The Next Generation of SOA Models: Development and Application J. H. Seinfeld 1,2, X. Zhang 1, C. L. Loza 2, J. S. Craven 2, L. D. Yee 1, and C. D. Cappa 3

1 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA 2 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA

3 Department of Civil and Environmental Engineering, University of California, Davis, CA, USA

Functional Group Oxidation Model (FGOM) Three recent studies, 2D-VBS (Donahue et al., 2012), SOM (Cappa and Wilson, 2012), and FGOM (Zhang and Seinfeld, 2012), have been directed at a next generation of SOA models. They represent SOA formation and evolution in terms of the competition between functionalization and fragmentation. Each contains a set of parameters that are to be determined by fitting of the model to laboratory chamber data. The FGOM model is developed based on explicit chemical information in terms of the types of functional groups that result from the oxidation of a parent VOC. Figure 1.

Sig -- The spectrum of gas-phase

semivolatiles at the ith generation.

Sip -- The spectrum of particle-phase

semivolatiles at the ith generation.

Np – Nonvolatile products in the particle phase.

Si

Fragmentation

Functionalization

Oligomerization

-  The probability of fragmentation of a compound is a function of its O:C ratio. -  Fragmentation is assumed to lead to one-carbon compound (C1), together with a co-product (Cn−1) that has the same collection of functional groups but one fewer carbon atom than the parent compound.

-  Addition of different combinations of four functional groups. -  Particle-phase oxidation reactions proceed in parallel with and via the same chemical mechanisms as in the gas phase.

-  Bimolecular reaction - The elemental composition of non-volatile species is a parameter .

Application of the FGOM to C12 Alkanes Free parameters in the FGOM

fv - Characterizing the probability of fragmentation.

rp -  The ratio of the particle-phase oxidative reaction rate constant to that in the gas phase.

ka - The accretion reaction rate constant in the particle phase.

[Cx Hy Oz] -  Carbon, hydrogen, and oxygen numbers of the non-volatile particle-phase products.

Figure 3. Simulated (colors) and observed (black) time-dependent total organic mass and O:C ratio from the photooxidation of four C12 alkanes under high (red) and low (green) NOx conditions. Note that “sim.1” represents the full fitting of the six free parameters in the FGOM to the chamber data and “sim.2” refers to fitting by only two parameters, rp and fv, to the total organic mass concentration.

Statistical Oxidation Model (SOM) The SOM model describes SOA formation as a statistical evolution in the space of numbers of carbon and oxygen atoms, nC and nO. It is assumed that the properties of the nC / nO pair can be represented by mean values that account for the actual distribution of functional groups within the group of molecules that make up an SOM species.

Free parameters in the SOM f - Characterizing the probability of fragmentation

dlVP -  The decrease of vapor pressure per generation

nO [% % % %] -  An array of the probabilities of adding 1, 2, 3 or 4 oxygen atoms per generation

Comparison of FGOM with SOM

HC S1g S2

g S3g

S1p S2

p S3p

Np

ox1g%%

NpNp

ox2g% etc.%

ox1p% ox2p% etc.%

ox0g%

02468101214161820−4

−3

−2

−1

0

1

2

3

4

Oligomerization Func

tiona

lizat

ion

Fragm

entat

ion

Number of carbon atoms (nC)

Aver

age

carb

on o

xidat

ion

stat

e (O

S C)

0 5 10 15 20 25 30 35CO2

CO

HCHO

CH3OH

CH4

C12H26

FGOM

SOM

Reaction time (h)

−6 −4 −2 0 2 4 6−2.0

−1.6

−1.2

−0.8

−0.4

0.0

log C*

OS C

−6 −4 −2 0 2 4 6−2.0

−1.6

−1.2

−0.8

−0.4

0.0

log C*−6 −4 −2 0 2 4 6

−2.0

−1.6

−1.2

−0.8

−0.4

0.0

log C*−6 −4 −2 0 2 4 6

−2.0

−1.6

−1.2

−0.8

−0.4

0.0

log C*

Molar Fraction

0

0.05

0.1

0.15

0.2

0.25

0.3ELVOC LVOC SVOC IVOC

(A)

ELVOC LVOC SVOC IVOC

(B)

ELVOC LVOC SVOC IVOC

(C)

ELVOC LVOC SVOC IVOC

(D)

−6 −4 −2 0 2 4 6−2.0

−1.6

−1.2

−0.8

−0.4

0.0

log C*

OS C

−6 −4 −2 0 2 4 6−2.0

−1.6

−1.2

−0.8

−0.4

0.0

log C*−6 −4 −2 0 2 4 6

−2.0

−1.6

−1.2

−0.8

−0.4

0.0

log C*−6 −4 −2 0 2 4 6

−2.0

−1.6

−1.2

−0.8

−0.4

0.0

log C*

Molar Fraction

0

0.05

0.1

0.15

0.2

0.25

0.3ELVOC LVOC SVOC IVOC

(A)

ELVOC LVOC SVOC IVOC

(B)

ELVOC LVOC SVOC IVOC

(C)

ELVOC LVOC SVOC IVOC

(D)

0 1 2 3 4 5 64

6

8

10

12

14

16

18

20

Saturation concentration in log scale (µg/m3)

Car

bon

num

ber r

ange

C=O

2*C=O

3*C=OOOH

C=O+OH

2*C=O+OHC=O+OOH

2*C=O+OOHOOH+OH

3*C=O+OOH2*OOHC=O+OOH+OH

Volatility Estimation

Figure 2. Carbon number range for products of OH oxidat ion of dodecane under low-NOx conditions. The grey shaded region defines the particle phase “boundary” for compounds having twelve or fewer carbon numbers. The lines define the carbon number range and types of functional groups for oxidation products of dodecane under low NOx conditions.

0.0 0.6 1.2 1.8 2.4 3.00

5

10

15

C OA (µ

g m

−3)

exp.sim.1sim.2

0.0 1.6 2.2 2.5 2.6 2.7

0.0 0.6 1.2 1.8 2.4 3.00

12

24

36

C OA (µ

g m

−3)

exp.sim.1sim.2

0.0 1.3 1.7 1.8 1.9 1.9

0.0 0.6 1.2 1.8 2.4 3.00

8

16

24

C OA (µ

g m

−3)

exp.sim.1sim.2

0.0 1.3 1.9 2.1 2.2 2.3

0.0 0.6 1.2 1.8 2.4 3.00

13

26

39

Reaction time (h)

C OA (u

g m

−3)

exp.sim.1sim.2

0.0 1.2 1.9 2.3 2.6 2.8

0 7 14 21 28 350

13

26

39

exp.sim.1sim.2

0.0 1.8 3.5 5.3 7.0 8.7

0 7 14 21 28 350

12

24

36

exp.sim.1sim.2

0.0 1.6 3.2 4.7 6.3 8.0

0 7 14 21 28 350

13

26

39

exp.sim.1sim.2

0.0 1.7 3.5 5.2 6.9 8.6

0 7 14 21 28 350

11

22

33

Reaction time (h)

exp.sim.1sim.2

0.0 1.8 3.6 5.5 7.3 9.1

dodecane

2−methylundecane

hexylcyclohexane

cyclododecane

dodecane

2−methylundecane

hexylcyclohexane

cyclododecane

High−NOx Low−NOx

(* 107 molec cm−3 hrs) (* 107 molec cm−3 hrs)OH exposureOH exposure

0.0 0.6 1.2 1.8 2.4 3.00.0

0.2

0.4

0.6

O:C

0.0 1.6 2.2 2.5 2.6 2.7

0.0 0.6 1.2 1.8 2.4 3.00.0

0.2

0.4

0.6

O:C

0.0 1.3 1.7 1.8 1.9 1.9

0.0 0.6 1.2 1.8 2.4 3.00.0

0.2

0.4

0.6

O:C

0.0 1.3 1.9 2.1 2.2 2.3

0.0 0.6 1.2 1.8 2.4 3.00.0

0.2

0.4

0.6

Reaction time (h)

O:C

0.0 1.2 1.9 2.3 2.6 2.8

0 7 14 21 28 350.0

0.2

0.4

0.60.0 1.8 3.5 5.3 7.0 8.7

0 7 14 21 28 350.0

0.2

0.4

0.60.0 1.6 3.2 4.7 6.3 8.0

0 7 14 21 28 350.0

0.2

0.4

0.60.0 1.7 3.5 5.2 6.9 8.6

0 7 14 21 28 350.0

0.2

0.4

0.6

Reaction time (h)

0.0 1.8 3.6 5.5 7.3 9.1

dodecane

2−methylundecane

hexylcyclohexane

cyclododecane

dodecane

2−methylundecane

hexylcyclohexane

cyclododecane

High−NOx Low−NOx

OH exposureOH exposure(* 107 molec cm−3 hrs)(* 107 molec cm−3 hrs)

Conclusions

Figure 4. Comparison of FGOM with SOM for low-NOx dodecane SOA formation. The upper panel is the FGOM predicted SOA composition represented by saturation concentration and average oxidation state after 7(A), 14(B), 21(C), and 28(D) hours of reaction, respectively. The middle panel is the SOM predicted SOA composition at same time points as FGOM. The lower panel is the simulated results of these two models in the nC vs. OSC space. Both FGOM and SOM attempt to reproduce the observed chamber generated SOA properties, but via different mechanisms incorporated in the models.

Organic aerosol yield depends most strongly on three variables: the probability of fragmentation to produce volatile compounds, oxidation reactions involving semi-volatile compounds in the particle phase, and irreversible particle-phase accretion reactions. The elemental composition of particles is dependent not only on the functionalization in the gas phase, but also, probably to as great an extent, on potential particle-phase chemistry. Comparison between the FGOM and SOM models reveals the potential importance of dehydration processes in shaping the aerosol chemical composition.