You are all invited to attend Li's PhD proposal defense.

Date: Thursday, June 11, 2009
Time: 2pm
Location: PHYS 401

TITLE:
Study of Aerosol Absorption with MODIS

ABSTRACT:
The long term recorded near-surface air temperature over land and sea shows a linear warming trend (1906 to 2005) of 0.74 ^oC. Most of this observed increase is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. In contrast, anthropogenic aerosols produce an average global cooling effect with a direct radiative forcing of -0.5 Wm^-2. More importantly, the aerosol radiative forcing still remains the dominant uncertainty in the radiative forcing. The aerosol direct forcing of climate depends significantly on the light absorption characteristics of the aerosol. Different kinds of aerosols have very different absorption. For example, dusts have much stronger spectral dependence than smoke, showing significant absorption in the ultraviolet (UV) and visible (VIS), and nearly zero absorption in the near infrared (NIR), while the smoke imaginary refractive index is usually flat over the solar spectrum, and the absorption spectral dependence is nearly proportional to the inverse of the wavelength. However, accurate spectral aerosol absorption measurements are still scarce. Current measurements of aerosol absorption from remote sensing data are limited. The AErosol RObotic NETwork (AERONET) can retrieve the aerosol single scattering albedo, ω_o = β_scatt / β_ext where β_ext and β_scatt are the extinction and scattering coefficients respectively, from the combination of sky and sun radiation measurements. However, high quality values of ω_o are reported only for aerosol optical depths larger than 0.4 (at 550nm) and for homogeneous sky conditions. The Ozone Monitoring Instrument (OMI) on the Aura satellite derives aerosol single scattering albedo only at UV wavelengths using a guess of the aerosol height. In situ measurements from field experiments and campaigns are limited in time and spatial coverage. There is a large need for global systematic derivation of aerosol absorption and direct aerosol radiative effect with satellite data.

I propose to systematically study the aerosol absorption properties, single scattering albedo, and their direct radiative forcing impact over selected regions with the “critical reflectance” technique, using MODIS and POLDER data. I will add surface BRDF (Bidirectional Reflectance Distribution Function) effect in our radiative transfer calculations by using POLDER BRDF data and the Ross-Li-HS BRDF model instead of the simpler assumption of a Lambertian surface albedo in the SBDART code. The sensitivity of the absorption retrievals and the aerosol forcing with the aerosol model (phase function, size distribution, refractive index) will be studied. The validation of the retrieved single scattering albedo will be performed against AERONET and other available in situ data. The aerosol direct radiative forcing connected with the AOD and single scattering albedo will also be studied to better understand the warming or cooling effect from different aerosols.