SCERP Project Number: A2
Principal Investigator: Henk L.C. Meuzelaar
University of Utah
OBJECTIVES OF THE PROJECT:
RESULTS AND ACTIVITIES:
Aerosol emissions arise from a large number of different types of sources.
Most of the source apportionment studies focus exclusively on specification
of inorganic components and information about the origin of organic aerosol
emissions is derived from bulk chemical analyses that quantify the total
amount of organic carbon present. Taking into account that inhalable particulate
(PM-10) carbon containing aerosols, i.e., particulates with aerodynamic
diameter smaller than 10 micrograms, are associated with harmful effects
caused by specific organic compounds that are known as toxic and even carcinogenic,
it is clear that chemical composition and structure of particulate organic
matter (POM) can provide important information necessary to asses air quality
and associated deleterious health and environmental effects. Additionally,
precise chemical analyses of POM can be used to characterize the origin,
distribution and fate of respirable aerosols in the environment, in a way
that reveals the distinctive features of the different source types. Consequently,
development of analytical methodologies capable of characterizing the chemical
composition and structure of the POM fraction became one of our main objectives
of the project.
DEVELOPMENT AND FIELD TESTING OF FULLY AUTOMATED INSTRUMENTATION FOR
MASS SPECTROMETRIC CHARACTERIZATION OF VOLATILE ORGANIC COMPOUNDS (VOC)
AND PARTICULATE ORGANIC MATTER (POM) IN AIR.
Although some information has been presented in the literature regarding the origin and chemical nature of organic aerosol components in urban and to some extent suburban and rural environments, the methods used are typical laboratory techniques requiring collection of field samples followed by transportation, storage, preparation and, finally, analysis of samples, often after a considerable amount of time. Moreover, since aerosol composition and concentrations tend to vary widely as a function of time of the day, it is clearly impractical to operate sequential samplers on a round the clock basis in different locations and analyze all of these samples in the laboratory. Therefore, a standard approach is based on relatively long sampling periods (12-24 hours). However, under such conditions semivolatile organic compounds that are adsorbed on particles are exposed for a relatively long time to reactive compounds like ozone, nitrogen oxide, sulfur trioxide and nitric acid that are always present in the atmosphere. Hence, artificial formation, e.g., of mutagenic oxides and nitroderivatives, as well other reactions give rise to changes in concentration and content compared to the original air mass. Therefore, it would be advantageous to have a mobile, field portable equipment that can be quickly transported to problem areas in order to provide rapid feedback regarding the origin and composition of PM-10.
The developed methodology for POM characterization is based on the very efficient thermal desorption of microgram amounts of PM-10 (e.g., collected on a quartz filter) and transferring of volatilized organic components to a very sensitive and specific detector, i.e., mass spectrometer. To facilitate the identification of numerous chemical components an additional discrimination unit, based on high performance gas phase chromatographic separation of desorbed components before entering a mass spectrometer was included into our system. This approach allows substantially shorter sampling times to be used, thus eliminating many drawbacks of the classical sampling methodology caused by long exposure to reactive species as well as obtaining more meaningful data in comparison to the results representing averages over long periods of time.
A mobile, pick-up truck transportable laboratory module equipped in the sampling port (raised approx. 5 ft above the roof of the unit) and meteorological sensors monitoring wind speed and direction as well as temperature, rainfall and humidity was field tested in the Nogales area between December 15 and 22, 1991 and at the US/Mexican border in Calexico/Mexicali area from December 13 to 19, 1992 (Figure 1).
Aerosol concentration and particle size distribution measurements (parallel to aerosol collection on quartz fiber filters) were performed at 10 minute intervals using a multichannel particle counter which allows counting over 8 size ranges of particulates covering 0.3 -10 microgram diameter size range and the data were continuously collected by computer.
The air stream entering a panoramic (360 deg), isokinetic sampling port was directed onto 2 stacked (primary and backup) quartz fiber filters using a small portable air sampling pump and mass flow controller, thus allowing precise sampling volume of air to be determined.
From the sampled filters 2 mm wide strips were cut and positioned inside of a special glass reaction tube lined with a ferromagnetic foil. Flash desorption of organic constituents was achieved by very fast inductive heating of the foil up to so called Curie-point temperature (315 deg C) under continuous flow of helium that carries analytes into a fused silica capillary column (Figure 2).
Volatile organic compounds were enriched on the Tenax-GR sorbent wrapped in a similar ferromagnetic foil with a 315 C Curie-point temperature and desorbed under identical conditions as quartz filter strips (Figure 2).
Subsequent to sample desorption, a capillary column was temperature programmed to separate a complex mixture of organic compounds, taking advantage of differences in physico-chemical properties of particular components. A commercially available mass spectrometer (Finnigan MAT) used in our studies was specially miniaturized and ruggedized for transportation and field operations. Organic compounds entering the mass spectrometer were identified on the basis of characteristic fragmentation patterns recorded by a computer and specific responses quantified for source apportionment studies.
The results obtained confirmed the feasibility of field operation of
a highly sophisticated, research grade instrumentation prototype developed
in our laboratory for characterization of organic pollutants in air.
COLLECTION AND ANALYSIS OF AEROSOL SAMPLES AT SELECTED POINTS ALONG THE U.S./MEXICAN BORDER
The analysis of samples collected in Nogales (Arizona) and in Calexico (California) revealed a complex suite of organic components present in the gas phase as well as adsorbed on inhalable particulates of airborne aerosols, as documented in the attached Quarterly Progress Reports and a copy of the paper presented during the Third International Symposium on Field Screening Methods for Hazardous and Toxic Chemicals (February 24-26, 1993, Las Vegas, NV).
This complexity is especially evident in a broad "unresolved complex mixture"(UCM) peak eluted after thermal desorption of sample strips obtained from quartz fiber filters (Figure 3). The UCM "hump" is a characteristic feature for gas chromatography/mass spectrometry profiles of PM-10 samples containing significant anthropogenic contributions from a range of different combustion/pyrolysis sources including automotive emissions, burning of biomass, emissions from industrial combustion furnaces and domestic stoves or even food preparations. In addition to these and other industrial and agricultural activities also a natural components derived from plants and soil contributed to the complexity of detected constituents. The UCM "hump" contains primarily straight chain, branched and alicyclic hydrocarbons containing 2030 carbon atoms. The other dominant components represented the ubiquitous alkyl-phthalates used widely as plasticizers as well as methoxyphenols, polycyclic aromatic hydrocarbons (PAH), terpenoids and their corresponding aromatized and decarboxylated derivatives and fatty acids.
Throughout the year, smoke from the intermittently burning garbage dump
generates more complaints about air quality than any other local source
in Nogales (Arizona). A filter sample submitted by the County Health Department
in Nogales with the urgent request to characterize the trapped organic
matter in view of serious air quality problems ascribed to the burning
garbage dump revealed that in addition to described above components several
aliphatic and aromatic polychlorinated organics as well as chloroprene
monomer and dimer signals were observed together with a suite of small
aromatics. Most of these signals are readily explainable as partial combustion
products of chlorinated rubber compounds (Figure 4).
DEVELOPMENT OF ADVANCED PATTERN RECOGNITION TECHNIQUES FOR STUDIES OF SOURCE CHARACTERIZATION, IDENTIFICATION AND APPORTIONMENT OF POM SAMPLES
High sensitivity of the developed analytical methodology permits to analyze samples collected at relatively short time intervals. The obtained so-called "time-series" allow to use an advanced pattern recognition techniques for data analysis. A developed in our laboratory software based on principal component analysis techniques in combination with graphical rotation methods permits to perform deconvolution of overlapping trends followed by "mathematical extraction" of the individual components patterns. In this way, the time dependent concentration profiles of each source can thus be reconstructed and the principal component analysis approach can provide important information about diurnal and nocturnal variations in the relative contributions of various PM-10 sources. Use of these multivariate statistical analysis tools permitted us, for example, to distinguish in Nogales area a dominating sources of air pollution, with automotive emissions (represented by a suite of aliphatic and polynuclear aromatic hydrocarbon signals) and wood burning sources (represented by characteristic lignin and resin related biomarker signals) being the main contributors. In addition time-series analysis allowed us to distinguish also additional sources contributing to the UCM hump. For instance, the absence of odd-to-even carbon predominance and the very low ratio of resolved to unresolved peaks composing the UCM suggest a petroleum origin of the hump components whereas the superimposed pattern of nalkanes is considered characteristic for diesel exhaust sources. On the other hand nocturnal samples were found to contain a series of alkanes exhibiting a dominant odd-carbon number hydrocarbon pattern thought to be typical for plant origin.
Aerosol samples collected in Calexico area over a 50 hr period between December 16 and 18 indicated a strong correlations between wind direction and velocity and aerosol concentrations. During this period aerosol concentrations varied between lows of 5-10 micrograms/cubic meter before dawn and high values of 300-500 micrograms/cubic meter during evening hours when low velocity southern wind was blowing highly concentrated aerosols apparently from the Mexicali area. A high contribution of exhaust emissions from gasoline and diesel-powered vehicles was reflected by high content of aliphatic and aromatic hydrocarbons including polycyclic aromatic hydrocarbons, carboxaldehydes, ketones and quinones. A more comprehensive data analysis and interpretation of the sample data obtained in Calexico are still pending due to the lack of continued funding for this project.
Results reported here were presented by principal investigator at the Third International Symposium on Field Screening Methods for Hazardous and Toxic Chemicals (Las Vegas, February 24-26, 1993), by Professor O. Lopez at the II Reunion de Estadistica y Medio Ambiente (Hermosillo, Mexico, May 16-28, 1993) and by Dr. J.P. Dworzanski at the 40th ASMS Conference on Mass Spectrometry and Allied Topics (Washington, DC, May 31-June 5, 1992).
Last updated 7/1/99