Understanding the Earth's Atmosphere: Composition, Gases, and Aerosols, Slides of Chemistry

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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 Africa

California fire plumes

ATMOSPHERIC GASES ARE “VISIBLE” TOO…

IF YOU LOOK IN THE UV OR IR

Nitrogen dioxide (NO

2

) observed by satellite in the UV

1.1 Mixing ratio or mole fraction

C

X

[mol mol

]

# moles of X

mole of air

X

C

remains constant when air density changes 

robust measure of atmospheric composition

SPECIES

MIXING RATIO(dry air)[mol mol

]

Nitrogen (N

2

)

Oxygen (O

2

)^

Argon (Ar)

Carbon dioxide (CO

2

)

380x

Neon (Ne)

18x

Ozone (O

3

)

(0.01-10)x

Helium (He)

5.2x

Methane (CH

4

)^

1.7x

Krypton (Kr)

1.1x

Tracegases

Air also contains variable H

2

O

vapor (

mol mol

) and

aerosol particles

Trace gas concentration units:

1 ppmv = 1 μmol mol

= 1x

mol mol

1 ppbv = 1 nmol mol

= 1x

mol mol

1 pptv

= 1 pmol mol

= 1x

mol mol

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ATMOSPHERIC CO

2

TREND OVER PAST 25 YEARS

IPCC [2007]

mol mol

is the proper SI unit; ppm, ppmv are customary units

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

THIS WEEK’S STRATOSPHERIC OZONE LAYER

Method: UV solar backscatter Scattering byEarth surfaceand atmosphere





Ozone layer

Ozoneabsorptionspectrum





http://jwocky.gsfc.nasa.gov/

1 “Dobson Unit (DU)” = 0.01 mm ozone at STP= 2.69x

16

molecules cm

THICKNESS OF OZONE LAYER IS MEASUREDAS A COLUMN CONCENTRATION

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US AREAS EXCEEDING THE PARTICULATE MATTER (PM

EPA AIR QUALITY STANDARD (2010)

PM

concentration of aerosol particles < 2.

m diameter)

U.S. air quality standard:

PM

g m

(annual mean)

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1.3 Partial pressure

P

x

[Pa]

Dalton’s law:

X

X

P

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

P

H2O

eventually, flux escaping = flux returning :

saturation (

P

H2O,SAT

cloud formation in atmoshere requires

P

H2O

> P

H2O,SAT

T

P

H2O,SAT

No lid: water molecules escape from pan to

atmosphere (evaporation)

CLAUSIUS-CLAPEYRON EQUATION:

P

H2O, SAT

f(T)

2

,

1

1

exp[

(

)]

H

O SAT

o

P

A

B

T

T





A

= 6.11 hPa B

= - 5310 K

T

o

= 273 K

(hPa)^ H2O,SAT P

T

(K)

RUNAWAY 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 itreached saturation and precipitated, forming the oceans

WHY CAN YOU SEE YOUR BREATH ON COLD MORNINGS?

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

Your breath 37

o

C,

100%RH

warm outside air

no cloud

cold outside air

cloud!

P

H2O

is plotted on linear

scale to draw the mixinglines

ICE

LIQ

GAS

RAOULT’S LAW

water saturation vapor pressureover pure liquid water surface

water saturation vapor pressureover aqueous solution of watermixing ratio

x

H2O

2

,

o H

O SAT

P

2

,^

2

2

,

o

H

O SAT

H

O

H

O SAT

P

x

P



An atmosphere of relative humidity

RH

can contain at equilibrium

aqueous solution particles of water mixing ratio

2

,

2

2

,^

100

H

O SAT

H

O

o H^

O SAT

P

RH

x

P





solutemoleculesin green

HOWEVER, AEROSOL PARTICLES MUST ALSO

SATISFY 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 2

"deliquescence RH"

s

RH

K

At lower RH, the particle is dry.