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Lecture 4

GEOG 221 Lecture 4.2.docx

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McGill University
GEOG 221
Nancy Ross

GEOG 221 Lecture 4.2 (Jan. 24 , 2013) Everything that you’re breathing is being affected by the air pollutants in the air Layer can vary from a few kilometers during the day, sometimes a few hundred meters at night: Temperature and mixing height: The dominant factor is the temperature profile:  Distribution of temperature with height  Either promotes or inhibits vertical mixing through convection  Atmosphere’s ability to mix is categorized by “stability” Adiabatic process  It doesn’t exchange energy with its surroundings  Air parcel: internal pressure > external pressure, expansion occurs  As the air parcel rises, it expands: it’s doing work, pushing outwards o Work requires energy o Uses internal energy o Cools down (temperature drops) o Rate of cooling is a constant: Dry Adiabiatic Lapse Rate (DALR)  Dry - not a 100% humidity so not saturated  Rate of temperature decrease in a non-saturated environment that does not exchange energy with its surroundings Lapse rates and Stability  Every km it rises up it loses 10°  Actual rate of increase or decrease is called the Environmental Lapse Rate (ELR)  Stability is the relative tendency for an air parcel to move vertically  If parcel is warmer than surrounding air it will be less dense, more buoyant and will continue to rise unaided  If parcel is colder it is more dense and will sink  If parcel and ambient air are at the same temperature, parcel has no net buoyancy and will remain in place Stability Conditions  ELR > DALR  UNSTABLE o Typical of sunny days near the ground o Surface heating  strong lapse rate o Parcel moving up is always warmer than ambient air and will continue to rise o Unstable: wants to move away from where it started  Air parcels rise and cool through expansion, sink and warm through compression  ELR < DALR  STABLE o ELR is negative or between 0 and 9.8°C km^-1 o Conditions typical of an inversion o Parcel is colder than surrounding air and will want to return to equilibrium position o Resists vertical motion  Stable atmospheres resist vertical motion, unstable atmospheres promote vertical motion  ELR = DALR  NEUTRAL o No difference between parcel and ambient air o Parcel will remain stationary o Typical of cloudy, windy conditions o Cloud restricts surface heating/cooling; wind helps homogenize temperature structure by mechanical convection o Atmosphere well mixed o Promotes air movement Types of temperature inversions  Surface emitting long wave radiation, losing energy  Layers of air, bottom one cools the most so air cools at the surface the most then starts getting hotter as your rise in layers  Daytime heating happens because of heating at the surface  At night, it cools at the surface and heat rises  Long clear, not windy, summer nights, longer time to cool - inversion  When there is wind, there is mixing: erodes the inversion  Topography: pockets of cold in a valley, so air above is warmer o Air at the bottom does not get moved out and mixed easily o Inversion comes in o Pollution stays in the valley because the atmosphere does not allow vertical motion (stable atmosphere)  Air goes up and keeps coming back down o If the mixing layer does not increase the concentration increases very quickly  INVERSIONS IMPORTANT IN AIR POLLUTION  Oceanic environments: ocean and land have different properties: o They heat and cool at different rates o Warm air on ground during the day rises and gets replaced by cold marine air o Warm air rises above cold air, causes inversion o Stable atmosphere: pollution remains over the city  Flat ground: Eastern US in the summer o Higher surface pressure, air slowly falling to the surface o Most of the atmosphere piled up at the surface (unevenly distributed) o As the air falls, it’s going to encounter more particles o The more distance you go down the more you compress and the more you warm o Temperature at the top becomes warmer than the temperature at the bottom because it travels more  Inversion  Not at the surface – starts somewhere up in the atmosphere  Most inversions are local inversions caused at the surface  This one is not  It acts like a lid  At the bottom the atmosphere could be very well mixed but at the point where the temperature starts rising is the mixing layer limit  The barrier within which the pollution is mixing is getting smaller  Concentration of pollution starts to increase  In winter, the mixing layer becomes very shallow, cars are still running, heaters, factories etc – smog warning Different conditions:  Strongly stable: inversion o High pressure systems or night o Turbulence weak or absent o Fanning – smoke spreading out horizontally, no vertical motion (suppressed) o Low concentration of pollutants as it is not dispersing downwards  Inversion starts above: worst possible case! o Inversion above traps plume (stable above neutral) o Neutral below allows much vertical mixing o Fumigation – smoke rises and is blocked by the lid, drops back down and encounters a neutral atmosphere which allows mixing so spreads out everywhere from the base inversion to the surface  Unstable o Typical of daytime, summer, clear, sunny conditions o Eddies larger than plume diameter o Minimal diffusion o Looping plume type – goes up and down, if it touches the ground it is very localized; not very well mixed but it can mix very well; no lid so eventually the smoke disperses and spreads everywhere  Neutral o Day/night, all seasons o Windy, cloudy – near neutral o Coning plume type o Equally good at mixing vertically and horizontally, so eventually it will reach the surface but not locally  Neutral above stable o Ideal conditions for dispersion o Early evening during transition from day to night – inversion starting to become the nighttime one o Lofting plume type: the parcel can’t go down because if it’s given a push downwards it will reach a stable atmosphere and want to rise again, so it will spread above the mixing layer and the surface will not really be touched The role of topography  At night, cold air tends to drain downhill settling in low-lying areas and valleys o Can strengthen existing inversions o Can transport pollutants downhill Country breeze  Thermally induced low pressure center over the city o Induces a convection cell which brings air in from surrounding rural area o If industry is located outside city, pollutants can be drawn into the city Sea breeze  Land and water have different heat capacities: it takes more energy to increase the temperature of the same amount of water than ground  Localized: air wants to move from high to low, causes a circulation, rising over the land, sinking over the water (cloud = typical of air rising) o Causes warmer air over cooler air = inversion Los Angeles inversion and pollution  Daytime: weak sea breeze, strong emission of primary pollutants, barrier at the east (mountains) and barrier at the west (sea), temperature inversion, clear skies (photochemical smog)  During the day: inversion height increases, strong sea breeze, smog accumulates at the mountains behind  Night: inversion height decreases during the night, land breeze (breeze inverses, ground cools faster, sea warmer at night), pollutants swept out to sea Huaibei, Central Chine, Jan. 14, 2013  Bright sunny day, but too much pollution to even see anything Domora, Pennsylvania, Oct. 28, 1948  Highly industrialized town, located in a valley, poor air quality, stable air acted as a lid, after 2 days visibility reduced, after day 3: people started to feel ill, day 4: 17 people dead, day 6: rain etc clear out the pollution (new weather system, plume washed out of the atmosphere, atmosphere cleaned, concentration goes down), 20 people dead Assessing pollution  Pollution potential o Probability for a high level of pollution in a given area if pollution sources were present  Soiling index o Measure of actual pollution levels experienced in a certain region  Terms used when assessing air quality in an urban environment or in “impact assessment” for planned industrial complexes The box model  Understanding pollution potential  Box has dimensions x, y and h(mixed layer or boundary layer)  Wind moves though the box at some rate (u)  A source of pollution at the bottom coming up (s)  Will result in some concentration (c)  Mass balance o Use a simple mass balance with the box model  Input of pollutants from sources (S per unit area) into box = Sxy (kgs^-1)  Pollutant output blown from the box by wind (u) is given by product of volume per unit time and concentration = yhuC (kgs^-1)  Assuming steady state conditions (no change with time) these must be equal: Sxy = yhuC o Solve for concentration ‘C’: o C= Sx/hu o Hu is called the “ventilation coefficient”  Higher means lower pollution concentration (air is better mixed above the source region)  The inverse of the ventilation coefficient is the pollution potential  PP = 1/(hu)  The more the windspeed: the more spreading out,
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