Modeling Dynamic Partitioning of Semi-volatile Organic Gases to Size-Distributed Aerosols

Rahul A. Zaveri Richard C. Easter

Pacific Northwest National Laboratory

1

International Workshop on Air Quality Forecasting Research November 29, 2011 Potomac, MD, USA

2

Motivation

SOA formation processes

SOA modeling challenges

MOSAIC aerosol modeling framework

Gas-particle partitioning of organic gases (new)

Sample results

Future Directions

Outline

3 Battelle Proprietary

Dust

Phytoplankton

Dimethylsulfide

H2SO4, MSA

Oxidation Soot

Sea-salt

Direct Radiative Forcing

Indirect Radiative Forcing

Terp

enes

Oxi

datio

n

Fore

st F

ires

Nucleation

Hydrocarbons, NOx, SO2, NH3, POA

H2SO4, HNO3, Organic aerosol

Activation

Resuspension

chemistry

Need to efficiently and reliably model aerosol size, number, mass, composition, and their climate related properties at urban to global scales

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Up to 90% of submicron aerosol mass is composed of organics (Kanakidou et al., 2005; Zhang et al., 2007)

SOA formation is quite rapid (within a few hours) during daytime (Volkamer et al., 2006; Kleinman et al., 2007; de Gouw et al., 2008)

SOA from oxidation of SVOCs from diesel exhaust may help explain some of the missing organic aerosol mass in models (Robinson et al., 2007)

Observed rapid growth of newly formed particles (via homogeneous nucleation) is thought to be by SOA condensation (Kuang et al., 2008)

Anthropogenic and biogenic SOA precursors may interact to enhance the overall SOA yield (Weber et al., 2007)

Particle-phase reactions of absorbed VOCs within inorganic particles can form SOA (Jang et al., 2003; Kroll et al., 2005; Liggio et al., 2005)

Accretion reactions, including aldol condensation, acid dehydration, and gem-diol condensation can transform VOCs into oligomeric compounds (Gao et al., 2004; Jang et al., 2003; Kalberer et al., 2004)

SOA Formation Processes

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Gas-particle partitioning processes are still poorly understood at a fundamental level

How do we handle the complexities in the gas-phase VOC chemistry?

Should we use Raoult’s Law or some sort of reactive uptake formulation as driving force for gas-particle mass transfer?

Should we use Henry’s Law if the organics are dissolved in the aqueous phase?

Are the organic particles liquid or solid? Virtanen et al. (2011) and Vaden et al. (2011) suggest that SOA particles are solid.

How do we treat organic-inorganic interactions and the associated phase transitions?

How do we treat particle-phase reactions? What are the time scales?

What are the anthropogenic-biogenic interactions? How do we reliably represent them in models?

SOA Modeling Challenges

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General Problem Solving Approach

Develop a comprehensive aerosol model framework that includes all the processes that we think are (or might be) important

Evaluate the roles of specific processes using appropriate laboratory and field observations

Simplify, parameterize, and optimize the process model as much as possible to increase computational efficiency and decrease memory requirements

MOSAIC Aerosol Module

Model for Simulating Aerosol Interactions and Chemistry (Zaveri et al., 2008) Comprehensive aerosol module for air quality and climate modeling Flexible framework for coupling various gas and aerosol processes Robust, accurate, and highly efficient custom numerical solvers for several processes Suitable for 3-D regional and global models Implementation in:

Weather Research and Forecasting Model (WRF-Chem) – done Global model: Community Atmosphere Model (CAM5) – in progress EPA’s CMAQ – planned

Thermodynamics & Mass Transfer Treatments in MOSAIC

H2SO4

HNO3 HCl

Solid Salts

Aqueous Phase

OM, BC

NH3

H2O

OH-

NO3-

Cl-

SO4-

Na+ NH4

+ H+

Ca2+ Particle Phase

Thermodynamic equilibrium within the particle phase (depends on composition and RH)

Kinetic mass transfer between the gas and size-distributed particles (1 to 10,000 nm)

Custom numerical techniques have been developed to solve these equations efficiently and accurately

Reversible Gas-Particle Reactions

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Organic-inorganic interactions within the particle to determine water uptake and phase separation (partially implemented)

Size-distributed, dynamic mass transfer between gas and particles

Raoult’s Law in the absence of aqueous phase (implemented)

Reactive uptake that instantly converts VOC to non-volatile products (implemented)

Henry’s Law in the presence of aqueous phase (future)

Particle-phase reactions (future)

Gas-Particle Partitioning of Organics

Organics H2SO4

HNO3 HCl

Salts, BC, Org

Particle Phase

NH3

H2O

OH-

NO3-

Cl-

SO4-

Na+ NH4

+ H+

Ca2+

Gas Phase

Org

Oligomer

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Raoult’s Law vs. Reactive Uptake

HC + oxidant α1 G1 + α2 G2

dGi dt

xi = mole fraction

vapor pressure

A1 A2

= -ki (Gi – xiPi0)

Raoult’s Law Based Mass Transfer

HC + oxidant α1 G1 + α2 G2

dGi dt

A1 A2

= -kiGi

Reactive Uptake Mass Transfer

Sample Results

Idealized Case

Sacramento Urban Air Case

Idealized Case

Diameter, Dp (µm)

0.01 0.1 1

Num

ber C

once

ntra

tion

(cm

-3)

0

20

40

60

80

100

120

140t = 0 h (initial)

Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1

Idealized Case

Diameter, Dp (µm)

0.01 0.1 1

Num

ber C

once

ntra

tion

(cm

-3)

0

20

40

60

80

100

120

140 t = 0 (initial)t = 9 h (Raoult's Law)

Time (h)

0 2 4 6 8 10

SOA

Mas

s (µ

g m

-3)

6

8

10

12

14

16

Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1

Idealized Case

Diameter, Dp (µm)

0.01 0.1 1

Num

ber C

once

ntra

tion

(cm

-3)

0

20

40

60

80

100

120

140

t = 0 h (Initial)t = 9 h (Raoult's Law)t = 9 h (Reactive Uptake)

Time (h)

0 2 4 6 8 10

SOA

Mas

s (µ

g m

-3)

6

8

10

12

14

16

Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1

Sacramento June 6, 2010

Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10

Diameter, Dp (µm)

0.01 0.1

Num

ber C

once

ntra

tion

(cm

-3)

0

200

400

600

800

1000

1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 am

Sacramento June 6, 2010

Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10

Diameter, Dp (µm)

0.01 0.1

Num

ber C

once

ntra

tion

(cm

-3)

0

200

400

600

800

1000

1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 amModel at 11:00 am (Raoult's Law)

Sacramento June 6, 2010

Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10

Diameter, Dp (µm)

0.01 0.1

Num

ber C

once

ntra

tion

(cm

-3)

0

200

400

600

800

1000

1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 amModel at 11:00 am (Raoult's Law)Model at 11:00 am (Reactive Uptake)

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Diameter, Dp (µm)

0.01 0.1

Num

ber C

once

ntra

tion

(cm

-3)

0

200

400

600

800

1000

1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 amModel at 11:00 am (Raoult's Law)

Sacramento June 6, 2010 Aitken mode mass ratio OA/(NH4)2SO4 = 10

Accumulation mode mass ratio OA/(NH4)2SO4 = 0.01

Diameter, Dp (µm)

0.01 0.1

Num

ber C

once

ntra

tion

(cm

-3)

0

200

400

600

800

1000

1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 amModel at 11:00 am (Raoult's Law)Model at 11:00 am (Reactive Uptake)

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Sacramento June 6, 2010 Aitken mode mass ratio OA/(NH4)2SO4 = 10

Accumulation mode mass ratio OA/(NH4)2SO4 = 0.01

Future Directions Perform additional constrained Lagrangian model analyses to test different SOA formation mechanisms

Use carefully designed chamber experiments to constrain and evaluate different formulations

Extend model analyses to mixtures of organic and inorganic species at different relative humidities

Implement and evaluate new SOA formulations in WRF-Chem using urban to regional field observations

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Thank you for your attention

Funding for this work was provided by Department of Energy (DOE) Atmospheric System Research (ASR) Program