Air Pollution as a Climate Forcing: A Workshop
Day 2 Presentations
AERONET: Ground-based Aerosol Characterization
Brent Holben+, Oleg Dubovik+, Tom Eck+, Alexander Smirnov+,
Anne Vermeulen+, I. Slutsker+, Philippe Goloub*
+ NASA Goddard Space Flight Center, Greenbelt, MD, U.S.A.
* Lab. d'Optique Atmos., CNRS, Univ. de Sciences et Techniques de Lille, France
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AERONET is a collaborative internationally federated network of globally distributed automatic sun-sky radiometers located in ~125 long-term key locations that are representative of diverse aerosol conditions (see NASA's MODIS "Blue Marble" map above showing AERONET sites in red). Both direct sun and diffuse sky directional radiance measurements taken throughout the daylight hours are utilized to characterize total column integrated aerosol optical properties. The spectral aerosol optical depths (340 to 1020 nm in 7 wavelengths) and radiatively effective column integrated size distributions, complex refractive indices, and single scattering albedo are retrieved from these measurements. The AERONET spectral aerosol optical depth (AOD, τaλ) measurements are highly accurate (~0.01 uncertainty) and are used to validate satellite retrievals of AOD from all the current satellite sensor systems, including MODIS, TOMS, SeaWiFS, MISR, and AVHRR. These data have also provided validation for global radiation and transport models. Additionally the aerosol size distributions and single scattering albedo (ω0λ) retrievals from AERONET have also been and continue to be utilized in algorithm development for some of these sensors, such as MODIS and TOMS.
Multi-year averages of aerosol optical properties for selected biomass burning aerosol sites are shown in the figures above as an example of the aerosol characterization that AERONET monitoring provides. More than 80% of global biomass burning occurs in the tropics and AERONET has long monitored sites in Brazil and Zambia that are located in major biomass burning regions. All sites shown in the figure exhibit peak biomass burning AOD for approximately 2-3 months, and levels above background for ~4-6 months. The background AOD levels in non-burning months are below 0.10 at most of these sites, very similar to the background AOD levels over many oceanic regions. The magnitude of aerosol absorption (single scattering albedo) of the smoke at these sites shows a wide range of regional variability, mainly due to the amount of black carbon (BC) in the smoke. The greatest absorption (lowest SSA) occurs at the southern African savanna site as a result of the high percentage of combustion that occurs in the flaming versus the smoldering phase (much grass burning) that results in greater BC production. The lowest absorption was measured in the two forest regions and was very similar even though these forest types are very different from each other (Amazonian forest versus Canadian boreal forest). It is likely that the smoldering phase of combustion is
more dominant in the production of smoke from the burning of woody fuels at these sites. The SSA is intermediate at the South American cerrado (savanna-like) site since the smoke at this site is a combination produced from both local burning (more flaming phase) and long-range transport from forest burning regions to the north. The aerosol size distributions from all of the biomass burning sites are dominated by accumulation mode particles, however the sub-micron particle size is somewhat smaller in the savanna and cerrado sites due perhaps as a result of the phase of combustion, but also possibly due to differences in aerosol aging, fire intensity, and ambient relative humidity and temperature.
AERONET's global distribution yields similar characterizations, aerosol climatologies and aerosol models for marine, urban/industrial, aeolian dust, biogenic, arctic and mixtures of these aerosol types. The oral presentation will focus on monthly climatology of aerosol optical depth (τaλ), single scattering albedo (ω0λ) and absorption optical depth [τaλ*(1-ω0.λ)] for regionally representative sites to illustrate the applicability to radiative forcing, public health and long term environmental impacts.
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