Seasonality

1. North African Dust Export and Impacts: An Integrated Satellite and Model Perspective.D. A. Ridley1, C. L. Heald1, B. Ford 11Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA

2. MISR AOD @ 550nm [2007] Seasonality • Similar picture from MODIS and MISR aerosol optical depth (AOD) • Both biomass and dust emissions in the Sahel during the winter season • Emissions from the Bodélédepression throughout the year, peaking during the spring season • Change in the latitude of the export with season

3. GEOS-Chem Dust Modeling • 2 x 2.5 resolution with 47 vertical levels • Dust emitted according to Zender et al. [2003] • Dust emitted into four size bins: • 0.1–1.0, 1.0–1.8, 1.8–3.0, and 3.0–6.0 µm radius • Optics improved using SHADE size distribution measurements

4. Mass Partitioning of Sub-Micron Dust • Equal vs realistic partitioning • GEOS-Chem Annual AOD (2006) • Dust size distribution from the SHADE campaign (Highwood et al., 2003) • AOD decreased by 10-25% • Due to size dependence of scattering • NB no change in total dust mass • AOD contribution from dust size bins Original uniform mass partitioning New dust mass partitioning

5. GEOS-Chem AOD MISR AOD • (@ 550nm) [2007] Horizontal Structure • The model captures the spatial distribution, magnitude, and the seasonality of the AOD • Statistical comparison with both MISR and MODIS yields R>0.63 over the region shown • AOD at the Bodélé Depression underestimated by the model • Comparison with AERONET also made at eight sites over the three year period

6. Vertical Structure • Aerosol extinction across the Atlantic averaged over 15 deg latitudal bands [2007] • Marine aerosol? Lapina et al. [2011] • Seasonality in vertical distribution reproduced well • Missing aerosol close to ocean surface – dust and marine?

7. Deposition of Dust • Annual deposition of phosphorus (dust) in the Atlantic, Caribbean, and Amazon • Annual average dust deposition totals [2006-2008] • Annual westward export 250±50Tg (240±80Tg from MODIS1) • Deposition to Atlantic is slightly higher than other studies1,2,3,4 • Deposition to the Amazon is lower than previously estimated1,5 Tg 1 Kaufman et al., [2005] 2 Ginoux et al., [2004] 3 Prospero et al., [1996] 4 Fan et al., [2004] 5 Mahowald et al., 2005]

8. Deposition of Dust to the Caribbean • Annual deposition of phosphorus (dust) in the Atlantic, Caribbean, and Amazon • Good correlation with Barbados deposition measurements (R=0.74) • Similar monthly variability between model and observations • Observations suggest more dust export during Fall season • Barbados dust deposition (in-situ data from J. Prospero) Tg Tg

9. Dust Sinks • Estimated dust lifetime from seasonal AOD transects • MODIS (Deep Blue) AOD @ 550nm [2007] • Seasonally averaged AOD along Atlantic-outflow transects • Modeled lifetimes are typically 30-50% shorter than from MODIS • Consistently 50% shorter in Spring • Switching off convective removal increases deposition to Amazon by ~50%

10. Dust Sources • GEOS-Chem – MODIS AOD (550nm) • Observed AOD (550nm) [2006] • Doubling emissions from Bodélé Depression improves agreement with MODIS and MISR • R increased by ~10% and RMS error decreased by ~35% • Dust deposited to Amazon increased by ~10% AOD AOD

11. Conclusions • GEOS-Chem predicts 17±5Tg/yr is deposited in the Amazon, less than half of the 50Tg/yr estimate from satellite • This contrasts with good model agreement with dust deposition measurements in Barbados (R=0.74) and westward dust export comparison with MODIS (250±50 and 240±80Tg/yr respectively) • Modeled dust loss due to wet deposition appears too rapid • A doubling of the dust from the Bodélé depression improves the agreement between the model and satellite observations • Even at the extreme the model and the satellite estimates of dust deposition to the Amazon cannot be reconciled

12. Time Series of AOD at AERONET sites New GEOSchem Old GEOSchem AERONET Range of obs