Atmospheric Dispersion, Transport and Deposition EOH 468 Spring 2008 Dr. Peter Bellin, CIH, Ph.D. Dispersion Atmospheric process affect dilution. Wind speed and lapse rate impact on emissions. Planetary Boundary Layer (PBL) Up to 2000 meters from surface Relatively stable layer in troposphere above PBL. Mixing in PBL is variable in short term. Wind Speed Varies with height. Profile is affected by topography. 1
Wind Speed The wind profile affects dilution (image from Boubel, 1994) Wind Direction Affected by meteorology, and topography. Depending on conditions, small changes can have big impact on pollution concentration. Quite variable Long range transport. Turbulence Mechanical Smaller in scale, regular pattern Work around edges of a plume Thermal Larger in scale, more variable Can transport larger bundles of air. 2
Mechanical turbulence Caused by air moving over and around structures/vegetation Increases with wind speed Affected by surface roughness Turbulence Turbulence will usually dilute pollutants Downwash is an exception. Thermal turbulence Caused by heating/cooling of the earth s surface Flows are typically vertical Convection cells of upwards of 1000-1500 meters 3
Thermal turbulence Effects Enhances mixing/pollutant dispersion Downwash may result from mechanical turbulence Increases ground level concentrations Atmospheric stability Back to temperature profiles Atmospheric stability affects plume behavior and dispersion. Dry adiabatic lapse rate is constant, air cools as it rises. -9.8 degrees C per 1000 meters. Air contains water; it will cool at dry lapse rate until water starts condensing, then cool at (variable) wet adiabatic lapse rate. -6.5 degrees C per 1000 meters. Atmospheric Stability Environmental Lapse rate refers to the actual temperature profile, as impacted by meteorological variables. This is what can lead to temperature inversions, and stable or unstable air. 4
Atmospheric Stability The relationship between environmental and dry lapse rates determines stability of air. Here, we see a mixing height. Atmospheric stability As parcels of air rise, they are cool less than the environment and buoyancy is enhanced. Unstable air. Sunny days with low wind speeds. Atmospheric Stability Neutral conditions. Environmental Lapse rate is equal to dry lapse rate. Dividing line between stable and unstable conditions. Windy days or cloud cover 5
Atmospheric Stability Stable conditions. Air rises, but then is cooler than surrounding air, so falls back down. Night time with little or no wind. Atmospheric Stability Radiational inversion. Ground cools at night, air is stable near the ground. Types Radiational Subsidence Frontal Advective Inversions 6
Inversions Frontal -warm air overrides cooler air Advective - warm air flows over a cold surface or cold air Radiational inversions Result from radiational cooling of the ground Occur at night -nocturnal Typically surface based Radiational inversions Occur on cloudless nights Are intensified in river valleys Cause pollutants to be trapped 7
Radiational inversion Radiational inversions Breakup after sunrise Breakup results in elevated ground level concentrations Breakup described as a fumigation Radiational inversions Elevated inversions are formed over urban areas Due to heat island effect Due to dust dome 8
Dust dome and inversion Subsidence inversion Associated with high-pressure systems Inversion layer is formed aloft Covers hundreds of thousands of square kms Persists for days Subsidence inversion 9
Subsidence inversion Migrating high-pressure systems Semi-permanent marine high-pressure systems Results in large number of sunny calm days Inversion layer closest to the ground on continental side Responsible for air stagnation over Southern California Mixing Height Height of air that is relatively vigorously mixed and where dispersion occurs Varies from one region to another Mixing Heights 10
Atmospheric Stability Associated with high pressure systems. Subsiding air warms, becomes warmer than air below: inversion forms Stable for longer periods than radiational inversions. Stability and plume behavior Stability and plume behavior 11
Stability and plume behavior Stability and plume behavior Stability and plume behavior 12
Plume height depends on temperature of plume, and air at top of stack. Plume Height Dispersion Modeling Models can be used to predict dispersion and transport of stack emissions. The figure represents a Gaussian plume Air Pollution Modeling Now essential for effective planning and control. See EPA web site: http://www.epa.gov/scram001/dispersionin dex.htm Modeling photochemical smog: http://www.epa.gov/scram001/photochemi calindex.htm 13
Urban Plume Consider all the sources in an urban region. Large geographic area Regional transport with weather systems. Photochemistry will occur in the plume. Long Range Transport Historical assumption: dispersion results in minimal effects distant from the source. 1970 s: observed long range transport of ozone, with elevated levels at night. Acid rain issue, for example in the NE US. Arctic haze resulted form pollution emitted in Europe and Asia. Natural and anthropogenic pollution. Long Range Transport 14
Removal/deposition Atmospheric lifetimes Average of the life histories of all molecules of a substance May also be described as residence time Can be characterized by half life Time to decrease to 50% of initial value Removal Mechanisms Chemical reactions do this, as substances are converted, for example, from gas to particle. Sedimentation < 20 µ treat as gases 20 to 100 µ move with the plume, but drop due to gravity > 100 µ particles particles fall rapidly Removal Mechanisms Dry Deposition refers to particles impacting on surfaces of vegetation. This is described by a deposition velocity, and can include chemical reactions. Wet Deposition refers to scavenging in clouds (cloud droplets), or below clouds (rain droplets). 15
Deposition processes Dry deposition Transfer of gas/particulate phase substances to ground, water, vegetation by: Impaction Diffusion Settling Physiological uptake Deposition rates Depositional processes Wet deposition Processes by which gases/particles are brought to the earth s surface in aqueous form Absorption in cloud/rain droplets Particles serve as condensation nuclei in in-cloud processes Described as rainout 16
Emission inventories Emission inventories are essential for input in air pollution modeling. Tutorial is here: http://www.epa.gov/air/oaqps/eog/course4 19a/index.html 17