1Steady State Box ModelA defined volume of air (the box) receives pollutionfrom a source, while pollution is removed at the same time by a sink processSourceSinkIn a “steady state” the concentration (and the totalamount) of the pollutant inside the box does notchange (is constant)Box Model FormulaSSqVVτSq×=V = Volume of boxS = Source rate= Sink rate τ = residence time (τ: tau)q = steady state concentrationof pollutant in boxIf we know the source rate of a pollutant and its residence time in the atmosphere, we can calculate its concentration in a given volume.Sources and SinksSources:Everything that introduces pollutants into the air in the box• direct emissions(cars, industry,…)• transport by wind• chemical formation Sinks:Processes that remove or convert pollutants• wind blows pollutants away (ventilation)• chemical conversion• pollutants are depositedon the ground (rainfall)2An Example: Your next partynumberof guestsq8pm10pm2am4amin:sourceguest arrivingguest leavingout:sink# guests insteady stateas many guest leave per hour as arriveBox Model FormulaSSqVVτSq×=V = Volume of boxS = Source rate= Sink rate τ = residence time (τ: tau)q = steady state concentrationof pollutant in boxSource/Sink Ratestintervaltimetintervaltimeinstemitted/losubstanceofAmountS =in a steady state the source and the sink ratesare equalParty: Source rate: People arriving per hourSink rate: People leaving per hourLet’s say S = 5 guests/hour3Residence timeτ = average period of time that a molecule of a pollutant is in the box before it is removedParty: How long does a guest on averagestayin your home.Let’s say τ = 2 hoursBox Model FormulaSSqVVτSq×=Your party at steady-state:S = 5 guests/hourτ= 2 hoursV = 1Average number of guests in your house? q=S x τ = 5 guests/hour x 2 hours = 10 guests4Sources of aerosols• Biological: seeds, pollen, spores (1-250 μm); bacteria, algae, fungi, viruses (<1 μm)• Solid Earth: dust, volcanoes• Oceans: sea-salt• Anthropogenic (~20% mass): fires (soot and ash); dust from roads; wind erosion of tilled land; fuel combustion; industrial processes• Chemical formation: gas (SO2, HNO3, hydrocarbons,) condensing onto existing particles, or forming new particles.SEA SPRAYDust Storm off West Africa (sept. 2005)http://visibleearth.nasa.gov/view_rec.php?id=202385Mount St. Helens (Fall, 1982)Peter Frenzen, available fromMount Saint Helens NationalVolcanic Monument Photo GalleryPrescribed Burn in Big Horn National Forest, Wyoming (1981)Fig. 5.7. U.S. Forest Service, available from National Renewable Energy Lab.Urban AerosolPower generationDieselConstructionAutomobiles6Dry DepositionHNO3H2SO4Aerosols are taken up by surfaces,i.e. ground, buildings,plantsFactors that govern dry deposition rates:• Level of atmospheric turbulence• Chemical properties of depositing species• Nature of surface itselfWet DepositionAerosols are taken up into water droplets, which are then deposited2H+SO4=rain orwash outfogClouds in contactwith mountains2H+SO4=2H+SO4=Particle concentrations in the atmospherePolluted environments PM10~ 100 μg/m3Take a volume of 1 m3of air (bathtub size)ÆMass of 100 μg of particles = 0.0001 grams (10 billion or more particles)Marine background ~ 10 μg/m3Arctic ~ 1 μg/m37Hazy day: VR=80kmClear day: VR=240kmVisibility impairment inMt. Rainier National ParkVR = visibility rangeWavelengths vary over many orders of magnitudeShort wavelengthHigh frequencyHigh energyLong wavelengthLow frequencyLow energyhttp://www.nrao.edu/whatisra/mechanisms.shtmlWavelengthsVisible light (0.4 - 0.7 μm)λ (μm)Ultraviolet0.1 - 0.4 μmblueEnergyInfrared (heat)0.7 - 100μmgreen yellow red0.4 0.650.570.58Gas (Rayleigh) ScatteringRedirection of radiation by a gas molecule without a net transfer of energy to the moleculeFigure 7.4Incoming energyProbability distribution of where a gas molecule scatters incoming lightWhat is white light?Sum of all wavelengths in the visible region.-+9Radiation Scattering by a SphereFigure 7.15A SidescatteringBCD SidescatteringEBackscatteringForwardscatteringVisibilityVisibility is defined as the ability to distinguisha perfectly black surface from a white backgroundExpressed asvisibility lengthGood VisibilityPoor VisibilityParticles decreasevisibility!Hazy day: VR=80kmClear day: VR=240kmVisibility impairment inMt. Rainier National ParkVR = visibility range10Processes Affecting Visibilityfrom “Introduction to Visibility” by William C. MalmParticles and Visibilityimage seen by observerPhysical targetLight reflectedby backgroundScattering byparticlesVisibility at National Parkshttp://vista.cira.colostate.edu/improve/Education/VisConcepts.swf11Visibility in the USTypical visual ranges• Western U.S.: 90-180 km (50-100 miles), ~ one-half of what it would be without human-made air pollution.• Eastern U.S.: 30-60 km (15-40miles), or about one-third of the visual range under natural conditions. Visibility in the US IMPROVE web site: http://vista.cira.colostate.edu/views/Web/GraphicViewer/seasonal.htmVisibility in the USfrom “Introduction to Visibility” by William C. Malm12Air Quality Standards for Particulate Matter (PM)PM10: (particles smaller than 10 μm)24 hour average 150μg / m3PM2.5: (particles smaller than 2.5 μm)24 hour average: 35μg / m3annual average: 15μg / m3PM = Particulate Matter = particles = aerosolsAir Quality Guide for Particle PollutionPeople with heart or lung disease, older adults, and children should avoid all physical activity outdoors. Everyone else should avoid prolonged or heavy exertion.201 - 300Very unhealthy (Alert)People with heart or lung disease, older adults, and children should avoid prolonged or heavy exertion. Everyone else should reduce prolonged or heavy exertion.151 – 200UnhealthyPeople with heart or lung disease, older adults, and children should reduce prolonged or heavy exertion.101 – 150 Unhealthy for sensitive groupsUnusually sensitive people should consider reducing prolonged or heavy exertion.51 – 100ModerateNone.0 – 50GoodHealth AdvisoryAir Quality Index (AQI)Air QualityAn AQI of 100 for particles up to 2.5 micrometers in diameter corresponds to a level of 40 micrograms per cubic meter (averaged over 24 hours). An AQI of 100 for particles up to 10 micrometers in diameter corresponds to a level of 150 micrograms per cubic meter (averaged over 24 hours). Health effects of aerosol particles“Our bodies natural defenses help us to cough or sneeze larger