Dr. Jun Wang

Geosciences-UNL

The Effect of Non-Sphericity on Geostationary Satellite Retrievals of Dust Aerosols

Collaborated with Drs. Xiong Liu (Harvard University), Jeff Reid (NRL), and Hal Maring (Miami Univ.), My advisor Dr. Sundar Christopher and I recently published an article in GRL on the dust non-sphericity effect in the satellite aerosol retrievals.

One advantage in using geostationary satellite data is its high temporal resolutions. It revisits the same area every 30 minutes, and the change of solar zenith angle for a certain location provides an opportunity to examine the satellite retrievals over a wide range of scattering angles, even for the same dust layer on the same day.(see conceptual figure on the left).

Details can be found in our paper and recent poster:

Wang, J., X. Liu, S.A. Christopher, J.S. Reid, E.A. Reid, and H. Maring, The effects of non-sphericity on geostationary satellite retrievals of dust aerosols, Geophys. Res. Lett., 30, doi:10.1029/2003GL018697, 2003.(pdf file)

Wang, J., S A. Christopher^, Xiong Liu,Jeffrey S. Reid^, Elizabeth Reid, Hal Maring^,The effect of non-sphericity on GOES-8 dust aerosol retrievals during PRIDE, Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract A11E-0011, 2003. (pdf file, download first and then open it)

1. Background

Dust affects visibility, human health, and the Earth energy budget. However, modeling of dust distribution and quantification of its radiative effects are difficult, simply because ground-based measurements for dust aerosols are limited in both space and time.
The satellite measurements have been considered as one of the best tools to characterize the high spatial-temporal variations of aerosols. However, the current dust retrievals from satellite measurements have large uncertainties, mainly because dust particles are non-spherical, and their phase functions can not be calculated/treated properly.
It has been shown theoretically that such uncertainties can be easily larger than 2. In the context of practical applications, few quantitative evaluations of non-spherical effect on satellite retrievals have been made, either due to the lack of in situ aerosol characterization measurements, or because most satellite measurements lack the capability to monitor the same dust layer from different angles with high temporal resolutions.

2. Objectives

Will consideration of non-spherical effects improve the satellite retrievals, if all the required data to characterize aerosol optical properties are given in the same temporal-spatial domain?

3. Methods/Strategies

Using all possible in situ data collected during the Puerto Rico Dust Experiment (PRIDE).

Measured dust size distribution (from 3 different sizers) and aerosol light scattering/extinction coefficients (from 3 Nephelometers) are combined together to infer the aerosol effective refractive index, and constrain aerosol properties in the retrievals. Inferred refractive index for dust particles is 1.53-0.0015i, and single scattering albedo is about 0.97~0.98.

Dust samples were collected from the aircraft measurements and then analyzed through the scanning electronic monograph (SEM). A statistical mode for dust morphologies is created based on the SEM analysis of 60, 500 particles. The model uses 6 size intervals and 15 aspect ratios (1.2~10) to describe the dust size and shape. In this study, we assume dust particles are oblate spheroid. The aerosol optical properties are then computed through the T-matrix calculations.

4. Results and Conclusion

The retrieval results using different phase functions are shown in the following figure. Here listed some conclusions.

Using SEM data and T-matrix calculations, the computed non-spherical phase function agrees well with synthetic phase function derived from independent measurements [Liu et al., 2003].

Applying purely non-spherical phase functions into the satellite retrieval algorithms only shows slightly improvement at certain scattering angles. However, using composite phase function by considering both spherical and non-spherical particles greatly improves the retrievals.

Further efforts are needed to combine the use of multi-angle, multi-channel, and polarization data sets to retrieve the morphologies of particles and to apply them in satellite retrievals.