Atmospheric chemistry

Regions of the Atmosphere

troposphere, stratosphere, traupopause

Troposphere

- the lowest atmospheric layer; from 4 to 11 miles high
- the temperature of this layer decreases as altitude increases
- Most of the atmospheres water is here

stratosphere

- the atmospheric layer between the troposphere and the mesosphere
- Most of the ozone is in the stratosphere and the most is 20-30km above the surface of the earth.
- IR Absorption occurs here and as a result in is very warm that increases as altitude increases
- because tropopause is very cold and troposphere decreases in temperature as altitude increases the increase in temperature in stratosphere is a temperature inversion
- very little water

tropopause

- boundary between troposphere and stratosphere
- Cold (~ -60 degrees celcius)

Light

Can be considered a wave but also have particle-like properties in that it is absorbed (or emitted) by matter only in finite packets now called photons.

Einstein-Planck Equation

E=hv=hc/wavelength

How does energy transfer change with wavelength?

The shorter the wavelength of light the greater the energy it may transfer to matter when absorbed.

How to X-Rays affect molecules?

causes ionization (ejection of core electrons) to occur

How do UV Rays affect molecules?

Bonds break and molecules dissociate

How does Visible light affect molecules?

Cause absoprtion/reflection of light/photons (you'll see colours)

How does Infrared affect molecules?

Molecules will vibrate causing release of heat.

How do microwaves affect molecules?

molecules will rotate

Absorption

When light absorbed in the volume of a material. It doesn't pass through the object.

Pressure in Atmosphere vs. Altitude (Draw)

Temperature in Atmosphere vs. Altitude (draw)

Ozone in atmosphere vs. Altitude (draw)

Mole fraction of water in atmosphere vs. Year (draw)

too variable because of positive and negative feedback it would be a crazy graph.

Mole fraction of CO2 in atmosphere vs. Year (draw)

Mole fraction of CH4 in atmosphere vs. Year (draw)

Mole fraction of N2O in atmosphere vs. Year (draw)

Mole fraction of SF6 in atmosphere vs. Year (draw)

Transmitted light

Light emerges propagating in the same direction as the incident light. It does pass through the object.

Scattered light

Emerges in a different direction from the incident light.

Beer-Lambert Law equation

I/I0=e^-sigma*xnxL
where, I= intensity transmitted
I0=intensity of incident light.
Sigma=absorption cross section
N=number density of absorbing molecule
L=optical path length
e=euler's number

Rayleigh scattering

- Scattering of radiation by gases, at low wavelengths light is scattered more efficiently than at high wavelengths
-O2 and O3 are big contributors to this

Solar radiation

Light from the sun from the atmosphere is mostly infrared then visible then ultraviolet.. Most of it is absorbed by the atmosphere but some transmits to earth and some gets scattered to earth at space.

Conditions for dissociation

1- enough energy to dissociate
2- molecules need to actually absorb the energy

Absorption

Atoms and molecules absorb light and electrons become excited but this is unstable and they will return to ground state. Or the molecule can undergo a chemical reaction that releases a photon or heat,

bond dissociation energy

the energy required to break the bond between two covalently bonded atoms. Like O2→ 20

Impact of excess UV radiation

DNA mutations, shin cancer (melanoma), and death.

Ratio for ozone to uv-b intensity on the ground

1% decrease in ozone results in 2% increase of UV-B

DNA mutations

DNA can absorb UV-B light

Skin cancer

Over exposure to UV-B light can lead to skin cancer

Death

Sun stroke, shin cancer, sun burns can all lead to death

Sunscreen

- Full spectrum sunscreen protects against UV-A and UV-B light.
- Sunscreens often contain organic substances that absorb or scatter light.
- zinc oxide and titanium dioxide are nano particles also included that appear transparent due to their size.
- these ingredients may produce byproducts like OH radicals
- SPF 15 means 15 minutes of protection against UV-B
- A new star rating is being introduced to define effectiveness against UVA.

Cataracts.

When the eye is exposed to uv light the cornea and lens will filter 99% of uv light. Absorption of uv can produce reactive molecules that attach to structural molecules in the eye and produce cataracts

Crop failure

Uv-b exposure can effect photosynthesis.

Reaction mechanism

A sequence of elementary reactions by which overall chemical change occurs (and allows to deduce the rate law)

Catalyst

A molecule that lowers the reaction energy barrier (and speeds up the reaction) without being consumed or created in the net reaction.

Catalytic cycle

A reaction sequence in which a chemical family, catalyst, catalizes a net reaction.

Chapman Mechanism

What does the Chapman mechanism do?

Qualitatively predicts altitude dependence of o3 but does not account for catalytic destruction of the stratospheric ozone

Catalyst in catalytic destruction of ozone

1. OH hydroxyl group
2.NO nitric oxide radical
3. Reactive halogen radicals chlorine and bromine

OH catalytic destruction of the ozone

Nitric oxide radicals catalytic destruction of the ozone

Halogen Radicals

General mechanism for destruction of Stratospheric ozone.

X: catalyst
X + O3 -> XO + O2
XO + O -> X + O2
Overall: O3 + O -> 2O2

Steady state

Rate of formation = rate of destruction
d[O]/dt = 0 = P-L or 2k_1[O_2] + k_3[O_3] = k_2[O_2][M][O] + k_4[O_3][O]

Inactive gases

HCl, ClNO3

Reservoir species

Catalytically inactive molecules containing chlorine available in the stratosphere. This accounts for most of the chlorine in the stratosphere. They are relatively long-lived and they tie up large amounts of active, short lived species but can slowly rerelease them when activated.

Chlorofluorocarbons (CFCs)

- A compound containing only C, F, and Cl
- Nontoxic
- Non flammable
- volatile
- nonbiocumulative
- inert
- widely used since the 1940s in fridges and spray cans

Chlorofluorocarbon story

- Back in the day, 1974, the solution to pollution was dilution so CFCs were spread around the were
- the lack of tropospheric sinks with a long lifetime which meant that they eventually mixed with the stratosphere.
- in the stratosphere there is a lot more uv light and CFCs would photo- dissociate
- in 1985 large losses of ozone in Antarctica
- in mid 1990s, holes were extending out to Australia and New Zealand
- the holes required sunlight to form
- the effect was seen at lower latitudes but they were less pronounced.

Dobson unit

Quantitative measure of ozone column abundance. The height of the layer of pure gaseous ozone in units of 10-5m that one would have, if one took all the O3 and compressed it to a column at 1 atm and 273K

How do ozone holes form

-they form at South Pole @ very cold temperatures
- pressure drop causes a polar vortex that isolates the south pole
- it is so cold that polar stratospheric clouds (PSCs) form made up of ice crystals with sulfuric and nitric acid the crystals serve as important catalytic surfaces.
- reservoir species like ClNO3 and HCl will react with PSC's surface to produce active chlorine that destroys the ozone
- catalytic destruction requires sunlight so this occurs in spring
-Only when the polar vortex ends and temperatures rise do the active chlorines convert back to inactive chlorine,

How to fix the ozone holes.

1- give up on refrigeration
2 - replace CFCs

Replacing CFCs

- If we stick with Cl and Br containing compounds we want ones with a shorter tropospheric lifetime than CFCs, less volatile but are still good refrigerants.
- add a functional group to the molecule that is prone to reaction
- main tropospheric sink for most compounds is reaction with hydroxyl radical so big driving force.
-Which functional group to add? Hydrogen to make hydrochlorofluorocarbon (HCFCs)

hydrochlorofluorocarbon (HCFCs)

- Compound containing H and only C, F, and Cl as other elements
- nontoxic
- nonflammable
-Volatile
- non bioaccumulative
- not as inert as CFCs, but still good refrigerants
- also known as class Ii ozone depleting substance.

Problem with HCFCs

Not quite short lived enough it would be better to avoid chlorine and bromine alto their

Hydrofluorocarbons (HFCs)

Tropospheric short lifetime

Problem with HFCs

- Their degradation products HF, and CF3COOH are toxic

International treaties:

-Montreal protocol (1987)
-purpose to ban CFCs
- gradual ban of HCFCS
- some countries initially exempted India and China
- halons contain Br, used in fire extinguishers, were also banned and phased out
- called for ozone assessments
- predicts ozone depletion to return to 1960 levels

Southern Hemisphere 2020

One of the largest and deepest holes since ozone monitoring occurred. The hole formation was driven by a strong, stable, and cold polar vortex.

Recent northern hemisphere.

First whole appeared in 2011 and Canada decided to reduce ozone monitoring we were the country with longest record of atmospheric ozone levels that is threatened by this new hole.

What reaction from Chapman does not happen in the stratosphere?

O2 -> 2O (wavelength

Can 03 be produced in the troposphere?

No there is not enough uv radiation in the troposphere to generate O3 with Chapman. Instead there is a low amount of transport of stratospheric 02 to troposphere. There are efficient sinks for 03 and therefore another way to produce it

Production of ozone in the stratosphere.

Production of ozone in the troposphere

Many roles of ozone

- Source of the hydroxyl radical (OH), the primary atmospheric oxidant (OH is the detergent of the atmosphere)
-oxidizes unsaturated VOCS (alkenes and alkines)
- is a greenhouse gas (contributes to warming of the surface)
- is toxic and an important pollutant in urban air
-easily quantified indicator of secondary air pollution. (stratospheric ozone = good, tropospheric ozone = bad)

Good vs. Bad ozone

stratospheric ozone = good, tropospheric ozone = bad

VOC

- Volatile organic Compound
- most have low molecular weight
- which means they have high vapour pressure

Sources of Natural VOC

Plants and trees

Anthropogenic

Man made

Sources of Anthropogenic VOC

gasoline and industry

Sources of Natural NOx

Lightning, bacteria in soil biomass, and biomass burning.

Secondary pollutants: aldehydes and peroxyacetyl nitrate (PAN)

Sources of anthropogenic NOx

combustion engines

Two types of smog

Reducing smog and oxidizing smog ( photochemical smog).

Reducing smog

Largely based on SO2 (London, 1950s) but has disappeared due to emissions regulations

Oxidizing smog.

- Known as photochemical smog, appeared first time in Los Angeles 1940-1950s.
- High levels of ozone.
- can cause eye irritation.

Conditions for photochemical smog.

- warm air (hotter than 18°c )
- significant sunlight (hv)
- hydrocarbons and NOx (many vehicles)
- stable air masses (city surrounded by mountains)

Inversion

Hot air above city (stratosphere) which traps the cold smog in the city (troposphere) near the surface of the earth.

Photochemical smog cycle.

- Vehicles exhaust produce VOCs, NO, and NO2 (where NO2 is photolyzed by incoming solar radiation to produce NO, O, O3)
- ozone is low because it is being destroyed O3+No->O2+NO2
- in mid-morning there's lower NO because traffic is lower.
- OH radicals increased sufficiently that VOCs begin to decline.
- 03 concentration begins to increase (low NO concentration)
- around noon, aldehyde production has taken over by free- radicals stock concentration until its consumed

Health effects of ground level ozone.

- Worsening symptoms for people with athsma and other lung diseases, and for people with cardiovascular (heart) disease
- swollen, irritated airways.
- irritation to your eyes, nose, and throat.
- Coughing wheezing
- Headaches
- over time, ozone can cause permanent lung damage.

Air qualify health indices.

Low risk: 1-3
Moderate risk: 4-6
High risk: 7-10
Very high risk: Higher than 10

What can we do to lower ozone levels?

- Lower NOx emissions from automobiles and industry (cars use mixture of 80% N2 / O2 ( ambient air ) as oxidant at high combustion temperatures )
- driving bans and / or temporary shutdown industry/ power plant.
- catalytic converters in vehicles
- lower combustion temperatures.
- re-circulate a fraction of the engine emissions back into flame ( less 02 = less heat)

Types of Catalytic converters

Two way or three way.

Two way converter

- Completes conversion of CO and other C-containing gases to CO2
2CO+O2->2CO2
CnHm + (n+m/4)O2->nCO2+(m/2)H2O
- air to fuel ratio is a critical parameter

Three way converter

- Same as two-way plus platinum- rhodium catalyst reduces nitrogen oxides back to N2 and O2
2NO+2H2->N2+2H2O
- H2 and CO are formed during combustion process and act as reducing agents.
- efficiency up to 80-90% when warm (300°c) most emissions come from cold starts
- catalyst also reduces SO2 to H2S (especially when converter is cold)
- air to fuel ratio is a critical parameter

When Catalytic converters are working...

CO and NOx are greatly reduced.

Henry's law, wet deposition

K=[X]aq/[X]g or H =[X]aq/Px
- A solution with [X]aq>HPx is supersaturated and will outgas X
- A solution with [X]aq- A solution with [X]aq~=HPx is at equilibrium; X is still exchanged but no net change in concentration results.

Acid rain

precipation that is slightly more acidic than natural unpolluted rain. pH natural ~ 5.6 hover

Two primary drivers of acid rain

SO2 anthropogenic emissions (coal burning, petroleum refining, and smelt processing) and NOx (NOx + H2O -> HNO3)

Predominant acids in acid rain

H2SO4 and HNO3

Major ions in rain

H+, SO4-, NO3-, NH4+

Acidification of droplets by sulfur dioxide.

In polluted air:
- SO2 gas oxidation by molecular oxygen is a very slow process
- Research in the early 1980s showed that most of the atmospheric oxidation of SO2 actually takes place in cloud droplets and in the raindrops themselves.
- dissolved aqueous SO2 in airborne droplets is oxidized to H2SO4 by dissolved ozone and hydrogen peroxide.

H2SO3 speciation at different pH

- pH 0-2: H2SO3
- pH 2- 7: HSO3-
- pH 7+: SO3-2

Rain water pH bad areas

Anywhere there is a lot of people like United States, Europe and Canada, mainly, first 2 though. Worse on east coast of US.

Process involved in acid rain

- SO2 and NOx are emitted into the air from human pollution.
- it can be dry dry deposited onto surface from there
-OR it can be oxidized by ozone and turn into acidic versions
- H2SO4 and HNO3 will dissociate in condensation in clouds
- the acidic rain call precipitate (wet deposit)

Impacts of acid rain.

- Acid rain can be neutralized by limestone
- it will undergo cation exchange and will remove Ca +2 an important nutrient from soils
- this induces stress to trees, crops, rivers and fish.
- acidified lakes will have high aluminum content ( dissolved from rocks) Al(OH)3 + H+ -> Al+3 +H2O
- acid rain affect regions with granite or quartz bedrock which has little capacity to neutralize acid rain
- SO2 and NOx emissions from tar sands to crude oil in northern Alberta will affect Manitoba and northern Saskatchewan.

Aerosol

A collection of particulates (solid or liquid) suspended in air
- negligible settling velocity
- small size ( < 100 mm diameter)