1. J.-F. Müller, K. Ceulemans, S. Compernolle
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. Secondary Organic Aerosol (SOA)
O3, OH, NO3
etc.
volatile compounds
semi-volatile compounds
Secondary Organic Aerosol (SOA)
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.)
Combination of 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. Aerosol formation: Partitioning theory (Pankow, 1994)
Gas- and particulate phase concentrations at equilibrium
Organic aerosol concentration
Adsorption and desorption rates
Saturation vapour pressure
Partioning coefficient:
obtained from a group contribution method (Capouet and Müller, 2006)
Aerosol radius
Accomodation coefficient (assumed > 0.1)
Adsorption rate:

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
KPP/Rosenbrock as chemical solver

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
ΔVOC
(ppb)
/ NOx
OH:O3:NO3
(% ) T
(K)
J(NO2)
(104 /s)
Nozière et al., 1999
(4 experiments)
300 -
1500
0.09–0.52
100:0:0
298
3.5 (lamp)
Kamens et al., 2001
(2 experiments)
~ 980
~ 2
42:44:13
12-35 (sun)
Hoffman et al., 1997
(7 experiments)
0.17–0.78
44:31:20
83 (sun)
Takekawa et al., 2003
(6 experiments)
55 –
196
53:42:4
40 (lamp)
Ng et al., 2006
(1 experiment)
108
1.1
62:22:16
293
10 (lamp)
Presto et al., 2005
(8 experiments)
11 –
205
6:82:11
295
300 (lamp)

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