1. K. Ceulemans, J.-F. Müller, S. Compernolle
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
KPP(Kinetic PreProcessor) /Rosenbrock as chemical solver

4. Explicit chemistry Oxidation by OH, O3 and NO3
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 + HO 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 + HO 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 + HO LXeOOH …
LXeOOH + OH LXeO
LXeO + O 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. 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
Activity coefficient

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
Exp
Model RH 1%
0.01%
Exp. Yield 2
0.16
0.35 3
0.18
0.36
0.38 4
0.015
0.20
0.15 5
0.19
0.37
0.43 6
0.21
0.46 7
0.08
0.28 8
0.14
0.34 9
0.17

15. Next step: model reduction
Currently BOREAM model contains about 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)
LXONO LXNO2p (partitioning)
Advantage: carbon balance conserved, some effects of aging are reproduced
Disadvantage: simplifications lead to errors compared
with full mechanism