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Learn about air pollution, turbulence, stability, and the atmospheric engine. Explore how the ideal gas law applies to air quality and how atmospheric factors impact pollution dispersion. Discover various plume types and terrain effects.
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Ideal gas law Although polluted air may not be ‘’ideal’ from the biological point view, we may treat is behavior with respect to temperature and pressure as if were ideal. We assume that at the same temperature and pressure, different kind of gases have different density proportional to their molecular weight may being writing in this form.
Unit measure Unit measure are used to indicate the concentration of a gaseous pollutant.
Convert 80g/m3 of SO2 to ppm at 25C and 101.325kPa • Convert 0.55 ppm of NO2 to g/m3 at -17.7C and 100 kPa • Convert 370 ppm of CO2 to g/m3 at 20C and 101.325 kPa
AIR POLLUTION METEREOLOGY • TURBULENCE • STABILITY • THE ATMOSPHERIC ENGINE • TERRAIN EFFECTS
TURBULENCE • MECHANICAL TURBULENCE • Turbulence is the addition of fluctuations in the wind velocity, as compared to the average wind velocity. • It is caused by fact that the atmosphere is sheared as it moves. • This shearing occurs because the air actually sticks to the ground (even though we may not feel it) due to friction. Therefore the wind velocity at the earth's surface is zero.
As the mass of air moves across the earth, the air on top moves faster than the air on the bottom and falls over the slower air. This "tumbling" creates a swirling motion. • The faster the average wind velocity, the more tumbling and swirling is created.
THERMAL TURBULENCE • When the earths surface is heated by the sun, it will also heat the air directly above it. • Since hot air is less dense than cool air, this heated air will rise from the earths surface to a higher elevation. • This movement forces a vertical rotation of the air because the cooler air sinks to the bottom as the warm air rises.
In the evening, the opposite occurs. The cold ground cools the air that is above is, causing it to become more dense. • This dense air will feel heavy and will sink even closer to the ground.
STABILITY • Stability is defined as the atmospheres ability to enhance or resist vertical motion. • The stability of the atmosphere is affected by the wind speed and by the lapse rate (the change in air temperature with height) of the atmosphere. • The atmosphere is classified as either stable, neutral, or unstable.
Neutral Stability • the temperature of air parcel moving up or down adjusts to that of its surrounding and the rate of cooling is the same as the adiabatic lapse rate of 1C/100m. • In other words, the temperature will drop by 1 degree C for every 100 meters we go up into the air.
UNSTABLE ATMOSPHERE • When the rate of air cooling with altitude is greater than >1C/100m, the air mass becomes unstable and rapid mixing and dilution of pollutants occurs. • If air is moving up, it is warmer than its surroundings and it will continue to climb • whilst conversely, if the air is moved down, it is cooler and denser than surroundings and it will continue to fall. • This steeper temperature gradient encourages greater thermal turbulence.
STABLE ATMOSPHERE • However, if the rate of cooling with altitude is slower than the adiabatic lapse rate of 1C/100m (ie<1C/100m ), the air will remain stable and pollutants will concentrate. • This occurs commonly at night and during winter.
PLUME TYPES • The smoke trail or plume from a tall stack located on flat terrain has been found to exhibit a characteristic shape that is dependent on the stability of the atmosphere • The 6 classical plumes are shown in figure along with the corresponding temperature profile
LOOPING • common in early afternoon • Require windy conditions which cause the plume can swirl up and down • Moderate and strong winds are formed on sunny days creating unstable conditions CONING • Happen at late morning • Require moderate winds and overcast days • wider than it is deep, and is elliptical in shape
FANNING • Common at night • Require stable air and slow vertical movement of the emission • temperature inversion limits the rise of the plume into the upper atmosphere FUMIGATION • Common in early morning • occur when the conditions move from stable to unstable • unstable air causes the plume to move up and down - can cause localised pollution
LOFTING • Common in late afternoon • When plume is above the inversion layer (or there is no inversion), it becomes a lofting plume • Normal wind direction and speed will disperse the plume into the atmosphere without effect from ground warming or cooling.
ATMOSPHERIC ENGINE • Atmospheric like an engine (continually expanding and compressing gases, exchanging heat and generally raising chaos) • Driving energy comes from the sun • Diff. in heat input between the equator and the poles provides initial overall circulation of the earth’s atmosphere. • Rotation of the earth coupled with different heat conductivities of the ocean and land produce weather.
TERRAIN EFFECT • HEAT ISLAND • LAND/SEA BREEZES • VALLEYS
HEAT ISLAND • absorbs and reradiates heat at greater than surrounding area. • Causes moderate to strong vertical convection currents above the heat island. Can be nullified by strong wind • Industrial complexes and cities
Depending upon location of the pollutant, it can be good or bad news. - good news: ground level sources such automobiles, the bowl of unstable air that forms will allow greater air volume for dilution of pollutant. -Bad news: stable conditions plume from tall stacks plumes from tall stacks will carried out over country Unstable conditionsheat island will mixes these plumes to the ground levels
LAND/SEA BREEZES • Stagnant anticyclone strong circulation will develop across the shorelineof large water bodies During night The land cools more rapidly than the water. Cooling air over the land flows toward the water. `land breeze’(bayu darat) During morningland heats faster than water. The air over the land become warm and rise. The rising air is replaced by air from over the water body. ‘lake breeze’(bayu laut)
Effect of lake breeze on stability: -imposed a surface based inversion on the temperature profile • Air moves from the water over the warm ground. Thus, stack plumes originating near the shoreline,stable lapse rate causes a fanning plume close to the stack. The lapse condition grows to the height of the stack as the air moves inland. At some point inland,a fumigation plume results
Valleys • Moderate to strong winds, valleys oriented at an acute angle to the wind direction channel the wind • The valleys peels off part of the wind and forces it follow the direction of valley flow (page 588,figure 7.19) • Valley will have its own circulation under stagnating cyclone. Valley air will be warmed by warming valley walls. It become more bouyant and flow up. At night, wind will flow down. • Valley walls protect the floor from radiative heating by sun. walls and floor are free to to radiate heat away to the cold night sky.
What is a dispersion model? Mathematical description of the meteorological transport and dispersion process that is quantified in terms of source and meteorologic parameters during a particular time
Basic Point Source Gaussian Dispersion Model • The model gives the ground level concentration (X) of pollutant at coordinate (x,y) downwind from a stack with an effective height (H) • The equation model is as follows:
Where; = Downwind concentration at ground level, g/m³ = emission rate of pollutant, g/s = plume standard deviations, m = wind speed, m/s = distances, m = exponential x is the crosswind distance from the centerline of the plume y is the downwind distance along plume mean centerline from point source
Value of effective stack height, H Where h = physical stack height ∆H = plume rise ∆H may be computed from Holland’s formula as follows;
Where; = stack velocity. m/s = stack diameter, m = wind speed, m/s = pressure, kPa = stack temperature, K = air temperature, K
A – Extremely unstable B – Moderately unstable C – Slightly unstable D – Neutral E – Slightly stable F – Moderately stable
A – Extremely unstable B – Moderately unstable C – Slightly unstable D – Neutral E – Slightly stable F – Moderately stable
TABLE 7-8 Key to stability categories
Algorithm to express stability class lines developed by D.O Martin (1976) where the constants a, c, d and f are defined in Table 7-9
Inversion Aloft • Vertical standard deviation, Sz = 0.47(L – H) Where L = Height to bottom of inversion layer, m H = Effective stack height, m • When the distance is > 2XL, the centerline concentration of pollution may be estimated using equation below:- -------- (7-25)
Example 7.4 It has been estimated that the emission of SO2 from a coal-fired power plant is 1656.2 g/s (E). At 3km downwind on an overcast summer afternoon, what is the centerline concentration of SO2 if the wind speed is 4.50 m/s (u)? (Note: “centerline” implies y = 0) Stack Parameters: Height, h = 120.0 m Diameter, d = 1.20 m Exit velocity, vs = 10.0 m/s Temperature, T = 315ºC Atmospheric conditions: Pressure, P = 95.0 kPa Temperature, Ta = 25.0ºC
Solution • Determine effective stack height (H) • Determine atmospheric stability class based on Table 7-8 Since it is stated in the question that it has overcast condition, thus class D is used.
iii) Determine plume standard deviations, Sy and Sz 2 ways to determine: 1st Graphical method ( Figure 7-22 & Figure 7-23) 2nd Equation 7-22 & 7-23 Hence; iv) Substitute all values into Eqn 7-19 to obtain centerline concentration of SO2
