2014 Progress Report: Understanding the Hygroscopic Properties of Black Carbon/Organic Carbon Mixing States: Connecting Climate and Health Impacts of Anthropogenic Aerosol

EPA Grant Number: R835040
Title: Understanding the Hygroscopic Properties of Black Carbon/Organic Carbon Mixing States: Connecting Climate and Health Impacts of Anthropogenic Aerosol
Investigators: Asa-Awuku, Akua
Institution: University of California - Riverside
EPA Project Officer: Chung, Serena
Project Period: October 1, 2011 through September 30, 2014 (Extended to September 30, 2016)
Project Period Covered by this Report: October 1, 2013 through September 30,2014
Project Amount: $449,925
RFA: Black Carbon's Role In Global To Local Scale Climate And Air Quality (2010) RFA Text |  Recipients Lists
Research Category: Global Climate Change , Climate Change , Air

Objective:

The objective of the study is to understand properties of black carbon (BC) containing aerosol that is well mixed with organic hygroscopic material. The mixtures of BC and organic material significantly affect water uptake and cloud nucleating ability. To address these scientific questions, the PI has: 1) identified unique and viable online and offline measurement techniques to characterize BC/organic carbon (OC) water uptake and droplet growth; 2) initiated a testing protocol to measure the in-situ cloud droplet ability of mixed BC/OC aerosol from biomass burning, alternative fuel emissions and advanced vehicle technologies; and 3) designed a novel apparatus to control and modify BC/OC aerosol mixing states for subsequent testing. BC and OC from two major sources (biomass burning and vehicular emissions) have been studied. The PI completed the construction of the mixing apparatus in the second year and has collected data with known compounds as proposed in the third year. The project has been extended to complete characterization of the mixing apparatus. The behavior of mixing state on cloud condensation nuclei (CCN) activation has been observed. The following briefly describes the progress and findings of these projects. 

Progress Summary:

1.1 Black Carbon/Organic Carbon Aerosol from Biomass Burning
 
This study evaluates the significance of surface tension effects on the hygroscopicity of biomass burning aerosol that has been aged under UV lights in a chamber. The surface tension measurement is an offline-measurement technique. Smoke from two species of biomass, Manzanita and Chamise, were injected in separate experiments into the chamber and filter samples and cloud condensation nuclei (CCN) measurements were taken. Analysis of the filter samples using a water soluble OC meter and tensiometer provided surface tension depression as a function of soluble carbon. These data were used in Köhler theory analysis to calculate the hygroscopicity parameter κ, corrected for surface tension effects. This correction lowers the measured κ by 100%, implying that the assumption of constant surface tension in the calculation of κ is too simplistic for a complicated system such as wood smoke. This study confirms that surface active materials may exist in atmospheric aerosols and will thus influence CCN activity. The ratio of BC/OC in the experiment decreases with time. The aerosol formed in this study demonstrates that droplet surface tension effects are real and can be observed in a complex aerosol mixture from semi-continuous measurements. If the dissolved material is soluble, the surface tension depression can be substantial. Köhler theory is combined with measurements of surface tension, water solubility, and CCN activity to yield a consistent method to account for surface activity of aerosols. The inclusion of surfactant parameters is in agreement with the consistent findings that the organic component of aerosol has soluble properties with single hygroscopicity parameter ~0.1. Experiments on both biomass species show a substantial fraction of biomass burning aerosol is water soluble and surface active. Köhler theory analysis on the systems studied shows that hygroscopicity can be overestimated by a factor of 2 if surface tension effects are not taken into account. This analysis also helps bridge the gap between hygroscopicities for single component studies and the complex system in this study. A key and surprising result is that the aged aerosol, can become less CCN active due to an decrease in surfactant material. The results of the study were recently published in Environmental Science & Technology (Giordano, et al., 2013, ES&T). 
 
 
Aerosol from biomass burning also is fractal. Black carbon is observed in the fractal back bone of the aerosol (Figure 1). Thus the selected mobility diameter used to predict CCN activation does not truly reflect the amount of soluble material available for activation. In our measurements, the electrical mobility of the aerosol is size selected and the aerosol then is measured with an aerosol particle mass analyzer (APM). The combination of the two instruments provides quantitative information about the particle effective density.
 
Mass selected aerosol also is deposited on a TEM Grid. TEM analysis is used to quantify the shape factor and confirm the fractal nature of the aerosol. The combination of the shape factor and particle effective density is used to correct hygroscopicity information of the biomass burning aerosol. Preliminary results find that the BC containing biomass burning aerosol is much more active than previously thought. As suggested in previous papers, the electrical mobility can overestimate the amount of solute required to activate the aerosol. The results of this research currently are being published and have been included in the thesis dissertation of the graduate student Michael Giordano who graduated in June 2014. 
 
1.2 Black Carbon/Organic Carbon Uptake from Vehicular Emissions
 
Several vehicles, fuel combinations and vehicle technologies have been tested in this first annual report period. Each vehicle is driven on either the Federal Test Procedure (FTP) cycle or the Unified Cycle (UC). Some vehicles also may be tested on both cycles. Both cycles have three phases. The first phase is known as a cold start phase; the test vehicle is started after 8 hours but before a 32 hour time lag. The first phase is known to emit greater gas and particle phase pollutants as the catalytic converter is cold and less efficient. The second phase is the stabilized phase because the catalytic converter is warm and the emissions have dropped significantly. Before the cycle starts the third and final phase, the car engine is turned off and remains off for 10 minutes, called the hot soak period. This conditions the warm start for the third phase. The UC has greater average speed than the FTP cycle. 
 
The light-duty chassis dynamometer allows for each vehicle to be run in a climate controlled environment, which will create replicated driving conditions. VERL also includes the constant volume sampler (CVS), which is a large dilution tunnel used for particle sampling. The CVS allows for fresh emissions to be diluted so that they may be accurately sampled. The amount of dilution depends primarily on the flow rate traveling through the tunnel at any particular moment. The higher the flow rate the more diluted the emissions. Once the emissions travel through the CVS, they are sampled. 
 
 
BC measurements were taken with a multi-angle absorption photometer (MAAP). The MAAP is a filter-based measurement that uses one light source at 670 nm to produce photons directed toward an accumulation of particles on Teflocarbon filter paper. The back scattering of these photons then is measured by four photo-detectors located at 45 degree intervals. In both the 2008 Dodge Ram and Honda Civic, BC concentrations are greater in the first phase of the cycle. The Dodge Ram produces significantly more BC particles than the Honda Civic. A novel online measurement system has been developed to observe real-time WSOC concentrations. Preliminary data suggest that butanol-based fuel, produces similar particle number as ethanol blends; however, more water-soluble materials are formed in butanol emissions. 
 
In Year 2, the MAAP instrumentation sustained considerable electronic damage and its use in vehicle studies was limited. However, the BC data collected before and after damage is slated for publication in Year 3. A paper entitled, “Evaluating the regulated emissions, air toxics, ultrafine particles, and black carbon concentrations from spark ignition port fuel injection (SI-PFI) and spark ignition-direct injection (SI-DI) vehicles operating on different ethanol and iso-butanol blends” has been submitted to Fuel. The submitted work explores the influence of different mid-level ethanol and iso-butanol blends on the regulated emissions, gaseous air toxics, and particle emissions from three SI-PFI vehicles and two SI-DI vehicles over triplicate FTP and UC cycles. This study utilized seven fuels with varying ethanol and iso-butanol contents, including E10, E15, E20, Bu16, B24, Bu32, and a mixture comprised of E10 and Bu8. Equivalent BC concentration measurements are reported. Figure 2 shows BC data and measurements (contributed to the submitted paper) supported by the EPA grant. The BC emissions from advanced vehicle technologies work has been led by the doctoral graduate student Daniel Short and has been included in his PhD thesis. Daniel Short graduated in August 2014. 
 
1.3 Mixing Tube Apparatus
 
A novel laminar flow tube mixer (OD=7.48" ID=7.08", Length: 78") with an adjustable concentric inlet system has been designed to better control and modify BC/OC aerosol mixing states for subsequent testing (Figure 3). SolidWorks 3D and 2D drawings of the flow tube accessories, such as the caps, U-shape supports, etc., were drawn and reviewed for machining. In Year 2, materials were ordered and collected for manufacturing in the UCR College of Natural and Agricultural Sciences Machine shop. A strong and steady movable frame made by 80/20 T-slotted extrusions was designed and assembled. 
 
Additional modeling simulations were added to the work from Year 1. Computational fluid dynamics simulation (CFD - Comsol) was employed to test and improve the aerosol mixing capabilities of the laminar flow tube mixer (Figure 3). 
 
 
 
According to the simulation results, several perturbation items (blocks) with optimized shapes and dimensions were designed to increase the flow velocities and thus the local Reynolds numbers to maximize the mixing behaviors of the two concentric flow streams introduced by the adjustable inlet system. It should be noted that CFD modeling by the mechanical engineering graduate students was not included in the original proposal and internal and separate budgets were used to fund modeling studies. 
 
 
 
In the mixing apparatus design, stream 1 is introduced into the flow tube mixer by a ¼" SS tube (wall thickness: 0.065", length: 80") with a flow rate of 1.5 lpm, stream 2 is introduced at the same flow rate by an outer concentric ¾" SS tube (wall thickness: 0.065" length: 80") from the side arm by using a SS Swagelok Tee connection. The two flows mix together at the exit of ¼" tube, where a converging and a diverging perturbation item are utilized together to increase the mixing. Then, the mixed flow stream will be diverged at the exit of the ¾" SS tube by another cone-shaped perturbation item, and then goes into the quartz tube. Residence time in the laminar flow tube mixer will be varied according to the desired mixing state fraction by changing the length of the inlet system in the quartz tube, and maximum residence time with both of the flow rate settings at 1.5 lpm can reach up to ~1 hour. Pressure gauges and thermal couples will be installed upstream and downstream of the flows through the ¼" NPT holes drilled on the caps, and thus the pressure and temperature changes can be monitored. The pressure will be maintained at around 1 atm by carefully tuning a needle valve, which is used to change the pumping speed of a mechanical pump connected at the exit of the flow tube mixer. 
 
In Year 2, the apparatus was constructed (Figure 4) and the initial CCN activity of known compositions has been tested. After several months of machine shop fabrication, the ability to measure and control size distributions within the flow tube was examined. The aerosol distribution was found to reach steady state after 1 hour with a total flow rate ~ 1.5 lpm; this is consistent with the long residence times suggested by the theoretical modeling of the apparatus. The size and number particle distributions are observed before and after the mixing tube apparatus. CCN activity shows multiple activation peaks consistent with known compounds (Figure 5). To our knowledge, this is the first time that experimental CCN data have been shown for controlled mixed aerosol states.
 
 
Year 3 of the project has focused on providing controlled information about BC/OC mixing states for CCN activation. Data sets yielding multiple activation curves were successfully recreated by mixing compounds of varying hygroscopicity in the flow tube. A slightly soluble organic aerosol, C4H6O4 (succinic acid, κ=0.231), and a soluble inorganic salt aerosol, (NH4)2SO4 (ammonium sulfate, κ=0.61), were selected. The data agreed well with Köhler Theory and single parameter (κ) thermodynamic predictions of droplet activation; activation curves were characteristic of ammonium sulfate (AS) and succinic acid (SA) (Figure 6). It also suggested that aerosol water is a significant factor. Under dry conditions, the aerosols maintained an external mixture, and activation curves remained separate and constant. However, humid conditions promoted internal mixing and the activation curves were observed to converge. 
 
We currently are in the process of adding a mixing state model to the data set to compare predictions of mixed composition size distributions with our inferred CCN data mixing state information. Once the work has been applied to known compounds, we will introduce distribution of organic BC compounds with other known and unknown organic species. 
 
 
 
 
Key Findings and Results
  • The contribution of BC to the total biomass burning aerosol mass decreases during aging and photochemistry, as additional organic material is formed during controlled aging environmental chamber experiments. 
  • In addition, the organic mixture has been shown to be surface active; hence, though black carbonaceous material often is considered insoluble, the mixture with a surface active organic composition can facilitate cloud nucleation, and promote CCN activity in biomass burning systems.
  • The changes in fractal nature and effective particle density have been observed and affect cloud nucleation predictions. The effective density of a biomass particle can enhance apparent hygroscopicity three-fold. Using a closer approximation of the spherical size with the use of effective density and TEM images shows that the CCN activity of BC containing biomass burning aerosol is much larger than expected. 
  • BC concentrations have been measured for light duty vehicles operated on the Federal Test Procedure and Unified Testing Cycle. In both cycles, BC concentrations are greatest in the first cold start phase. 
  • The PI and her research group have developed a viable online technique to understand the nucleating properties of BC- aerosol compositions. The technique has been applied to vehicular studies that investigate the emissions from alcohol (ethanol – E10, E15, E20 and butanol- Bu16, Bu24, Bu32) gasoline fuel blends in commercial light duty vehicles. 
  • The PI completed the construction of the mixing apparatus in the second year and has collected preliminary data with known compounds as proposed. The behavior of mixing state on CCN activation has been observed. Multiple activation peaks correspond to mixed aerosol. 

Future Activities:

We plan to continue with work as proposed. An emphasis will be placed on completing and conducting experiments with the unique mixing apparatus. More data will be gathered from the apparatus and more complex BC containing aerosol particles will be introduced to the device. 


Journal Articles on this Report : 7 Displayed | Download in RIS Format

Other project views: All 34 publications 9 publications in selected types All 9 journal articles
Type Citation Project Document Sources
Journal Article Giordano MR, Short DZ, Hosseini S, Lichtenberg W, Asa-Awuku AA. Changes in droplet surface tension affect the observed hygroscopicity of photochemically aged biomass burning aerosol. Environmental Science & Technology 2013;47(19):10980-10986. R835040 (2013)
R835040 (2014)
R835040 (2015)
R835040 (Final)
  • Abstract from PubMed
  • Full-text: ES&T-Full Text HTML
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  • Abstract: ES&T-Abstract
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  • Other: ES&T-Full Text PDF
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  • Journal Article Giordano MR, Asa-Awuku A. Rebuttal to correspondence on "Changes in droplet surface tension affect the observed hygroscopicity of photochemically aged biomass burning aerosol". Environmental Science & Technology 2014;48(3):2084-2085. R835040 (2014)
    R835040 (2015)
    R835040 (Final)
  • Abstract from PubMed
  • Full-text: ES&T-Full Text HTML
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  • Abstract: ES&T-First Page Preview
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  • Other: ES&T-Full Text PDF
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  • Journal Article Giordano M, Espinoza C, Asa-Awuku A. Experimentally measured morphology of biomass burning aerosol and its impacts on CCN ability. Atmospheric Chemistry and Physics 2015;15(4):1807-1821. R835040 (2014)
    R835040 (2015)
    R835040 (Final)
  • Full-text: ACP-Full Text PDF
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  • Abstract: ACP-Abstract
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  • Other: ResearchGate-Abstract & Full Text PDF
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  • Journal Article Karavalakis G, Short D, Vu D, Villela M, Russell R, Jung H, Asa-Awuku A, Durbin T. Regulated emissions, air toxics, and particle emissions from SI-DI light-duty vehicles operating on different iso-butanol and ethanol blends. SAE International Journal of Fuels and Lubricants 2014;7(1):183-199. R835040 (2014)
    R835040 (2015)
    R835040 (Final)
  • Full-text: University of California-Full Text PDF
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  • Abstract: SAE International-Abstract
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  • Journal Article Karavalakis G, Short D, Vu D, Villela M, Asa-Awuku A, Durbin TD. Evaluating the regulated emissions, air toxics, ultrafine particles, and black carbon from SI-PFI and SI-DI vehicles operating on different ethanol and iso-butanol blends. Fuel 2014;128:410-421. R835040 (2014)
    R835040 (2015)
    R835040 (Final)
  • Full-text: ScienceDirect-Full Text HTML
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  • Abstract: ScienceDirect-Abstract
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  • Other: ScienceDirect-Full Text PDF
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  • Journal Article Short D, Giordano M, Zhu Y, Fine PM, Polidori A, Asa-Awuku A. A unique online method to infer water-insoluble particle contributions. Aerosol Science and Technology 2014;48(7):706-714. R835040 (2012)
    R835040 (2014)
    R835040 (2015)
    R835040 (Final)
  • Full-text: Taylor&Francis-Full Text PDF
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  • Abstract: Taylor&Francis-Abstract
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  • Journal Article Short D, Vu D, Durbin TD, Karavalakis G, Asa-Awuku A. Particle speciation of emissions from iso-butanol and ethanol blended gasoline in light-duty vehicles. Journal of Aerosol Science 2015;84:39-52. R835040 (2014)
    R835040 (2015)
    R835040 (Final)
  • Full-text: ScienceDirect-Full Text HTML
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  • Abstract: ScienceDirect-Abstract
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  • Other: ScienceDirect-Full Text PDF
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  • Supplemental Keywords:

    atmospheric pollution, black carbon, BC, organic carbon, OC, climate, aerosol-indirect effect, tropospheric aerosol, mixing state, particle hygroscopicity. 

    Relevant Websites:

    UC-Riverside Aerosol-cloud Climate Research Group

    Progress and Final Reports:

    Original Abstract
    2012 Progress Report
    2013 Progress Report
    2015 Progress Report
    Final Report