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Transport of Air Pollutants Marti Blad, Ph.D., P.E. Yavapai Apache Nation What is the Difference? 2 What we will learn Pollutants move and spread in air Up and down (vertical) Away and outward (horizontal) Dispersion vs Diffusion Slowly “spread” out in all directions (diffusion) 3 Diffusion & dispersion 4 Transport of Air Pollutants Weather conditions are important Temperature Affect Molecule dance Pressure Affects density Volume Temperature and Pressure related 5 3 parts to puzzle Source of pollutant Stationary vs Mobile Control technologies Physical, Chemical, Biological How transported Fate of pollutant Toxicology & Chemistry Receptor or recipient 6 Pollutants moving through air 2 ways to look at mathematically Box: Mass Balance Flux = mass / (time x area) Follow one particle X, y, z, and time Mass Transport Pollutant has mass so can be tracked Models use mass 7 Momentum transfer Air pollutants can move horizontally As fast as wind wind speed Flow = advection, In direction of wind (wind direction) Wind Rose activity later w James Pressure currents in atmosphere Diffusion different than Dispersion 8 Heat transfer Air pollutants can move vertically Convection Cities as “Heat Island” Air temperature changes with altitude Ambient air temperature decreases as you go up Colder on mountain than in Phoenix Pressure changes with altitude Pressure decreases as you go up Less Molecules on top of you at 7000 ft 1 atm = sea level 9 Changes in meteorology and climatology Molecules have mass and are transported Heat transfer Temperature difference Momentum Wind speed Wind direction Pressure systems 10 Atmospheric stability When air overhead is cold compared to air near ground, vertical motion stronger. (Unstable) When air overhead is closer to ground temperature prevents air from moving vertically. (Stable) When air is warmer overhead than near ground, “inversion” (molecule dance lid) 11 How does stability affect what I can see? Smoke plume behavior – a useful indicator Predict Good burning days Models use “stability class” How can pollutants concentrate? Under an inversion Maximum mixing height Mixing height: Height plume will rise to given prevailing atmospheric conditions 12 Predict Stability by slope temperature change altitude change Can be positive or negative slope Ambient Lapse Rate Recorded by weather stations Dry Adiabatic Lapse Rate (DALR)? Theoretical line with constant slope Slopes are rates (per time) How dry air rises and falls Cools 1° C for each 100 meters rise Warms 1° C for each 100 meters fall 13 DALR Dry adiabatic lapse rate Air expands as pressure decreases Function of elevation Rate at which dry air cools at it rises Adiabatic = no heat exchange Approx. 1° C for every 100 meters Speed pollution disperses & diffuses Based on “Air stability” Relationship between ambient & DALR Compare slopes 14 Mixing Height: Adiabatic compared to ambient 15 Atmospheric stability See Pictures comparing Slopes Neutral= DALR slope Superadiabatic Unstable air favors dispersion Molecules moving Subadiabatic Stable air so poor dispersion Inversion Warm air over cold air 16 Slopes of different conditions DALR subadiabatic Inversion superadiabatic Dry adiabatic lapse rate = neutral 17 Stability affects plume shape Series of pictures to help you understand new vocabulary smoke stacks image from Univ. of Waterloo Environmental Sciences 18 19 20 21 Stability affects plume shape Superadiabatic looping plume Adiabatic coning plume Inversion Fanning plume Inversion over superadiabatic fumigation 22 Atmospheric Stability Classes How is stability classified? Stability classifications A = strongly unstable C = slightly unstable E = slightly stable B = moderately unstable D = neutral F = moderately stable How does stability relate to air pollution? UNSTABLE Good vertical mixing & dispersion of pollutants STABLE or INVERSION Poor vertical mixing & dispersion of pollutants 23 24 Vertical Dispersion & Diffusion Worst pollution episodes often correspond to inversion trapping pollution near ground during calm periods Shallow inversions common at night & in winter (can be especially strong in geographical basins where cold air pools) Deeper inversions can be caused by large-scale subsidence of air. As air moves toward ground, compressed and heated. Can lead to a capping inversion layer 3000 to 6000 feet off ground 25 What do you think is happening here? 26 Another Example http://eol.jsc.nasa.gov/debrief/Iss007/topFiles/ISS007-E-13281.htm 27 Upper Air Data Soundings Radiosondes Dropsoundes Rocketsondes Isotherms Isobars Isohumes What We Just Covered Pollutants move and spread in air Diffusion and Dispersion Vertically and horizontally Transport phenomena Weather conditions dictate transport Actual temperature profile=ambient DALR = theoretical comparison Speed pollution disperses & diffuses Based on Air stability class Source height & mixing height 30 Laboratory: Create an Inversion Hands-on exercise in stability (see manual)