1. SATELLITE OBSERVATIONS OF ATMOSPHERIC AEROSOLS:
Awash in uncertainties or a useful constraint on atmospheric composition?
Colette L. Heald
David A. Ridley, Bonne Ford
University of Washington
November 18, 2011

2. ATMOSPHERIC AEROSOLS ARE LINKED TO MAJOR ENVIRONMENTAL ISSUES
CLIMATE
AIR QUALITY / HEALTH
FERTILIZATION

3. AEROSOLS: A MAJOR CLIMATE FORCER
GHGs dominate mass budget in atmosphere…
800+ Pg
10’s of Tg
BUT one “molecule” of aerosol is >1x1018 times more effective than a molecule of CO2 in terms of radiative forcing. Even 1x1011 times more effective than CFCs…
[Heald and Worsnop, in prep]

4. CHALLENGES OF CHARACTERIZATION: HETEROGENIETY, COMPLEXITY
Image from:
C. Leck
Diversity of sources types
Size, shape, chemical composition, radiative properties…
Aerosol lifetime ~ days  not well mixed!

5. Aerosol Distributions SURFACE (variable reflectance properties)
CHALLENGE: OBSERVING AEROSOL COMPOSITION FROM SPACE TO QUANTITAVELY INVESTIGATE AEROSOL LOADING
A tough
measurement
to make!
What we are
comparing!
SIMULATED
AOD
SATELLITE
AOD
Assumptions:
Optical Properties
Size Distributions
Aerosol Distributions
etc.
*DIFFERENT*
Assumptions:
Optical Properties
Size Distributions
etc.
AEROSOL SPECIATED MASS CONCENTRATIONS
What we want
to estimate!
Dust
Carbonaceous
aerosols
Sulfate
Sea Salt
Nitrate
SURFACE (variable reflectance properties)

6. SATELLITE OBSERVATIONS: DIFFERENT PLATFORMS, TECHNIQUES, PHYSICS…DIFFERENT AODs!
MODIS (Aqua)
CALIOP
GEOS-Chem
GEOS-Chem (w/ DL)
Spring
Summer
MODIS and CALIOP (both in A-Train) observe very different AODs.
Moving model target?
Accounting for CALIOP detection limits important for model comparison…
[Ford and Heald, submitted]

7. FOCUS ON TODAY: (1) Dust from North Africa (2) Aerosol in the SE US
(3) The global budget of organic aerosol

8. DUST FROM NORTH AFRICA: IMPACTING AQ AND THE BIOSPHERE DOWN-WIND
More than half of dust emitted globally from N. Africa
TOMS: June 13-21, 2001
Miami ( )
[Prospero et al., 1999]
summer
winter/spring
French Guiana ( )
[Prospero et al., 1981]

9. SATELLITE CONSTRAINTS ON DUST SOURCE & TRANSPORT
GEOS-Chem overestimates observed AOD in source region, underestimates winter/spring export

10. REALISTIC APPORTIONMENT OF SUB-MICRON DUST MASS
Observed Saharan dust size distribution
Simulated decrease in AOD
[Highwood et al., 2003]
OLD
NEW
Shifted mass to larger sub-micron sizes (less optically efficient). Reduces AOD in better agreement with satellite & AERONET observations

11. SATELLITE CONSTRAINTS ON DUST SOURCE & TRANSPORT
Sub-micron size apportionment significantly improves model agreement with observations.
Remaining challenges: Bodélé Depression and outflow

12. DUST REMOVAL MOVING ACROSS THE ATLANTIC
Seasonally averaged AOD along Atlantic-outflow transects
Assuming first order loss can estimate lifetime from slope of log-plots…
Model removal too strong, especially in winter-spring (transport towards Amazon).
Likely excessive removal along the ITCZ.
We estimate dust lifetimes in model (~3 days) in spring about 50% of satellite-based estimate (> 6 days).

13. MODEL CAPTURES AIR QUALITY IMPACTS IN THE CARRIBEAN
Barbados
R2=0.74
Measurements: J. Prospero
(Surprisingly) good simulation of dust transport over 5000 km to the Caribbean!

14. DEPOSITION OF AFRICAN DUST & PHOSPHOROUS IN THE AMAZON
ANNUAL
We estimate 17 Tg/yr of dust transported to Amazon annually (lower limit). This is ~10-25% of the P supply for the Amazon.
Otherwise from fires and biogenic particles [Mahowald et al., 2005].
Impact of greening of the Sahel on productivity of the Amazon?
[Ridley et al., JGR, in press]

15. FOCUS ON TODAY: (1) Dust from North Africa (2) Aerosol in the SE US
(3) The global budget of organic aerosol

16. IS AEROSOL CONTRIBUTING TO CLIMATE TRENDS IN SOUTHEASTERN US?
Summertime trends
Data: Global Historical Climate Network Daily (GHCND)
“Although clearly speculative, increasing biogenic secondary organic aerosol/cloud effects linked to forest regrowth and/or interactions with anthropogenic pollution
is one possibility that is qualitatively consistent, not only with the spatial structure, but also with the seasonality of the correlation of the unusual negative temperature trends with precipitation found in the southeastern United States.”
[Portmann et al., 2009]

17. SEASONAL AEROSOL MAXIMUM IN THE SOUTHEASTERN US
Annual Mean AOD (MISR)
Summer-Winter AOD (MISR)
Summer-Winter AOD (MODIS)
AOD at Walker Branch
MODIS
MISR
Seasonal maximum in AOD consistent with biogenic emissions (implication: biogenic SOA)
[Goldstein et al., 2009]

18. IS THIS CONSISTENT WITH OUR UNDERSTANDING OF AEROSOL IN THE REGION?
Spring
(MAM)
Summer
(JJA)
NO! Model does not reproduce the Southeastern maximum.
Bonne Ford, CSU

19. BUT, IT’S ALSO INCONSISTENT WITH SURFACE DATA
Observed Seasonality of Surface PM
Preliminary Comparison of Vertical Profile in the SE US
[Malm et al., 2003]
Surface data does not indicate that organic aerosol dominates at surface.
Also see stronger season cycle in Northeast.
Possibly aerosol aloft?

20. FOCUS ON TODAY: (1) Dust from North Africa (2) Aerosol in the SE US
(3) The global budget of organic aerosol

21. ORGANIC AEROSOL MAKES UP AN IMPORTANT/DOMINANT FRACTION OF OBSERVED AEROSOL
Sulfate
Organics
[Zhang et al., 2007]
Globally makes up 25-75% of total fine aerosol at the surface (ignoring dust here)

22. MODELS UNDERESTIMATE OBSERVED ORGANIC AEROSOL OA
Mass
(fine)
2-10!
Model underestimate observed OA concentrations by factor of 2-10 in the mean.
Big Issue in the community: What is the source of “missing OA”.
Global Model: GEOS-Chem(2x2.5)
[Heald et al., submitted]

23. WHY DON’T MODELS GET IT RIGHT….
10,000’s of (unidentified?) compounds with variable properties
Continuing Oxidation/Partitioning in the Atmosphere
Uncertain Formation (Missing sources? Poorly understood processes?)
Hydrocarbons
(gas-phase & particulate)
Terpenes
(gas-phase)
PBAP

24. HOW LARGE A MISSING SOURCE OF ORGANIC AEROSOL?
Goldstein and Galbally [2007] suggest that SOA source may be anywhere from TgC/yr.
For comparison, current global model (GEOS-Chem) estimates total ~50 TgC/yr
Can total aerosol optical depth (AOD) measurements shed any light on the total budget of OA?

25. IF ONLY AEROSOL IN THE ATMOSPHERE WAS OA, WHAT LOADING IS IMPLIED BY SATELLITE AOD?
Calculate the “hypothetical” AOD implied by a constant 1 g/sm3 profile over the land, and see how we need to scale this locally to make up ENTIRE AOD reported by MISR.
Inverted total MISR AOD: Surface OA concentrations
Inverted OA loading is 3.5 TgC over land.
Assume a 6 days lifetime = 215 TgC/yr
 extrapolate to include outflow ~430 TgC/yr. (middle of Goldstein & Galbally range)

26. A MORE REALISTIC POSSIBILITY: REMOVE CONTRIBUTIONS FROM DUST, BC, INORGANICS (assuming all the negative bias in the model is ONLY OA)
DJF
JJA
Estimate that ~150 TgC/yr source is required to close the MISR-GEOS-Chem* discrepancy.
MISR
GEOS-Chem*
MISR-
GEOS-Chem*
*excluding OA

27. UNCERTAINTY ANALYSIS (boring but important!)
Estimated uncertainty on OA budget due to:
Uncertainty on OA optical properties
Aerosol optical properties
Size parameters
Refractive indices
Aerosol water uptake (growth factor)
Relative humidity (assuming 5% uncertainty in GEOS-5 fields)
50%
20%
10%
6%*
Conversion from burden to source
Aerosol lifetime (including effects of vertical profile and export fraction)
Global budget of “other” aerosols simulated in GEOS-Chem
25%
MISR AOD measurements
Total Error (added in quadrature)
80%
Assumed optical properties based on GADS database and log-normal size distribution recently evaluated by Drury et al. [2010]
* Except over high RH regions
Uncertainty on estimated OA source = 80%

28. HAVE WE REDUCED THE UNCERTAINTY ON THE OA BUDGET?
910
Range estimated by: Goldstein and Galbally [2007]
This is more than THREE TIMES what is currently included in global models….
BUT at the low end of Goldstein & Gallbally [2007] range.
140
150
Our satellite top-down estimate 47
Existing GEOS-Chem sources
All units in TgCyr-1
[Heald et al., 2010]

29. Global satellite measurements fill much needed observational gap.
WRAPPING UP
Global satellite measurements fill much needed observational gap.
But quantitative comparisons (moving beyond pretty pictures) can be challenging