1 2 3 4 5 6 7 8 9 10 11 120

0.02

0.04

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0.08

0.1

0.12

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Month (J anuary-December)

AO

D

MODIS

AEROCOM

MODIS-AEROCOM

AEROCOM AODs are systematically smaller than MODIS, with slightly larger/smaller differences in winter/summer.

85S 75S 65S 55S 45S 35S 25S 15S 5S 5N 15N 25N 35N 45N 55N 65N 75N 85N0

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0.15

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0.25

Latitude (degrees)

Aero

sol O

pti

cal D

epth

(A

OD

)

AEROCOM

MODISZonal variation

Aerosol optical properties are difficult to measure globally using ground-based instruments, due to limited number of stations. At the same time, their retrieval through satellites is subject to certain uncertainties and limitations. AEROCOM (Kinne et al., 2006) is a blend of ground-based observations and global modeling output, offering aerosol optical properties globally at 1°x1° resolution. In the present study AEROCOM is evaluated against MODIS retrievals, by comparing aerosol optical depth (AOD), single scattering albedo (ω) and asymmetry parameter (gaer), as well as radiative forcing simulations. For the simulations, a spectral radiation transfer model has been used (Hatzianastassiou &Vardavas, 1999). The vertical distribution of AEROCOM (model ECHAM output) is also compared with CALIOP version 2 data. AEROCOM data include AERONET quality observations from 1998 to 2007. These data are available as a climatological set and not as a time series. Hence, for the comparison, the mean value of MODIS data from 2000 to 2007 has been used. All AEROCOM data are freely available at the ftp site :ftp://ftp-projects.zmaw.de/aerocom/climatology/

•AEROCOM overestimates AOD in areas with large aerosol load. •Most grids of larger AEROCOM AOD are over land, implying an overestimation of AOD in AEROCOM or an underestimation of MODIS land values. •AEROCOM overestimates sea salt AOD at the latitude band between 45 ° and 55° S, probably due to large value of prescribed effective radius (Kinne et al., 2006).

(TOTAL)

•For both datasets, the season with the largest AOD values is spring and summer. However, while for MODIS the highest values appear during spring with summer coming second , for AEROCOM this is reversed. •The differences between the two datasets are largest in autumn.

Summer and spring are the seasons with the best agreement for AOD, while the worst is during autumn.Radiative forcings using AEROCOM data have smaller magnitude than those based on MODIS, mainly because of smaller AEROCOM AODs. However, differences at surface and in atmosphere cancel out so that the difference at TOA is small (3%).In places where AEROCOM AOD is larger than MODIS, Net Radiative forcing estimated by AEROCOM is larger.In the global perspective, RFTOA only differs by 3.1%. On a global annual mean basis, AEROCOM overestimates g and ω by 16.6% and 4.1%, respectively. Vertical distribution of AEROCOM aerosols is similar to the one of CALIOP version 2 data. This is expected to be improved with the implementation of CALIOP version 3 data by the AEROCOM team. In the total, AEROCOM is a valuable tool for studies using aerosol optical properties, given the relatively small differences in magnitude and spectral profiles of AOD, SSA and gaer with MODIS. The good performance of AEROCOM maximizes its usefulness based on advantages like the separation between natural and anthropogenic aerosols and the spectral resolution of the 1°x1°aerosol optical properties.

4 sites characteristic of different types of aerosols (pollutants, sea salt spray, biomass, dust) have been selected.•Generally AOD compares well with AEROCOM underestimating.•ω is available by MODIS only over land•g is available by MODIS only over oceans

2. Global and hemispherical view of AOD

1. Introduction-Motivation 3. Spatial variability

4. Seasonal variability

5. Radiative forcing (RF)(plots for [RFAEROCOM-RFMODIS/RFMODIS])

6. Vertical distribution

8. Conclusions

References

7. Spectral variation of AOD, g, ω

0 10 20 30 40 50 60 70 80 90 1000

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Accumulated fraction (%)

Alti

tude

(km

)

Accumulated fraction of columnar AOD

CALIOP (Mediterranean)

AEROCOM (Mediterranean)

•50 % of total columnar AOD is due to aerosols below 1.50 / 1.44 km for AEROCOM /CALIOP.•85 % of total columnar AOD is due to aerosols below 2.73 / 3.2 km for AEROCOM/CALIOP.

•Kinne et al., 2006: An AeroCom initial assessment optical properties in aerosol component modules of global models. Atmos. Chem. Phys., 6, 1-20•Hatzianastassiou, N., and I. Vardavas, 1999: Shortwave radiation budget of the Northern Hemisphere using International Satellite Cloud Climatology Project and NCEP/NCAR climatological data. J. Geophys. Res., 104, 24401-24421.

AEROCOM AEROCOM

AEROCOM

AEROCOMAOD: 0.137MODISAOD: 0.164

AEROCOMAOD: 0.144MODISAOD: 0.162

AEROCOMAOD: 0.123MODISAOD: 0.151

AEROCOMAOD: 0.117MODISAOD: 0.144

•For the AEROCOM RF computations, AEROCOM AOD/SSA/gaer have been used, while for MODIS RF, MODIS AOD and GADS SSA/gaer have been used.•Very good agreement of global mean TOA forcing, but large difference in S. Hemisphere oceans with AEROCOM underestimating it.•Overestimation of normalised RF (radiative forcing efficiency) up to 50 Wm-2 in areas with industrial type of aerosol . Underestimation in the largest part of oceans.

Wavelength (nm) Wavelength (nm)

Atmospheric forcing relative difference (%)