Grantee Research Project Results
Final Report: Black Carbon, Air Quality and Climate: From the Local to the Global Scale
EPA Grant Number: R835035Title: Black Carbon, Air Quality and Climate: From the Local to the Global Scale
Investigators: Pandis, Spyros N. , Robinson, Allen , Donahue, Neil , Adams, Peter
Institution: Carnegie Mellon University
EPA Project Officer: Chung, Serena
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
Objective:
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:
- 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.
- Improve our understanding of the atmospheric processing of BC particles.
- 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.
- Quantify the contributions of the different BC source sectors (including long-range transport) to BC mass and number concentrations.
- Quantify the contributions of the same source sectors to the direct, indirect and semi-direct effects of BC on climate.
- Elucidate the role of BC in local and regional meteorology, including temperature and the hydrological cycle.
- 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.
- 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.
Fig. 7. Predicted fractional source contributions to primary ultrafine particle number concentrations (cm-3) at the ground level for July 2001.
The source contributions to primary particle number concentrations are on average similar to those of their source emissions contributions: gasoline is predicted to contribute 36% of the total particle number concentrations, followed by industrial sources (31%), non-road diesel (18%), on-road diesel (10%), biomass burning (1%), and long-range transport (4%) (Fig. 7). For this summertime period in Pittsburgh, number source apportionment predictions for particles larger than 3 nm in diameter (traffic 65%, other combustion sources 35%) are consistent with measurement-based source apportionment (traffic 60%, combustion sources 40%).
We have developed a novel approach for using long-term size distribution observations to evaluate an aerosol model’s ability to predict formation rates of CCN from nucleation and growth events (Westervelt et al., 2013). We derived from observations at several locations nucleation-relevant metrics such as nucleation rate of particles at diameter of 3 nm, diameter growth rate, particle survival probability, condensation and coagulation sinks, and CCN formation rate. These quantities were also derived for a global microphysical model, GEOS-Chem-TOMAS, and were compared to the observations on a daily basis. Using GEOS-Chem-TOMAS, we simulated nucleation events predicted by ternary or activation nucleation over 1 year and found that the model slightly understated the observed annual average CCN formation mostly due to bias in the nucleation rate predictions, but by no more 4 than 50% in the ternary simulations. Model-predicted annual-average growth rates were within 25% across all sites but also show a slight tendency to underestimate the observations, at least in the ternary nucleation simulations. These results add support to the use of global models as tools assessing the contributions of primary sources and microphysical processes such as nucleation to the total number and CCN budget.
1.5 Changes in BC Emissions and Climate
We have investigated the effect of climate change on BC concentrations in the Eastern Unites States assuming constant emissions but a changing climate (Day and Pandis, 2015). We used the coupled global-regional atmospheric chemistry and climate modeling system GRE-CAPS to compare current (2000s) BC concentration fields with those predicted for the 2050s. The effect is predicted to be quite variable in space with decreases in the northeast and increases in the south (Fig. 8). GRE-CAPS predicts that precipitation changes will dominate the overall change with changes in wind speed and mixing heights also contributing to this complex picture. While the average changes will be modest, the local changes can be quite significant.
Fig. 8. Predicted changes in average summertime BC concentrations due to climate change (2050s compared to 2000s). A positive number corresponds to an increase.
We have tested the effects of a number of control strategies of PM emitted by diesel sources. The results of one of these tests in which all particle emissions by on- and off-road diesel sources were reduced by 50% are shown in Fig. 9. The EC concentrations were reduced by approximately 40% over most of the continental Eastern Unites States as expected from the significant contributions of these sources to the Elemental Carbon (EC) levels during summertime. The response of particle number concentrations was more complex. The concentrations of the smallest particles actually increased in a lot of areas (Fig. 9b) by as much as 20%. This was due to increased nucleation rates due to the reduction of the aerosol surface area. For particles in the 10-50 nm range, the response was mixed: decreases as much as 10% in urban areas and increases in other areas. For larger particles, the PMCAMx-UF CTM predicted decreases over the entire domain. The predicted decreases for particles larger than 100 nm are shown in Fig. 9d. However, these increases are 50-100% than the expected reductions based on the contributions of the diesel sources in this size range. These additional reductions in this critical range (these particles are CCN) are due to the increase of the concentrations of the smaller particles, which share the same amount of condensing vapors and grow less. Therefore fewer particles grow into the CCN range.
Fig. 9. Predicted fractional changes in particle concentrations during a summertime period due to a 50% reduction of particulate emissions from diesel sources: (a) EC mass concentration; (b) number concentration of particles with diameters between 3 and 10 nm; (c) number concentration of particles in the 10-50 nm range; and (d) number concentration of particles larger than 100 nm.
Similar results were observed in other tests. Reductions of the diesel particulate emissions lead to increases of the total particle number and the smallest ultrafine particles, but also super-linear increases of the CCN.
Details about the project activities, technical aspects of the project, results and conclusions can be found in the project publications. A list of these publications can be found in the next section.
Conclusions:
This work presents the first direct evidence that SOA produced in aged biomass-burning emissions is absorptive.
These results add support to the use of global models as tools assessing the contributions of primary sources and microphysical processes such as nucleation to the total number and CCN budget.
Reductions of the diesel particulate emissions lead to increases of the total particle number and the smallest ultrafine particles, but also super-linear increases of the CCN.
References:
Tasoglou A., Saleh R., Subramanian R., and S. N. Pandis (2015) Investigation of the chemical aging and absorption of carbonaceous aerosol from wood burning, in preparation.
Fountoukis C. and S. N. Pandis (2015) A computationally efficient module for the explicit simulation of the black carbon mixing state in atmospheric particles, in preparation.
Journal Articles on this Report : 11 Displayed | Download in RIS Format
Other project views: | All 25 publications | 11 publications in selected types | All 11 journal articles |
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Type | Citation | ||
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Day MC, Zhang M, Pandis SN. Evaluation of the ability of the EC tracer method to estimate secondary organic aerosol carbon. Atmospheric Environment 2015;112:317-325. |
R835035 (Final) R835405 (2014) R835405 (Final) |
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Day MC, Pandis SN. Effects of a changing climate on summertime fine particulate matter levels in the eastern U.S. Journal of Geophysical Research: Atmospheres 2015;120(11):5706-5720. |
R835035 (2013) R835035 (Final) R833374 (Final) |
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Gkatzelis GI, Papanastasiou DK, Florou K, Kaltsonoudis C, Louvaris E, Pandis SN. Measurement of nonvolatile particle number size distribution. Atmospheric Measurement Techniques 2016;9(1):103-114. |
R835035 (Final) R835405 (Final) |
Exit Exit |
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Posner LN, Pandis SN. Sources of ultrafine particles in the Eastern United States. Atmospheric Environment 2015;111:103-112. |
R835035 (2013) R835035 (Final) R833374 (Final) R835405 (2014) R835405 (Final) |
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Saleh R, Hennigan CJ, McMeeking GR, Chuang WK, Robinson ES, Coe H, Donahue NM, Robinson AL. Absorptivity of brown carbon in fresh and photo-chemically aged biomass-burning emissions. Atmospheric Chemistry and Physics 2013;13(15):7683-7693. |
R835035 (2013) R835035 (Final) R833747 (Final) |
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Saleh R, Robinson ES, Tkacik DS, Ahern AT, Liu S, Aiken AC, Sullivan RC, Presto AA, Dubey MK, Yokelson RJ, Donahue NM, Robinson AL. Brownness of organics in aerosols from biomass burning linked to their black carbon content. Nature Geoscience 2014;7(9):647-650. |
R835035 (Final) |
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Saleh R, Marks M, Heo J, Adams PJ, Donahue NM, Robinson AL. Contribution of brown carbon and lensing to the direct radiative effect of carbonaceous aerosols from biomass and biofuel burning emissions. Journal of Geophysical Research-Atmospheres 2015;120(19):10285-10296. |
R835035 (Final) |
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Shamjad PM, Tripathi SN, Pathak R, Hallquist M, Arola A, Bergin MH. Contribution of brown carbon to direct radiative forcing over the Indo-Gangetic Plain. Environmental Science & Technology 2015;49(17):10474-10481. |
R835035 (Final) R835039 (2015) R835039 (Final) |
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Westervelt DM, Pierce JR, Riipinen I, Trivitayanurak W, Hamed A, Kulmala M, Laaksonen A, Decesari S, Adams PJ. Formation and growth of nucleated particles into cloud condensation nuclei: model-measurement comparison. Atmospheric Chemistry and Physics 2013;13(15):7645-7663. |
R835035 (2013) R835035 (Final) R833374 (Final) |
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Westervelt DM, Pierce JR, Adams PJ. Analysis of feedbacks between nucleation rate, survival probability and cloud condensation nuclei formation. Atmospheric Chemistry and Physics 2014;14(11):5577-5597. |
R835035 (Final) R833374 (Final) |
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Tasoglou A, Saliba G, Subramanian R, Pandis SN. Absorption of chemically aged biomass burning carbonaceous aerosol. Journal of Aerosol Science 2017;113:141-152. |
R835035 (Final) R835438 (Final) |
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Supplemental Keywords:
Air quality modeling, smog, PM, general circulation modelProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.