1. MEASURES OF ATMOSPHERIC COMPOSITION

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

The atmosphere seen from space

Pollution off U.S. east coast Dust off West AfricaCalifornia fire plumes

WHAT ABOUT CLOUDS?

Clouds are made up of water droplets or ice crystals (1-100 m), much larger than typical aerosols (0.01-10 m). They are technically aerosols but have unique properties and are in practice considered separately.

Vis0.4-0.7 µm

Atmospheric gases are “visible” too… if you look in UV or IR

Nitrogen dioxide (NO2 ) observed by satellite in the UV

% radiationabsorbed byatmosphericgases

UV IR

1.1 Mixing ratio or mole fraction CX [mol mol-1]# moles of X

mole of airXC remains constant when air density changes 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

Tracegases

Air also contains variable H2O vapor (10-6-10-2 mol mol-1) and aerosol particles

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

Hindenburg disaster movie

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 EPA SURFACE OZONE AIR QUALITY STANDARD ““8-hour8-hour average of 0.08 ppmv not to be exceeded more than 3x/year”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]

# molecules of X

unit volume of airXn Proper measure for• reaction rates• optical properties of atmosphere

0

Column concentration = ( )X Xn z dz

Proper measure for absorption or scattering of radiation by atmosphere

nX and CX are related by the ideal gas law:

vX a X X

A Pn n C C

RT

Also define the mass concentration (g cm-3):

mass of X

unit volume of air X XX

v

M n

A

na = air densityAv = Avogadro’s numberP = pressureR = Gas constantT = temperatureMX= molecular mass of X

THIS WEEK’S STRATOSPHERIC OZONE LAYER

Method: UV solar backscatter

Scattering by Earth surface and atmosphere

Ozone layer

Ozoneabsorptionspectrum

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

http://ozoneaq.gsfc.nasa.gov

US AREAS EXCEEDING THE PARTICULATE MATTER (PM2.5) EPA AIR QUALITY STANDARD (2010)

PM2.5 ≡ concentration of aerosol particles < 2.5 m diameter)

U.S. air quality standard:PM2.5 = 15 g 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

Typical U.S. aerosol size distributionsby volume

URBAN

RURAL

1.3 Partial pressure Px [Pa]

Dalton’s law:X XP C P Proper measure for phase change

(such as condensation of water vapor)

Evaporation of liquid water from a pan:

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 PH2O,SAT

No lid: water molecules escape from pan to atmosphere (evaporation)

CLAUSIUS-CLAPEYRON EQUATION: PH2O, SAT = f(T)

2 ,

1 1exp[ ( )]H O SAT

o

P A BT T

A = 6.11 hPaB = - 5310 KTo = 273 K

PH

2O,S

AT

(hP

a)

T (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

Relative humidity (%) = 100(PH2O/PH2O,SAT)

Dew point: Temperature Td such that PH2O = PH2O,SAT(Td)

triple pointgas-liquid

metastableequilibrium

http://weather.unisys.com

RUNAWAY GREENHOUSE EFFECT ON VENUSRUNAWAY GREENHOUSE EFFECT ON VENUS

EARTH 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

WHY CAN YOU SEE YOUR BREATH ON COLD MORNINGS?WHY CAN YOU SEE YOUR BREATH ON COLD MORNINGS?

Draw mixing lines (dashed) to describe dilution of your breath plume w/outside air

Your breath37oC, ≈ 100%RH

warm outside air

no cloud

cold outside air

cloud!

PH2O is plotted on linearscale to draw the mixinglines

ICE

LIQ

GAS

AIR POLLUTION HAZEAIR POLLUTION HAZE

•

“clean” day “moderately polluted” day

http://www.hazecam.net/

Views of Acadia National Park

Visibility is limited by high concentrations of aerosol particles that have swollen to large sizes due to high (but <100%) relative humidity

RAOULT’S LAWRAOULT’S LAW

water saturation vapor pressureover pure liquid water surface

water saturation vapor pressureover aqueous solution of water

mixing ratio xH2O

2 ,oH O SATP 2 , 2 2 ,

oH O SAT H O H O SATP x P

An atmosphere of relative humidity RH can contain at equilibrium aqueous solution particles of water mixing ratio 2 ,

22 , 100

H O SATH O o

H O SAT

P RHx

P

solutemoleculesin green

HOWEVER, AEROSOL PARTICLES MUST ALSO HOWEVER, AEROSOL PARTICLES MUST ALSO SATISFY SOLUBILITY EQUILIBRIASATISFY SOLUBILITY EQUILIBRIA

Consider an aqueous sea salt (NaCl) particle: it must satisfy

2

(solubility equilibrium)

(electroneutrality)

1 (closure)

Na Cl s

Na Cl

Na Cl H O

x x K

x x

x x x

This requires:1

2100(1 2 ) "deliquescence RH"sRH K

At lower RH, the particle is dry.

UPTAKE OF WATER BY AEROSOLS: HAZEUPTAKE OF WATER BY AEROSOLS: HAZE

Deliquescence RH;depends on particlecomposition

NaCl/H2O