Composition of Tropospheric Air Sources, Transport, and Sinks of Trace Gases

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PHYS-575/CSI-655PHYS-575/CSI-655Introduction to Atmospheric Physics and ChemistryIntroduction to Atmospheric Physics and Chemistry

Lecture Notes #5: Lecture Notes #5: Atmospheric ChemistryAtmospheric Chemistry

1. Composition of Tropospheric Air2. Sources, Transport, and Sinks of Trace Gases3. Some Important Tropospheric Trace Gases4. Tropospheric Aerosols5. Air Pollution6. Tropospheric Chemical Cycles7. Stratospheric Chemistry

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Atmospheric ChemistryAtmospheric ChemistryAtmospheric chemistry concerns the sources, properties, and effects of themany chemical species that exist in the atmosphere.

Effects: Acid Rain (deposition) Ozone Depletion (Antarctic Ozone Hole) Air Pollution (Photochemical Smog) Trace gases and aerosol formation (Climate effects) Oxidation State of the Atmosphere Chemical Destruction of Greenhouse Gases

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Tropospheric Gases - CategoriesTropospheric Gases - Categories

(1)Non-Reactive Species: Ar, Ne, He(2) Long-Lifetime Molecules: N2, O2, CO2

(3) Disequilibrium Gases: CH4, N2O, SO2, H2S, NH3,

CS2, DMS(4) Radicals: OH, HO2, NO, CH3

(5) Photochemical Species: O3, H2, NOy

(6) Biogenic Gases: CH4, NH3, H2S, DMS

NB – A given gas maybelong to more than one category.

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Atmospheric AbundancesAtmospheric AbundancesAbundances by Volume of Dry Air:Nitrogen (N2) = 78.084%Oxygen (O2) = 30.946%Argon (Ar) = 0.934%Carbon Dioxide (CO2) = 0.03%Total: = 99.99%

The volumes occupied by different gases at the same temperature and pressureare proportional to the numbers of molecules of the respective gases, i.e., eachatom or molecule occupies the same average volume.

At standard temperature (273oK) and pressure (1bar = 1.013 x 105 Pa) thenumber of molecules in 1 m3 of air is given by the ideal gas law P = nkT,and is known as Loschmidt’s Number.

31932523

5

10687.210687.2273)10381.1(

10013.1 cmxmx

x

xno

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Concentration and Residence TimeConcentration and Residence Time

Loschmidt’s Number No = 2.687 x 1019 molecules cm-3

Example:N2O occupies 310 ppbv (parts per billion by volume) of air.Number Density of N2O = 310 x 10-9 x No = 8.33 x 1012 cm-3

Residence Time:

F

M

M = mass/volume of constituent in the atmosphere (kg m-3 or gm cm-3)F = rate/volume of its removal from the atmosphere (kg m-3 s-1 or gm cm-3 s-1)

Note carefully that the residence time a more general concept than thechemical lifetime. The chemical lifetime is the loss timescale due to onlychemical processes. The residence time includes all loss/or removalprocesses that affect a gas.

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Spatial and Temporal Variability - ScalesSpatial and Temporal Variability - Scales

Radicals

Stable Molecules

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2. Sources and Sinks of Trace Gases2. Sources and Sinks of Trace Gases

Major Categories of Sources and Sinks• Biogenic• Solid Earth• Oceanic• In situ formation/loss

Biogenic examplesPhotosynthesis: CO2(g) + H2O(l) + hν CH2O(s) + O2(g)Methanogenic Bacteria (Cow stomachs & termites): CO, CO2 CH4

Nitrogen Fixing Bacteria: N2 NH3, N2O, NOPhytoplankton: CH4, SO2, COS DMS, dimethyl disulfide (DMDS)Seaweed: NaCl, Br, I, CH4 CH3Cl, CH3Br, CH3IVOCs: several thousand Volatile Organic Compounds (plants & human activity)Biomass burning: CO2

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Solid Earth Sources & SinksSolid Earth Sources & Sinks

Volcanoes: H2O, CO2, SO2, H2S, COS, HCl, etc.Radiogenic elements: He, Ar, RnWeathering of Rocks: O2, CO2 loss

Oceanic Sources & SinksSoluble gases: CO2, N2, O2

In situ Formation/loss (formation of chemicals by reactionsdriven by sunlight, lightening, solar ions, etc.)Ozone formation: O + O2 + M O3 + MWater Photolysis: H2O + hν H + OHCarbon monoxide destruction: CO + OH CO2 + HMethane photolysis: CH4 + hν CH3 + H

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Chemical ReactionsChemical Reactions

In situ formation is a major source of many important trace gases such as ozone (O3) and a major loss mechanism for other gases such as methane (CH4).

Much of atmospheric chemistry is initiated and driven by the absorption of solar UV radiation (λ < 3000 Angstroms = 300nm).

If the photon energy (hν) is high enough, it will break apart molecules into more reactive components (e.g. radicals).

Example: Formation of the Hydroxyl Radical (OH)Above ~60 km altitude: H2O + hν H + OH

Below ~60 km: O3 + hν O2 + O* (λ < 3200 Angstrom)O* + H2O 2 OH

Net result: O3 + H2O + hν O2 + 2OH

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3. Important Tropospheric Trace Gases3. Important Tropospheric Trace Gases

The hydroxyl radical OH is powerful oxidizing agent that reacts with most trace gases containing H, C, N, O, & S.

Reactions with OH control the loss of numerous atmospheric pollutants and control the buildup of some GH gases.

Because of its role in removing many pollutants, OH has been called the atmosphere’s detergent.

The dominant loss of OH is via:

OH + CO CO2 + HOH + CH4 CH3 + H2O

OH is so intensively reactive that its steady state abundance is extremely low, of order 10-4 ppbv, or mixing ratio ~10-13. Only in the past decade have techniques been developed to measure such low abundances.

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Important Hydroxyl (OH) as an OxidantImportant Hydroxyl (OH) as an Oxidant

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Carbon Monoxide (CO)Carbon Monoxide (CO)

CO is produced by the oxidation of CH4 or Non-methane hydro-carbons (NMHC) such as isoprene.

Fossil fuel combustion and the burning of biomass are also very significant. The loss of CO

CO + OH CO2 + H

is also the dominant loss process of OH in non-urban and non-forested areas. So the distribution of OH is sometimes inferred by the abundances of CO.

CO has a seasonal cycle, with high abundances during winter when OH concentrations are low, and low abundances in spring and summer when OH concentrations are high.

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Carbon Monoxide (CO)Carbon Monoxide (CO)

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Types of Atmospheric ComponentsTypes of Atmospheric Components

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Atmospheric Ozone (OAtmospheric Ozone (O33))

Ozone – O3

Source:O2 + photon O + O (photolysis)O + O2 O3 (chemical reaction)

Ozone Vertical Distribution

Ozone “layer”

Approximately 90% of Earth’s ozone is inthe stratosphere. The Ozone “layer” absorbssolar UV radiation that can produce cellulardamage. However, ozone in direct contact with cells is a poison because of its oxidizing nature.

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Ozone Spatial Variation: Altitude vs. LatitudeOzone Spatial Variation: Altitude vs. Latitude

Ozone “layer”

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Ozone Seasonal Variation: Latitude vs. Month of YearOzone Seasonal Variation: Latitude vs. Month of Year

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Seasonal Variation of Tropospheric OzoneSeasonal Variation of Tropospheric Ozone

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Ozone: Human InfluencesOzone: Human Influences

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The Basics of Chemical KineticsThe Basics of Chemical Kinetics

2-Body Chemical Reaction: A + B C + D (k = rate coefficient, units: cm3 s-1)A, B: ReactantsC, D: ProductsReaction Rate: Rate = k [A] [B] (molecules cm-3 s-1)

where [X] denotes the concentration of reactant X (cm-3)

Homogeneous Chemistry: Reaction occurs entirely in gas phase.If reaction occurs in gas phase, but products later condense, it is stillconsidered a homogeneous gas phase reaction.

Heterogeneous Chemistry: Reaction occurs on surface or in liquid background.Chemistry occurring inside liquid aerosols are considered heterogeneous.

Photolysis: A + hν B + C (J photolysis rate coefficient, units: s-1)Where hν denotes a photon of frequency ν, and energy E = hν.

Photolysis Rate: Rate = J [A] (molecules cm-3 s-1)

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Basics - continuedBasics - continuedSometimes a reaction of the form A + B C + D is extremely slow becauseof the necessity of conserving both energy and momentum during the reaction.In those cases a nearby atom or molecule can act as a “third body” to speedup the reaction, but without being consumed during the reaction. The rateof the reaction in that case is dependent upon the density of the third body Mwhich we denote, by convention, as [M].

3-Body Chemical Reaction: A + B + M C + M (k = rate coefficient units: cm6 s-1)

3-Body Rate: k [A] [B] [M] (cm -3 s-1)

An example of a 3-body reaction is the stratospheric formation of ozone (O3):

O + O2 + M O3 + M

where M may be a background N2 molecule, or even another O2 molecule.

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Photolysis LifetimePhotolysis LifetimeThe chemical lifetime is the timescale for chemical process to change the abundanceof a certain species by a factor of e-1.

First example: photolysis A + hν B + CPhotolysis Rate: J [A] (cm-3 s-1)

The rate of change of the concentration of A is then, since photolysis is a loss of A:

d[A]/dt = -J[A]

Which gives d[A]/[A] = - J dt

This can be integrated from starting concentration [A]o to [A] as time goes from 0 to t.

[A](t) = [A]o e-Jt = [A]o e-t/τ

So the chemical loss (1/e) time scale is τ = 1/J, which has units of time.

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Chemical Reaction LifetimeChemical Reaction LifetimeSecond example: 2-body reaction A + B C + D

Reaction Rate: Rate = k [A] [B] (cm -3 s-1)

The rate of change of the concentration of A is then:

d[A]/dt = -k [A] [B]

Which gives d[A]/[A] = - k [B] dt

If we assume [B] is constant, then the solution for [A](t) is:

[A](t) = [A]o e-k[B]t = [A]o e-t/τ

So the chemical loss timescale is τ = 1/k[B]But every time an A is destroyed, a B and a C are produced, so

-d[A]/dt = d[B]/dt = d[C]/dt

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Stratospheric OzoneStratospheric Ozone

Ozone “layer”

Production of “active” oxygen: O2 + hν O + O

O3 source: O + O2 + M O3 + MO3 loss: O3 + hν O2 + ONet result: nothing

However, the absorption of solar UV radiation leads to atmospheric heating,even though it is a chemical “do nothing” cycle.

The loss of “active” oxygen is accomplished by catalytic cycles of the form

H + O3 OH + O2

OH + O H + O2

Net result: O + O3 2 O2

General catalytic cycle:

X + O3 XO +O2

XO + O X + O2

Net result: O + O3 2O2

Where X = H, OH, Cl, N, NO, Br, etc.

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Tropospheric OzoneTropospheric Ozone

Let [Y] denote the number density of gas Y:

Catalytic Cycle:

NO2 + hν NO + O Rate Coefficient J; Rate = J[NO2]O + O2 + M O3 +M Rate Coefficient k1 Rate = k1[O][O2]MO3 + NO NO2 + O2 Rate Coefficient k2 Rate = k2[O3][NO]Net: “do nothing” cycle

These reactions are extremely fast. So the rate of the last reaction is the same as that of the first reaction. Thus we can write:

J[NO2] = k2[O3][NO] and solve for the abundance of [O3]

[O3] = (J/k2) ([NO2]/[NO]) Photostationary State Relation

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Hydrogen Radicals and Ozone ChemistryHydrogen Radicals and Ozone Chemistry

[O3] = (J/k2) ([NO2]/[NO]) Photostationary State Relation

The O3 abundance predicted by this cycle is far below observed values. The reason is that the OH radicals reduce the abundance of NO and increase the abundance of NO2 via

OH + CO + O2 HO2 + CO2

HO2 + NO OH + NO2

The loss of OH: OH + CO CO2 + H (still leaves a radical) OH + HO2 H2O + O2 (removes radicals)

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Methane and Photochemical SmogMethane and Photochemical Smog

CH4 – methane: biomass burning, cows, termites, ride paddies, etc.

Oxidation: OH + CH4 H2O + CH3

OH + CH3CHO H2O + CH3COetc

Net result: CH4 + 2O2 + 2NO H2O + 2NO2 + HCHO (formaldehyde)

CH3CO + … CH3COO2 + NO2 CH3COO2NO2 (PAN)

Formaldehyde is an eye irritant. Peroxyacetyl nitrate (PAN) is an important component of photochemical smog.

Most of the complex hydrocarbons condense onto aerosols, there initiatingeven more complex liquid phase chemistry.

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Photochemical SmogPhotochemical Smog

Diurnal Variation:

Hydrocarbons, NOx build up in the morning rush hour.

Ozone, PAN, aldehydes form during the day and build up until the afternoon.

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Sulfur Gases and Acid RainSulfur Gases and Acid Rain

Sulfur is assimilated by living organisms and is released as various gases as an end product of metabolism. Some important sulfur gases are:

H2S – hydrogen sulfide: blue-green algae in marshlands, soils, ocean, volcanoesCH3SCH3 – dimethyl sulfide (DMS): from seaweed and phytoplanktonSO2 – sulfur dioxide: oxidation of DMS and H2S, volcanoes, biomass burningCOS – carbonyl sulfide: biomass decay, fossil fuel, oxidation of CS2

CS2 – carbon disulfide: biomass decay & burning, fossil fuel

OH + H2S H2O + HSHS + O3 HSO + O2

HS + NO2 HSO + NOHSO + O3 HSO2 + O2

HSO2 + O2 HO2 + SO2

OH + SO2 + M HOSO2 + MHOSO2 + O2 HO2 + SO3

Sulfuric Acid SO3 + H2O H2SO4 (absorbed in aerosols, deposition)

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4. Tropospheric Aerosols4. Tropospheric Aerosols

Atmospheric aerosols are suspensions of small solid and/or liquid particles(excluding water cloud particles) in air that have negligible terminal fall speeds.

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What are Aerosols?What are Aerosols? Atmospheric aerosols (or particulate matter) are solid or liquid particles or both suspended in air with diameters between about 0.002 μm to about 100 μm.

Aerosol particles vary greatly in size, source, chemical composition, amount and distribution in space and time, and how long they survive in the atmosphere.

Primary atmospheric aerosols are particulates that emitted directly into the atmosphere (for instance, sea-salt, mineral aerosols (or dust), volcanic dust, smoke and soot, some organics).

Secondary atmospheric aerosols are particulates that formed in the atmosphere by gas-to-particles conversion processes (for instance, sulfates, nitrates, some organics).

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Why are Aerosols Important?Why are Aerosols Important?

A significant fraction of atmospheric aerosols are anthropogenic.

Importance of aerosols: heterogeneous chemistry air quality and human health visibility reduction acid deposition cloud formation climate and climate change

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Atmospheric Aerosols in the Upper AtmosphereAtmospheric Aerosols in the Upper Atmosphere

http://www.misu.su.se/~gumbel/norfa/maa_scheme.gif

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Atmospheric Particles Size DistributionAtmospheric Particles Size Distribution

http://jan.ucc.nau.edu/~doetqp-p/courses/env440/env440_2/lectures/lec35/Fig35_10.gif

Diameter and radius of a particle are both used to characterized its size. If a particle is non-spherical, its equivalent radius is used. There are several ways to define particle equivalent radius (for instance, aerodynamic equivalent radius, which is radius of a sphere that experiences the same resistance to motion as the non-spherical particle).

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Marine AerosolsMarine Aerosols

http://www.atmos.washington.edu/~beckya/ALKCartoon.gif

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Surface BubblesSurface Bubbles

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Sources of Atmospheric Sources of Atmospheric ParticulatesParticulates

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In SituIn Situ Sources of Atmospheric Particulates (in Tg/year) Sources of Atmospheric Particulates (in Tg/year)

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Number Distribution Number Distribution of Tropospheric of Tropospheric

AerosolsAerosols

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Number Distributions of Tropospheric AerosolsNumber Distributions of Tropospheric Aerosols

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5. Air Pollution5. Air PollutionIn Urban and industrialized locations, anthropogenic emissions lead to large concentrations of undesirable chemical species which can adversely affect air quality, visibility, and pose threats to human health.

Severe air pollution episodes occur when the rates of emissions or formation of pollutants greatly exceed the rates at which the pollutants are dispersed by winds, chemical reactions, or deposition.

Severe air pollution episodes tend to occur in association with extended periods of light winds and strong static stability.

Emissions from Fossil Fuel Combustion:CO, CO2, NOx (radicals of N), SO2, hydrocarbons, etc.

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Urban, Continental, and Urban, Continental, and Marine ParticulatesMarine Particulates

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Anthropogenic Greenhouse GasesAnthropogenic Greenhouse Gases

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Nitrogen-containing GasesNitrogen-containing Gases

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Sulfur-containing gasesSulfur-containing gases

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7. Stratospheric Chemistry7. Stratospheric ChemistryOzone Vertical DistributionOzone Vertical Distribution

It forms a protective shield that reduces the intensity of UV radiation (with wavelengths between 0.23 and 0.32 μm) from the sun that reaches the earth’s surface.

Because of the absorption of UV radiation by O3, it determines the vertical profile of temperature in the stratosphere.

It is involved in many stratospheric chemical reactions.

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The Ozone LayerThe Ozone LayerThe ozone layer, or ozonosphere layer (rarely used term), is that part of the Earth’s atmosphere which contains relatively high concentrations of ozone (O3). "Relatively high" means a few parts per million - much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere.

The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. Its properties were explored in detail by the British meteorologist G. M. B. Dobson, who developed a simple spectrophotometer that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations which continues to operate today.

The "Dobson unit", a convenient measure of the total amount of ozone in a column overhead, is named in his honor.

http://www.search.com/reference/Ozone_layer

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Solar UV Radiation and OzoneSolar UV Radiation and Ozone

http://en.wikipedia.org/wiki/Image:Ozone_altitude_UV_graph.jpg

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The Sun and OzoneThe Sun and Ozone

http://en.wikipedia.org/wiki/Image:Ozone_cycle.jpg

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Techniques for Measuring OzoneTechniques for Measuring Ozone

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Processes Influencing OzoneProcesses Influencing Ozone

http://earthobservatory.nasa.gov/Library/Aura/Images/StratosphericChemical_HiRes.jpg

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Chapman Chemistry and OzoneChapman Chemistry and Ozone

O2 + hν 2O ja SlowO + O2 + M O3 + M kb Fast O3 + hν O2 + O jc FastO + O3 2 O2 kd Slow

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Solution of Chemical Equations (Chapman Chemistry)Solution of Chemical Equations (Chapman Chemistry)

3132121 2 nnknjnnnknjdt

dndcMba

313213 nnknjnnnkdt

dndcMb

3121322 nnknnnknjnj dMbca


Let:n1 = [O] Atomic Oxygenn2 = [O2] Molecular Oxygenn3 = [O3] Ozone

3123 22 nnknjdt

dnda

Atomic Oxygen

In steady state:

Ozone

These last two equations imply:

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3132121 2 nnknjnnnknjdt

dndcMba

Mbc nnnknj 213 3121322 nnknnnknjnj dMbca



Mb

c

nnk

njn

2

31

Atomic Oxygen

In steady state:

Using only the Fastest Two Reactions

The last relation implies:

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3123 22 nnknjdt

dnda

Mb

c

nnk

njn

2

31



Ozone Abundance

Mb

cda nnk

njknj

dt

dn

2

23

23 2

2 Finally:

In Steady State, we cansolve algebraically for n3

in terms of background atmospheric properties and known rate coefficients.

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Ozone and Catalytic CyclesOzone and Catalytic Cycles

O + O2 + M O3 + M O3 + hν O2 + ONet: Nothing (no net change)

OH + O3 HO2 + O2

HO2 + O OH + O2

Net: O + O3 2 O2

NO + O3 NO2 + O2

NO2 + O NO + O2

Net: O + O3 2 O2

Cl + O3 ClO + O2

ClO + O Cl + O2

Net: O + O3 2 O2

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Anthropogenic Effects on OzoneAnthropogenic Effects on Ozone

CFCl3 + hν CFCl2 + Cl

CF2Cl2 + hν CF2Cl + Cl

Cl + O3 ClO + O2

ClO + O Cl + O2

Net: O + O3 2 O2

Industrially manufactured chlorofluorocarbons (CFCs): compounds containingCl, F, and C. They were produced for non-toxic, non-flammable refrigerants.

CFC-11: CFCl3CFC-12: CF2Cl2

In the troposphere CFCs have residence times of hundreds of years.But in once they are transported into the stratosphere, UV radiationphoto-dissociates them, releasing highly reactive Cl.

By 1990, ~85% of the chlorine in thestratosphere was anthropogenic.

The first measurements indicating aglobal effect of anthropogenic Cl onozone were in the Antarctic.

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Antarctic Ozone HoleAntarctic Ozone Hole

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The Ozone HoleThe Ozone Hole

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Aerial Extent Aerial Extent of the of the

Ozone HoleOzone Hole

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The Antarctic Polar VortexThe Antarctic Polar Vortex

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Ozone Hole FormationOzone Hole Formation

During Polar Night:

1) Cold, stagnant, isolated air leads to formation of polar stratospheric clouds (PSCs, ice particles of H2O and HNO3).2) Heterogeneous chemistry inside the PSCs produce and release the molecules Cl2 and HOCl.

During Polar Spring:

3) Immediately upon sunrise, photolysis of HOCl and Cl2 produces free Cl atoms.4) Free Cl atoms rapidly catalyzes the destruction of O3, leading to extreme depletion of lower stratospheric ozone.

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Formation Formation and and

Evolution Evolution of the of the Ozone Ozone HoleHole

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Northern Hemisphere Ozone DepletionNorthern Hemisphere Ozone Depletion

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Montreal Protocol on Substances that Montreal Protocol on Substances that Deplete the Ozone LayerDeplete the Ozone Layer

1987 – Montreal Protocol signed1992 – Copenhagen amendment called for complete elimination of CFC by 1996.1995 – Vienna Amendment called for complete elimination of HCFCs by 2020

2001 – First evidence that stratospheric levels of anthropogenic Cl was leveling off.2005 – First evidence that stratospheric levels of Cl were decreasing.

Several decades will be required for all stratospheric Cl of anthropogenic origin to disappear.

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Recovery of the Ozone LayerRecovery of the Ozone Layer

http://www.nasa.gov/centers/goddard/images/content/139177main_ozone_recover_hurst.jpg

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Stratospheric Stratospheric

AerosolsAerosols

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Stratospheric Stratospheric AerosolsAerosols

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Stratospheric Sulfur Layer: Volcanic PerturbationsStratospheric Sulfur Layer: Volcanic Perturbations

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Stratospheric Aerosols; Sulfur in the StratosphereStratospheric Aerosols; Sulfur in the Stratosphere

Stratospheric sulfate aerosols are produced primarily by oxidation of SO2 to SO3, followed by formation of H2SO4 that condenses with H2Oto form high altitude stratospheric haze.

OH + SO2 + M HOSO2 + M

HOSO2 + O2 HO2 + SO3

O + SO2 + M SO3 + M

SO3 + H2O H2SO4 (condenses onto aerosols)

Stratospheric Aerosols:The Junge Layer

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Discussion QuestionsDiscussion Questions1) What is it about the atmosphere that makes atmospheric

chemistry so complex?2) Does atmospheric chemistry drive climate or vice versa?3) Would chemistry occur in the atmosphere without sunlight?4) Could the ozone hole “grow” with time to eventually cover the entire globe?5) If the Earth was completely cloud covered, would tropospheric chemistry cease?6) In what situations do transport effects dominate over chemical

effects for a given trace gas?7) If the Earth was in a global ice age (Snowball Earth episode) with complete ice cover and hence high surface albedo, would stratospheric chemistry be substantially different?

Documents

Composition of Tropospheric Air Sources, Transport, and Sinks of Trace Gases