Determining the impact of soot on climate

点击翻译按钮获取中文摘要

Climate change is an important issue for Canadians as it has the potential to drastically affect their quality of life. It is generally accepted that global warming can mostly be attributed to changes in the radiative forcing (changes to the heat balance between incoming solar radiation and Earth’s outgoing thermal radiation) due to greenhouse gases and aerosols. Aerosols directly affect global climate by scattering and absorbing solar radiation, while they indirectly affect climate by changing the properties of clouds (i.e. their reflectivity and lifetime). Light-absorbing particles like black carbon or ‘soot’ have a warming effect and increase the amount of energy retained in the atmosphere. Recent estimates suggest that the atmospheric forcing due to soot is the second most important component of global warming after CO2 - even greater than that of methane. Importantly, the relative uncertainty in the radiative forcing estimate for soot is much higher than that of CO2 or methane. It is important that the role of soot in climate change is understood so that the uncertainties in the forcing estimates can be reduced. This information is essential for governments as they develop appropriate climate policies that may have a large impact on economic development. A large portion of the uncertainty in the soot forcing estimates is due to the evolution of soot particles in the atmosphere. It has been observed that after a few hours in the atmosphere, gas-phase material can condense on soot particles causing an internal mixture (i.e. a particle composed of a mixture of soot and condensed material). It has been observed that this internal mixture can take two physical forms: i) the condensed material is uniformly distributed, forming a ‘coating’ on the soot particle, or ii) the soot particle is located on the edge of a droplet of the liquid material, which is sometimes referred to as an ‘associated’ particle. If the material is coating the soot, then the condensed material may cause the collapse of the soot’s fractal structure due to the surface tension of the coating. This can have a profound effect on the particle’s ability to absorb solar radiation due to two effects: i) the collapse of the particle will result in a decrease in its cross-section and the particle absorption will decrease but ii) the optically-transparent coating on the particle will cause an increase in the absorption because additional light is refracted to the absorbing core of the particle. Thus, these two effects may compete against each other in terms of the total particle absorption as the particle evolves. However, if the soot particle is located at the edge of the condensed material (an associated particle), then the absorption would not be as efficient compared to a coated soot particle. Also, it is unclear if this type of internal mixing would result in a collapse of the soot particle. The goal of the applicant’s research program is to understand the mechanisms effecting soot absorption and to reconcile the apparent differences between laboratory and field studies. This will be done by trying to replicate atmospheric coating processes in the lab by using a soot ageing chamber and producing condensable material through the photo-oxidation of organic precursors, and also by participating in field studies to compare laboratory and field results. Also, a novel instrument (the Aerodynamic Aerosol Classifier) will be further developed and used in these experiments to improve the accuracy of these experiments.