernolle et al. ACPD 2008) Saturated vapor pressure

7. Oligomerization reactions: gas-phase reactions of Criegee intermediates • Observed in several recent studies (Tobias & Ziemann 2001, Heaton et al. 2007) • Example: SCI + pinic acid: produces a very condensable product

8. Oligomerization reactions: gas-phase reactions of Criegee intermediates • Tobias and Ziemann (2001) investigated the relative reaction rates of water vapour and other molecules with Stabilized Criegee Intermediates from tetradecene ozonolysis • We take these rates and apply them to the most important species in alpha-pinene ozonolysis

9. Results: photo-oxidation: SOA yields • Capouet et al. JGR 2008 • 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

10. Model Validation: Dark ozonolysis experiments • New simulations for dark ozonolysis • About 150 smog chamber experiments from 10 different studies were simulated • Typical experimental conditions • Excess ozone + OH-scavenger • Very low or no NOx • Temperatures generally between 0°C and 45°C • RH variable, but many dry experiments ( < 10%)

11. Dark ozonolysis: modelled versus experimental SOA yields • SOA yield is predicted within a factor 2 for majority of experiments • Some overstimations for Cocker et al. and Iinuma et al. at colder temperatures • Some very serious underestimations for Hoffmann et al.1997: at high temperature (45°C)

12. Pathak et al. 2007: modelled versus experimental SOA • Dry, RH<10% but not exactly determined. • Clear temperature dependence in model performance • Overestimations of about factor 2 for 0-20°C • Some very serious under-estimations at 30°C and 40°C with low initial VOC • Example: • Pathak01:(40°C,14.3 ppb) • experimental yield: 9% modelled yield: 0.001% modelled with stabilized Criegee oligomers: 0.4 %

13. Results: temperature dependence of SOA yields is problematic Experimental yields do not decrease strongly with temperature Modelled yields strongly decrease with temperature Serious underestimations at high temperature and no seed aerosol

14. Possible Importance of SCI oligomers at low RH • Song et al. 2007: Dark ozonolysis • RH < 2% in all experiments • Temperature 28°C • Assuming very low RH significantly improves modelled yields, due to decreased competition of water vapor in formation of Criegee Intermediate oligomers • Therefore: • At very low RH gas phase reactions of Stabilized Criegee intermediates with acids and alcohols can significantly influence SOA yields • Precise measurements of RH in smog chambers are important • SOA yields deduced in very dry ozonolysis experiments might not be representative for real atmospheric conditions

15. Next step: model reduction • Currently BOREAM model contains about 10000 reactions and 2500 species • Global models: chemical reactions consume large amount of CPU time • Model reduction is needed: • At most a few hundred reactions • Less than 100 species • Work in progress…

16. Conclusions • Validation of dark ozonolysis experiments: majority of SOA yields reproduced up to factor 2 • Overall temperature influence not well reproduced • Oligomerization of Stabilized Criegee Intermediates can be important at very low RH Thank you for your attention!

17. Model reduction: requirements • Reduced model should be able to reproduce: • Inorganics (NOx, HOx, O3) • Small organics (CH2O, acetone, PAN) • Some important products: Pinic, pinonic acid, pinonaldehyde • SOA • Validation through comparison with full mechanism • Focus on atmospherically relevant scenarios

18. Reduction Techniques: Removing negligible reactions • Identify negligible reactions in atmospheric conditions • Branching can depend strongly on NOx-regime • Example: Peroxyradical in alpha-pinene + OH

19. Reduction Techniques: product merging • Products with • Similar reactivity • Similar products can be merged • Example: in OH-addition on alpha-pinene • The resulting peroxy radicals lead to similar products (nitrates, hydroperoxides and pinonaldehyde) • Use of averaged reaction rates for the merged species

20. Reduction Techniques: Reducing length of long radical reaction chains • Some reactions produce a sequence of several peroxyradicals • Radical reactions are very fast: considered instantaneous • The chain ends through radical termination • Is replaced by a single equation yielding • LXO2, represents the peroxy radicals • Stable endproducts

21. Reduction Techniques: Lumping • Not all different products can be treated explicitly in a reduced mechanism. • Use generic species • Example: generic nitrate • LXONO2 + OH LXCHO + NO2 (OH oxydation) • LXONO2 + hv LXO2 + NO2 (photolysis) • LXONO2 LXNO2p (partitioning) • Advantage: carbon balance conserved, some effects of aging are reproduced • Disadvantage: simplifications lead to errors compared with full mechanism