1. MEASURES OF ATMOSPHERIC COMPOSITION
The atmosphere seen from space
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
California fire plumes
Pollution off U.S. east coast
Dust off West Africa
WHAT ABOUT CLOUDS?
Clouds are made up of water droplets or ice crystals
(1-100 mm), much larger than typical aerosols (0.0110 mm). They are technically aerosols but have
unique properties and are in practice considered
separately.
Atmospheric gases are “visible” too… if you look in UV or IR
UV
Vis
0.4-0.7 µm
IR
% radiation
absorbed by
atmospheric
gases
Nitrogen dioxide (NO2 ) observed by satellite in the UV
1.1 Mixing ratio or mole fraction CX [mol mol-1]
CX 
Trace
gases
# moles of X
mole of air
remains constant when air density changes
e robust measure of atmospheric composition
SPECIES
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)
(0.01-10)x10-6
Helium (He)
5.2x10-6
Methane (CH4)
1.7x10-6
Krypton (Kr)
1.1x10-6
Air also contains variable H2O
vapor (10-6-10-2 mol mol-1) and
aerosol particles
Hindenburg disaster movie
Trace gas concentration units:
1 ppmv = 1 µmol mol-1 = 1x10-6 mol mol-1
1 ppbv = 1 nmol mol-1 = 1x10-9 mol mol-1
1 pptv = 1 pmol mol-1 = 1x10-12 mol mol-1
ATMOSPHERIC CO2 INCREASE OVER PAST 50 YEARS
Intergovernmental Panel on Climate Change (IPCC), 2014
Concentration units: parts per million (ppm)
number of CO2 molecules per 106 molecules of air
CO2 CONCENTRATION IS MEASURED AS MIXING RATIO
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 0.075 ppm or 75 ppb
1.2 Number density nX [molecules cm-3]
Proper measure for
• reaction rates
• optical properties of atmosphere
# molecules of X
nX 
unit volume of air

Column concentration  X =  nX ( z)dz Proper measure for absorption or
0
scattering of radiation by atmosphere
nX and CX are related by the ideal gas law:
Av P
n X  naC X 
CX
RT
Also define the mass concentration (g cm-3):
X 
M n
mass of X
 X X
unit volume of air
Av
na = air density
Av = Avogadro’s number
P = pressure
R = Gas constant
T = temperature
MX= molecular mass of X
THIS WEEK’S STRATOSPHERIC OZONE LAYER
http://ozoneaq.gsfc.nasa.gov
Method: UV solar backscatter
l1
Ozone layer
Scattering by
Earth surface
and atmosphere
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
Ozone
absorption
spectrum
l1 l2
l2
US AREAS EXCEEDING THE PARTICULATE MATTER (PM2.5)
EPA AIR QUALITY STANDARD (2010)
PM2.5 ≡ concentration of aerosol particles < 2.5 mm diameter)
U.S. air quality standard:
PM2.5 = 15 mg m-3
(annual mean)
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 is a continuous function
URBAN
RURAL
Typical U.S. aerosol
size distributions
by volume
1.3 Partial pressure Px [Pa]
Dalton’s law:
PX  CX P
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
•Tk
e PH2O,SAT k
CLAUSIUS-CLAPEYRON EQUATION: PH2O, SAT = f(T)
PH2O,SAT (hPa)
PH 2O ,SAT
1 1
 A exp[ B(  )]
T To
T (K)
A = 6.11 hPa
B = - 5310 K
To = 273 K
Phase rule of physical chemistry
The number n of independent variables determining the equilibrium partitioning
of c species between p phases is given by
n=c+2-p
Questions
1. Oxygen has a fixed mixing ratio in the atmosphere. How would you
expect its number density measured in surface air to vary between day and
night? How would you expect its partial pressure measured in surface air to
vary between day and night?
2. Give a rough order of magnitude for the number of molecules present in a
typical 1 micrometer aerosol particle.
3. In an atmosphere with fixed mixing ratio of water vapor, what two
processes can cause an increase in relative humidity?
4. We saw that a cloud in the atmosphere can remain liquid at temperatures
below freezing. At a given temperature below freezing, and for a given total
amount of water in an air parcel, will a cloud contain more condensed water
if it is liquid or solid?
PHASE DIAGRAM FOR WATER
http://weather.unisys.com
gas-liquid
metastable
equilibrium
triple
point
Relative humidity (%) = 100(PH2O/PH2O,SAT)
Dew point: Temperature Td such that PH2O = PH2O,SAT(Td)
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
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
cold outside air
GAS
AIR POLLUTION HAZE
Views of Acadia National Park
http://www.hazecam.net/
•
“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
RAOULT’S LAW
o
H 2O, SAT
P
PH 2O,SAT  x
o
H 2O H 2O,SAT
P
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
PH 2O,SAT
aqueous solution particles of water mixing ratio
xH 2O 
PHo 2O,SAT

RH
100
HOWEVER, AEROSOL PARTICLES MUST ALSO
SATISFY SOLUBILITY EQUILIBRIA
Consider an aqueous sea salt (NaCl) particle: it must satisfy
xNa  xCl   K s (solubility equilibrium)
xNa   xCl  (electroneutrality)
xNa   xCl   xH 2O  1 (closure)
This requires:
1
2
RH  100(1  2Ks ) "deliquescence RH"
At lower RH, the particle is dry.
UPTAKE OF WATER BY AEROSOLS: HAZE
NaCl/H2O
Deliquescence RH;
depends on particle
composition