An Exploration of Model Concentration Differences Between CMAQ and CAMx

Brian Timin, Karen Wesson, Pat Dolwick, Norm Possiel, Sharon Phillips

EPA/OAQPSTimin.brian@epa.govOctober 3, 2007

Introduction OAQPS conducted 2001 base case modeling with

CMAQ and CAMx Both models used the same “raw” emissions and

meteorological data Large differences were seen in predicted ozone

concentrations as well as other precursors and species

We conducted several analyses to help examine differences in the models Sensitivity model runs with CAMx and CMAQ Analysis of existing information

2001 Model Platforms Both models were run with a very similar

setup

CMAQ CAMxModel version 4.5 4.31Number of vertical layers 14 14Emissions processing SMOKE SMOKEMeteorological processing MCIP 3.1 MM5CAMXBoundary conditions GEOS-CHEM GEOS-CHEM12 km domain 188 X 212 220 X 273Chemistry CB-IV CB-IV (+)

CMAQ vs. CAMx- Ozone Concentration

July 17, 2001 at 21Z

CAMx (w/O’Brien) CMAQ

CMAQ vs. CAMx- CO Concentration

July 17, 2001 at 21Z

CAMx (w/O’Brien) CMAQ

CMAQ vs. CAMx- FORM

July 17, 2001 24-hour average

CAMx (w/O’Brien) CMAQ

CMAQ vs. CAMx- Sulfate Concentration

July 19, 2001 24-hour average

CAMx (w/O’Brien) CMAQ

Analyses

Chemical mechanism Photolysis rates Cloud attenuation of radiation Vertical mixing Dry deposition

Analysis of Chemistry and Clouds

CAMx (mechanism 4) uses a hybrid version of CB-IV which contains additional reactions (CB-IV+) compared to CMAQ CB-IV

Photolysis rates are generally higher in CAMx and with CB05 compared to CMAQ CB-IV

Cloud attenuation of radiation differs between the models These differences between the models were judged not

likely to cause significant regional ozone differences between the models

Vertical Mixing

Vertical mixing is governed by vertical diffusion coefficients (Kv) CMAQ v4.5 used “ACM” mixing CAMx used “O’Brien” Kv’s

There is an option in MM5CAMX to generate “CMAQ like” Kv’s Comparison of actual CMAQ Kv’s and “CMAQ like” Kv’s confirmed

similar magnitudes and spatial patterns We conducted a CAMx sensitivity run which used “CMAQ like”

Kv’s and compared the results to O’Brien CMAQ like Kv’s (and actual CMAQ Kv’s) are generally much higher

than O’Brien Kv’s Expect higher ozone with CMAQ like Kv’s in NOx limited areas

CAMx Ozone Change-“CMAQ-like” Vs. O’Brien KV’s

Change in CAMx hourly ozone at 15Z on July 17, 2001

Blue= lower ozone with “CMAQ-like” Kv’s

Change in CAMx hourly ozone at 20Z on July 17, 2001

CAMx KV’s and Ozone- Atlanta Example

CAMx Kv's in Atlanta on 7/19/01

123456789

1011121314

0 100 200 300 400Kv's (m2/s)

La

ye

r In

de

x

O'Brien

CMAQ-like

CAMx/CMAQ O3 in Atlanta on 7/19/01

0

500

1000

1500

2000

2500

40 60 80 100 120Ozone (ppb)

La

ye

r H

eig

ht

(m)

O'Brien 15:00

CMAQ-like 15:00

CMAQ 15:00

O'Brien 20:00

CMAQ-like 20:00

CMAQ 20:00

CMAQ-like Kv’s are (almost) always higher than O’Brien and tend to drop off at a higher layer

Ozone concentrations in CAMx and CMAQ are similar at 15z, but CAMx becomes much higher at 20z

Maximum Daytime PBL Comparison We compared maximum PBL heights in Atlanta from observations,

predictions from MM5 (MCIP), and from CMAQ and CAMx CMAQ tends to mix to a higher layer compared to the PBL heights

from MCIP This example for Atlanta is not representative of all days and areas

CAMx/CMAQ Layer Height (m)14 1567613 962612 615411 421210 31089 22128 16577 11306 7125 4694 3103 1542 761 38

Atlanta

0

500

1000

1500

2000

2500

3000

3500

7/1

7/2

001

7/1

8/2

001

7/1

9/2

001

Date

Dail

y M

ax P

BL

Heig

ht

(m)

CMAQ

CAMx CMAQ-like

CAMx O-Brien

Observed

MM5/MCIP

Note: CAMx and CMAQ mix up to the top of discrete model layers (as defined in the table above)

Dry deposition CAMx uses a Wesely based dry deposition scheme CMAQ uses the M3Dry scheme

Closely tied to the Pleim-Xiu land surface model Accounts for enhanced deposition to wetted surfaces (soluble

species) Contains more recent science RADM dry deposition scheme (similar to Wesely) is

optional in CMAQ (MCIP 3.2 and prior) Examination of dry deposition velocities (Vd)

revealed large differences between models

Dry Deposition VelocitiesCAMx vs. CMAQ- Ozone

CAMx ozone Vd at 16Z

on July 17, 2001

CMAQ (M3Dry) ozone Vd at 16Z

on July 17, 2001

Dry Deposition VelocitiesCAMx vs. CMAQ- CO

CAMx CO Vd at 16Z

on July 17, 2001

CMAQ (M3Dry) CO Vd at 16Z

on July 17, 2001

Dry Deposition VelocitiesCAMx vs. CMAQ- NO

CAMx NO Vd at 16Z

on July 17, 2001

CMAQ (M3Dry) NO Vd at 16Z

on July 17, 2001

Dry Deposition VelocitiesCAMx vs. CMAQ- NO2

CAMx NO2 Vd at 16Z

on July 17, 2001

CMAQ (M3Dry) NO2 Vd at 16Z

on July 17, 2001

Dry Deposition VelocitiesCAMx vs. CMAQ- FORM

CAMx FORM Vd at 16Z

on July 17, 2001

CMAQ (M3Dry) FORM Vd at 16Z

on July 17, 2001

Dry Deposition Sensitivities

Two CMAQ sensitivity runs were conducted to examine dry deposition issues Alternative mesophyll resistance values with M3Dry Alternative dry deposition scheme (RADM)

The platform for these CMAQ runs was CMAQ v4.6 with CB05 chemistry Ran CMAQ for 2 weeks in August 2002 (plus 7 day

ramp-up)

CMAQ Dry Deposition Sensitivity No. 1 Mesophyll Resistance

M3Dry Vd values for CO, NO, and NO2 were found to be too high Added a mesophyll resistance value in MCIP* for:

NO = 9400 S/M NO2= 500 S/M CO = 100,000 S/M

Ran MCIP and CMAQ with the new values August 2002 period

*The mesophyll resistance values for NO, NO2, and CO were later incorporated into MCIP 3.3

CMAQ CO Vd- M3Dry vs. M3Dry w/modified Mesophyll Resistance

M3Dry CO Vd at 16Z on August 10, 2002

M3Dry CO Vd at 16Z on August 10, 2002 (w/mesophyll resistance)

CMAQ CO concentration on August 5, 2002 (24 hour avg.)

Change in CMAQ CO concentration on August 5, 2002 (w/mesophyll resistance) (24 hour avg.)

CMAQ CO Concentration and Change in Concentration Due to Mesophyll Resistance

CMAQ Ozone Concentration and Change in Concentration Due to Mesophyll Resistance

CMAQ ozone concentration on August 5, 2002 (8 hour max.)

Change in CMAQ ozone concentration on August 5, 2002 (w/mesophyll resistance) (8 hour max.)

CMAQ Dry Deposition Sensitivity No. 2 RADM Dry

The RADM dry deposition routine is an option in MCIP* Formulation is based on Wesely, 1989

Very similar to CAMx

Ran MCIP and CMAQ with RADM dry August 2002 period

*RADM Dry was removed from MCIP v3.3

CMAQ Ozone Vd- M3Dry vs. RADM

M3Dry ozone Vd at 16Z on August 10, 2002

RADM Dry ozone Vd at 16Z on August 10, 2002

CMAQ FORM Vd- M3Dry vs. RADM

M3Dry FORM Vd at 16Z on August 10, 2002

RADM Dry FORM Vd at 16Z on August 10, 2002

CMAQ Ozone Concentration and Change in Concentration Due to RADM Dry Deposition

CMAQ ozone concentration on August 5, 2002 w/M3Dry (8 hour max.)

Change in CMAQ ozone concentration on August 5, 2002 (w/RADM Dry) (8 hour max.)

CMAQ Sulfate Concentration and Change in Concentration Due to RADM Dry Deposition

CMAQ sulfate concentration on August 5, 2002 w/M3Dry (24 hour avg.)

Change in CMAQ sulfate concentration on August 5, 2002 (w/RADM Dry) (24 hour avg.)

Conclusions We examined numerous differences between

CMAQ and CAMx The majority of the ozone differences can be

attributed to different implementations of vertical diffusion and dry deposition Numerous other smaller differences were also identified

CO concentrations were too low in CMAQ due to high CO Vd (corrected by adding a mesophyll resistance value)

Other species (including secondary aerosols) are also affected by mixing and dry deposition

Recommendations Further testing of vertical mixing is needed in both

models Need more comparisons between observed PBL and

CMAQ/CAMx mixing Does CMAQ “overmix” compared to MM5 predicted PBL? Does O’Brien have too little mixing?

More vertical layers may be needed in the AQM boundary layer

Further examination of dry deposition velocities is needed Evaluate diurnal pattern of Vd

Are afternoon Vd values too high in CMAQ? Does the Wesely scheme need to be replaced?

Recommendations

Various combinations of chemical mechanisms (CB-IV, CB05, SAPRC), vertical diffusion (O’Brien, ACM, ACM2) and dry deposition (M3Dry, Wesely, AERMOD) can give very different results Each process needs to be individually evaluated Operational ozone evaluation should not be used to

determine the “best” model formulation