1999 Progress Report: Atmospheric Fate and Dry Deposition of Urban Soot to Great Waters Using a Novel, State-of-the-Art Isotopic Particulate TracerEPA Grant Number: R825247
Title: Atmospheric Fate and Dry Deposition of Urban Soot to Great Waters Using a Novel, State-of-the-Art Isotopic Particulate Tracer
Institution: University of Maryland - College Park
EPA Project Officer: Chung, Serena
Project Period: December 6, 1996 through December 5, 1999 (Extended to December 5, 2000)
Project Period Covered by this Report: December 6, 1998 through December 5, 1999
Project Amount: $454,976
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
The objectives of this project are to: (1) determine the principle sources of soot depositing to the Chesapeake Bay; (2) characterize the evolution of the size-distribution and composition of tagged soot aerosol; (3) develop and test a high-volume sampling system for making gradient flux measurements of tagged soot particles; and (5) use the system to determine the flux and deposition velocity of carbon soot particles depositing onto the Chesapeake Bay during a well-characterized set of meteorological conditions.
In the first year of the project, mass spectrometry procedures for highly precise and accurate determinations of Ir were developed for application to dry deposition flux measurements using difference methods (i.e., relaxed eddy accumulation and the gradient method) and for determining changes in the size distribution and atmospheric concentration of urban diesel soot tagged with an iridium tracer. Measurement precision of ±, 0.2 percent for ambient aerosol samples containing 1 to 10 pg quantities of Ir (detection limit 150 fg) was achieved; precision of ±, 0.5 percent is achieved by neutron activation analysis with a detection limit of 90 fg. In addition, we assembled and performed some initial laboratory tests of a high-precision flow rate sampling system (240 LPM) for the difference methods. A screen sampler for measuring deposition to the surface microlayer was constructed, tested, and found to be too irreproducible for our application. Lastly, Iridium background concentrations in fine particles (i.e., < 2.5 um particle diameter) collected previously (November 1995, July 1996, and October 1996) in Baltimore were determined.
During the second project year, we developed and tested a pair of systems for precise volumetric control of our MOI flowrates and a system for sampling at elevated (i.e., 90%) relative humidity (RH). The flow control system limits flow rate fluctuations to ±, 0.12 percent during sampling with Micro-Orifice impactors. The system for RH control permitted sampling at 90 percent RH with an average relative precision of ±, 1.7 percent during field measurements. These systems were deployed in and around Baltimore from July to December 1998, to determine differences in the size distributions of trace elements and Ir-tagged soot (emitted from Baltimore City sanitation trucks) at high and low relative humidities and after transport up to 30 km. Additionally, airfoil deposition collectors and dichotomous samplers also were frequently operated. Sampling sites included Lake Clifton Park, slightly northeast of downtown Baltimore, Ft. Howard, BWI, Oregon Ridge, and Fallston Airport.
During the third project year, all of the samples collected during this period were analyzed for Ir and up to 20 elements, including Ce, Cr, Cs, Fe, Sb, Sc, Se, and Zn, by neutron activation. We are just beginning to evaluate the data. The Ir data clearly confirm the multimodal nature of soot aerosol emissions from diesel sanitation trucks. Size distributions for Ir, Fe, Zn, Sb, Cr, Se and Cs were plotted for each day of sampling. Mass geometric mean diameters (gmmad) for elements concentrated in fine particles collected with ambient and humidified MOI were examined to determine particle growth due to elevated humidities. Those elements were Se (nine samples), Sb (seven samples), Zn (three samples), and Cs (two samples). The results indicate that average growth for Se was 22.4 ±, 3.7% (average change in RH 29.4 ±, 9.8%), 17.8 ±, 6.5% for Sb (averageRH change of 30.0 ±, 11%), and 14.0 ±, 2.0% for Zn (averageRH change of 36.4 ± 9.2%). For Cs the growth was 26.9% (RH change of 39.6%) for our first sample, and only 3.7% when the difference between ambient and humidified MOI was very small (i.e., 7.5%). We found that the gmmads of humidified (typically 90%) and nonhumidified aerosol bearing the various elements are well-correlated with linear fit for all elements (i.e., humidified gmmad = 0.03858 + 1.0776*dry gmmad, with an R2 of 0.9740). A CMBD model (Caffrey et al., Environ. Sci. Technol 32:1615-1622, 1998) is being used to derive empirical deposition velocities from simultaneous MOI and airfoil deposition data. Deposition velocities determined for the Lake Clifton site ranged from 0.01 to 0.2 cm s-1 for submicrometer particles, and from 0.07 to 9 cm s-1 for supermicrometer particles. The latter were four-fold greater than their terminal settling velocities, in agreement with literature values (Davidson and Wu, Dry Deposition of Particles and Vapors, Springer-Verlag, New York, 1989).
Reduction and interpretation of field data from the 1998 campaigns will be finished by the end of April 2000. This includes application of the Ir-tagged soot size distributions in a gradient deposition model to permit estimation of urban-scale deposition velocities for supermicrometer particles bearing soot. A final tracer release will be completed in May/June 2000 to permit any accumulation/gradient measurements to be completed.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
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|| Maciejczyk PB, Kidwell C, Ondov JM. System for precise control of volumetric flow rate during sampling with a cascade impactor. Aerosol Science and Technology 2002;36(4):397-406.