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The Atmosphere Composition and Structure. Outline Intro to air pollution Atmospheric Composition...
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Transcript of The Atmosphere Composition and Structure. Outline Intro to air pollution Atmospheric Composition...
The Atmosphere
Composition and Structure
Outline
• Intro to air pollution• Atmospheric Composition
– Measures of concentration– Concentration of gases– Concentration and composition of PM
• Structure of the Atmosphere– Thermal stratification– Characteristic vertical and horizontal mixing times– Spatial variability of atmospheric composition
• Light– Nature of light– Interaction of light and matter– Sunlight and its propagation through the atmosphere
Problems due to Air Pollution
• Question– What are the major problems due to chemicals discharged into
the atmosphere? List them.• Stratospheric ozone depletion (due to CFCs, HCFCs, etc)
• Global climate change (due to GHGs, etc)
• Acid deposition (SO2, NOx)
• Smog (VOCs, NOx)
• Particulates (PM, especially “fine PM”)
• Other toxic air pollutants (eg, CO, Pb, Hg, PAHs and other toxic organics, etc)
• Lecture Question– What are the criteria pollutants?
• Carbon monoxide, CO
• Nitrogen dioxide, NO2
• Ozone, O3
• Lead, Pb
• Particulates, PM10 and PM2.5
• Sulfur dioxide, SO2
• Lecture Question– What are the four most abundant components of dry air at sea
level?
– Nitrogen (N2), oxygen (O2), argon (Ar), carbon dioxide (CO2)
Atmospheric CompositionChemical Species Concentration Source
N2 78.08% volcanic, biogenic
O2 20.95% biogenic
H2O (gaseous) up to 4% (avg ~2.5%) volcanic, evaporation
Ar 0.93% radiogenic
CO2 0.037% (370 ppmv) volcanic, biogenic, anthropogenic
Ne 18 ppmv volcanic (possibly)
He 5.2 ppmv radiogenic
Kr 1 ppmv radiogenic
CO 50 – 200 ppmv biogenic, anthropogenic, photochemical
CH4 1.7 ppmv biogenic, anthropogenic
NMHC 5 – 20 ppbv biogenic, anthropogenic, photochemical
CH2O 0.1 ppbv photochemical
N2O 310 ppbv biogenic, anthropogenic
NH3 0 – 0.5 ppbv biogenic, anthropogenic
NOx 0 – 0.5 ppbv biogenic, anthropogenic, lightning
OCS 0.5 ppbv volcanic, biogenic, anthropogenic
H2S 0 – 0.5 ppbv biogenic, anthropogenic
SO2 0.01 – 1 ppbv volcanic, anthropogenic, photochemical
DMS 0.01 – 0.1 ppbv biogenic
Measures of Concentration• Relative concentration: volume mixing ratio (VMR)
– What is it? The mole (or volume) fraction.
– Units: %, ppmv, ppbv, etc
– Easy to understand
– Constant with altitude for inert gases like N2
Measures of Concentration
• Lecture Question– List the current atmospheric concentration of CO2 in air in units
of (a) % and (b) ppm.
– Answer in book:• 375 ppm = 0.0375% by volume.
– According to NOAA (Jan 2007):• Current average CO2 concentration (at Mauna Loa) is 383 ppm
• 383 ppm = 0.0383% by volume.
Measures of Concentration
• Absolute concentration– Typical units
• Mass/volume (eg g/L)
• Number density (cm-3), particularly for low concs
• Pressure units (torr, atm, bar, etc)
z , km P z /P 0 (= rz / r 0 ) 1 - P z /P 0 (%)
0 1.000 0.0%10 0.259 74.1%15 0.132 86.8%25 0.034 96.6%50 0.001 99.9%100 0.000 100.0%
/0 7.4 kmat 250Kz H
za
RTP Pe H
M g
Note: diameter of the Earth is 6400 km.
Particular Matter Concentration
Background PM: 300/cm3
PM Mass Concentration
Background PM: 1 g/m3
Thermal Stratification of the Atmosphere
• Lecture Question– List the major regions (layers) of the atmosphere, along with the
typical altitudes for each region.• Troposphere (0 – 15km)
– Upper limit (the tropopause) varies between 9 – 16 km depending on lattitude and season
• Stratosphere (15 – 50 km)– Contains the stratospheric ozone layer, which (mostly) shields us from
harmful uv light
• Mesosphere (50 – 100 km)
• Thermosphere (above 100 km)– Above 60 km is the ionosphere, where there is a significant
concentration of ions and electrons
Thermal Stratification of the Atmosphere• Troposphere is heated by the ground
• Stratosphere and mesosphere are directly heated by ozone chemistry (solar energy)
• Tropopause varies in height (9–16 km) depending on latitude and season
• Vertical mixing in troposphere is rapid, but stratosphere is fairly stagnant
• Lower 1 – 3 km called the planetary boundary layer (PBL), which is rapidly mixed and often topped by a local inversion
Time Scales of Vertical Mixing
Time Scales of Horizontal Mixing
Mixing vs Removal• Two competing processes
– Effect on atmospheric composition• Depends on relative rates of mixing and removal• Species with higher rates of removal are more concentrated near
their sources
– Removal mechanisms• Water scavenging
– Dissolution into suspended water droplet (or water bodies on ground)– Rate depends on solubility
• Reaction/decomposition– Rxn with a reactive species (often OH)– Photodissociation: absorption of light, followed by breaking of bond
• Adsorption to solid surface– Either PM or surface on the ground
– Mixing mechanisms• Buoyant mixing (vertical)• Atmospheric circulation – ie, wind (horizontal)
Light: Electromagnetic Radiation
• Plays a critical role– The vast majority of our energy arrives as sunlight
• Drives global circulation of atmosphere and water
– Many air pollution problems directly involve light• Ozone depletion
• Global climate change
• Photochemical smog
– Drives atmospheric chemistry• Through photodissociation
• Through generation of reactive species (OH, Cl, NO3, O3)
Light: Electromagnetic Radiation
• What is light?– Light acts like a wave
• With oscillating electric and magnetic fields
• Can propagate through a medium or through vacuum
• animation here
– Light also acts like a particle• Localized energy packet: a photon
• Massless...but has momentum
• Lecture Question– What is the relationship between wavelength, frequency, and the
speed of propagation?
c
Light: Electromagnetic Radiation• Lecture Question
– What is the relationship between photon energy, wavelength, and frequency?
hcE h
• Photon energy– Is the minimum energy available in its interaction with matter
The Electromagnetic Spectrum
increasing energy
decreasing energy
The Electromagnetic Spectrum
• Can further subdivide the UV region– UV-C is 200-280 nm (most energetic, completely blocked by ozone layer)– UV-B is 280-320 nm (ground-level UV-B intensity most affected by ozone depletion)– UV-A is 320-280 nm (least energetic, almost all gets through the ozone layer)
least energetic portion of solarradiation; absorbed by greenhousegases
770 nm – 100 minfrared (ir)
atmosphere is mostly transparent tovisible light
380 – 770 nmvisible (vis)
only partially blocked by theatmosphere
200 – 380 nmultraviolet (uv)
most energetic portion of solarradiation; absorbed completely by theupper atmosphere
10 – 200 nmfar ultraviolet (“vacuum uv”)
CommentsWavelength RangeSpectral Region
Incident SunlightTop & Bottom of Atmosphere
Lecture Question
What fraction of incident sunlight is in the uv, visible and ir regions?
• 8% ultraviolet• 39% visible• 53% infrared