ATMOSPHERIC CONDITIONS WHICH AFFECT AIR POLLUTION • Wind direction and speed • Cloud cover radiation reaching earth temperature profile turbulence • Precipitation Neutral Stability Unstable Stable Inversion in Los Angeles, CA Inversion at Four Corners power plant Fig 7-17: Land Breeze during night Fig 7-18: Land Breeze during day Fig 7-19: Effect of lake breeze on plume dispersion Fig 7-21: Plume dispersion coordinate system Fig 7-22: Horizontal dispersion coefficient Fig 7-23: Vertical dispersion coefficient Building Downwash Building Downwash Building Downwash Building Downwash Plume Rise • H is the sum of the physical stack height and plume rise. H = Δhplume rise + hactual stack http://www.air-dispersion.com/briggs.html -1 Rural Terrain Urban Terrain Stability Exponent n Stability Exponent n A 0.10 A 0.15 B 0.15 B 0.15 C 0.20 C 0.20 D 0.25 D 0.25 E 0.25 E 0.40 F 0.30 F 0.60 Example: A given windspeed of 5 m/s measured at 10 meters above the ground and a stability class of B in rural terrain, calculate the windspeed at 500 meters above ground: 0.15 uz = (5)(500/10) = 9 m/s Table 1 Table 2 For Use In Rural Terrain For Use In Urban Terrain Stability Exponent n Stability Exponent n A 0.10 A 0.15 B 0.15 B 0.15 C 0.20 C 0.20 D 0.25 D 0.25 E 0.25 E 0.40 F 0.30 F 0.60 Gaussian Dispersion Models • Most widely used • Based on the assumption – plume spread results primarily by molecular diffusion – horizontal and vertical pollutant concentrations in the plume are normally distributed (double Gaussian distribution) • Plume spread and shape vary in response to meteorological X conditions Z Q u Y H Fig 7.11 Model Assumptions • Gaussian dispersion modeling based on a number of assumptions including – Steady-state conditions (constant source emission strength) – Wind speed, direction and diffusion characteristics of the plume are constant – Mass transfer due to bulk motion in the x-direction far outshadows the contribution due to mass diffusion – Conservation of mass, i.e. no chemical transformations take place – Wind speeds are >1 m/sec. – Limited to predicting concentrations > 50 m downwind Gaussian Dispersion Equation Q ⎡ 1 ⎛ y 2 (z − H )2 ⎞⎤ C(x, y, z) = exp⎢− ⎜ + ⎟⎥ 2 u 2 ⎜ 2 2 ⎟ πσ yσz ⎣⎢ ⎝ σ y σz ⎠⎦⎥ Why isn’t x in the equation? Atmospheric Stability Classes Table 7.4 Dispersion Coefficients: Horizontal Fig 7.12 Dispersion Coefficients: Vertical Fig 7.13 Gaussian Dispersion Equation If the emission source is at ground level with no effective plume rise then Q ⎡ 1 ⎛ y 2 z 2 ⎞⎤ C(x, y, z) = exp⎢− ⎜ + ⎟⎥ u 2 ⎜ 2 2 ⎟ πσ yσz ⎣⎢ ⎝ σ y σz ⎠⎦⎥ Wark & Warner, “Air Pollution: Its Origin & Control” 2 2 Q ⎡ y 2 ⎤⎪⎧ ⎡ (z − H ) ⎤ ⎡ (z + H ) ⎤⎪⎫ C(x, y, z) = exp⎢− 2 ⎥⎨exp − 2 + exp − 2 ⎬ 2 u 2 ⎢ 2 ⎥ ⎢ 2 ⎥ πσ yσz ⎣⎢ σ y ⎦⎥⎩⎪ ⎣ σz ⎦ ⎣ σz ⎦⎭⎪ What if the surface is absorbing? How should the concentration profile look like w/ reflection? Maximum Ground Level Concentration Under moderately stable to near neutral conditions, σ y = k1σ z The ground level concentration at the center line is Q ⎡ H 2 ⎤ C x,0,0 exp ( )= 2 ⎢− 2 ⎥ πk1σ z u ⎣ 2σ z ⎦ The maximum occurs at H dC / dσ = 0 ⇒ σ = z z 2 Once σz is determined, x can be known and subsequently C. Q Q C(x,0,0)= exp[−1]= 0.1171 πσ yσzu σyσzu σ yσ z Q ⎛ y 2 ⎞ ⎛ (z − H 2 ) ⎞ C(x, y, z) = exp⎜− ⎟exp⎜− ⎟ 2 U ⎜ 2 2 ⎟ ⎜ 2 2 ⎟ πσ yσ z ⎝ σ y ⎠ ⎝ σ z ⎠ ⎛ 2 ⎞⎧ ⎛ 2 ⎞ ⎛ 2 ⎞ Q ⎜ 0 ⎟⎪ (2 − H ) (2 + H ) C(400,0,2) = exp − 2 ⎨exp⎜− 2 ⎟ + exp⎜− 2 ⎟ 2 U ⎜ 2 ⎟ ⎜ 2 ⎟ ⎜ 2 ⎟ πσ yσ z ⎝ σ y ⎠⎩⎪ ⎝ σ z ⎠ ⎝ σ z ⎠ Q ⎪⎧ ⎛ (2 − H 2 ) ⎞ ⎛ (2 + H 2 ) ⎞ = ⎨exp⎜− 2 ⎟ + exp⎜− 2 ⎟ 2 U ⎜ ⎟ ⎜ ⎟ πσ yσ z ⎩⎪ ⎝ 2σ z ⎠ ⎝ 2σ z ⎠ σ y ,σ z : 0.894 d σ y = a*x ,σ z = c*x + f σ y σ z Q ⎪⎧ ⎛ (2 − H 2 ) ⎞ ⎛ (2 + H 2 ) ⎞ C(400,0,2) = ⎨exp⎜− 2 ⎟ + exp⎜− 2 ⎟ 2 U ⎜ ⎟ ⎜ ⎟ πσ yσ z ⎩⎪ ⎝ 2σ z ⎠ ⎝ 2σ z ⎠ Determine the ground level concentration (Cx) at some downwind distance (x). For the following conditions, calculate the ground level concentrations at 10 km directly downwind. A power plant burning 9 tons of 2.5% sulfur coal/hr emits SO2 at a rate of 113 g/sec. The effective stack height is 100 m, and the wind speed is 3 m/sec. It is 1 hour before sunrise, and the sky is clear. Since the off centerline distance (Y) in this case is equal to O, the following equation reduces to: From table, the atmospheric stability classes for the condition described is F. It represent a nighttime condition with <37.5% cloud cover. The horizontal dispersion coefficient σy for a downtime distance of 5 km for atmospheric stability class F is approximately 90 m (from figure, next slayt); the vertical dispersion coefficient σz is approximately 20 m (from figure) The ground level concentration of SO2 from this source would be 3 90 approximately 44 μg/m under the conditions given. 5 20 5 1. A power plant burns 100 tons of coal per hour containing 2%sulfur. On a sunny summer afternoon the wind at 10 m above the ground is 4 m/s from NE. The morning radiosonde taken at a nearby weather bureau station has indicated that a frontal inversion aloft will limit vertical mixing to 1500 meters. The effective height of emission is 150 meters. Find the distance to the maximum ground level concentration. What is the concentration at this point? 2. Solve the above problem for overcast conditions. All other data are same as before. 3. For the condition given in problem 1, tabulate ground-level centerline SO2 concentration with distance from 100 meters to 1000 kilometers 100 tons/h, 2%sulfur, U10= 4 m/s , NE, Inv= 1500 m He= 150 m Example • An industrial boiler is burning at 12 tons (10.9 mton) of 2.5% sulfur coal/hr with an emission rate of 151 g/s. The following exist : H = 120 m, u = 2 m/s, y = 0. It is one hour before sunrise, and the sky is clear. Determine downwind ground level concentration at 10 km. Stability class = σy = σz = C(10 km, 0, 0) = Exercise • If emissions are from a ground level source with H = 0, u = 4 m/s, Q = 100 g/s, and the stability class = B, what is downwind concentration at 200 m? At 200 m: σy = σz = C(200 m, 0, 0) = Example • Calculate H using plume rise equations for an 80 m high source (h) with a stack diameter = 4 m, stack velocity = 14 m/s, stack gas temperature = 90o C (363 K), ambient temperature = 25 oC (298 K), u at 10 m = 4m/s, and stability class = B. Then determine MGLC at its location. F = h plume rise = H = σz = σy = Cmax = Residents around Florida Rock Cement Plant are complaining its emission being violating its allowed level. The plant has its facility within 0.5 km diameter. Its effective stack height is 60 m. You are a FLDEP environmental specialist. Where are you going to locate your air quality monitors? Why? Atmospheric dispersion models ADMS 3 AERMOD ATSTEP CALPUFF DISPERSION21 ISC3 NAME MERCURE RIMPUFF SAFE_AIR PUFF-PLUME Converting Atmospheric Pollutant Concentrations: from mg/m3 to ppmv The conversion factor depends on the temperature at which you want the conversion (usually about 20 to 25 degrees Centigrade). At an ambient pressure of 1 atmosphere, the general equation is: ppmv = (mg/m3)(273.15 + °C) / (12.187)(MW) ppmv = ppm by volume (i.e., volume of gaseous pollutant per 106 volumes of ambient air) mg/m3 = milligrams of gaseous pollutant per cubic meter of ambient air MW = molecular weight of the gaseous pollutant °C = ambient air temperature in degrees Centigrade As an example, for gaseous pollutant NOx, convert 20 mg/m3 to ppmv at 25 °C: ppmv = (20)(273.15 + 25) / (12.187)(46.01) = 10.6 where: 46.01 = molecular weight of NO2 (i.e., NOx expressed as nitrogen dioxide) NOTES: (1) The pollution laws and regulations in the United States typically reference their pollutant limits to an ambient temperature of 20 to 25 °C as noted above. However, in other nations, the reference ambient temperature for pollutant limits may be 0 °C or other values. (2) 1 percent by volume = 10,000 ppmv (i.e., parts per million by volume). (3) For all practical purposes, degrees Centigrade and degrees Celsius are synonymous. Converting Atmospheric Pollutant Concentrations: from ppmv to mg/m3 The conversion factor depends on the temperature at which you want the conversion (usually about 20 to 25 degrees Centigrade). At an ambient pressure of 1 atmosphere, the general equation is: mg/m3 = (ppmv)(12.187)(MW) / (273.15 + °C) mg/m3 = milligrams of gaseous pollutant per cubic meter of ambient air ppmv = ppm by volume (i.e., volume of gaseous pollutant per 106 volumes of ambient air) MW = molecular weight of the gaseous pollutant °C = ambient air temperature in degrees Centigrade As an example, for gaseous pollutant NOx, convert 20 ppmv to mg/m3 at 25 °C: mg/m3 = (20)(12.187)(46.01) / (273.15 + 25) = 37.6 where: 46.01 = molecular weight of NO2 (i.e., NOx expressed as nitrogen dioxide) Effect of Altitude on Atmospheric Pollutant Concentrations: Atmospheric pollutant concentrations expressed as mass per unit volume of atmospheric air (e.g., mg/m3, ug/m3, etc.) at sea level will decrease with increasing altitude because the atmospheric pressure decreases with increasing altitude.