Air Quality Exam 2
Boundary layer process
Day time data does not change as much
OO UTC
Daytime boundary layer
How to determine which conditions are present
Density is key Must always compare parcel to its environment
Avogadro's Hypothesis
Equal volumes of all gases, measured at the same T and P, contain the same # of molecules
Types of chemical transformation
Oxidation processes Photodissociation
Atmospheric boundary layer
Part of troposphere Varies in direct response to surface forces Free atmosphere Strong day/ night variation in troposphere
Signs that the atmosphere has been mixed
Pressure, temp, density NOT the same Virtual potential temp IS the same
The Gaussian Plume Model
Provides mathematical method of estimating averages, used for point sources, generally assumes continuous emissions
Oxidation processes
Rate of a chemical reaction (k) depends on: 1. Activation energy 2. Temperature
Emission Factor Rating
Rate quality of emission factor A- excellent B- above average C- average D- below average F- poor
Richardson Number (Ri)
Ratio of stability compared to vertical wind shear. The smaller the number the more turbulent. Laminar flow becomes turbulent when Ri < 0.25
Dispersion is transported by _______________
Wind -many scales of motion in the atmosphere -winds regularly and accurately observed -DOESN'T necessarily reduce the pollutant concentration -can vary dramatically in short distances, especially vertically
SD depends on what
Wind and time of day
Transformation
- Chemical reaction (oxidation) - Photo-discoloration --> sunlight can split molecules - Gas to particle conversion - Solution (can lead to formation of acids)
Dispersion
Advection (transport) and diffusion Includes: -transport -diffusion -transformation -deposition
Components of CAMEO
1. CAMEO: chemical database manager 2. ALOHA: air dispersion model 3. MARPLOT: mapping application
Historical pollution episodes
1. Meuse VALLEY, Belgium. December. Fog, anticyclone, inversions. PM SO2 --> H2SO4 (63 deaths) 2. Donora, PA. October. Polar H over northeast PA. Very light winds in a river valley 3. Chernobyl 4. Kuwaiti Oil Fires
To calculate density you need 2 things
Air Parcel Density Environment
Main parts of boundary layer
A very turbulent mixed layer Residual layer containing less turbulence (left over from previous days mixed layer) Nocturnal boundary layer of sporadic turbulence
Net effect of automobile exhaust?
Absorbed a form of sunlight to create third molecule. Production of ozone, with the presence of hydrocarbons, there will be chemical reactions that take place to cause formation of ozone
Weaknesses of the Box Model
Assumes pollutant mixes uniformly through volume, result is an average concentration
ALOHA
Atmospheric dispersion model for SURFACE releases Designed to produce results quickly for first responders
AERMOD
Based on gaussian plume model Designed to model SHORT RANGE dispersion form stationary industrial sources Primary model used for regulation Strength: the way they represent the sources
Why are air pollution episodes significant
Because they provide solid scientific documentation that exposure to elevated ambient pollutant levels can cause acute illness and even death
Gaussian Distribution
Bell shaped curve, normal curve. Models mimic natural systems well and can therefore be used as a predictive model. Need to know mean, variance, and SD.
Puff
Big emission and then nothing & so on
Wind shear generates vertical motion, but ________ often dampens it
Buoyancy
Atmospheric Dispersion Models
CAMEO and HYSPLIT
Strengths of the Box Model
Can accommodate many sources, ideal for urban air quality analysis
Air pollution illnesses characterized by
Cough Shortness of breath Chest pain Eye and nose irritation
Tv ______ as parcels rise and sink
Changes
Air Pollution Episodes
Characterized by significant short term increases in atmospheric pollutant concentrations above normal daily levels
Urban Air Pollution
Commonly referred to as smog
CAMEO
Computer Aided Management of Emergency Operations Designed to estimate local impact Weather conditions don't vary Gaussian Plume Model
Simplified Gaussian Plume Model tells what
Concentration at point
Properties of Gaussian Plume Model
Concentration proportional to emission rate Concentration diluted by wind Time averaged concentrations have bell- shaped distributions SDs (lateral and vertical) of concentrations - Related to turbulence - Increase with distance from source
Most common plume shape
Conical
HYSPLIT
Designed to estimate LONG RANGE transport and dispersion Uses operational NWP (National Weather Predictions) models as input: computer simulation of atmosphere Gradient transport model Can run forwards and backwards- "where was this weather at yesterday?"
Uses for Virtual Potential Temperature
Determine stability and thus potential for mixing Identifying the depth of the near- surface layer through which pollutants will be mixed
Nonlocal technique using virtual potential temperate (θv)
Displace parcels a small distance upwards AND downward from ALL PARTS of the virtual potential temperature profile
Automobile exhaust
Emits carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC), NO, and NO2
The Box Model
Examines a fixed model of air (like a watershed for the atmosphere)
Subsidence inversions
Formed over large geographical areas as the result of the subsidence of air in high- pressure systems. As air subsides to lower altitudes, it compresses the air beneath it, causing temperatures to rise. Generally between 850- 500mb
Surface forces
Frictional drag Evapotranspiration Heat transfer Terrain Pollutant emissions
Lofting plume
Growing more upwards
Contributing factors to air pollution episodes
Increased industrialization Limited pollution control efforts Population growth
Virtual Potential Temperature (θv)
Is the Tv (K) that a parcel of air would have if you expanded it or compressed it (without adding or removing heat) from its existing pressure to 1000mb. BEST measure of density difference between an air parcel and its environment
Residual layer
Left over layer from yesterday Turbulence decays as thermals dissipate Virtual potential temp. becomes nearly constant with height
Wind rose
Length of petals indicates frequency of wind direction
Moist air is _____ dense than dry air
Less
Which historical pollution episode led to the British Clean Air Act?
London in December. Fog, anticyclone, light winds. Incredibly low visibility with PM and SO2. About 4000+ deaths. Lots of sickness
Cities with a history of smog
Los Angeles, Houston, London
Wind
Mean + eddy
Morons are comparable to _________ because they move in an unpredictable manner
Molecular diffusion
The Gradient Transport Model
Most sophisticated way to measure
Mixed layer (ML) Daytime feature
Near surface layer where pollutants are mixed Turbulence driven by convection produced by warming at surface (solar radiation) and cooling at top of cloud layer (emitted radiation) Turbulence mixes heat, momentum, and moisture so vertical profile is uniform
12 UTC
Nighttime boundary layer
Eddy
Refers to an infinite variety of turbulent motions Responsible for much of the viscosity at scales larger than the molecular Transfer properties "from rich to poor" Transport properties "down the gradient"
Roughness length
Related to how "rough" the surface is DOES NOT mean how tall the obstacle is Greater roughness= greater amount of eddies trees> corn> grass> snow
Deposition
Removal from the air Gravitational setting Impaction/ interaction with surface features Serve as cloud condensation nuclei Scavenging by precipitation
The Gaussian Equations
Requires input of SD (lateral) and SD (vertical) -both are functions of: dispersive nature of atmosphere and distance downwind
Lewis F. Richardson and the Cascade of Energy
Researched eddies motion at larger scale transferred to smaller scales
Plume
Results from a continuously emitted source
Vertical mixing of Air Parcels by Eddies
STABLE conditions RESIST vertical mixing UNSTABLE conditions ENHANCE vertical mixing Neutral conditions don't offer resistance
Factors that contribute to pollution episodes
Slow winds Presence of many sources Warm- core high pressure systems- deep features tend to last long time Subsidence inversions
Coning plume
Spread uniform laterally and vertically
Diffusion
Spreading out of pollutants in all directions by molecular and turbulent motion (random motion of molecules). Reduces concentration of pollution
The average plume
Spreads out like a cone until it hits the ground or a stable layer in the atmosphere Has max pollutant concentration in the center and at the source Becomes more dilute both away from the source and towards the edge of the cone
Atmospheric Stability Classes
Stability classes relate lateral and vertical spreading of a plume to wind speed and solar intensity / cloudiness
Factors that contribute to turbulent flow in a layer
Stability- turbulence favored by unstable or neutral conditions Vertical wind shear- turbulence favored by large vertical wind shear
Turbulent motion can occur with both ______ and ________ conditions
Stable and unstable
Small SD means
Stable conditions
Photodissociation
Sunlight is source of photons
Virtual Temperature (Tv)
The temp that dry air must have in order to have the same density of moist air at the same pressure. Accounts for the effect of water vapor on density by adjusting the air temperature to a warmer value
Drunks are comparable to _____________ because they move in a manner that has some pattern to it
Turbulence
Nocturnal Boundary Layer
Turbulence becomes sporadic Virtual potential temps become weakly stable Oscillatory motions are common (up and down)
Looping plume
Turbulence in boundary layer Good for air quality conditions
Large SD means
Turbulent conditions, increases with distance from the source
Fumigating plume
Unstable beneath plume, stable above
General Gas Law for Dry Air
Warmer air= less dense Colder air= more dense
3D display of Gaussian Plume Model
on x (downwind) and y (lateral, crosswind) plot, z is up and can't ever be negative