Module 9 Atmospheric StabilityPhotochemistryDispersion Modeling
MCEN 4131/5131 Preliminaries • FCQs? • Seminar Monday DLC 11am “Air quality in California” • Pizza Party! May 2 6pm, the Sink • 3rd midterm • here in this classroom • Thursday May 4 9:30am - 10:45 am
MCEN 4131/5131 Gaussian Plume • Transport of contaminants downwind of their point of discharge • Predict ground-level concentrations • Problem statement • Emissions from a point source: buoyant pollutant stream exiting a stack of geometric height, h • Plume rises a distance, dh (plume rise) • Stack gas transported downwind, pollutants dispersed vertically and horizontally • Horizontal dispersion by molecular and eddy diffusion • Vertical dispersion by atmospheric stability
MCEN 4131/5131 Advection-Diffusion Equation Conservation of Mass equation: Generation loss accumulation Molecular flux due to concentration gradient Advective flux due to fluid flow Assume fluid incompressible,wind blows constantly in X direction, use K theory for turbulent diffusion, steady state: Solution to the above equation is called Gaussian Plume equation
MCEN 4131/5131 Group Clicker Question • The Gaussian Model of plume dispersion tells us that the concentration at some point downwind from the stack is linearly related to the ______. • average wind speed at stack height • emissions rate • vertical distance from ground level
MCEN 4131/5131 Stability Classes • The tendency of the atmosphere to resist or enhance vertical motion and thus turbulence is termed stability • Stability is related to both the change of temperature with height (the lapse rate) and wind speed • Quiz question: Which stability class depicts the most unstable atmosphere? • A • C • E
MCEN 4131/5131 Project Prairie Grass
MCEN 4131/5131 Project Prairie Grass • Conducted during the summer of 1956 in flat, open prairie country in northcentral Nebraska • 70 trials, released sulfur dioxide continuously for a 10-minute period • Sulfur dioxide in the cloud measured by sensors located along arcs from 50 to 800 meters downwind of the point of release
MCEN 4131/5131 Project Prairie Grass • Observations used from this and similar experiments to see how quickly a gas cloud spreads outwards under different atmospheric stability classes. Pasquill, Gifford, Briggs, and others then developed equations to describe this spreading • Note that during the experiments, most measurements made at < 1 km, only a few made at 10 km. So…?
MCEN 4131/5131 Group clicker question? • Which atmospheric stability produces the highest ground-level centerline concentration nearest to the stack using the Gaussian model? • stable • neutral • unstable
MCEN 4131/5131 Hospital Waste Example • It is common practice for the infectious waste collected at hospitals to be burned in on -site incinerators. The presence of chlorinated plastics in these wastes lead to the production of polychlorinated dioxins (PCDD) that are released to the atmosphere from the exhaust gases. Because hospitals are centrally located in densely populated areas, the potential for human exposure to these toxic chemicals is large. As sources of air pollution, these incinerators are largely unregulated and uncontrolled.
MCEN 4131/5131 Hospital Waste Example • Provide an assessment of the total human exposure to PCDD using the Gaussian plume model. a) Determine for typical meteorological conditions, the location of the maximum ground-level concentration downwind of the incinerator b) Calculate the ground-level concentration at the position determined in part a c) Assuming that incineration is the best disposal method for infectious waste, discuss options for reducing the maximum PCDD concentrations downwind.
MCEN 4131/5131 Example Cont’d • Note: although the waste bags are fed to the incinerator intermittently, you may assume for this analysis that the emission rate is constant. • You may treat the ground as a perfectly reflecting source • Average occupancy is 350 beds • Production rate of waste to be burned in 1135 kg/mo per 100 beds • 10% of waste by weight consists of chlorinated plastics • 0.1% of the chlorinated plastic mass is released as PCDD • Stack height is 15 m • Mean wind speed is 5 m/s • The dispersion coefficients may be represented by (stability class D):where for y, a = 0.219, b= 0.764 and for z, a = 0.140, b = 0.727 and the dispersion coefficients are in meters