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Concentration Fluctuations and Averaging Time in Vapor Clouds

Concentration Fluctuations and Averaging Time in Vapor Clouds

David J. Wilson

ISBN: 978-0-816-90679-6 December 1995 181 Pages


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This book contributes to more reliable and realistic predictions by focusing on sampling times from a few seconds to a few hours. Its objectives include developing clear definitions of statistical terms, such as plume sampling time, concentration averaging time, receptor exposure time, and other terms often confused with each other or incorrectly specified in hazard assessments; identifying and quantifying situations for which there is no adequate knowledge to predict concentration fluctuations in the near-field, close to sources, and far downwind where dispersion is dominated by atmospheric turbulence; identifying areas where further information is required to define concentration variability statistics; and formulating an operation model for concentration fluctuations based on the current state of knowledge of dispersion processes.


1. Background and Objectives.

2. Sampling and Averaging Time Definitions.

3. Effect of Averaging Time on Mean Calculations.

4. Concentration Fluctuation Modeling.

5. Probability Distributions.

6. Release Height and Source Size Effects on Fluctuation Intensity.

7. Source Density Effects on Fluctuations.

8. Buildings and Obstacles.

9. Threshold Crossing and peak Levels.

10. Framework for an Operational Model.

Appendix A. Averaging and Sampling Time Effects on Plume Spread Velocity and Concentration Fluctuations.

Appendix B. Peak Values and Threshold Crossing Probability.

Appendix C. Eulerian and Lagrangian Turbulence Scales.