The same haze that paints beautiful sunsets might also be heating our planet in unexpected ways.
Imagine a scene from August 2000 in southern Africa: vast savannas engulfed in seasonal fires, sending massive plumes of smoke high into the atmosphere. This smoky haze, known as the "African haze," travels thousands of kilometers in a counter-clockwise gyre across the subcontinent. While creating spectacular sunsets, this haze contains a scientific mystery that could reshape our understanding of climate change.
During the dry season biomass burning in southern Africa, scientists noticed something puzzling about how these smoke particles interact with sunlight. Depending on who was measuring and how, the same smoke appeared to have dramatically different properties, particularly in how much sunlight it absorbed. This uncertainty had enormous implications—aerosols that absorb more sunlight can actually warm the atmosphere rather than cool it, potentially reversing the expected climatic effect of pollution.
An international team of over 200 scientists from eighteen countries converged on southern Africa to study the complex relationships between land, atmosphere, and human activity.
Conducted from 1999-2001
Why did the same smoke appear to have dramatically different light-absorbing properties depending on measurement techniques?
This question had significant implications for understanding whether biomass burning aerosols cool or warm our planet.
To understand the significance of the SAFARI 2000 discoveries, we first need to grasp some key concepts about how tiny aerosol particles interact with sunlight:
What makes biomass burning aerosols particularly tricky to study is their complex composition. They contain both scattering components (like organic carbon) that cool the atmosphere by reflecting sunlight back to space, and absorbing components (like black carbon) that warm the atmosphere by absorbing sunlight 5 .
The SAFARI 2000 campaign represented an unprecedented coordinated effort to study these aerosol properties from every conceivable angle. What made this research initiative particularly powerful was its comprehensive approach:
Five research aircraft with different altitude capabilities participated in the campaign, each equipped with specialized instruments :
Flying at approximately 70,000 feet, acting as a simulated satellite with specialized remote sensing equipment.
Conducting detailed atmospheric measurements with in-situ instruments to characterize aerosol properties.
Collecting regional aerosol data across multiple flight paths to map the distribution of smoke plumes.
Providing additional atmospheric sampling with comprehensive instrument payload.
A key objective was validating measurements from NASA's newly launched Terra satellite, part of the Earth Observing System . Research missions were carefully timed for synchronous over-flights of the satellite, allowing scientists to compare airborne measurements with satellite observations.
Throughout the subcontinent, numerous ground validation activities supplemented the airborne measurements, creating a comprehensive three-dimensional picture of aerosol distribution and properties .
Among the many studies conducted during SAFARI 2000, one crucial investigation led by Kuzmanoski and colleagues focused specifically on modeling aerosol optical properties from actual field measurements 2 5 . This case study exemplifies how scientists tackled the central mystery of biomass burning aerosols.
The research team selected two distinct atmospheric layers with different aerosol characteristics—one with high aerosol loading (dense smoke) and another with lower loading and different particle sizes 2 . They then employed a sophisticated multi-step methodology:
Using NASA's Ames Airborne Tracking 14-channel Sunphotometer (AATS-14) aboard research aircraft to measure aerosol layer optical thickness spectra.
Mathematically deriving size distributions of aerosol particles from light measurements.
Determining the refractive index of particles using available chemical composition data.
Calculating three critical optical properties across the wavelength range of 0.3-1.5 μm.
The most innovative aspect of their approach was testing two different models for how absorbing and non-absorbing components mix within individual aerosol particles 2 5 :
Visualize a tiny particle with an absorbing black carbon core surrounded by a shell of non-absorbing organic compounds.
Core-shell structure
Imagine the absorbing and non-absorbing components thoroughly mixed together like a microscopic fruitcake.
Homogeneous mixture
| Property | Definition | Climate Impact |
|---|---|---|
| Single Scattering Albedo | Ratio of scattering to total extinction (scattering + absorption) | Determines whether aerosols cool (high values) or warm (low values) the atmosphere |
| Asymmetry Parameter | Directional preference of scattered light (forward vs. backward) | Affects how much sunlight reaches the surface vs. returned to space |
| Lidar Ratio | Ratio of aerosol extinction to backscatter | Critical for accurate interpretation of satellite lidar measurements |
The findings from this case study revealed both expected patterns and surprising complexities:
The two mixing models produced noticeably different results for the aerosol optical properties, particularly the single scattering albedo 2 5 . This wasn't just an academic exercise—the choice of model directly influenced whether the aerosols appeared to have a net cooling or warming effect on the atmosphere.
When the researchers adjusted the volume fraction of black carbon—the primary absorbing component—they found that even small changes created significant variations in single scattering albedo values, ranging from 0.81 to 0.91 at a wavelength of 0.50 μm 2 5 . This sensitivity explained why different measurement techniques might yield dramatically different results.
Single Scattering Albedo: 0.89
Climate Effect: Cooling Dominant
The most striking finding emerged when the team calculated the radiative effect of these differences in aerosol absorption 2 5 . The variation in single scattering albedo values resulted in significant differences in the instantaneous radiative forcing at both the surface and the top of the atmosphere. Most dramatically, this difference could actually change the sign of the aerosol forcing at the top of the atmosphere from negative to positive—meaning the same smoke layer could switch from cooling to warming the climate system depending on its exact absorption properties 2 5 .
| SSA Value | Surface Forcing | Top-of-Atmosphere Forcing | Net Climate Effect |
|---|---|---|---|
| 0.81 (More absorption) | Strong cooling | Potentially positive | Warming dominant |
| 0.91 (More scattering) | Moderate cooling | Negative | Cooling dominant |
The SAFARI 2000 campaign, including the detailed case study on modeled aerosol properties, left a lasting legacy in atmospheric science. The research demonstrated that the climatic impact of biomass burning aerosols is far more nuanced than previously thought—small differences in composition and mixing state can dramatically alter whether these particles cool or warm our planet 2 5 .
If biomass burning aerosols have a stronger warming effect than previously accounted for, then our climate models may need revision to accurately represent these complex interactions.
As climate change potentially increases the frequency and intensity of wildfires in many regions, we now understand that the smoke from these fires creates a complex feedback loop that could either partially mitigate or additionally amplify warming.
"The SAFARI 2000 initiative exemplified how international scientific collaboration, cutting-edge technology, and multifaceted approaches can unravel even the most complex environmental mysteries. As we face continuing climate challenges, the lessons learned from those smoky African skies continue to inform our understanding of the delicate balance between humanity, fire, and climate."
The groundbreaking findings from SAFARI 2000 relied on an impressive array of specialized equipment deployed across multiple platforms:
| Instrument | Platform | Function | What It Measured |
|---|---|---|---|
| AATS-14 (Airborne Tracking Sunphotometer) | Aircraft | Measure aerosol optical thickness | Sunlight attenuation at 14 wavelengths |
| Passive-Cavity Aerosol Spectrometer Probe (PCASP) | Multiple aircraft | Size distribution of aerosol particles | Particle counts in specific size ranges |
| Nephelometer | Aircraft | Measure light scattering by aerosols | Total and backward scattering coefficients |
| Particle Soot Absorption Photometer (PSAP) | Aircraft | Measure light absorption by aerosols | Absorption coefficient for calculating SSA |
| Cloud Absorption Radiometer (CAR) | CV-580 aircraft | Measure spectral radiance | Radiation fields for closure studies |
| Solar Spectral Flux Radiometer (SSFR) | ER-2 and CV-580 | Measure upwelling and downwelling radiation | Spectral irradiance for radiative forcing |
For those interested in exploring further, the full data from SAFARI 2000 remains available through NASA's Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) 1 .