ATMOSPHERIC CHEMISTRY

MOPITT and aircraft pictures from http://www.ncar.ucar.edu/ncar/, cloud and surface T figures from http://earthobservatory.nasa.gov/, Hurricaine Alberto (Aug. 2000) and Europe’s lights from http://eospso.gsfc.nasa.gov/, haze off of Eastern US, TOMS Ozone hole (Oct 2001) courtesy http://visibleearth.nasa.gov/ , smokestack from http://www.plantexpansion.org

ATS 621: ATMOSPHERIC CHEMISTRYATS 621: ATMOSPHERIC CHEMISTRYFall 2008Fall 2008

Instructor: Colette L. Heald (heald@atmos.colostate.edu), Rm. 107

TA: Mandy Holden (aholden@atmos.colostate.edu) , Rm. 111

Class Hours: Tues/Thurs 10-10:50am

Text: Introduction to Atmospheric Chemistry (D.J. Jacob)Complementary Text: Atmospheric Chemistry and Physics: from Air Pollution to Climate Change (Seinfeld and Pandis)

Acknowledgements for class materials: Daniel Jacob and Lyatt Jaegle

IMPORTANCE OF ATMOSPHERIC CHEMISTRYIMPORTANCE OF ATMOSPHERIC CHEMISTRY

Climate ChangeClimate Change(…1990s…)

Regional Air PollutionRegional Air Pollution(…1950s…)(…1950s…)

Acid rainAcid rain(1970s…)

Stratospheric Stratospheric Ozone depletionOzone depletion

(1985…)

AtmosphericCHEMISTRY

URBAN/REGIONAL AIR POLLUTIONURBAN/REGIONAL AIR POLLUTION

Global air pollution

Tropospheric ozone column: Jun-Aug

1948 – noontime (PA)

LA

ACID RAINACID RAIN

Tropospheric ozone column: Jun-Aug

1989-1991 1997-1999

Wet sulfate depositionGetting better

in the U.S.

… but worsening in Asia

STRATOSPHERIC OZONE DEPLETIONSTRATOSPHERIC OZONE DEPLETION

Tropospheric ozone column: Jun-Aug

Downward trends in stratospheric ozone column on a global scale

Large increases in greenhouse gases and aerosolssince pre-industrial times

CLIMATE CHANGECLIMATE CHANGE

IMPACTS OF ATMOSPHERIC CHEMISTRY GO IMPACTS OF ATMOSPHERIC CHEMISTRY GO BACK EVEN FURTHER….BACK EVEN FURTHER….

[Zerefos et al., 2007]

Paintings by J.M.W. Turner show light scattering from volcanic aerosol

Non-volcanic sunset (1828) Volcanic sunset (1833 Babuyan 1831)

BUT REMAINS A CONTEMPORARY ISSUE…BUT REMAINS A CONTEMPORARY ISSUE…

1. MEASURES OF ATMOSPHERIC COMPOSITION1. MEASURES OF ATMOSPHERIC COMPOSITION(CHAPTER 1)(CHAPTER 1)

WHAT IS THE ATMOSPHERE?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 gasWe will use “species” as a generic term to describe a constituent of the atmosphere

Aerosols are the “visible” components of the atmosphere

Pollution haze over East Coast Dust off West Africa

BUT ATMOSPHERIC GASES ARE “VISIBLE” TOO…BUT ATMOSPHERIC GASES ARE “VISIBLE” TOO…IF YOU LOOK IN THE UV OR IRIF YOU LOOK IN THE UV OR IR

NONO22 Columns Observed from the SCIAMACHY Satellite Instrument Columns Observed from the SCIAMACHY Satellite Instrument

Fundamental quantities of physics

MKS cgs ()

Time s s 1

Length m cm 100

Mass kg g 1000

MKS units = "SI" (standard international units)

Once we have defined units (standard measures) for these quantities we can derive for all other quantities of physics.

Derived quantities: important combinations of fundamental quantities

Quantity, definition Formula Dimensions (units)

velocity= distance

time

L/t

m s-1

acceleration= change of velocity

time

v/t

m s-2

force=massacceleration F=m a kg ms-2 Newtons (N) weight= massacceleration of gravity = force exerted by gravity on an object

F=mg

(same as force; g=9.8 ms-2)

work = forcedistance E=F L kg m2 s-2 Joules (J)

kinetic energy = energy of motion E=12 mv2

(same as work)

Temperature (related via Boltzmann's constant, k, to the mean kinetic energy of a gas molecule)

12 mv2 = kT

[non-dimensional, units=degrees Kelvin; k=1.38 10-23 J/K]

pressure = force per unit area P = F/A kg m-1 s-2 ≡ Pascal (Pa) The relationships between fundamental and derived quantities are intimately tied to the laws of physics. For example, Newton's laws relate familiar quantities (force, energy) to the fundamental quantities.

vector

PREDICTING ATMOSPHERIC CONCENTRATION OF SPECIES X:PREDICTING ATMOSPHERIC CONCENTRATION OF SPECIES X:solve mass balance (or “continuity”) equation for [X](xsolve mass balance (or “continuity”) equation for [X](x ,,tt))

[ ]( [ ])X X X X

XE X P L D

t

U

local change in concentration

with time

transport(flux divergence;U is wind vector)

chemical production and loss(depends on concentrations of other species)

[ ] is our general notation for concentration, but “concentration” is an ambiguous word – we will use three different measures:

1. Mixing ratio (or mole fraction)2. Number density, mass concentration3. Partial pressure

emissiondeposition

1.1 Mixing ratio or mole fraction 1.1 Mixing ratio or mole fraction CCXX [mol mol[mol mol-1-1]]

# moles of X

mole of airXC remains constant when air density changes 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) 365x10-6

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 = 1x10-6 mol mol-1

1 ppbv = 1x10-9 mol mol-1

1 pptv = 1x10-12 mol mol-1

ATMOSPHERIC COATMOSPHERIC CO22 INCREASE OVER PAST 1000 YEARS INCREASE OVER PAST 1000 YEARS

Intergovernmental Panel on Climate Change (IPCC) document, 2005

Concentration units: parts per million (ppm)number of CO2 molecules per 106 molecules of air

CO2 CONCENTRATION IS MEASURED AS MIXING RATIO

ATMOSPHERIC COATMOSPHERIC CO22 TREND OVER PAST 25 YEARS TREND OVER PAST 25 YEARS

IPCC [2005]

mol mol-1 is the proper SI unit; ppm, ppmv are customary units

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”

"Perfect Gas" Law

PV = N' kT or PV = N RT

N' = number of molecules in the air parcel

N = number of moles; N' = N x Av

k = Boltzmann constant

R = Universal Gas Constant; R = k x Av

=================================

(In meteorology texts: P = RT - different "R“ = 287 J/kg/K)

k = 1.3806503 × 10-23 m2 kg s-2 K-1

R = 8.31 J mole-1 K-1

1.2 Number density 1.2 Number density nnXX [molecules cm[molecules cm-3-3]]

# 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 of radiation by atmosphere

nX and CX are related by the ideal gas law:

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

mass of X

unit volume of airX

na = air densityAv = Avogadro’s numberP = pressure [Pa]R = Universal gas constant = Av k k=Boltzmann cnstT = temperature [K]MX= molecular weight of X [g/mol]

LAST WEEK’S STRATOSPHERIC OZONE LAYERLAST WEEK’S STRATOSPHERIC OZONE LAYER

Method: UV solar backscatter

Scattering by Earth surface and atmosphere

Ozone layer

Ozoneabsorptionspectrum

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

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

ANNUAL MEAN PARTICULATE MATTER (PM) ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, 1995-2000CONCENTRATIONS AT U.S. SITES, 1995-2000

EPA particulate matter assessment document (NARSTO), 2003EPA particulate matter assessment document (NARSTO), 2003

PM2.5 (aerosol particles < 2.5 m diameter)

U.S. air quality standard:PM2.5 = 15 g m-3

(annual mean)

Red circles indicate sites in violation of the standard

STANDARD IS EXPRESSED AS A MASS CONCENTRATION PER UNIT VOLUME

SPECIFIC ISSUES FOR AEROSOL CONCENTRATIONSSPECIFIC 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

Typical U.S. aerosol size distributionsby volume

URBAN

RURAL

1.3 Partial pressure 1.3 Partial pressure PPxx [Pa][Pa]

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

(such as condensation of water vapour)

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 atmosphere requires PH2O > PH2O,SAT

• T PH2O,SAT

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

Evaporation and condensation are dynamic processes always taking place at the liquid-air interface. When the rate of evaporation equals the rate of condensation, the air and water are in equilibrium, and the air is said to be saturated with water vapour. The relationship between water vapour pressure and temperature is the Clausius-Clapeyron equation.

ΔHvap = molar enthalpy of vaporization

Vapour Pressure of Water: The pressure of H2O vapour in equilibrium with liquid water.

Clausius-Clapeyron relation:

Vapour pressure increases sharply with temperature, due to the large latent heat.

A=6.11 mbar (= Pvap at 0C)B=5308 K

PHASE DIAGRAM FOR WATERPHASE DIAGRAM FOR WATER

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

gas-liquidmetastableequilibrium

Latest Observation for Fort Collins, CO (80523)

           

              

Site: on CSU campus Time: 9 AM MST 25 AUG 08 Temp: 72.6 F (22.5 C) Dewpt: 54.4 F (12.4 C) Rel Hum: 53% Winds: 1 m/s (from W)Pressure: 849.4 mb

2

2 ,

(%) 100( )

H O

H O sat

PRH

P T

triple point of water(n=0)

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

cold outside air

PH2O is plotted on linearscale (vs. log on previousplots) to draw the mixinglines

ICE

LIQ

GAS

CONVERTING BETWEEN DIFFERENT MEASURES CONVERTING BETWEEN DIFFERENT MEASURES OF CONCENTRATIONOF CONCENTRATION

Cx

Pxnx

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 ,o

H O SATP 2 , 2 2 ,o

H 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

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 relative humidity

water droplet, pwater= p0water

lower humidity evaporates

water+salt droplet, pwater< p0water

lower humidity gets smaller

higher concentration of salt, lower concentration of water

The water+salt droplet will shrink when RH declines, but there is a limit…

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