Grantee Research Project Results
Final 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 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 were to develop computer modeling techniques to simulate more accurately the advection and dispersion of toxic pollutants in urban and regional scale environments. The research focused on two areas:
- further development and refinement of a highly accurate and computationally efficient algorithm to simulate the advection of poorly resolved point sources of toxic pollution in urban environments within Eulerian models;
- and more accurate understanding of the role that shear (changing directions and speeds of winds with height) plays in enhancing horizontal dispersion of pollutants.
Summary/Accomplishments (Outputs/Outcomes):
Under this research grant, a highly accurate and computationally efficient algorithm for calculating the advection of pollutants in atmospheric models was refined, tested, and published. This algorithm is starting now to become more widely used within the air pollution modeling community and other disciplines of the physical sciences. This computational scheme is absolutely monotonic, mass conserving, and capable of advecting poorly resolved features with errors that are appreciably smaller than the best algorithms used previously in Eulerian models. The scheme is being tested in the developing Community Multiscale Air Quality (CMAQ) and Weather Research and Forecasting (WRF) modeling efforts, and numerous modeling groups have contacted the principal investigator of this project for the published code for incorporation in their models. The innovative feature of this algorithm that enhances its accuracy is the use of a relatively minor flux adjustment near local extremes of a tracer distribution to reduce numerical diffusion.
An outstanding problem that was addressed during this research effort is the failure of advection schemes (all schemes, including the scheme developed here) to advect steep gradients embedded within tracer distributions accurately. During the proposal phase of this research effort, it was hypothesized that the highly successful flux adjustments used near local extremes also could be applied near steep gradients to alleviate errors associated with advecting gradients.
Unfortunately, after considerable effort and investigations of numerous methods of identifying and “steepening” gradients, we could not find a generally applicable method to alleviate the errors inherent in advecting steep gradients without seriously compromising other desirable features of this innovative advection algorithm. It was concluded that the simple and accurate linear scheme would have to be made much higher order (and thus overly complex and computationally expensive) to advect embedded gradients accurately. Thus, we encountered an inevitable trade-off: this highly desirable, accurate, and simple scheme that works so well for a wide variety of atmospheric and test applications would have to become unwieldy, overly complex, and computationally expensive to address a relatively minor problem of advecting steep embedded gradients that is sometimes encountered.
During the later portion of this research effort, emphasis has shifted to studying the effects of shear on horizontal transport and diffusion of pollution. Following publication of a paper showing the effects of shear on the dispersion of pollution “puffs,” we derived an explicit method to account for shear in standard Gaussian plume models and have published a more scientifically accurate and less empirical Gaussian plume model that accounts for both turbulence (quantified with eddy diffusion coefficients) and shear (∂v/∂z). To use this more accurate shear-dispersion plume model, measurements of shear in the Planetary Boundary Layer (PBL) are required as input, so considerable efforts were devoted to analyzing newly available profiler measurements to extract the shear information required for this model. The results of this research effort show that Lagrangian methods of defining transport and dispersion, especially those that track only individual “puffs” (no puff “splitting” allowed, as default in the CALPUFF model) will contain serious deficiencies in quantifying downwind impacts from pollution sources. Currently, shear is not even considered in the latest EPA guideline plume dispersion models (e.g., AERMOD).
The advection scheme resulting from this research effort sets a new standard for numerical advection schemes in terms of accuracy, mass consistency, computational efficiency, and zero nonmonotonic oscillations. One outstanding problem remains in the area of numerical advection schemes: the ability to simulate accurately the transport of embedded steep gradients. Despite exhaustive efforts and “blind alley” approaches to solving this minor shortcoming, we were not able to upgrade our superior numerical advection algorithm to alleviate this minor deficiency. The ability to advect an embedded gradient accurately remains a challenge for future developments in this area.
Using this new, accurate, advection algorithm, we studied the effects of shear on the dispersion of puffs and plumes of pollution in the PBL. Based on results of Eulerian simulations, we mathematically derived a new and improved, more accurate Gaussian Plume Dispersion Model for simulating concentrations downwind of point sources that accounts for the effects of both turbulence and shear. The model is less empirical, and more based on first principles of science and math than the standard EPA-approved Gaussian plume models (e.g., AERMOD). Based on an exhaustive analysis of remotely sensed PBL winds derived from National Oceanic and Atmospheric Administration (NOAA) profilers, we find that shearing motions are ubiquitous within the PBL, and the magnitudes of the shear will cause it to be the dominant factor influencing horizontal plume spreads at distances beyond a few kilometers from a point source. Horizontal plume dispersion is specified in highly empirical fashion in current plume models, and this research suggests that shear can explain a significant fraction of the observed behavior of plumes downwind of point sources.
Conclusions:
The state-of-the-science has been advanced for quantifying advection and diffusion of pollutants in the atmosphere computationally. Within Eulerian models, the advection algorithm developed under the support of this research effort represents the most accurate and absolutely monotonic advection scheme available today, and it is being incorporated rapidly into numerous air pollution models around the world. An improved Gaussian Plume Dispersion Model has been rigorously derived using a mathematical solution to the steady-state advection-diffusion equation for point sources of pollution emitted into an environment containing turbulence and wind shear. It is found that shearing motions perpendicular to the mean flow significantly enhance horizontal plume dispersion in a manner consistent with observations. According to this mathematical derivation, shearing effects lead to significantly larger plume dispersions (∫) than can be explained by turbulence alone, and plume spread increases with powers of downwind distance (x) ranging from ~ x 0.5 to ∫ ~ x 1.5, depending on the distance from the release location, and the relative magnitudes of turbulence and shear in the flow. This new theory, therefore, is able to quantitatively explain both the observations of larger plume spread and larger plume growth rates.
Two publications are in preparation. One describes the mathematical and physical basis for an improved Gaussian plume dispersion model that explicitly accounts for the effects of shear of the mean flow on plume dispersion. A second publication presents for the first time an explicit summary of over a year’s worth 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 the shear dominate the dispersion relative to dispersion caused by 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 measure this important physical quantity directly for the first time. Currently, shear is not even considered in the latest EPA guideline plume dispersion models (e.g., AERMOD).
Our analysis of observations of shear shows that in the lowest kilometer, wind shear fluctuates widely and is influenced strongly by synoptic-scale weather disturbances and diurnally forced processes. Shear perpendicular to the mean wind reaches its greatest magnitude of slightly less than -6 m/s/km during about 3:00-9:00 a.m. local time, and is lowest near sunset (-2 m/s/km). Generally winds “veer” with height (wind direction turning clockwise with increasing height above the surface) in the lowest kilometer, although about 20 percent of the time, winds “back” with height (counterclockwise rotation with height). On average, we see about -3.7 m/s/km wind shear perpendicular to the mean wind in the lowest km, although the 90th and 10th percentile shears are -10 m/s/km and +2 m/s/km. Even during daytime periods when the lowest kilometer is clearly within the “well mixed” PBL, “well mixed” wind speeds and directions are not observed. Shear magnitudes are considerably greater than calculated thermal wind or Ekman shear would dictate, suggesting that shear in the PBL is not in a balanced dynamic state with respect to the forces acting on air in the lowest kilometer.
Journal Articles on this Report : 3 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. 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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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,, 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.