Grantee Research Project Results

2006 Progress Report: Development and Application of a Mass Spectra-Volatility Database of Combustion and Secondary Organic Aerosol Sources for the Aerodyne Aerosol Mass Spectrometer

EPA Grant Number: R831080
Title: Development and Application of a Mass Spectra-Volatility Database of Combustion and Secondary Organic Aerosol Sources for the Aerodyne Aerosol Mass Spectrometer
Investigators: Ziemann, Paul J. , Worsnop, Douglas R. , Jimenez, Jose-Luis
Institution: University of California - Riverside , Aerodyne Research Inc. , University of Colorado at Boulder
EPA Project Officer: Chung, Serena
Project Period: October 1, 2003 through August 14, 2006 (Extended to September 30, 2007)
Project Period Covered by this Report: October 1, 2005 through August 14, 2006
Project Amount: $409,922
RFA: Measurement, Modeling, and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5) (2003) RFA Text |  Recipients Lists
Research Category: Air , Air Quality and Air Toxics , Particulate Matter

Objective:

In this 4-year experimental research program, we are developing and applying a thermodenuder-Aerodyne Aerosol Mass Spectrometer (TD-AMS) technique for ambient organic fine particle analysis. The objectives of the project are to: (1) construct and couple a TD to the AMS and evaluate and optimize its performance; (2) use the TD-AMS technique in laboratory studies to develop a mass spectra-volatility database for the major atmospheric sources of combustion aerosol and secondary organic aerosol (SOA); and (3) apply the database to a TD-AMS study of organic aerosol in the Los Angeles Air Basin.

The TD-AMS system will be evaluated and optimized using standard particles and then used to develop a database of mass spectra-volatility signatures for interpreting ambient organic aerosol TD-AMS data. TD-AMS measurements will be made on laboratory-generated organic aerosols from the major primary sources (combustion of gasoline, diesel fuel, and wood, and meat cooking) and the major sources of fine particle organics (oxidation of aromatics, alkanes and alkenes, and biogenic compounds by OH and NO₃ radicals and O₃). A field study then will be carried out in the Riverside area of the Los Angeles, California metropolitan area under conditions when both primary and secondary organic aerosols are expected to be present, and the TD-AMS data analyzed using the source database developed in this study. The results of this project will lead to a powerful new technique for the chemical characterization of atmospheric organic fine particulate matter (PM_2.5), which through its application will improve the understanding of organic aerosol formation mechanisms and sources, and add valuable new data for use in source apportionment modeling.

Progress Summary:

During Years 1 and 2 of this project, two TD systems were designed, constructed, and tested at Aerodyne, and then further evaluated and improved upon by the University of Colorado at Boulder (CU) group. The final design employs less insulation than the original and a fan to shorten the cool-down period, as well as a valve and 3-zone temperature control system in software (Labview and Visual Basic 5.0). The valve system allows for switching between ambient air that either bypasses or passes through the TD, for constant time periods. The temperature control system is typically programmed to either step through several temperatures repeatedly (e.g., 50, 75, 100, 125, 150, 175, and 200°C in 20-minute intervals) or to ramp the temperature continuously. This system was employed for preliminary measurements in the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign at Chebogue Point, Nova Scotia in Year 1 and in the Study of Organic Aerosol in Riverside (SOAR-1) in Year 2. The SOAR-1 study was organized by Professors Jimenez and Ziemann to evaluate the performance of the TD-AMS. It was carried out at the University of California at Riverside (UCR), which is in a polluted region downwind of Los Angeles that is heavily impacted by both primary and secondary aerosol. Although it was originally intended that this study would include only the UCR and CU groups, in the end, ~50-60 scientists from 15 universities and research institutes and companies participated in what is probably the most complete analysis of organic aerosols performed to date. More detailed information on the participants and measurements can be found at http://cires.colorado.edu/jimenez-group/Field/Riverside05/ Exit .

During SOAR-1, the TD system was used by the CU group in front of a new high-resolution time-of-flight aerosol mass spectrometer (ToF-AMS) and a scanning mobility particle sizer (SMPS). This system was used to continuously analyze ambient particles, and to analyze SOA formed from the oxidation of alkanes, alkenes, aromatics, and gasoline in an environmental chamber in the Ziemann laboratory. In addition, a stepped AMS vaporizer temperature method of operation was developed and applied to both ambient and chamber aerosols, with the objective of reducing organic fragmentation and assessing whether volatility information also can be obtained in this way. Measurements also were made by the UCR group with their thermal desorption particle beam mass spectrometer (TDPBMS) system (with and without a versatile aerosol concentration enrichment system particle concentrator developed and operated by the Sioutas group from the University of Southern California) and by the Prather group from the University of California at San Diego who used the TD in front of a new Aerosol Time-of-Flight Mass Spectrometer (ATOFMS). In addition to these field studies, the UCR group has used a TD-TDPBMS system (equivalent to TD-AMS) to analyze organic and inorganic aerosol standards and SOA formed in an environmental chamber in order to determine the extent to which aerosol components can be separated by the TD on the basis of volatility, develop methods for extracting information on component vapor pressures using volatility data, and create a mass spectra-volatility database for SOA.

In Year 3 of this project (the period of this report), the UCR group carried out both experimental and modeling studies on particle evaporation in the TD. Carboxylic acid standards were analyzed using the same flow conditions as those employed by the CU group for ambient studies, along with careful measurements of the temperature profile within the TD. Studies of the effect of mass loading on particle evaporation indicated that, for ambient mass loadings, organic vapor saturation is probably not reached within the TD. Vapor concentrations in the TD should, in fact, be far enough below saturation such that the evaporation process is not significantly impacted by mass loading. This condition is important for understanding the performance of the TD. The evaporation of organic compounds in the TD also was modeled assuming free-molecule conditions in an attempt to extract component vapor pressures. If this approach worked, it would provide a convenient simplification to the complex transition-regime conditions under which the particles actually evaporate. Unfortunately, this method was not sufficiently accurate for our purposes, and so we have decided to instead develop an empirical calibration to relate component TD desorption temperatures to their vapor pressures. The UCR group has for many years used a similar approach to estimate vapor pressures of SOA compounds analyzed by temperature-programmed thermal desorption (TPTD) with the TDPBMS. The calibration is being developed using TD measurements for alkanes, monocarboxylic acids, and dicarboxylic acids. These compounds cover the range of properties from the nonpolar hydrocarbons that comprise most primary emissions to the polar oxygenates that comprise SOA. The AMS data analysis procedure will involve differentiation of the TD profiles of total organic aerosol, as well as oxidized organics (OOA) and hydrocarbon-like organics (HOA), which can be distinguished by the AMS. These curves then will be converted to distributions of organic mass with respect to vapor pressure using the TD calibration.

During Year 3 of the project, the CU group:

1. Continued the work on characterization of the passing efficiency of the TD, which is found to be similar to that of the Wehner design on which our design is based.
2. Participated in the SOAR-2 field study in Riverside (November 1-15, 2005) during which the TD-AMS was deployed at UCR under less polluted conditions than during SOAR-1, and alongside the ATOFMS and thermal desorption aerosol GC/MS-FID instruments from the Prather and Goldstein groups respectively.
3. Successfully deployed the TD in front of two high-resolution (HR)-ToF-AMSs and an SMPS for a month at the “T0 Supersite” in the Mexico City field campaign (MILAGRO/MCMA-2006, March 2006).
4. Deployed a TD-HR-ToF-AMS + SMPS system as part of the FLAME-1 study at the U.S. Forest Service fire laboratory in Missoula, MT, where mass spectra-volatility signatures were obtained for the combustion of 18 biomasses, with large differences in composition and volatility between them that were correlated with burning regime (flaming vs. smoldering) and biomass origin. During this deployment a faster method of TD-AMS operation was developed, which allows for obtaining full TD profiles in shorter times (< 1 hour) and with more resolution in temperature space.
5. Continued work on the stepped AMS vaporizer temperature method first developed during SOAR-1, with acquisition of signatures from many pure compounds in the laboratory.

Our work to date is successfully meeting the original goals of the project. We have completed Task 1 of this project, which was to construct two TD systems. We also have completed most of Task 2, which was to characterize the TD. Evaluations of particle passing efficiencies are ongoing and nearly completed. Task 3 also has been completed, as a number of pure organic compounds have been analyzed using the TD and TPTD methods, and the TD (interfaced with both the TDPBMS and AMS) and TPTD methods have been used to analyze complex SOA mixtures formed from a series of environmental chamber reactions. Tasks 4-6 are all underway. For Task 4, a large database of TDPBMS mass spectra and volatility profiles are being compiled for SOA formed from reactions of alkanes, alkenes, and aromatics with OH radicals, NO₃ radicals, and O₃ under high and low NO_x conditions. There also are a few chamber reactions that still need to be performed. This database will be made available to the public through the Jimenez group Web site. For Task 5, analyses of diesel lubricating oil have been used as a surrogate for organic combustion aerosol, and profiles have been obtained from burning of 18 different biomasses, with additional experiments planned for Year 4 by the CU group. The SOAR-1 campaign was the focus of Task 6, and additionally the ICARTT, SOAR-2, and MILAGRO campaigns were targets of opportunity. The large amounts of data from that study are now being analyzed and prepared for publication in what will be the first field determination of chemically-resolved aerosol volatilities. The data will be cast in a form that is compatible with upcoming models for treatment of semivolatile organics, following the approach of Donahue, et al. Data analyzed from both the SOAR-1 and MILAGRO campaigns show several consistent trends. Typical OOA mass fragments show considerably lower volatility than HOA mass fragments. Sulfate masses are consistently the least volatile, while conversely, nitrate shows consistently high volatility. Ammonium and nitrate were more volatile in the SOAR-1 campaign than in MILAGRO, and chloride was significantly more volatile in each. These later effects may be due to chemical differences or matrix effects. To date, no significant diurnal volatility pattern was evident from these data. Analysis of the listed field campaigns is, however, in its early phase. Much effort was spent developing a software package to efficiently aid in the analysis of TD-AMS field data, and this product is becoming very useful.

The results of this project are demonstrating that the TD-AMS technique will be a powerful new approach for characterizing the chemical composition of atmospheric organic PM_2.5. We have demonstrated the TD can provide a significant degree of separation of components based on volatility, which improves mass spectral identification and yields information on component vapor pressures. It also is expected that it will be possible to use TD profiles to estimate the distribution of vapor pressures of aerosol components. The information obtained by the AMS has previously come from the dependence of aerosol mass spectra on particle size and time. Addition of the desorption temperature, as a new correlation parameter for aerosol characterization and a means of estimating component vapor pressures, will significantly enhance the power of the AMS method. Through its application, it is expected to provide improved data for understanding organic aerosol formation mechanisms and sources, and for use in aerosol source apportionment modeling. Such data can aid in the evaluation of the effects of fine particulate matter on human health and the environment, and can be used to develop more accurate air pollution models. This will allow for more efficient targeting of aerosol sources in pollution control strategies.

It also is worth noting an important practical application of this project. One of the original reasons the TD technique was selected as the means for gaining aerosol volatility information with an AMS was that a TD is easily interfaced to an AMS. It was expected that once the power of the TD-AMS technique was demonstrated, the TD would become a desired addition to current and future AMS instruments. Because of the widespread use of the AMS (~50 currently in use), the impact of the TD technique (and this project) would therefore be greatly amplified. The TD technique has already generated considerable interest in the AMS community, especially after results were presented by Professor Jimenez at the AMS Users Meeting. Aerodyne reports that four groups have inquired about the possibility of ordering a TD from Aerodyne to interface with their AMS. Two groups are interested in the application of the TD-AMS technique during aircraft sampling.

Data from the SOAR-1 and SOAR-2 studies, which were motivated and partially supported by this grant, were included in 13 additional presentations (besides the three listed below) at the 2006 International Aerosol Conference. Data from these campaigns have been used in three published papers to date, with at least 10 other papers being in preparation by different groups.

Future Activities:

The plans for Year 4 for the CU group include: (1) finishing the TD lab characterization, mainly verifying the detailed mapping of the passing efficiency versus particle size and temperature for non-volatile aerosols, and publication of a paper describing and demonstrating the TD-AMS system; (2) TD-AMS characterization of combustion particles generated with a burner, as well as car exhaust and meat cooking; (3) finishing data analysis and publishing results from TD-AMS field deployments during the ICARTT, SOAR-1, SOAR-2, and MILAGRO field experiments, relating them to the data obtained for SOA in the UCR chamber; and (4) finishing experiments and writing a paper on the demonstration of stepped-vaporizer temperature operation of the AMS, comparing it to the TD-AMS and TPTD approaches. During Year 4 the UCR group will complete TD measurements on standard compounds and develop the empirical calibration and data analysis procedures needed to convert TD desorption profiles into vapor pressure distributions. They also will perform the remaining SOA chamber experiments and finish compiling the database of TDPBMS SOA mass spectra and volatility profiles for the Jimenez group Web site. One important goal is to determine whether a particular SOA system can be identified as the source of the large m/z 44 signal that generally appears in AMS ambient mass spectra and is indicative of a large contribution from oxidized organic aerosol. This work then will be prepared for publication.

Journal Articles:

No journal articles submitted with this report: View all 39 publications for this project

Supplemental Keywords:

ambient air, tropospheric, air pollution, particulates, environmental chemistry, monitoring, carbonaceous particles, combustion aerosols, source apportionment,, RFA, Scientific Discipline, Air, Waste, Ecosystem Protection/Environmental Exposure & Risk, Air Quality, particulate matter, air toxics, Environmental Chemistry, Monitoring/Modeling, Environmental Monitoring, Incineration/Combustion, Engineering, Chemistry, & Physics, Environmental Engineering, carbon aerosols, air quality modeling, particle size, atmospheric particulate matter, combustion byproducts, particulate organic carbon, aerosol particles, atmospheric particles, mass spectrometry, carbon, chemical characteristics, PM 2.5, air modeling, air quality models, airborne particulate matter, air sampling, gas chromatography, thermal desorption, carbon particles, air quality model, emissions, secondary organic aerosol, particulate matter mass, ultrafine particulate matter, PM2.5, modeling studies, mass spectra volatility database, particle dispersion, aerosol analyzers, aerosol mass spectrometry, measurement methods, combustion contaminants, chemical speciation sampling

Relevant Websites:

http://cires.colorado.edu/jimenez Exit
http://www.chem.ucr.edu/index.html?main=faculty&facsort=profile&faculty=ziemann Exit
http://www.aerodyne.com Exit
http://cires.colorado.edu/jimenez-group/Field/Riverside05/ Exit

Progress and Final Reports:

Original Abstract

2004 Progress Report

2005 Progress Report

Final Report