1. 1. MEASURES OF ATMOSPHERIC COMPOSITION

2. WHAT IS THE ATMOSPHERE? Gaesous envelope surrounding the Earth
Mixture of gases, also contains suspended solid and liquid particles (aerosols)
Aerosol = dispersed condensed phase suspended in a gas
Aerosols are the “visible” components of the atmosphere
Pollution haze over East Coast
Dust off West Africa

3. ATMOSPHERIC GASES ARE “VISIBLE” TOO… IF YOU LOOK IN THE UV OR IR
NO2 Columns Observed from the SCIAMACHY Satellite Instrument

4. WHAT ABOUT CLOUDS?
Clouds are made up of water droplets (1-100 mm), much larger than typical aerosols ( mm). They are technically aerosols but have unique properties and are in practice considered separately.

5. 1.1 Mixing ratio or mole fraction CX [mol mol-1]
remains constant when air density changes
e robust measure of atmospheric composition
GAS
MIXING RATIO
(dry air)
[mol mol-1]
Nitrogen (N2)
0.78
Oxygen (O2)
0.21
Argon (Ar)
0.0093
Carbon dioxide (CO2)
380x10-6
Neon (Ne)
18x10-6
Ozone (O3)
( )x10-6
Helium (He)
5.2x10-6
Methane (CH4)
1.7x10-6
Krypton (Kr)
1.1x10-6
Air also contains variable H2O vapor ( mol mol-1) and aerosol particles
Trace gas concentration units:
1 ppmv = 1x10-6 mol mol-1
1 ppbv = 1x10-9 mol mol-1
1 pptv = 1x10-12 mol mol-1
Trace
gases

6. ATMOSPHERIC CO2 INCREASE OVER PAST 1000 YEARS
Intergovernmental Panel on Climate Change (IPCC), 2007
Concentration units: parts per million (ppm)
number of CO2 molecules per 106 molecules of air
CO2 CONCENTRATION IS MEASURED AS MIXING RATIO

7. ATMOSPHERIC CO2 TREND OVER PAST 25 YEARS
IPCC [2007]
mmol mol-1 is the proper SI unit; ppm, ppmv are customary units

8. EPA SURFACE OZONE AIR QUALITY STANDARD “8-hour average of 0
EPA SURFACE OZONE AIR QUALITY STANDARD “8-hour average of 0.08 ppmv not to be exceeded more than 3x/year”
EPA air quality standard for surface ozone is 75 ppb

9. 1.2 Number density nX [molecules cm-3]
Proper measure for
reaction rates
optical properties of atmosphere
Proper measure for absorption or scattering of radiation by atmosphere
nX and CX are related by the ideal gas law:
na = air density
Av = Avogadro’s number
P = pressure
R = Gas constant
= Av k k=Boltzmann cnst
T = temperature
MX= molecular mass of X
Also define the mass concentration (g cm-3):

10. TUESDAY’S STRATOSPHERIC OZONE LAYER
Method: UV solar backscatter l1 l2
Ozone layer
Scattering by
Earth surface
and atmosphere
Ozone
absorption
spectrum
1 “Dobson Unit (DU)” = 0.01 mm ozone at STP = 2.69x1016 molecules cm-2
THICKNESS OF OZONE LAYER IS MEASURED AS A COLUMN CONCENTRATION l1 l2

11. U.S. air quality standard:
ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, EPA particulate matter assessment document (NARSTO), 2003
PM2.5 (aerosol particles < 2.5 mm diameter)
U.S. air quality standard:
PM2.5 = 15 mg m-3
(annual mean)
Red circles indicate sites in violation of the standard
STANDARD IS EXPRESSED AS A MASS CONCENTRATION PER UNIT VOLUME

12. SPECIFIC ISSUES FOR AEROSOL CONCENTRATIONS
A given aerosol particle is characterized by its size, shape, phase, and chemical composition – large number of variables!
Measures of aerosol concentrations must be given in some integral form, by summing over all particles present in a given air volume that have a certain property
The aerosol size distribution must be treated as a continuous function
URBAN
Typical U.S. aerosol
size distributions
by volume
RURAL

13. 1.3 Partial pressure Px [Pa]
Dalton’s law:
Proper measure for phase change
(such as condensation of water vapor)
Evaporation of liquid water from a pan:
No lid: water molecules escape from pan to atmosphere (evaporation)
Add a lid:
escaping water molecules collide on lid and return to surface; collision rate measures PH2O
eventually, flux escaping = flux returning :
saturation (PH2O,SAT)
cloud formation in atmoshere requires
PH2O > PH2O,SAT
T k e PH2O,SAT k

14. CLAUSIUS-CLAPEYRON EQUATION: PH2O, SAT = f(T)
A = 6.11 hPa
B = K
To = 273 K
PH2O,SAT (hPa)
DEMO: the freezing experiment. ask for student predictions.
Explain about nucleation of the bubbles. Why are bubbles formed in the liquid and what are they made from?
freezing: more energy in the ordered lattice, latent heat of "fusion" = 0.34 x 106 J/kg (less than evaporation, but still considerable).
relate to boiling – what is boiling?
condensation—as in a shower stall. Is heat released on condensation?
To evaporate 1 kg of ice, = 2.6 kJ/kg.
Clausius-Clapeyron relationship—
Psat = A exp [B( 1/ – 1/T)] A=6.11 mbar, B= 5308K. A=water vap. pressure at 0C.
T (K)

15. SHORT QUESTIONS
1. Give a rough order of magnitude for the number of molecules present in a typical 1 micrometer aerosol particle Consider harmful UV radiation for which the ozone layer has an optical depth of 10. The ozone layer has thinned by 6% since 1970, with a corresponding 6% decrease in optical depth. What is the resulting percent increase in this UV radiation reaching the surface of the Earth? 3. In an atmosphere with fixed mixing ratio of water vapor, what two processes can cause an increase in relative humidity?

16. PHASE DIAGRAM FOR WATER
Latest Observation for Cambridge, MA (02138)
Site: KBOS Time: 12 PM EDT 23 SEP 08 Temp: 61 F (16 C) Dewpt: 46 F (7 C) Rel Hum: 58% Winds: E at 10 knt Pressure: mb (30.52 in)
gas-liquid
metastable
equilibrium
Relative humidity (%) = 100(PH2O/PH2O,SAT)
Dew point: Temperature Td such that PH2O = PH2O,SAT(Td)

17. RUNAWAY GREENHOUSE EFFECT ON VENUS
due to accumulation of water vapor from volcanic outgassing early in its history
…did not happen on Earth because farther from Sun; as water accumulated it reached saturation and precipitated, forming the oceans
EARTH
VENUS

18. WHY CAN YOU SEE YOUR BREATH ON COLD MORNINGS?
Draw mixing lines (dashed) to describe dilution of your breath plume w/outside air
PH2O is plotted on linear
scale to draw the mixing
lines
LIQ
Your breath
37oC, ≈ 100%RH
ICE
cloud!
no cloud
warm outside air
GAS
cold outside air

19. Views of Acadia National Park
AIR POLLUTION HAZE
Views of Acadia National Park
“clean” day
“moderately polluted” day
Visibility is limited by high concentrations of aerosol particles that have swollen to large sizes due to high (but <100%) relative humidity

20. RAOULT’S LAW solute molecules in green water saturation vapor pressure
over pure liquid water surface
water saturation vapor pressure
over aqueous solution of water
mixing ratio xH2O
An atmosphere of relative humidity RH can contain at equilibrium aqueous solution particles of water mixing ratio

21. HOWEVER, AEROSOL PARTICLES MUST ALSO SATISFY SOLUBILITY EQUILIBRIA
Consider an aqueous sea salt (NaCl) particle: it must satisfy
This requires:
insert clapeyron eq
At lower RH, the particle is dry.

22. UPTAKE OF WATER BY AEROSOLS: HAZE
NaCl/H2O
Deliquescence RH;
depends on particle
composition

23. SHORT QUESTIONS
What is the fractional increase in mass of water-soluble aerosol particles when relative humidity increases from 90% to 95%? (assume that the aerosols are mainly water). Assuming that visibility degradation is proportional to the cross-sectional area of the particles, what is the effect of this mass increase on visibility?