Lecture 14, Fate and Transport of Contaminants in the Atmosphere, I

Transcription

1 Lecture 14, Fate and Transport of Contaminants in the Atmosphere, I Conrad D. Volz, DrPH, MPH-Course Director Bridgeside Point 100 Technology Drive Suite 564, BRIDG Pittsburgh, PA office : cell cdv5@pitt.edu Assistant Professor, Environmental and Occupational Health, University of Pittsburgh, Graduate School of Public Health ; Director-Center for Healthy Environments and Communities ;

2 Vertical structure of the atmosphere (Figure 1-19, p. 20 in Lutgens and Tarbuck's The Atmosphere, 2001)

3 Definitions and Types of Air Pollutants Aerosols- An aerosol is a suspension of solid particulates and/ or liquid droplets in a gas. The term aerosol, derives from the observation that matter diffusing or settling in air is a suspension in a fluid-the fluid here being air. Smoke-Extremely small solid particles, generally produced during incomplete combustion processes, such as from the burning of coal. Smokes also carry ash particles in suspension and can include soot, cinders, tar materials, and the gaseous material they are suspended in. Dusts These are solid particles projected into air by natural or mechanical forces. Most dusts are formed by the breakdown of larger particles. A dust is usually assumed to be less than 100 microns in diameter. Dusts can be of mineral sources (metals, rocks, sand etc) or even from animal or vegetable origin (grain, flour, pollen or even lint). A dust is an aerosol. Fiber in Air-a fiber is a dust of a special case. It is generally understood that a fiber has a length to width aspect ratio of 3:1 or greater. An example is asbestos because it splits longitudinally (as well as horizontally) making fibers of greater aspect ratio in these splits. Fibers in air are aerosols other fibers can be cotton fibers. Mists-Suspended liquid droplets in air generated through condensation from the gaseous state or generated through agitation or atomization of a liquid by mechanical means. A mist is an aerosol.

4 Definitions and Types of Air Pollutants Fog-A fog is closely associated with a mist but it is usually limited to airborne droplets which form due to condensation from the vapor state. The most important natural fog is of course water droplets formed by the condensation of water vapor in air. A fog is an aerosol. Smog- this is a composite of fog, smoke, fumes and /or mists that is anthropogenic. Smog is an aerosol. Fumes Fumes are solid particles that result from condensation from the vapor state of material which are usually solid. This can occur of course in smelting operations when you have molten metals and the resulting fume is a metal oxide because of a reaction of the metal with oxygen in the atmosphere. Fumes often aggregate into clumps. Vapors A vapor is the gaseous phase of a substance that is generally a liquid at STP (could be a solid). Examples are water, mercury, and volatile organic compounds. Solids like camphor, dry ice, water-ice and iodine produce vapors by sublimation-going directly from a solid to a vapor. A vapor acts like a gas when in the vapor state. A vapor could be part of an aerosol if it also contains suspended in it liquid or solid particulates. Gas formless fluids that occupy a space completely and diffuse freely in a space by diffusion and advection. Gases are really vapors which are at STP far above their boiling point. All gases can be made into liquids or solids if temperature is reduced sufficiently and pressures are made higher. Gases can be part of an aerosol in the same way that vapors may be a component of the aerosol.

5 PM 10 and PM 2.5 How Big Are PM10 and PM Compared to a Human Hair?

6 PM 2.5 Composition/Emissions Pie Chart Category Percent Elemental and Organic Carbon 1 (Combustion Related) 30% - 50% Sulphate 4 30% - 40% Nitrate 2 10% - 20% Soil 3 3% - 10% Notes:

7 Atmospheric Processes: Transport of Air Toxics, Source-Receptor Relationships Dr. Mark Cohen, NOAA Air Resources Laboratory

8 Source US EPA

9 Source; UNEP The reduction of the direct (unscattered) solar radiation at the Earth s surface is related to the aerosol optical depth (AOD). The transmission of the direct solar radiation aerosol optical decreases depth (AOD). exponentially The transmission with increasing of the direct optical solar depth. AOD is the best measure for column loading of aerosols. The reduction of the direct (unscattered) solar radiation at the Earth s surface is related to the radiation decreases exponentially with increasing optical depth. AOD is the best measure for column loading of aerosols.

10 The regional map of aerosol visible optical depth (AOD). The AOD values over ocean are retrieved from AVHRR data (Rajeev et al., 2000) and over land are estimated using a 4-D assimilation model (Collins et al., 2000). The figure illustrates the north-south gradient in AOD with values around 0.5 around the coast and less than about 0.1 south of equator. As described in Rajeev and Ramanathan (2001), the standard error of the seasonal average shown in this figure is about ± 0.02 or 15 %, whichever is greater. Jan-March

11 Satellite images of the INDOEX pollution cloud over the Indian Ocean. These observations were performed by NASA with the SeaWiFS instrument during INDOEX in the period January-March 1999.

12 Fractional contribution of chemical components to the INDOEX aerosol, as measured over the Indian Ocean by aircraft in February and March The total mass concentration is 22 μg/m3. MIS is minor inorganic species. These are free troposphere measurements and do not include boundary layer sea-salt.

13 Potential Transcontinental Nature of the "Haze" Forward Trajectories from 700 mb, March 14-21, 1999 Trajectories are from India, China, Mexico, US east and west coasts, London, Paris and Berlin (courtesy of T.N. Krishnamurti). UNEP Report

14 Stunning Picture of Advective Transport of Any Entrained Contaminant and Possible Wet Deposition Pattern

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19 The Standard Atmosphere Defined by International Civil Aviation Organization to represent typical atmospheric conditions at midlatitudes.(see 4-1 on page 286). At Sea level the Std. Atmosphere exerts a pressure of 760 torr (1 atm.) (14.7 #/in 2 ) and has a temperature of 15 C-59 F. Pressure decreases approximately exponentially with increasing altitude. This occurs because of the compressibility of air (while water is incompressible and thus hydrostatic pressure changes linearly with depth).

20 Balance of Forces in Air See figure 4-3-described in pressure and density forces. P*A-(P+(dP/dh)Δh)A-A Δhρg=0 Where P is pressure [M/LT 2 ], A is area in [L 2 ], ρ is air density [M/L 3 ], and g is acceleration due to gravity [L/T 2 ]. ρ =n/v*mw(air) {28.96 g/mol} and inserting into the ideal gas law we can derive the Pressure P at any height above Po by; P=P o e -(MW*g/(RT))h in practice temperature changes with altitude (6.5 C per 1000m) up to the lower limit of the stratosphere (37,000 ft). In the stratosphere the temperature is constant at -55 C-this is why the stratosphere inhibits vertical mixing-it causes stratification!

21 Pressure variations caused by altitude (Figure 1-17, page 18 in Lutgens and Tarbuck's The Atmosphere, 2001)

22 Local Scale Outdoor Air Pollution Smallest level of analysis is pollution of an air volume (remember control volume) from a single point source such as a coal fired electrical plant smokestack. So chemicals are carried downwind by advection and are also spread by Fickian forces in all dimensions. If we assume a continuous emission rate-we can employ a steady state model where; 1. The plume continues to disperse downwind. 2. At some point downwind the plume disperses in enough depth to reach the ground. 3. The point at which this happens and the concentration at each point is given by the advection-dispersion-reaction equation dc/dt=-v dc/dx +d/dx (D dc/dx)+r (eq. 1-5) This means the rate of change of chemical storage at any point, dc/dt, equals the sum of both the rates of chemical input and output by physical means and the rate of internal net production (sources-sinks). V is fluid velocity, D is the diffusion-dispersion coefficient and r is associated with the inputs or outputs from sources or sinks.

23 Smokestack Plume Modeling The modeling is analogous to the groundwater plume model except that the Fickian mixing mechanism is turbulent diffusion---not mechanical dispersion. Figure 4-24 shows cross sections of pollutant concentrations at two distances downwind of a stack. Please note the effective stack height is higher than the stack itself. All concentration versus distance plots are normally distributed. Pasquill-Gifford Model-used if there is flat terrain, steady emissions, and no mixing height (no inversion exists) and no sinks (nothing to degrade the chemical) C=Q/u (g 1 g 2 /2π σy σz) -C here is a time averaged value where C is the concentration in air; Q is the rate of chemical emission [M/T] ; u is the wind speed [L/T] in x direction;g1 is the horizontal gaussian distribution factor;g2 is the vertical gaussian distribution factor; and σy and σz are the standard deviation [L] of the pollutant concentrations in their respective directions.

24 Smokestack Plume Modeling Continued Calculation of Gaussian Distribution Factors g1 = exp(-0.5y 2 /σy 2 ) g2= exp (-0.5 (z-h )2 / σz 2 ) + exp (-0.5 (z+h )2 / σz 2 ) Where y is the distance along a horizontal axis perpendicular to the wind, z is the distance along a vertical axis with its origin on the ground surface. H is the effective stack height. H-the effective stack height is the physical stack height plus the plume rise (upward momentum and positive buoyancy of gases). ΔH is often given by 1.6 Fb 1/3 x 2/3 / u where ΔH is the plume rise [L], Fb is the buoyancy flux factor [L 4 /T 3 ], x is downwind distance [L], and u is the wind speed [L/T]. Fb = g (d 2 V/4) (Ts-Ta)/Ts) here g is acceleration due to gravity, d is the stack diameter {L}, V is the stack gas velocity [L/T], Ts is the absolute stack gas temperature and Ta is the absolute ambient air temperature

25 Example 4-7 Furnace emits 0.8 m3/sec of exhaust gas at 80 degrees C. Outside air is 10 degrees C and wind speed is 2 m/sec. The stack is 20m tall and diameter is 40 cm. What is plume height at boundary 30 m downwind? 1. Estimate stack gas velocity V= 0.8 m3/sec/π(0.2m) 2 = 6.4 m/sec 2. Use 4-19 to get Fb the buoyancy flux factor Fb=9.81m/sec2*(.4m) 2 /4* 6.4m/sec*(353K-283K/353K) =0.50m4/sec3 Use 4-18 to estimate the plume rise 1.6 Fb 1/3 x 2/3 / u so ΔH = (1.6) (.5m4/sec3) 1/3 (30m) 2/3 (1/2 m/sec) =6m So 30 m from the emission the plume height is the stack height of 20m+ plume rise 6m=26m