Assessing Dust And Biomass Aerosol Radiative Effects At Ilorin, Nigeria, From Instruments And Models

Ilorin, Nigeria is strongly affected by annual intrusions of desert dust and biomass burning aerosols. An overlap in the time periods covered by surface radiometer measurements and the NASA/GEWEX Surface Radiation Budget (SRB) projects from 1992-1994 allows a comparison of point measured surface shortwave fluxes with those from a global gridded product. An underestimation by SRB of the effects of the dust aerosol during the dry season is noted and explained. Optical depth calculations from the Fu-Liou model using assumptions of aerosol composition and size distributions, along with the 3-minute cloud screened surface radiometer measurements indicate optical depths approaching 2 during periods of heavy dust intrusion. Correlations of implied optical depth with column water vapor from the GEOS-1 and ERA-40 meteorological data sets are given to provide an estimate of the effects of hygroscopic growth. These optical properties will be compared against constituents projected as being over the site by GOCART (Georgia Tech/Goddard Ozone Chemistry Aerosol Radiation and Transport) and MATCH (Model of Atmospheric Transport and Chemistry) aerosol assimilations. Calculations from the NASA Langley Trajectory Model (LTM) give source regions for air masses over Ilorin. Periods when the Sahara and known biomass burning areas are the sources correlate well with periods of strong surface radiative forcing from the radiometer measurements. Comparisons with other aerosol and radiation data sets including the TOMS Aerosol Index, AERONET, and the NASA/GISS Surface And Atmosphere Radiative Fluxes FD data product are given.