1. SCALE ISSUES IN MODELING INTERCONTINENTAL TRANSPORT
Daniel J. Jacob, Rokjin Park, Lin Zhang, Colette L. Heald
with support from NASA, EPA, EPRI
Issue 4 of Background Document: “What is the effect of spatial resolution of the model on estimates of source-receptor relationships? Is it necessary to nest regional models within global models, and what difficulties must be overcome to perform this nesting? Is one way nesting sufficient, or is two-way nesting necessary?”

2. regional statistics and events surface stratification
INTERCONTINENTAL TRASPORT AS WEDDING OF GLOBAL AND REGIONAL MODEL CULTURES
GLOBAL MODELS
REGIONALMODELS
Priorities
regional statistics and events
global budgets
and distributions
local events
Scales
global
(~1000 km, months)
synoptic
(~100 km, days)
meso
(~10 km, hours)
Scaling nightmares
boundary conditions
surface stratification
BL dynamics
topography
fine variability
convection
scavenging
strat-trop exchange
NEED TO SHARE
OUR
NIGHTMARES

3. TRANSPACIFIC TRANSPORT OF OZONE AND AEROSOLS
HEMISPHERIC POLLUTION
PAN
VOCs
(long-lived)
warm conveyor belts,
convection
subsidence O3 OC
aerosol
PAN
plume
transport
Free
troposphere
Sulfate
OC aerosol
NOx O3
2 km
entrainment
Boundary layer
NOx, SO2, VOC
aerosols,
HNO3
ozone, sulfate, OC
ASIA PACIFIC NORTH AMERICA

4. SURFACE DATA FOR OZONE AT REMOTE SITES SUGGEST LITTLE VARIANCE IN INTERCONTINENTAL INFLUENCE*
CASTNet observations (13-17 LT)
GEOS-Chem model
Standard
Background
Natural
Stratospheric
Intercontinental ozone enhancement of 5-10 ppbv, driven by large-scale subsidence }
D =
Regional
pollution + X }
D =
Intercontinental
pollution
Voyageurs NP, Minnesota, 2001
Fiore et al. [2003]

5. SURFACE OZONE AT TRINIDAD HEAD, CALIFORNIA, Apr-May 2002
Observations: 41 ± 5 ppb (filtered against local influence)
GEOS-Chem model: 39 ± 5 ppb (Asian 4.5 ± 1.1 ppb)
MOZART model: 37 ± 9 ppb (Asian 4.2 ± 1.3 ppb)
Synoptic variability is weak in both model and observations,
but local nighttime depletion (stratification) is not resolved in global models
Goldstein et al. [2004]

6. WHY ARE INTERCONTINENTAL OZONE POLLUTION PLUMES OBSERVED IN FREE TROPOSPHERE BUT NOT AT SURFACE?
Aircraft observations during NOAA/ITCT-2K2 out of Monterey (April-May 2002)
Asian pollution plume at 2-4 km CO O3
May 17
This is consistent with aircraft vs. surface observations of transpacific dust plumes,
which show 10x dilution between lower free troposphere and surface –
but can models reproduce this? (they can’t resolve the free tropospheric plumes)
Hudman et al. [2004]

7. but global models cannot deal properly with this
INTERCONTINENTAL OZONE POLLUTION ASSESSMENTS MUST ACCOUNT FOR LOCAL TITRATION/STRATIFICATION
Ozone background is depleted under stagnant/stratified/titrated conditions…
but global models cannot deal properly with this
CASTNet sites
GEOS-Chem Model
Ozone background *
Ozone (ppbv)
Cumulative Probability
Fiore et al.
[2003]
Addressing this issue requires downscaling, e.g., global g regional nesting

8. 1-WAY NESTING OF GEOS-Chem AND CMAQ: PRELIMINARY RESULTS
CMAQ domain stretched from Asia to the North Atlantic
April 2001 surface ozone
CMAQ (GEOS-Chem boundary conditions)
GEOS-Chem
CMAQ and GEOS-Chem have comparable surface ozone over continents but
CMAQ is lower across Pacific and transpacific pollution influence is half that in GEOS-Chem
Rokjin Park (Harvard) and Carey Jang (EPA/OAQPS)

9. CMAQ vs. GEOS-Chem TRANSPACIFIC TRANSPORT
Ozone concentrations at 25-50oN vs. pressure and longitude (April 2001)
Evaluation w/ mean TRACE-P
obs in Asian outflow (<140E)
CMAQ
Ozone, ppbv
ASIA
N. AMERICA
GEOS-Chem
TRACE-P
CMAQ
GEOS-
Chem
Low free tropospheric ozone in CMAQ likely due to processes usually neglected
in regional models: STE, lightning, …
Rokjin Park (Harvard) and Carey Jang (EPA/OAQPS)

10. Ozone-CO relationship
USING SPACE-BASED TES OZONE-CO DATA TO OBSERVE INTERCONTINENTAL OZONE POLLUTION
602 hPa TES observations, July 2005
Ozone-CO relationship
downwind of Asia
TES
R=0.64
Slope=0.62
Ozone
Ozone (ppbv)
ppbv
GEOS-Chem
Model
R=0.84
Slope=0.50
Carbon monoxide (pppbv) CO
ppbv
Lin Zhang (Harvard)

11. Ozone-CO relationship downwind of eastern N. America
USING SPACE-BASED TES OZONE-CO DATA TO OBSERVE INTERCONTINENTAL OZONE POLLUTION
618 hPa TES observations, July 2005
Ozone-CO relationship
downwind of eastern N. America
TES
R=0.38
Slope=0.75
Ozone
Ozone (ppbv)
ppbv
GEOS-Chem
model
R=0.79
Slope=0.67
Carbon monoxide (pppbv) CO
ppbv
Lin Zhang (Harvard)

12. TRANSPACIFIC ASIAN AEROSOL POLLUTION AS SEEN BY MODIS
Heald et al. [2006]
X1018 [molecules cm-2]
Quantitative comparison to models must use MODIS reflectances (not AODs)

13. CMAQ vs. GEOS-Chem TRANSPACIFIC TRANSPORT OF SULFATE (April 2001)
CMAQ GEOS-Chem
Sulfate
In surface
air
Column
Asian pollution influence in U.S. surface air in CMAQ is 5x that in GEOS-Chem
Rokjin Park (Harvard) and Carey Jang (EPA/OAQPS)

14. EVALUATING ASIAN SULFATE OUTFLOW:
TRACE-P aircraft observations (Mar-Apr 2001, <140oE)
Concentration, mg m-3
Altitude, km
Sulfate
SOx
Suggests insufficient scavenging in CMAQ during venting to free troposphere
Rokjin Park (Harvard) and Carey Jang (EPA/OAQPS)

15. TESTING TRANSPACIFIC SULFATE IN GEOS-Chem WITH SURFACE OBSERVATIONS IN NORTHWEST U.S.
Model Asian sulfate
Model Asian dust
Observed sulfate
Observed dust
Model sulfate
Northwest U.S. IMPROVE sites (2001)
Mean Asian pollution enhancement in NW U.S. in spring: 0.16 ± 0.08 mg m-3
Heald et al. [2006]

16. CONCLUSIONS AND RECOMMENDATIONS
Global models have capability to describe intercontinental transport on synoptic scales ( km)
For policy applications, need coupling with regional models to downscale the information to meso and local scales (1-10 km)
focus regional models on synoptic-to-meso downscaling
2-way nesting of course nice but 1-way nesting should be OK
Apply regional models to improve simulation of continental venting
requires new thinking in regional models
could be exploited in 2-way nesting
Improve simulation of free tropospheric pollution plumes in global models
this may be tied to convective parameterizations, model resolution, advection algorithms
Need to investigate model ability to describe subsidence and boundary layer entrainment