Thumbs Up or Thumbs Down for Soot Mitigation?
Attention has recently been drawn to black carbon aerosols, or soot particles, as a target for short-term mitigation of climate warming. This measure seems attractive because soot is assumed to warm the atmosphere and at the same time has a lifetime of just a few days. Therefore regulating soot emissions could, as a short-term action, potentially buy time by slowing global warming until regulations for longer lived greenhouse gases are set in place.
But would the regulation of soot sources increase or decrease overall the absorption of solar energy in the atmosphere, and consequently warm or cool our planet?
Black carbon particles are emitted into the atmosphere during fossil fuel combustion, bio-fuel combustion, and biomass burning. Fossil fuel soot is mainly emitted during diesel and coal combustion, whereas bio-fuel soot is emitted primarily during burning of wood and organic waste for home heating and cooking. All of these sources emit a suite of chemical species. Hence, we can't think about soot as an isolated species.
Updates to the GISS climate model have made it capable of simulating the direct, radiative effect of aerosols on climate and also take into account the chemical composition of soot particles and their mixing state with other particles. For example, freshly emitted diesel soot particles are black and strong absorb sunlight. In contrast, bio-fuel soot generally appears brown because it contains a higher ratio of organic carbon to black carbon than does fresh diesel soot.
The model also simulates the indirect effect of aerosols, the effect that they have on cloud microphysical properties and lifetime. Semi-direct effects, which occur as aerosols heat or cool the surrounding air and influence the presence of clouds, are included as well.
The effect of aerosols on clouds is very important as "clean" clouds cool the atmosphere by 13 W/m2, approximately 13 times more effectively than aerosols themselves. Small variations in cloud cover and lifetime induced by the presence of aerosols can change the radiative balance of the atmosphere significantly. At the same time these processes are extremely complicated and simulations of such effects are rather uncertain.
In a recent study, we tested the effect of reducing fossil fuel emissions, focusing on the removal of on-road and off-road diesel emissions of black carbon (BC) and organic carbon (OC). We also explored the effect of another scenario in which of all bio-fuel emissions were reduced by 50%.
Particles emitted from diesel engines have the highest BC fraction of any major source, and the OC to BC ratio of 2:1 is common to all engines. Thus, this diesel experiment reflects the likely impact of any control strategy that reduces an emission source that contains a larger fraction of BC than OC. Bio-fuel sources have different OC:BC ratios, ranging from 3:1 or 4:1 for wood cooking stoves to 6:1 for fireplaces or cooking with animal waste. The average OC:BC reduction of 5.6:1 in this experiment provides an estimate of the response to reductions in high-OC sources.
Both soot emission reduction scenarios lead to less warming via the aerosol direct effect. The global mean direct effect is reduced by -0.05 W/m2, although the regional distributions differ.
However, the aerosol indirect aerosol effects are opposite in sign between these two experiments. In the diesel case, which involves mainly a reduction in BC, we find a global increase in cloud droplet number concentration (CDNC), the fraction of the aerosol concentration that grow into cloud droplets and influence cloud processes. However, areas with high BC loads like India and South East Asia show a decrease in CDNC.
In the bio-fuel case, which involves a reduction in BC combined with an even larger reduction in OC, we observed a decrease in cloud droplet number concentration.
Those opposite changes in cloud droplet number concentration lead to positive cloud forcing numbers in the bio-fuel reduction case and negative forcing numbers in the diesel case.
Thus, our study suggests that black carbon mitigation generally seems to be beneficial when sources with a large proportion of black carbon, such as diesel, are reduced. But reducing sources with a larger organic carbon component as well, such as bio-fuels, does not necessarily lead to climate benefits.
Black carbon is a toxic air pollutant, so there is no doubt that emission control measures should be applied as soon as possible. However, we have to be aware of the role and climate feedbacks of the co-emitted species when discussing certain emission source reduction scenarios.
Bauer, S.E., S. Menon, D. Koch, T. C. Bond, and K. Tsigaridis, 2010: A global modeling study on carbonaceous aerosol microphysical characteristics and radiative effects. Atmos. Chem. Phys., 10, 7439-7456, doi:10.5194/acp-10-7439-2010.
Bauer, S.E., D. Wright, D. Koch, E.R. Lewis, R. McGraw, L.-S. Chang, S.E. Schwartz, and R. Ruedy, 2008: MATRIX (Multiconfiguration Aerosol TRacker of mIXing state): An aerosol microphysical module for global atmospheric models. Atmos. Chem. Phys., 8, 6603-6035, doi:10.5194/acp-8-6003-2008.
Please address all inquiries about this research to Dr. Susanne Bauer.