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Ohio Northern University CE 3231 - Introduction to Environmental Engineering and Science Introduction Chemistry, Microbiology & Material Balance Water & Air Pollution Atmospheric Dispersion (Modeling) The atmosphere is a complex and dynamic system. Under certain conditions, the atmosphere can trap pollution at the Earth surface and lead to adverse health effects. Understanding near surface atmospheric conditions and their effect on pollution dispersion is the point of this lecture. Env Risk Management Readings for This Class: 5.5-5.6 Lecture 27 Atmospheric Dispersion (Modeling) (Air Quality II) Introduction to Air Quality Air Quality II Topics Covered Include: Adiabatic Lapse Rate Atmospheric Stability Inversions Plume Shapes and Dynamics Gaussian Plume Modeling Simplified Model to Predict Downwind Ground Level Pollution Adiabatic Lapse Rate • Temperature drops as you increase in altitude Γdry = 9.76 °C km Γsaturated = 6 °C km or 5.4 in tropopause °F 1,000 ft Atmospheric Stability • Stable Air – Air mass remains at set altitude – Problematic for pollution • Unstable Air – Air mass changes altitude – Favors mixing and pollution dispersion Temperature Inversions – – – – – Stable atmosphere Nocturnal cooling Earth cools rapidly, traps colder air on bottom Pollution can get trapped near surface Daytime sunlight radiates ground and breaks inversion http://www.metoffice.gov.uk/education/secondary/students/smog.html Plumes and Adiabatic Lapse Rates Plumes and Adiabatic Lapse Rates Plumes and Adiabatic Lapse Rates Gaussian Plume Modeling Gaussian Plume Modeling æ -y 2 ö æ -H 2 ö C ( x, y) = exp ç 2 ÷ exp çç 2 ÷÷ p uH s ys z è 2s z ø è 2s y ø Q Gaussian Plume Modeling æ -y 2 ö æ -H 2 ö C ( x, y) = exp ç 2 ÷ exp çç 2 ÷÷ p uH s ys z è 2s z ø è 2s y ø Q where: C (x,y) = downwind conc. at ground level(z=0), g/m3 Q = emission rate of pollutants, g/s sy, sz = plume standard deviation, m uH = wind speed, m/s x,y, and z = distance, m H = stack height How will the model change for ground-level along the plume? æ -y 2 ö æ -H 2 ö C ( x, y) = exp ç 2 ÷ exp çç 2 ÷÷ p uH s ys z è 2s z ø è 2s y ø Q Simplified Plume Modeling (Downwind Ground-Level Concentration) æ -H 2 ö C ( x, 0) = exp ç 2 ÷ p uHs ys z è 2s z ø Q Gaussian Plume Modeling æ -H 2 ö C ( x, 0) = exp ç 2 ÷ p uHs ys z è 2s z ø Q where: C (x,0) = downwind conc. at ground level (z=0, y=0), g/m3 Q = emission rate of pollutants, g/s sy, sz = plume standard deviation, m uH = wind speed, m/s x,y, z and H = distance, m Dispersion Constants A – very unstable; B – moderately unstable; C- slightly unstable, D – neutral, E – slightly stable; F – stable Gaussian Plume Modeling æ -H 2 ö C ( x, 0) = exp ç 2 ÷ p uHs ys z è 2s z ø Q A new power plant releases SO2 at a legally allowable rate of 6.5x108 mg SO2 /sec. The stack has an effective height of 300 m. An anemometer on a 10 m pole measures 2.5 m/s of wind, and it is a cloudy summer day. Predict the groundlevel concentration of SO2 4 km directly downwind. Gaussian Plume Modeling æ -H 2 ö C ( x, 0) = exp ç 2 ÷ p uHs ys z è 2s z ø Q Q = 6.5x108 mg SO2 /sec H = 300 m. U = 2.5 m/s @ 10 m X= 4 km Atmospheric condition = ? uH = ? sy , sz = ? Finding the Atmospheric Condition Atmospheric Condition A – very unstable, sunny day with wind speed < 3 m/s B – moderately unstable, sunny day clear night with winds between 3 – 5 m/s C- slightly unstable, sunny day with winds > 5 m/s D – neutral cloudy or overcast day; cloudy night winds > 3 m/s; clear night winds > 5 m/s E – slightly stable cloudy or overcast night winds < 3 m/s F – stable clear night winds < 5 m/s Finding uH Atmospheric Condition (p value) A – (0.15) B – (0.15) C- (0.2) D – (0.25) E – (0.4) F – (0.6) æH ö uH = ua ç ÷ è za ø p p 0.25 æH ö æ 300 ö uH = ua ç ÷ = 2.5ç ÷ = 5.85m / s è 10 ø è za ø Finding sy , sz D – neutral @ 4 km sy = 250 m and sz = 80 m Gaussian Plume Modeling æ -H 2 ö C ( x, 0) = exp ç 2 ÷ p uHs ys z è 2s z ø Q Q = 6.5x108 mg SO2 /sec H = 300 m. U = 2.5 m/s @ 10 m X= 4 km Atmospheric condition = D uH = 5.85 m/s sy = 250 m sz = 80 m Gaussian Plume Modeling A new power plant releases SO2 at a legally allowable rate of 6.5x108 mg SO2 /sec. The stack has an effective height of 300 m. An anemometer on a 10-mile pole measures 2.5 m/s of wind, and it is a cloudy summer day. Predict the ground-level concentration of SO2 4 km directly downwind. æ -300 2 ö 650g / s C ( x, 0) = exp ç 2÷ p 5.85(250)(80) è 2(80) ø C ( x, 0) =1.56ug / m3