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PhD Proposal Defense - Li Zhu

Li successfully defended her PhD proposal on Monday, April 12, 2010.

Aerosol Absorption Measurements Using Satellite Remote Sensing

Aerosols, the solid or liquid particles suspended in the atmosphere, directly affect the
energy balance of the Earth’s climate system by scattering and absorbing solar radiation.
Aerosol absorption can warm the atmosphere and cool the Earth’s surface, hence aerosols
can affect the atmospheric temperature profile, boundary layer evolution, convection,
cloud formation, and precipitation, particularly over the regions where significant amount
of aerosol absorption occurs.

Much research has focused on studying aerosol absorption properties. One method of
studying aerosol absorption properties is called the critical reflectance technique. The
critical reflectance technique has the unique advantage of providing continuous
measurements and global coverage when it is applied to satellite data. Specifically, it uses
satellite data from two days (a clean day and a polluted day with the same observing
geometry) to retrieve aerosol single scattering albedo (SSA) - the ratio of the aerosol
scattering coefficient to the sum of the aerosol scattering and absorption coefficient.

Using the critical reflectance technique we have investigated how sensitive the retrieved
SSA is to the following factors: aerosol optical depth (AOD), the real part of the
refractive index, detector zenith angle, AOD difference between the polluted day and the
clean day, and changing aerosol types between both days. In addition, we applied this
technique to Moderate Resolution Imaging Spectroradiometer (MODIS) data over South
Africa and South America. The validation results show that SSA from MODIS retrieval
is in good agreement with AERONET SSA within one standard deviation.

I propose to continue studying aerosol absorption properties retrieved from MODIS data
using the critical reflectance technique and its application to climate forcing. I will
quantify the uncertainty of critical reflectance and SSA and extend the SSA retrieval to
all MODIS wavelengths with enough signal and other AERONET stations. I will also
validate retrieval results with AERONET and other available measurements.
Furthermore, to better understand how aerosol absorption affects the Earth’s energy
balance, I will determine the aerosol radiative forcing at the Earth’s surface and the top of
the atmosphere. In addition, I will produce regional SSA maps and provide aerosol
absorption information to aid in aerosol semi-direct effect studies.