Extending Size-Dependent Composition to the Modal Approach:

A Case Study with Sea Salt Aerosol

Uma Shankar and Rohit Mathur

The University of North Carolina Carolina Environmental Program

Presented at the 2003 Models-3 Users’ Workshop, October 27-29, 2003

Acknowledgments

• Studies performed under EPA STAR Grant No. GR826773 (PI: R. Mathur)

• Jeff Vukovich: Sea salt emissions data

• Frank Binkowski and Shawn Roselle for useful feedback in the early stages of the development

Outline

• Motivation for Development• MAQSIP Modifications• Study Description• Analysis of Results

– Inter-model and model-obs comparisons• the effect of coarse mode chemistry and sea salt species

on other secondary inorganic aerosol constituents• modeled aerosol concentrations vs. CASTNet and

IMPROVE observations

• Conclusions• Next steps

Motivation for Development• The modal aerosol modules in the MAQSIP and CMAQ

models have to date used a bulk equilibrium approach to determine aerosol composition.

• The semi-volatile species concentrations after equilibrium are partitioned among the fine modes proportional to the sulfate concentration in each mode.

• The coarse mode has been treated as chemically non-interactive with the fine modes.

• This is not an inherent limitation of the modal approach, but a limitation of the model implementation

Motivation (cont’d)

• Sea salt particles emitted in coastal areas can interact with anthropogenic secondary aerosol species (SO4, NO3)

• Much of the sea salt is emitted in the coarse particle size range

• Chemistry in the coarse mode needs to be included in the model

MAQSIP Modifications: First Cut

• HCl in the gas phase and fine and coarse Na+ and Cl- in the aerosol/droplet phase have been added

- HCl produced by heterogeneous reaction of NaCl with HNO3

• Aqueous chemical mechanism expanded to include dissolution/dissociation of HCl in cloud water and the entrainment of Na+ and Cl- in cloud droplets

MAQSIP Modifications: First Cut (cont’d)

• The ISORROPIA thermodynamic model is used in the bulk equilibrium sense to determine aerosol composition

• A mass transfer scheme has been added for condensation or evaporation of semi-volatile species in each mode

• The Whitby scheme for partitioning condensing sulfate and organic mass among the modes has been extended to partition mass transferred from/to particles among all modes (Whitby et al, EPA Report # NTIS PB91-161729/AS, 1991; Binkowski and Shankar, JGR 1995).

• Partitioning factors use a surface-area based growth law similar to that used in sectional models

• MAQSIP simulations over the eastern U.S. for a 12-day episode (June 19-30, 1996) including a 3-day spin-up period

• Episode characterized by relatively dry conditions – used in NC O3 SIP modeling

• Domain consists of a 72 x 75 horizontal grid at 36-km resolution and 26 vertical layers

• Meteorological inputs are from MM5• Emissions inputs are from the NEI 1996 inventory

processed by SMOKE• Sea salt emissions modeled according to the method

of Monahan (in Oceanic Whitecaps, 1986) and Gong et al. (JGR, 1997); fine-coarse splits as in AER’s EPRI BRAVO study

Study Description

Fine and Coarse Sodium (g/m3)

Fine Modes Coarse Mode

Fine and Coarse SO4 (g/m3)

(Sea Salt – No Sea Salt)

Fine Modes Coarse Mode

Fine and Coarse NO3 (g/m3)

(Sea Salt – No Sea Salt)

Fine Modes Coarse Mode

NH4 (g/m3) and HNO3 (ppbV)

(Sea Salt – No Sea Salt)

NH4 HNO3

Measurement Networks

CASTNet IMPROVE

Evaluation Against Network Data

• Spatially complementary distribution of CASTNet and IMPROVE sites, many more CASTNet sites in the eastern U.S.

• CASTNet samplers non-size selective• CASTNet measurements are weekly averages

for total SO4, NO3 and NH4

• IMPROVE measurements are 24-hr averages reported twice weekly for all fine PM species modeled except NH4

• Model results compared on an event-average basis with the measurements

Aerosol Species Fractions of Total vs. CASTNet

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

SulfateNitrateAmmonium

y = 0.29362 + 0.5419x R2= 0.10139

y = 0.084876 + 0.27669x R2= 0.03951

y = 0.073174 + 0.63174x R2= 0.17124

Observed

w/ Sea Salt

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

SulfateNitrateAmmonium

y = 0.29548 + 0.53294x R2= 0.09532

y = 0.085562 + 0.24284x R2= 0.029805

y = 0.10229 + 0.52674x R2= 0.12325

Observed

w/o Sea Salt

Aerosol Species Concentrations vs. CASTNet

0

5

10

15

20

0 5 10 15 20

Sulfate

With s.sNo s.s.

CASTNet

0

1

2

3

0 1 2 3

Nitrate

With s.sNo s.s.

CASTNet

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7

Ammonium

With s.sNo s.s.

CASTNet

BWR

ARE

BEL

Fine Aerosol Concentrations vs. IMPROVE

0

5

10

15

20

0 5 10 15 20

Sulfate

With s.sNo s.s.

IMPROVE

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5

Sodium

IMPROVE

0

1

2

3

0 1 2 3

Nitrate

With s.sNo s.s.

IMPROVE

JEFF

EVEROKEF

Analysis• Reasonable agreement between predicted aerosol

compositional characteristics and CASTNet• Total nitrate in the system is over predicted, while

sulfate and ammonium are under predicted both with and without sea salt emissions

• Sulfate is non-volatile, thus it changes very little due to introduction of sea salt / coarse mode chemistry

• Fine sodium severely under predicted at almost all IMPROVE sites → possible sources of the bias could be the sea salt emissions and/or BL meteorology at the land/water sites

• Nitrate response to sea salt and coarse mode chemistry is a little more pronounced in coastal locations due to some HNO3 mass transfer occurring to coarse mode particles

• Fine mode ammonium reductions collocated with coarse sodium nitrate formation

Conclusions• Model behaves self-consistently, but

performance needs improving relative to observations, particularly in the prediction of total nitrate

• Effects of sea salt chemistry are slight possibly because the region is dominated by sulfate; evaluation could benefit from studies where nitrate is the dominant aerosol component

• Adaptation in CMAQ’s modal aerosol module should be straightforward

Next …

• Adapt the hybrid approach of Capaldo et al. (AE 2000) to improve simulation of condensation/evaporation for coarse particles relative to the strict bulk equilibrium approach of this study

• Address the HNO3 over prediction• Examine meteorological and BC influences

on production of coarse mode nitrate in the interior of the domain

• Examine the emissions vs. meteorological influences on the under prediction of fine mode sodium