Proposed Short-Lived Climate Pollutant Reduction Strategy

A. Black Carbon
Airborne particulate matter (PM) varies in its composition and plays a significant role in
human health and the climate system. Particulate matter is emitted from a variety of
natural processes and human activities, and tends to remain in the air for only a few
days to about a week, resulting in extreme spatial and temporal variability. Among
different types of particles, carbonaceous particles (those that contain organic and black
carbon) are particularly important because of their abundance in the atmosphere. With
respect to climate impact, black carbon is the principal absorber of visible solar radiation
in the atmosphere while organic carbon is often described as a light-reflecting
compound.
Black carbon is emitted from burning fuels such as coal, diesel, and biomass. Black
carbon contributes to climate change both directly by absorbing sunlight and indirectly
by depositing on snow and by interacting with clouds and affecting cloud formation. In
addition to its climate and health impacts, black carbon disrupts cloud formation,
precipitation patterns, water storage in snowpack and glaciers, and agricultural
productivity.
Scientists have known for some time that sources that emit black carbon also emit other
short-lived particles that may either cool or warm the atmosphere. Lighter colored
particles, for example, tend to reflect rather than absorb solar radiation and so have a
cooling rather than warming impact. Until recently, it had been thought that the impact
of lighter colored and reflecting organic carbon from combustion sources largely offset
the warming impact of black carbon from this source. However, new studies have
suggested that certain fractions of organic carbon known as “brown carbon” could be a
stronger absorber of solar radiation than previously understood. 72,73 The warming effect
of brown carbon may offset the cooling impact of other organic carbon particles; hence,
quantification of that absorption is necessary so that climate models can evaluate the
net climate effect of organic carbon.
To help characterize and differentiate sources of brown carbon from black carbon and
understand their climate impact in California, a current ARB-funded research project is
applying advanced measurement methodology along with regional and global climate
modeling simulations to characterize the extent to which brown carbon contributes to
climate forcing in California. This project will improve our understanding of the
fundamental processes that dominate brown carbon formation, and help to determine
the potential climate benefit of mitigating sources of brown carbon emissions in
California.
72 Jacobson, M. Z. (2014), Effects of biomass burning on climate, accounting for heat and moisture fluxes,
black and brown carbon, and cloud absorption effects, J. Geophys. Res. Atmos., 119, 8980–9002,
doi:10.1002/2014JD021861 http://onlinelibrary.wiley.com/doi/10.1002/2014JD021861/pdf
73 Kodros, J. K., Scott, C. E., Farina, S. C., Lee, Y. H., L'Orange, C., Volckens, J., and Pierce, J. R.:
Uncertainties in global aerosols and climate effects due to biofuel emissions, Atmos. Chem. Phys., 15,
8577-8596, doi:10.5194/acp-15-8577-2015, 2015. http://www.atmos-chem-phys.net/15/8577/2015/acp-
15-8577-2015.pdf
36 April 11, 2016