Final Report: Eddy-Correlation Measurement of Size-Segregated and Composition-Resolved Aerosol Depositional Flux Using an Aerosol Mass SpectrometerEPA Grant Number: R828172
Title: Eddy-Correlation Measurement of Size-Segregated and Composition-Resolved Aerosol Depositional Flux Using an Aerosol Mass Spectrometer
Investigators: Smith, Kenneth A.
Institution: Massachusetts Institute of Technology
EPA Project Officer: Hahn, Intaek
Project Period: July 24, 2000 through July 23, 2002 (Extended to July 23, 2004)
Project Amount: $225,000
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Engineering and Environmental Chemistry , Water , Land and Waste Management , Air
Aerosol dry deposition is the transfer of particulate matter (PM) from the atmosphere to terrestrial surfaces in the absence of precipitation. The rate of input of particle-borne contaminants to terrestrial surfaces by dry deposition is comparable to that by wet deposition (Davidson and Wu, 1989). The rate of dry deposition of fine particles, those with particle diameters (Dp) less than 2.5 mm, governs the concentration of a number of environmentally significant species. Some examples include:
- Fine particulate matter (PM2.5) concentrations are affected by dry deposition, an important removal mechanism for PM, (Kleeman and Cass, 1998); this is especially relevant in arid locations like Los Angeles.
- In the Northeastern United States, sulfate ions are associated primarily with fine aerosol particles and make up a large fraction (20-26%) of the fine particle mass (Salmon, et al., 1999). The deposition of sulfuric acid results in the acidification of surface waters and terrestrial environments (reviewed in Davidson and Wu, 1989).
- Toxic compounds like polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, and polychlorinated dibenzodioxins are deposited with particles to farmlands and surface waters (Koester and Hites, 1992; Jones and Duarte-Davidson, 1997). These compounds, especially the low vapor pressure species, are associated with fine aerosol particles (Allen, et al., 1996).
Estimations of the concentrations of PM2.5, inorganic ions, and toxic organic compounds in the atmosphere, therefore, require an understanding of fine particle dry deposition. A corollary of this statement is that the flux of these species to terrestrial environments and surface waters depends on fine particle dry deposition rates.
Despite the importance of accurate techniques to estimate the deposition flux of fine particles, the literature includes a wide range of sometimes contradictory particle deposition velocity estimates. One strategy to estimate fine particle deposition velocities has been to measure fluxes of well-characterized particles interacting with ideal surfaces in wind tunnels, and then to construct semi-empirical models to predict deposition velocities in natural environments (Slinn, 1982; Williams, 1982). The wind tunnel data show that the rate of dry deposition is a strong function of particle size. For example, the deposition velocities to water of Dp in the range 0.1-1 mm, are approximately 0.01 cm/second (Slinn, et al., 1978). The deposition velocities of particles larger than 1 μm rise from 0.1 cm/second for 3 μm particles to 30 cm/s for 30 mm particles—a variation of more than 4 orders of magnitude.
A second strategy is to measure fine particle fluxes by well-established eddy-correlation techniques (Businger, 1986) coupled to aerosol measurement instruments. Buzorius, et al. (1998) measured the particle flux over a forest by eddy correlation with a condensation particle counter. These measurements determined the particle number flux, which is not directly related to the flux of aerosol mass. A number of research groups have measured aerosol fluxes by using optical particle counters (summarized in Gallagher, et al., 1997). In these studies, particle size and mass are inferred from light scattering intensities, which are known to depend on particle composition and morphology (Hering and McMurry, 1991). Figure 1 shows that measured deposition velocities for fine particles, as obtained by eddy correlation methods, consistently are approximately an order of magnitude greater than those predicted by semi-empirical models.
Aerosol Deposition Velocity to Forest Canopies
Figure 1. Summary of Particle Deposition Velocities as a Function of Size to Forest Canopies (see Gallagher, et al., 1997, for observation symbols).
A number of explanations have been advanced to account for differences between measured and predicted dry deposition velocities; these include:
- Differences between surrogate and natural deposition surfaces.
- Extrapolation of micrometeorological data above the canopy (Slinn, 1983).
- Large uncertainties in measurements of coarse particles that are present in low concentrations but contribute significantly to depositional flux (Milford and Davidson, 1985).
- Rapid flow near the fine structure in natural canopies aids particle impaction (Wesley, et al., 1985).
The eddy-correlation mass spectrometry (ECMS) method has the potential to provide direct measurements of speciated and size-segregated fine particle deposition velocities by coupling the eddy-correlation technique with fast response measurements of fine particle size and composition. The ECMS system uses an Aerodyne aerosol mass spectrometer (AMS) to make 10 Hz measurements of speciated fine particles. The instantaneous vertical velocity, w, was measured with a sonic anemometer. The flux can be calculated as
where S is the AMS signal, which is proportional to the concentration of the aerosol species monitored (Jayne, et al., 2000). The deposition velocity, ud, for particles is then calculated as
The objective of this research project was to develop and demonstrate the ECMS method to measure the flux of speciated fine particles directly.
Two field studies were completed as part of this work. The Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (known as PROPHET 2001) study was conducted at the University of Michigan Biological Station during the summer of 2001. During this study, the eddy correlation mass spectrometry data were collected from a 31 m tower over an approximately 20 m forest canopy. The second study, Salt River 2005, was conducted at an agricultural site on the Salt River Pima-Maricopa Indian Community, in the Phoenix, Arizona metropolitan area. Data were collected from a 7 m tripod above a broccoli crop with a mean height of approximately 0.5 m.
In both cases, detailed meteorological data were collected in concert with the deposition data.
Fine particle deposition velocities were in the range of 0.1-2.0 cm/second at both sites. These are in agreement with prior experiments but differ from many predictions. These are the first direct field measurements of speciated fine particle deposition velocities. As such, they enhance the confidence with which regulators can predict deposition rates and the attendant consequences.
Allen JO, Dookeran NM, Smith KA, Sarofim AF, Taghizadeh K, Lafleur AL. Measurement of polycylic aromatic hydrocarbon associated with size-segregated atmospheric aerosol in Massachusetts. Environmental Science & Technology 1996;30(3):1023-1031.
Businger JA. Evaluation of the accuracy with which dry deposition can be measured with current micrometeorogical techniques. Journal of Climate and Applied Meteorology 1986:25(8):1100-1124.
Buzorius G, Rannik U, Makela JM, Vesala T, Kulmala M. Vertical aerosol particle fluxes measured by eddy covariance technique using condensational particle counter. Journal of Aerosol Science 1998;29(1):157-171.
Davidson CI, Wu Y-L. Dry deposition of particles and vapors. In: Lindberg SE, Page AL, Norton SA, eds. Acid Precipitation Volume 3, 1989.
Gallagher MW, Beswick KM, Duyzer J, Westrate H, Choularton TW, Hummelshoj P. Measurement of aerosol fluxes to speulder forest using a micrometerological technique. Atmospheric Environment 1997;31(3):359-373.
Hering SV, McMurry PH. Optical counter response to monodisperse atmospheric aerosols. Atmospheric Environment 1991;25A:463-468.
Jayne JT, Leard DC, Zhang X, Davidovits P, Smith KA, Kolb CE, Worsnop DR. Development of an aersol mass spectrometer for size and composition analysis of submicron particles. Aerosol Science and Technology 2000;33:49-70.
Jones KC, Duarte-Davidson R. Transfers of airborne PCDD/Fs to bulk deposition collectors and herbage. Environmental Science & Technology 1997;31(10):2937-2943.
Koester CJ, Hites RA. Wet and dry deposition of chlorinated dioxins and furans. Environmental Science & Technology 1992;26(7):1375-1382.
Kleeman MJ, Cass GR. Source contributions to the size and composition of urban particulate air pollution. Atmospheric Environment 1998;32(6):2803-2816.
Milford JB, Davidson CI. JAPCA 1985;35:1249-1260.
Salmon LG, Cass GR, Pedersen DU, Durant JL, et al. Determination of fine particle and coarse particle concentration and chemical composition in the Northeastern United States, 1995. Report to NESCAUM, 1999.
Slinn WGN, Hasse L, Hicks BB, Hogan AW, Lal D, Liss PS, Munnich KO, Schmel GA, Vittori O. Some aspects of the transfer of atmospheric trace constituents past the air-sea interface. Atmospheric Environment 1978;12:2055-2087.
Slinn WGN. Predictions for particle deposition to vegetative canopies. Atmospheric Environment 1982;16:1785-1794.
Wesley ML, Cook DR, Hart RL. Measurements and parameterizations of particulate sulpher dry deposition over grass. Journal of Geophysical Research 1985;90:2131-2143.
Williams RM. A model for the dry deposition of particles to natural water surfaces. Atmospheric Environment 1982;16(8):1933-1938.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 8 publications||2 publications in selected types||All 2 journal articles|
||Jayne JT, Leard DC, Zhang XF, Davidovits P, Smith KA, Kolb CE, Worsnop DR. Development of an aerosol mass spectrometer for size and composition analysis of submicron particles. Aerosol Science and Technology 2000;33(1-2):49-70.||
||Wu J, Lurmann F, Winer A, Lu R, Turco R, Funk T. Development of an individual exposure model for application to the Southern California Children's Health Study. Atmospheric Environment 2005;39(2):259-273.||