Grantee Research Project Results
2002 Progress Report: Improved Simulation of Advection and Dispersion of Urban Air Toxics
EPA Grant Number: R827929Title: Improved Simulation of Advection and Dispersion of Urban Air Toxics
Investigators: Walcek, Chris
Institution: The State University of New York
EPA Project Officer: Chung, Serena
Project Period: December 1, 1999 through December 1, 2002 (Extended to August 2, 2004)
Project Period Covered by this Report: December 1, 2001 through December 1, 2002
Project Amount: $347,991
RFA: Urban Air Toxics (1999) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
The overall objectives of this research project are to develop computer modeling techniques to more accurately simulate the advection and dispersion of toxic pollutants in urban and regional scale environments. The two broad research areas that we are pursuing are to: (1) further develop and refine a highly accurate and computationally efficient algorithm for simulating the advection of poorly resolved point sources of toxic pollution in urban environments; and (2) more accurately understand the role that shear (changing directions and speeds of winds with height) plays in enhancing horizontal dispersion of pollutants.
The approach during the first 2 years of this research project was to refine a recently developed, highly accurate, and computationally efficient algorithm for calculating the advection of pollutants in atmospheric models. This scheme is absolutely monotonic, mass conserving, and is capable of advecting poorly resolved features with errors that are appreciably smaller than the best algorithms used today. The innovative feature of this algorithm that enhances its accuracy is the use of a flux adjustment near local extremes of a tracer distribution to reduce numerical diffusion. Unfortunately, most algorithms, including this scheme, fail to accurately advect tracer distributions containing steep gradients. As shown in this proposal, this algorithm can advect some sharp gradients with high accuracy, and it is hypothesized that it is possible to "adjust fluxes" near large gradients in a manner similar to the "peak" adjustment to limit numerical diffusion around steep gradients. An appropriate algorithm for identifying and correcting fluxes around sharp gradients must be derived.
During the third year of this research project, emphasis has shifted to studying the effects of shear on horizontal transport and diffusion. This highly accurate numerical advection algorithm was used within an urban-scale 3-dimensional (3-D) model of the planetary boundary layer (PBL) to simulate the transport and shear-induced diffusion of point sources of pollutants in urban areas. It is hypothesized that small amounts of vertical shear of the horizontal wind direction coupled with small amounts of isotropic turbulence will induce substantial horizontal dispersion that currently is poorly understood and simulated by urban-scale dispersion models, which assume "uniform" horizontal dispersion coefficients without recognizing that there is a preferential direction of horizontal dispersion aligned with the vertical wind shear vector. The results of this research project will be a highly accurate numerical advection algorithm for use in many applications, as well as a more thorough understanding of dispersion within the PBL. Methods and algorithms developed by this project could be used by other models to provide more accurate exposure and risk assessments of toxic pollutants, and also could be used to improve the accuracy of source-apportionment investigations.
Progress Summary:
During the third year of this research project, we are close to finishing an exhaustive investigation into refining a technique to more accurately advect gradients in Eulerian numerical models. We basically tried applying the "sharpening" technique that proved highly successful in our earlier study (Walcek, 2000) to higher order advection schemes. The Walcek method applies "sharpening" to an extremely simple low-order scheme, and the effect of "sharpening" near local extremes is to improve the accuracy of the low-order scheme so that it is comparable and even (most of the time) significantly more accurate than extremely high-order schemes.
We hypothesized that the improvement achieved by sharpening the low-order scheme also could have a similar effect on a higher order scheme. For example, we found that by sharpening the low-order scheme, errors were reduced by 70 percent: from root-mean-sequence (RMS) errors of 10 percent to 3 percent. A higher order (more accurate but complex) scheme has RMS errors of 4 percent, so our "sharpened" low-order scheme was simpler, yet more accurate than the higher order scheme. Therefore, we thought that by "sharpening" the high-order scheme, we might be able to similarly remove approximately 70 percent of the errors of that scheme. For the example above, maybe absolute RMS errors of the high-order scheme could be reduced from 4 percent to 1 percent, which would represent a significant advance over all existing advection algorithm techniques.
Unfortunately, we could not find a method to sharpen the higher order schemes that produced results superior to the sharpened low-order scheme. Although minor improvement could be achieved (in the example above, high-order errors could be reduced from 4 percent to 3.5 percent), the resulting "sharpened" high-order scheme was still worse than the already excellent performance of the sharpened low-order scheme.
During the third year of this research project, we have devoted a majority of our efforts to studying the effects of wind shear on the horizontal dispersion of pollution in the lower troposphere. A research publication entitled "Effects of wind shear on pollutant dispersion" was published in the journal Atmospheric Environment during this year, which summarizes our initial efforts in this area. Coincident with this research is the fortuitous availability of "profiler" instruments that are capable of accurately measuring hourly averaged profiles of wind speed and direction in the lower few kilometers of the atmosphere. These unique measurements represent an excellent archive of the measurements needed to quantify the absolute amount of shear present in the polluted planetary boundary layer.
Two publications are in preparation. One will describe the mathematical and physical basis for an improved Gaussian plume dispersion model that explicitly accounts for the effects of shearing of the wind perpendicular to the mean flow on plume dispersion. A second publication presents, for the first time, an explicit summary of the first several months' of observations of shear in the Northeastern United States PBL. Both of these papers show that: (1) shear is important for spreading pollutants that are released into the lower troposphere; and (2) shear is always present in the lower troposphere at a magnitude that makes it dominate the dispersion relative to dispersion caused by PBL turbulence alone. These results suggest that all future assessments of pollution dispersions must somehow account for the shear experienced by puffs or plumes of pollution in the lower troposphere. Recent technological developments allow us to directly measure this important physical quantity for the first time. Currently, shear is not even considered in the latest U.S. Environmental Protection Agency (EPA) guideline plume dispersion (e.g., AERMOD) models. In summary, progress on the major tasks of this research effort during this year include the following:
· The Search for a Method To Identify and More Accurately Simulate Embedded Gradients. Despite a comprehensive and arduous search, we could not define a generally applicable method for identifying regions of steep gradients. We adjusted slopes in the vicinity of steep gradients to improve advection performance. The major reason for this problem is that although "sharpening" effects are applied only near local extremes, there appears to be too much sharpening for tracer distributions being advected in 2-D and 3-D. There is one last attempt that will be pursued during the coming year, where we will approach the problem from a 2-D perspective and apply sharpening based on 2-D definition of local extremes.
· The Application of Advection Algorithm To Simulate Diffusion in Sheared Environments. A study using our improved advection algorithm was published, where "typical" observed boundary layer winds are used to advect puffs of pollution within a typical planetary boundary layer. We find that dispersion of pollution puffs inherently is "nondiffusive" and heavily controlled by small amounts of ambient shear that usually is present in the winds of the lower troposphere. In the future, we will use longer term winds to define concentrations downwind of individual point sources.
· The Development of Plume Models That Account for Shear. We have found a mathematically exact solution to the combined advection and diffusion problem, where there is mean shearing wind flow perpendicular to the mean motion of a plume, and we find that this slightly more complex Gaussian plume model can reasonably represent the observed dispersion behavior of plumes in the PBL.
· The Analysis of Available Measurements of Shear in the Eastern United States PBL. We are in the process of gathering an archive of the National Oceanic and Atmospheric Administration (NOAA) profiler wind measurements gathered at numerous sites across the Eastern United States. The NOAA profiler measures vertical profiles of the wind speed and direction in the lowest several thousand meters of the atmosphere averaged over hourly periods. Such measurements are the first high-quality measurements of true shear in the lower atmosphere that are virtually perfect for assessing the effects of shear on pollutant dispersion. Individual wind profiles derived from pibals or radiosonde launches are inherently limited by the fact that they can be unduly influenced by the effects of individual turbulent eddies during their ascent, and therefore cannot accurately measure the mean wind motions without being tainted by small-scale turbulent noise.
At the end of the third year of this 3-year research project, we qualitatively estimate that about 70 percent of the overall research objectives and efforts have been fulfilled. During the 4th year, we anticipate the completion of the full research agenda, and possibly the completion of the graduate student's studies.
Future Activities:
We will pursue one additional objective as far as further refinements of our advection algorithm, but most of the efforts in the coming year will be devoted to refining our understanding of shear and its effects on plumes of toxic pollution. We will perform the following:
· The Development of a Method of Preserving Local Steep Gradients. We find that we can improve the performance of higher order algorithms by using small amounts of "steepening" or "sharpening" to those schemes. We find that more accuracy can be obtained by severely limiting the number of cells that are subjected to steepening during advection calculation. For general 1-D advection, using a low-order scheme greatly improved performance results by only applying steepening at local extremes, which are a few grid cells per line being advected. However, we find that this produces too much steepening for 2-D calculations. We will attempt to restrict the number of cells experiencing steepening by defining the local extremes in 2-D, rather than the current 1-D method.
· The Application of the Advection Algorithm to Assess Urban-Scale Pollutant Transport and Dispersion. One limitation of existing plume models used to assess pollutant impacts is that plume models can only accurately assess impacts 10-20 km downwind of individual point sources. As more sophisticated longer range Eulerian or Lagrangian models are used to assess longer range impacts, numerous shortcomings of these models seriously limit the accuracy of these models. Because the advection algorithm developed here is computationally cheaper and significantly more accurate than existing advection algorithms, we plan to explicitly simulate fairly long-range transport of atmospheric mercury using the Eulerian approach with our extremely accurate advection algorithm. Specifically, a series of carefully designed studies to investigate and quantify the effects of shear on pollutant dispersion will be undertaken.
· The Use of Recent NOAA Profiler Measurements, and Quantification of the Magnitudes of Shear in the PBL. We will continue to gather an archive of NOAA profiler wind measurements at numerous sites across the Eastern United States. The NOAA profiler measures vertical profiles of the wind speed and direction in the lowest several thousand meters of the atmosphere averaged over hourly periods. We will analyze these measurements to define the wind shear parallel to and perpendicular to the mean PBL winds and publish these important physical parameters.
· The Development of a Gaussian Plume Model That Accounts for Turbulent Diffusion and Shearing Effects. We have derived an analytical solution to the combined advection and diffusion problem, where mean shearing motions perpendicular to the mean flow are included. We will publish these results along with measurements of shear described above so that future Gaussian plume calculations can explicitly account for the effects of shear on pollutant dispersion.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 16 publications | 3 publications in selected types | All 3 journal articles |
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Kreidenweis SM, Walcek CJ, Feingold G, Gong W, Jacobson MZ, Kim C-H, Liu X, Penner JE, Nenes A, Seinfeld JH. Modification of aerosol mass and size distribution due to aqueous-phase SO2 oxidation in clouds: Comparisons of several models. Journal of Geophysical Research 2003;108(D7):4213, doi:10.1029/2002JD002697. |
R827929 (2002) R827929 (Final) |
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Walcek CJ. Effects of wind shear on pollution dispersion. Atmospheric Environment 2002;36(3):511-517. |
R827929 (2001) R827929 (2002) R827929 (Final) |
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Supplemental Keywords:
advection, transport, long-range dispersion, air, ambient air, atmosphere, troposphere, exposure, risk, physics, engineering, environmental chemistry, ozone, acid deposition, precipitation, chemical transport, exposure, oxidants, sulfates, mathematics, modeling, general circulation models, climate models, air toxics, climate change, tropospheric ozone, Lagrangian approach, volatile organic compounds, VOCs, air pollutants, air pollution models, air quality, air quality models, atmospheric models, atmospheric pollutant loads, circulation model, climate variability, climate variations, dispersion modeling, fate and transport, plumes, stratospheric ozone, urban air, urban air pollutants., RFA, Scientific Discipline, Air, air toxics, Environmental Chemistry, climate change, Chemistry, tropospheric ozone, fate and transport, urban air toxics, Lagrangian approach, urban air, stratospheric ozone, air pollutants, plumes, air quality models, ozone, climate variations, VOCs, urban air pollutants, air pollution models, circulation model, atmospheric pollutant loads, Volatile Organic Compounds (VOCs), air quality, atmospheric models, climate variabilityProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.