06.3.airpollt3.aos104f08.sld9 - Vertical Air Movement...

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Unformatted text preview: Vertical Air Movement Volume per unit mass and density per unit volume cancel each other: Air Pollution Meteorology • Does hot air sink or rise? • What is the change in temperature with • ! Start with the 1st law of thermodynamics and the ideal gas law dQ! heat added to air parcel (J/kg) Cp! specific heat dT ! temperature change V! volume per unit mass dP! incremental pressure in parcel elevation? How can we show this mathematically? !d = " dT g =! dz Cp This is defined as the Dry Adiabatic Lapse Rate g = 9.8 K km # 10 K km Cp dQ = C pdt ! VdP This is the rate of temperature change that would be experienced by a dry parcel of air moving up or down in ideal conditions. B 1 B 4 B 7 Air Pollution Meteorology • Air Pollution Transport by Convection ! Adiabatic Lapse Rate ! Atmospheric Stability ! Air Parcel Buoyancy Assume change in parcel is adiabatic ! no heat is transferred across boundary (dQ = 0) In English units = 5.4 °F/ 1000 ft) dT V = dP Cp Need to relate temperature change to elevation, not pressure A “slice” of air that is dz thick and has area A and weight of g!Adz g = acceleration due to gravity ! = density of air Example At 30,000 ft (~ 9 km), air is ~ –40°F. Does an airplane that uses it for fresh air have to heat it or cool it after compressing it to sea level pressure? • Chimney Plumes ! Plume Shape vs. Atmospheric Stabilty ! Plume Spread • Temperature Inversions B 2 B 5 B 8 Air Pollution Meteorology • To discuss air pollution we need to understand the physical state of the atmosphere. Relating the pressure above and below this parcel: g ! Adz P (z ) = P (z + dz ) + A dP ! P (z + dz ) " P (z ) Saturated Adiabatic Lapse Rate z + dz z Pz+dz ! Are there strong winds? Long-range transport Advection by winds = " g #dz ! Pz ! Is the air moving vertically? sunlight? Buoyant plume Rainout, washout Emission Turbulence (eddy mixing) Convection Surface deposition Source S urce Receptor-1 Receptor-1 Receptor-2 eceptor-2 Receptor-3 Rece t r-3 dP = "g # dz The adiabatic lapse rate for saturated air is (smaller? larger?) than the lapse rate for dry air. The Cp for water undergoing a phase change is much larger than that of air. If air is saturated, any cooling will result in condensation. Condensation heat raises temperature inside the air parcel, offsetting some loss of temperature from adiabatic expansion Take the earlier equation dT V = dP Cp ! Is there a lot of ! When was the last precipitation event? But, we are really interested in dT dz B 3 dT dT dP V = = (! g " ) dz dP dz Cp 5.x B 6 B 9 • The actual rate of temperature change with elevation is the ambient lapse rate or environmental lapse rate ! Depends on the effects of wind, sunlight, water vapor. and clouds on temperatures near the ground and aloft ! Global average environmental lapse rate near the ground is about 6.5°C/km • Since the (adiabatic) lapse rate for the air parcel is fixed and the ambient lapse rate is variable, the tendency for a lifted air parcel to be warmer or colder than the surrounding air will depend on the value of the ambient lapse rate z Equilibrium States of Stability Neutral equilibrium states neither suppress nor encourage changes " " ! >" ! <" ! =" B 10 T B 13 An atmosphere with an adiabatic ambient lapse rate will mix vertically B 16 Relating the Ambient Lapse Rate to Stability Equilibrium States of Stability Stable equilibrium states resist perturbations Chimney Plumes vs. Atmospheric Stability Chimney plume dispersion depends on atmospheric stability—the less stable the atmosphere, the quicker the plume disperses However, this has the opposite effect on air quality at the ground, since the plume starts above ground and must disperse downward to reach people • • To determine the stability of an air parcel, we compare the ambient lapse rate " to the adiabatic lapse rate ". Imagine a parcel of air that starts out at the same temperature and pressure of air around it. " ! <" ! If the parcel is forced upward, it will expand adiabatically ! This will decrease the internal energy and hence the temperature of the air parcel. B 11 An atmosphere with a sub-adiabatic ambient lapse rate will tend not to mix vertically B 14 B 17 ! If the air parcel is now cooler than the air around it, the parcel will have the tendency to sink back downward ! If the air parcel is warmer than the air around it, it will continue moving upward Equilibrium States of Stability Unstable equilibrium states easily accelerate away • While we preferred unstable atmospheres to help disperse pollutants away from the ground, we need a stable atmosphere to keep elevated plumes away from the ground! " ! >" B 12 An atmosphere with a super-adiabatic ambient lapse rate will mix vertically B 15 B 18 Temperature Inversions The atmosphere is heated from below, but still it can have layers that are thermally stratified. a) Cool marine air OCEAN OCEAN Warm air from land Warm air from land Prevailing winds Prevailing winds LAND Indoor Air Pollution Inversion height increases during the day Smog accumulates here High pressure system High pressure system c) Stagnant, polluted air Stagnant, polluted air Ground IInversiion base nvers on base • Large-scale Inversion Infrared radiation from surface Infrared radiation from surface Marine Layer Inversion Marine Layer Inversion Secondary pollutants peak in afternoon (what time?) near foothills Nighttime land breeze sweeps pollution out to sea Strong sea breeze • Background, sources, definitions • Indoor air pollution concentration calculations Maximum heating of land Maximum heating of land Mid-afternoon b) High deserts Air warmed Air warmed by compression by compression Inversion base Inversion base d) co ld air ld co air • • Radon gas ! Background and health effects ! Sources ! Mitigation Inversion height decreases during the night Cool, polluted marine air Basin Regional Inversion Radiation Inversion Radiation Inversion a) Cool marine air undercuts warm air over land b) Flow of air from a high plateau descends and warms by adiabatic compression, creating a cap above a cooler air mass in a basin below the highlands. c) Sinking air adiabatically compresses and warms aloft d) Ground cools at night, creating cool layer of air at the ground Pollutants swept out to sea Land breeze Maximum cooling of land Maximum cooling of land Late night • Combustion sources B 22 25 B 19 B The atmosphere undergoes a temperature inversion at ~12km. Below this is the troposphere. Above this is the stratosphere. Large thunderstorms often “bump” into this region. Indoor Air Pollution Fundamentals • Indoor pollutants may create largest portion of health risks from air pollution. ! We spend a lot of time indoors ! We are not far from the sources ! The biggest of these (on average) is from smoking and radon – Yet health effects correlate strongly with outdoor pollutant concentrations. B 20 B 23 B 26 A Smoggy Day in L.A. • • • Primary pollutants emitted in the morning over the city Ocean breeze sweeps primary pollutants toward foothills Inversion layer keeps pollution trapped in valleys, sunlight helps photochemical buildup of ozone Temperature inversion Strong emission of primary pollutants Weak sea breeze Ocean Los Angeles Basin Small land-ocean temperature contrast Mountains INDOOR AIR QUALITY Most Americans spend the majority of their time indoors. Pollutant Source Asbestos fireproofing, insulation, vinyl floors combustion particle board, carpet, foam insulation Mid-morning Indoor Air Pollution CO Formaldehyde Particles NOx O3 wood stoves, “pig pen” effect space heaters, stoves electrostatic cleaners, dog brushes (??!!??) Inversion height increases during the day Smog accumulates here Strong sea breeze Radon SO2 diffusion from soils, groundwater kerosene space heaters cooking, cleaning products, showers Maximum heating of land Maximum heating of land Mid-afternoon VOCs B 21 B 24 B 27 More Fundamentals • How does the exchange of indoor air and outside air happen? windows are closed: Modelling Indoor Air Quality Use a material balance “box model” to get indoor concentration accumulation rate = input rate +sources – output rate – decay If all of the air is replaced each hr, the exchange rate is 1 ach. A high ach is 3 or 4; typical value 0.5 to 1, new, energy efficient construction can be 0.1 ach. !"#$%&'()&'#""*) +&*),-+%&./) C (t ) = S 1 ! e It IV ( ) ! Infiltration—air exchange when doors and We can build on thenot separable in this equation, so we will have box model to examine indoor air to guess and do it iteratively. pollutants. !+00)1+%+'2$) Cannot solve for the infiltration rate since I is Know C(t) = 10 – Air moves through cracks around doors and windows, through gaps around where plumbing and wires enter house, and through the foundation-wall connection. Evaluate right hand side with I = 0.5 ach – The average house has ~ 1 m2 of holes… presumably more in Southern California! B 28 dt C = indoor concentration (mg/m3)! V = volume of conditioned space in building (m3/air change) I = Q/V = ach = infiltration rate! ! atmos_104_051707.doc S = pollutant source strength (mg/hr) Ca = ambient (=outside) concentration of pollutant (mg/m3) K = decay rate or reaction rate of pollutant (hr–1) accumulation rate = input rate +sources – output rate – decay dC V = S + C a IV ! CIV ! KCV S dC dt V C (t ) = 1 ! e ! It # S " Ca IV ! CIV ! KCV IV ( )= 2000 1 ! e 0.5 " 5 = 18.8 mg/m3 0.5 " 65 " 3 ( ) The air exchange rate must be higher if the 30 actual concentration is 10 mg/m3. B 31 B 34 The steady-state solution can be found by setting • How does the exchange of indoor air and outside air happen? ! Natural ventilation: open the doors and windows to let fresh air in. ! Forced ventilation: mechanical air handling systems brings in outside air. dC =0 dt S + CaI ! C (") = V I +K OR, notice that the exponential term is small and first evaluate without it: I= S 2000 = = 1.03 ach CV 10 ! 65 ! 3 and the general solution is: C (t ) = S V So try 1.0 ach + CaI 1 ! e ! ( I + K )t + C ( 0 ) e ! ( I + K )t I +K ( ) C = 10.2 mg/m3 (good enough)! B 29 where C(0) is the initial concentration of the pollutant in the building. B 32 B 35 Definitions • Air changes per hour (ach): a number that tells how many times per hour a volume equal to the house’s volume gets exchanged. the exchange rate is 1 ach. to 1 Example Carbon monoxide is emitted from your gas oven at 2000 mg/hr. You plan to cook your turkey for 7 hours. Assume you live in a small apartment (65 m2) with 3 meter ceilings. COi = 0 when you started cooking. Your CO detector went off after 5 hours. Its alarm is set at 10.0 mg/m3, the 8-hr ambient standard. Find the air exchange rate in your apartment. Assume that the outside [CO] = 0 as a first approximation; CO is fairly unreactive (K = 0) Radon (222Rn) • Radon is an indoor air pollutant that has received a lot of attention recently are known carcinogens • If all of the air is replaced each hour, • A high ach is 3 or 4; typical value 0.5 • New, energy efficient construction can be 0.1 ach. ! Radon gas and its radioactive daughters ! It is estimated that this is the second leading cause of lung cancer, after smoking. B 30 B 33 B 36 Other Radon Sources Lung Cancer Deaths, 1986 Smoking history Never smoked Former smoker Light smoker* Heavy smoker Total * less than 25 cig/day Mitigation Radon mitigation techniques have been developed for airborne radon. Most of these focus on ventilating basements or subslab regions of houses in areas that are radon-rich. • Radon can also be captured on 500 6,400 4,500 4,200 15,700 Population (million) 145 43 38 14 241 All Causes 5000 57,000 37,600 30,800 130,400 Radon Attributable groundwater, which is released during aeration (showering, cleaning). B 37 B 40 B 43 Radon forms in soil from decay of radioisotopes T38U # 226Ra # of 2 here are two routes exposure to radon. 222Rn Radonit seeps then can be emitted from soils and earth upward and is derived materials. sucked into the Radon can also be house The radon potential is a function of soil and rock types. Combustion and Indoor Air Quality • Heating and cooking appliances are sources of indoor air pollutants such a CO, NOx and VOC’s. This figure nicely illustrates a step function input and the decay that followed. Most experiments have shown that emission levels are very variable with equipment type, maintenance, and operation. !"#$%&'(")*+),*(),""-*+(-*.%+/('0* Heating and cooking appliances are sources of indoor air pollutant such a CO, NOx and VOC’s. This figure nicely illustrates a step function input and the decay that followed. • captured on groundwater, which is released during aeration (showering, cleaning). • atmos_104_051707.doc 39 atmos_104_051707.doc B 38 35 B 41 B 44 These geographic differences are reflected in These geographic differences are reflected in radon concentrations radon concentrations in groundwater supplies in groundwater supplies Radon mitigation techniques have been developed for air borne radon. Most of these focus on ventilating basements of houses in areas that are radon rich. atmos_104_051707.doc 39 Add the complications of air exchange rates and decay mechanisms, and indoor problems might look like this! B B 42 38 B 45 ...
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