2012 Progress Report: Black Carbon, Air Quality and Climate: From the Local to the Global ScaleEPA Grant Number: R835035
Title: Black Carbon, Air Quality and Climate: From the Local to the Global Scale
Investigators: Pandis, Spyros N. , Adams, Peter , Donahue, Neil , Robinson, Allen
Institution: Carnegie Mellon University
EPA Project Officer: Ilacqua, Vito
Project Period: September 1, 2011 through August 31, 2014
Project Period Covered by this Report: September 1, 2011 through August 31,2012
Project Amount: $900,000
RFA: Black Carbon's Role In Global To Local Scale Climate And Air Quality (2010) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Climate Change , Air
Reduction of black carbon (BC) emissions represents a potential win-win strategy in our effort to improve air quality while limiting climate change. However, the magnitude of the benefits remains quite uncertain because of our limited understanding of the contributions of the various source sectors to the BC mass and number concentrations, the atmospheric processing of black carbon particles including their physical and chemical changes, the role of other absorbing organics (brown carbon), the contributions of the various source sectors (and long range transport) to the direct and indirect effects of BC on climate, and the effect of BC on local and regional meteorology. Control strategies resulting in changes to BC emissions will often result in changes to emissions of various co-pollutants (primary and secondary organic aerosol, sulfur, particle number concentration) and may have significant effects on the aerosol and cloud droplet number concentrations. Reduction of the above uncertainties and quantification of the effects of the various BC control strategies on both air quality and climate change in the US are the main objectives of the proposed study.
We have developed new size- and composition-resolved number emission inventories for the United States with emphasis on black carbon sources. Gasoline, on-road and off-road diesel emissions are the most important sources of ultrafine particles for the Eastern United States during the summer representing together more than 70% of the corresponding number emissions. These inventories are used by the Chemical Transport PMCAMx-UF to simulate the ambient ultrafine particle levels in the atmosphere. The predictions of the model for aerosol number concentrations and size distributions have been compared against measurements from the EPA PM Supersites and the agreement has been quite encouraging.
A new technique has been developed for the source apportionment of the number concentration of particles of different sizes. The technique is based on simulations where the particles emitted by each source are removed and the results are compared to those of the base case simulation where all the sources are contributing. The novel aspect of the approach is that only particles smaller than approximately 150 nm are removed thus preserving practically all the mass concentration contributed by each source. This minimizes the nonlinear interactions in the system (due to nucleation and coagulation) and allows the estimation of the contribution of each source with an accuracy of 10%.
Using this technique we estimated that, for the summer, nucleation is the most important source of particle number in the Northeast United States contributing more than 50% of the particles. It is an important but not a dominant source in the South United States. Transportation and off-road diesel engines are the dominant sources of primary particles during the summer.
We have measured the absorptivity of organic aerosol (OA) in biomass burning emissions, both fresh and photo-chemically aged in an environmental chamber. We considered household wood burning and fuels commonly consumed in wild and prescribed fires in the Unites States. Using a core (BC) – shell (organic) Mie-theory model, we determined the imaginary part of the refractive index of the shell that best fits light absorption measured using a 7-wavelenght aethalometer. Both primary organic aerosol (POA) in the fresh emissions and secondary organic aerosol (SOA) produced by photo-chemical aging absorb light, and are categorized as brown carbon with imaginary part of the refractive indices comparable to BC at short visible wavelengths. Neglecting the absorptivity of POA and/or SOA may thus result in underestimation of the contribution of biomass-burning aerosols to climate forcing by approximately 0.1 W m-2.
During the second year of the project we will:
- Improve and evaluate the existing mass inventories using a consistent definition of BC.
- Improve the ability of the existing chemical transport and climate models to simulate the BC mass and number concentrations and their effects on climate.
- Quantify the contributions of the different BC source sectors (including long-range transport) to BC mass and number concentrations.