Grantee Research Project Results
2000 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, 1999 through December 1, 2000
Project Amount: $347,991
RFA: Urban Air Toxics (1999) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
The objective of this study is to 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. Current Lagrangian and Eulerian pollution models use advection algorithms that can, under many conditions, inaccurately simulate calculated impact areas. Attempts to "source apportion" measurements of toxic pollutants will contain large errors if advection and diffusion are inaccurately simulated. The approach is to first 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. This highly accurate numerical advection algorithm will then be used within an urban-scale 3-dimensional 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 is currently 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 effort 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 could also be used to improve the accuracy of source-apportionment investigations.Progress Summary:
During the first year of this research effort, the underlying advection algorithm that will be used for all of the subsequent diffusion and advection studies of this research effort was refined and simplified. The original advection algorithm (Walcek and Aleksic, 1998, "A simple but accurate mass conservative, peak preserving, mixing ratio bounded advection algorithm with FORTRAN code," Atmospheric Environment, vol. 32, pp. 2863-3880) has been simplified so that it is even more computationally efficient, yet at the same time the accuracy of the scheme has been significantly improved. The improved scheme has been published in the Journal of Geophysical Research (2000, see reference below), and is now being used in the preliminary assessment of the effects of shear on the transport and dispersion of pollutants from individual point sources. The major research areas focused on during the first year of the research effort are summarized below:- Refinement and simplification of advection algorithm. The accuracy of this advection algorithm for advecting numerous "test shapes" is usually superior to the highest-order advection algorithms available today (Prather, Bott L=8), but the computational requirements of this scheme are appreciably smaller than these algorithms.
- Application of advection algorithm to simulate diffusion in sheared environments. A journal publication is being prepared 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 is inherently "non diffusive" and heavily controlled by small amounts of ambient shear that is usually present in the winds of the lower troposphere.
- Search for a method to identify and more accurately simulate embedded gradients. One outstanding problem of all advection algorithms (including the scheme developed here) is that they cannot accurately simulate the advection of small steep gradients.
Future Activities:
The original innovation of the advection algorithm under development and testing was to identify local extremes of tracer distributions and perform minor adjustments during flux calculations to aggregate mass near the extremes to counter numerical diffusion that normally limits the accuracy of Eulerian methods for assessing pollutant transport. Similar problems also occur when local steep gradients are advected. During the remaining portion of this research effort, this limitation will be addressed.- Try to develop a method of preserving local steep gradients. We find that all existing advection algorithms (including the algorithm developed using EPA support) have trouble advecting tracer distributions containing steep gradients. We hope to develop methods for identifying local steep gradients, then performing mass adjustments in the vicinity of local gradients to increase the accuracy of transporting steep, small gradients. During the coming year, we will continue to search for a method of identifying gradients that can be used in conjunction with mass flux adjustments that can preserve embedded gradients. Thus far, we have had only limited success in solving this problem, and we have not yet identified a generally applicable technique of improving the advection of gradients. We have basically found that techniques to identify and improve gradient advection adversely and negatively impact the ability of the scheme to advect tracer distributions NOT containing gradients. So, while under limited conditions it is possible to vastly improve gradient advection, the adjustments necessary to preserve gradients degrade the advection other tracer distributions.
- Apply 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 out to 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. Since 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 using the Eulerian approach with our extremely accurate advection algorithm. Wind fields will be derived from observations, and individual puffs and plumes of pollution will be simulated. Specifically, a series of carefully designed studies to investigate and quantify the effects of shear on pollutant dispersion will be undertaken.
- Begin to study long-range pollution impact assessments using refined advection algorithms. We have gathered more than 150 days of extremely detailed, 80-km resolution, 3-D wind fields covering the domain of the Eastern North America. We plan to use these model-interpolated wind fields to construct long-range and longer-term impact assessments of individual point sources where synoptic-scale meteorological factors become extremely important in determining maximum impacts. Annual aggregation techniques used in previous studies of acid precipitation assessments will be used to assess annual impacts.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 16 publications | 3 publications in selected types | All 3 journal articles |
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Type | Citation | ||
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Walcek CJ. Minor flux adjustment near mixing ratio extremes for simplified yet highly accurate monotonic calculation of tracer advection. Journal of Geophysical Research 2000;105(D7):9335-9348. |
R827929 (2000) R827929 (Final) |
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Supplemental Keywords:
advection, transport, long-range dispersion, air ambient air, atmosphere, troposphere, exposure, risk, physics, engineering, environmental chemistry., 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.