Final Report: Black Carbon, Air Quality and Climate: From the Local to the Global Scale

EPA 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 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 United States are the main objectives of the proposed study. More specifically focusing on the United States, we will: 

  1. Develop size- and composition-resolved number emission inventories for BC-containing sources for the Unites States and also improve the existing mass inventories using a consistent definition of BC.
  2. Improve our understanding of the atmospheric processing of BC particles.
  3. Improve the ability of the existing regional and global chemical transport and climate models to simulate the BC mass and number concentrations and their effects on climate.
  4. Quantify the contributions of the different BC source sectors (including long-range transport) to BC mass and number concentrations.
  5. Quantify the contributions of the same source sectors to the direct, indirect and semi-direct effects of BC on climate.
  6. Elucidate the role of BC in local and regional meteorology, including temperature and the hydrological cycle.
  7. Quantify the effectiveness of various U.S. and global strategies of reducing BC on BC mass and particle number concentrations, direct, indirect and semi-direct radiative forcing and climate change.
  8. Identify and quantify major uncertainties in emissions, atmospheric processing, and climate impacts of BC mitigation.

Summary/Accomplishments (Outputs/Outcomes):

1.1 Laboratory Studies of Aging of Primary Emissions

Atmospheric particulate matter plays an important role in the Earth’s radiative balance. Over the past two decades, it has been established that a portion of particulate matter, black carbon, absorbs significant amounts of light and exerts a warming effect rivaling that of anthropogenic carbon dioxide. Most climate models treat black carbon as the sole light-absorbing carbonaceous particulate. However, some organic aerosols, dubbed brown carbon and mainly associated with biomass burning emissions, also absorbs light. Unlike black carbon, whose light absorption properties are well understood, brown carbon comprises a wide range of poorly characterized compounds that exhibit highly variable absorptivities, with reported values spanning two orders of magnitude. We performed smog chamber experiments to characterize the effective absorptivity of organic aerosol from biomass burning under a range of conditions (Saleh et al., 2014). We showed that brown carbon in emissions from biomass burning is associated mostly with organic compounds of extremely low volatility. In addition, we found that the effective absorptivity of organic aerosol in biomass burning emissions can be parameterized as a function of the ratio of black carbon to organic aerosol (Fig. 1) indicating that aerosol absorptivity depends largely on burn conditions, not fuel type. Brown carbon from biomass burning can be an important factor in aerosol radiative forcing.

Fig. 1. Dependence of the imaginary component (absorption) of the refractive index of OA at 550 nm on BC-to-OA ratio. Filled diamonds and open squares correspond to fresh and chemically aged emissions, respectively. Colours correspond to different fuels: black, black spruce; magenta, ponderosa pine; cyan, rice straw; forest green, organic hay; light green, saw grass; and blue, wire grass.

The photochemical aging of smoke emitted from the burning of biofuels commonly used for residential heating (oak) or consumed in wild-land and prescribed fires in the United States (pocosin pine and gallberry) was investigated in a smog chamber (Saleh et al., 2013). These experiments focused among others on the light absorption of organic aerosol (OA) in photochemically aged biomass-burning emissions. We constrained the effective light-absorption properties of the OA using conservative limiting assumptions, and found that both primary organic aerosol (POA) in the fresh emissions and secondary organic aerosol (SOA) produced by photo-chemical aging contain brown carbon, and absorb light to a significant extent. This work presents the first direct evidence that SOA produced in aged biomass-burning emissions is absorptive (Fig. 2). For the investigated fuels, SOA is less absorptive than POA in the long visible, but exhibits stronger wavelength-dependence and is more absorptive in the short visible and near-UV.

Fig. 2. Measurement-constrained absorption coefficients (black diamonds), and model calculations using different assumptions on OA absorptivity (solid lines) for (a) fresh oak emissions assuming external mixing (limiting case 1), (b) aged oak emissions assuming external-mixing (limiting case 1), fresh oak emissions assuming core-shell morphology (limiting case 2), and (d) aged oak emissions assuming core-shell morphology (limiting case 2).

The effect of anthropogenic secondary organic aerosol (SOA) coatings on BC particles was investigated in a third set of smog chamber experiments (Tasoglou et al., 2015). Soot particles consisting mainly of BC (>90% by mass) were produced by burning of white birch bark. These particles were then coated with toluene SOA, leading to larger particles that consisted of roughly half BC and half anthropogenic SOA. The absorption of the particles increased by a factor of 2 (Fig. 3). This increase was dominated by the lensing effect of the SOA coating. The O:C atomic ratio changed from 0.1 to 0.6 in these experiments, but it did not appear to have an important effect on the absorption of the particles.

Fig. 3. Measured absorption enhancement of BC particles as a function of time in a typical smog chamber experiment. The chamber was dark during shaded periods while the UV lights were on during the rest of the time. Nitrous acid (HONO) was injected three times and was photolyzed producing OH radicals that led to the chemical aging of the OA. Toluene was also injected and the SOA produced after 14:15 coated the BC particles leading to significant enhancement of their absorption.

1.2 Emission Inventories for Black Carbon Number

We have developed new size- and composition-resolved number emission inventories for the United States with emphasis on BC sources (Posner and Pandis, 2015). Gasoline, on-road and off-road diesel emissions are the most important sources for the Eastern United States during the summer representing together more than 70% of the corresponding number emissions (Fig. 4). The contribution of each source is different for each size range examined (Table 1). For example gasoline vehicles are estimated to emit 66% of the particles in the 3-10 nm range but only 20% of the particles larger than 100 nm. These inventories have been discretized into the 42 size bins used by the Chemical Transport PMCAMx-UF. 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 (Fig. 5).

Fig. 4. Average total particle number emission rate (hr-1) for major source types for July. Please note that the scale for the dust emissions is different.



Fig. 5. Comparison of predicted N3 (cm-3) (particles larger than 3 nm) by PMCAMx-UF in Pittsburgh using the new emissions against the measured concentrations. The high concentration periods are due to nucleation events.

1.3 Model Development

Most regional chemical transport models assume that aerosols are internally mixed with a size-dependent composition. As a result, all BC containing particles in a given size bin (or mode) have the same composition irrespectively of their age. While this approach is a reasonable approximation for the description of BC mass concentrations in polluted urban areas, it introduces biases in the calculation of the radiative effects of BC and its interactions with climate. We have developed a new aerosol module that it can simulate the mixing state of BC by introducing several size/composition distributions in the Chemical Transport Model (CTM) corresponding to different ages and mixing states of BC (Fountoukis et al., 2015). The model framework is based on a two dimensional representation of the aerosol size composition distribution (Fig. 6): one axis is the diameter of the particles and the second axis the diameter of their BC core. Ten size bins are currently used for the former and eight for the latter. Coating thickness is explicitly tracked, permitting online a calculation of coating enhancements to BC absorptivity. 

Fig. 6. Schematic of the two-dimensional representation of the new aerosol module that describes explicitly the BC mixing state. The aerosol composition is described as a function of the aerosol size and the BC core size. With black we depict the BC and with blue the rest of the aerosol components.

A mixing state parameter F(Dp) has been developed to describe the mixing state of the particles as a function of their diameter Dp given the two dimensional distribution shown in Fig. 6. The parameter has been defined so that F=0 corresponds to externally mixed particles and F=1 to internally mixed particles of size Dp. Testing of the new module suggests that over urban areas F increases as particle size increases and that the F values increase away from major sources of particles. Addition of the new module to regional CTMs is expected to improve their ability to estimate the BC optical properties and atmospheric lifetime.

1.4 Black Carbon Emissions, Condensation Nuclei (CN) and Cloud Condensation Nuclei (CCN) Concentrations

A new technique has been developed for the source apportionment of the number concentration of particles of different sizes (Posner and Pandis, 2015). 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 Eastern Unites States. The importance of primary particles and hence black carbon emissions increases as the particle size increases. For particles larger than 100 nm, the primary particles are responsible for around 80% of the particle number.