GACP Projects
The Direct Radiative Forcing of Biomass Burning Aerosols: Investigations During SCAR-B and ZIBBEE
Sundar A. Christopher, PI
Brent N. Holben, Collaborator
Thomas Eck, Collaborator
Abstract: Biomass burning aerosols have a large spatial distribution that is governed by the geographic distribution of their source regions and tropospheric circulation. Annually, more than 114 teragrams of smoke aerosols are produced in the tropics. Depending upon their size and chemical composition, these aerosols have atmospheric residence times ranging from days to weeks. They reflect the incoming solar directly which is called the "direct effect" and they also modify the shortwave reflective properties of clouds, which is called "indirect effect". The global top of atmosphere (TOA) direct radiative forcing (DRF) of aerosols range between -0.2 to -1.1 W/m^2. The uncertainties in this estimate are due to the various assumptions involved in the calculations. While the TOA radiative forcing estimates provide valuable information on the effects of aerosols on climate, equally important are the downward shortwave irradiances (DSWI) at the surface and the associated atmospheric impacts. The DSWI is an important component of the surface radiation balance and the atmospheric heating rates due to absorption by aerosols govern both local and regional circulation patterns. A recent study (Konzelmann et al. 1997) has shown large differences on the order of 40-80 W/m^2 between measured and calculated DSWI values in the presence of biomass burning aerosols. These differences are attributed to the inaccurate characterization of aerosols in radiative transfer models. Most satellite detection algorithms classify aerosols as clouds, which could lead to errors in radiative forcing calculations. Very few studies have attempted to estimate the surface and atmospheric effects of biomass burning aerosols largely due to the lack of information on the microphysical properties and chemical composition.
In 1995 a major field program called the Smoke Clouds and Radiation-Brazil (SCAR-B) was conducted in Central Brazil to understand the effects of biomass burning aerosols on radiation and climate. In 1997 another field experiment was conducted over Zambia called the Zambian International Biomass Burning Experiment (ZIBBEE) that also studied the effects of biomass burning on climate. A wide range of insitu and surface measurements are available from both of these field projects that will help reduce the uncertainties in the DRF estimates of biomass burning aerosols. The major focus of this proposed effort is to estimate the direct TOA, surface and atmospheric effects of biomass burning aerosols during SCAR-B and ZIBBEE in order to reduce the uncertainties in the aerosol radiative forcing values.
Using near-coincident coupled measurements from satellites, aircraft, and ground-based measurements from SCAR-B and ZIBBEE, a radiative transfer model will be used to estimate the DRF of biomass burning aerosols. Using satellite measurements from the Clouds and the Earth's Radiant Energy System (CERES) instrument from the Tropical Rainfall Measuring Mission (TRMM) platform, this proposed effort will also test and validate the algorithms during the upcoming Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) experiment over South America. The algorithm and products developed through this research proposal will be directly applicable to the CERES Surface Radiation Budget (SARB) efforts.
This proposed effort also brings together an interdisciplinary team that provides expertise in different areas. 1) Sundar A. Christopher (satellite retrievals and flux calculations), 2) Brent Holben (Sunphotometer measurements and DRF issues), 3) Thomas Eck (pyranometer measurements and radiative transfer calculations). Both Brent Holben and Thomas Eck have agreed to collaborate in this research effort.
