Atmosphere

How is atmospheric chemistry defined in the unit introduction?

The study of the complex chemical reactions that occur in the Earth’s atmosphere.

How do topography and meteorology influence pollution?

Determine the rate at which pollutants are transported away or converted into other compounds

Define atmospheric inversion.

A deviation from the normal thermal lapse rate where air temperature decreases with altitude

Radiation inversion

Occurs primarily in summer and autumn when the ground cools quickly at night

Subsidence inversion

Occurs when cloud cover is associated with stagnant air, preventing vertical mixing

Topographical effects

Cities in valleys or topographic bowls are more susceptible to smog due to mountains and inversions acting as barriers, preventing pollutant transport

4 determining factors for pollutant potential.

1. Rate of emissions

2. Downwind distance mass of air moves through an area

3. Average wind speed

4. Mixing height

Proportional laws

Conc. $\alpha$ (1/wind speed) and (1/mixing height)

Conc. $\alpha$ emission rate and downwind travel distance

Hydroclimate whiplash

Volatility characterised by sudden large or frequent transitions between very wet and dry effects

What impacts are associated with rapid transitions from wet to dry conditions?

Crop and hydropower loss

What impacts are associated with rapid transitions from dry to wet conditions?

Wildfires, flooding and landslides

What are the two types of smog?

Photochemical and sulphurous

When does NO peak?

Early morning.

When does $O_3$ peak?

With solar radiation, peaking around midday

When does $NO_2$ and HCs peak?

Specific trends with traffic and solar cycles

What country was cited as the most deadly in terms of Air Quality in 2016

China

Define concentration.

Amount of a specific air pollutant per unit area volume of air

Define exposure

Contact between an airborne contaminant and a surface of the human body

Define dose.

The amount of pollutant that crosses one of the bodies boundaries and reaches the target tissue

How do you calculate pH?

pH = - log[H+]

What is the concentration of secondary pollutants a function of?

Primary concentration, temperature and other species

According to James Lovelock, what is the weight of the atmosphere compared to the ocean?

The atmosphere weighs a little more than a billion megatons, while the ocean is a thousand times more massive.

What are the four regions of the atmosphere in order from the Earth's surface upward?

The Troposphere, the Stratosphere, the mesosphere, and the thermosphere.

Contrast the temperature lapse rates of the Troposphere and the Stratosphere.

The Troposphere has a positive lapse rate (temperature decreases with altitude), while the Stratosphere has a negative lapse rate (temperature increases with altitude).

What is the Tropopause?

The boundary separating the Troposphere and the Stratosphere, varying in depth from $10-17\,km$ depending on latitude and time of year.

What are free radicals?

Atoms/groups with an odd number of electrons, they have highly reactive indeterminates.

What is initiation in terms of radicals?

Non-radical + hv → radical + radical

What is propagation in terms of radicals?

Non-radical + radical → radical + non-radical

What is termination in terms of radicals?

Radical + radical → non-radical

How does the troposphere remove pollutants?$\cdot$

CO: reacts with $\cdot$ OH to form $CO_2$ and H$\cdot$. H$\cdot$ then forms hydroxyl radicals.

$SO_2$: wet solubility rains out as sulphuric acid and dry removal via mechanism with $\cdot$ OH + $SO_2$ → $HOSO_2$ → $SO_3$ + $H_2 O$ → $H_2 SO_4$

What goal for GHGs did the Climate Change Act (2008) set?

Net 80% reduction in emissions by 2050

What percentage of the atmosphere's mass and aerosols is contained within the Troposphere?

Approximately three-quarters of the atmosphere's mass and $99\%$ of its aerosols and water vapour.

According to the World Meteorological Organization, what is the formal definition of the Tropopause?

The lowest level at which the lapse rate decreases to $2\,^{\circ}C/km$ or less, provided the average lapse rate between this level and all higher levels within $2\,km$ does not exceed $2\,^{\circ}C/km$.

What is the average lapse rate in the Troposphere in the absence of inversions?

Approximately $6.5\,^{\circ}C\,km^{-1}$.

List the top three gases by volume percentage in dry, unpolluted air at sea level.

Nitrogen ($78.08\%$), Oxygen ($20.94\%$), and Argon ($0.93\%$).

Why is the Stratosphere described as stratified in temperature?

It has warmer layers higher up and cooler layers farther down, which inhibits vertical mixing.

What critical function does ozone serve in the Stratosphere regarding solar radiation?

It absorbs ultraviolet radiation, preventing lethal $UV-C$ and reducing harmful $UV-B$ from reaching the Earth's surface.

Which scientists published research in 1974 suggesting that CFCs catalytically destroy stratospheric ozone?

Mario Molina and F. Sherwood Rowland.

Who reported the first clear sign of the Antarctic 'ozone hole' in 1982?

Joe Farman of the British Antarctic Survey.

Define a Chlorofluorocarbon (CFC).

An organic compound containing only carbon, chlorine, and fluorine, produced as a volatile derivative of methane, ethane, and propane.

Explain the numbering system for a fluorinated alkane like Freon-12.

The rightmost digit is the number of fluorine atoms ($2$), the next left is hydrogen atoms plus $1$ ($0+1=1$), and the third left is carbon atoms minus $1$ ($1-1=0$, not stated), result: $CCl_2F_2$.

What is the projected timeline for the complete recovery of the ozone layer across different regions?

By $2040$ for most regions, $2045$ over the Arctic, and $2066$ over the Antarctic.

Characterise the lower, middle, and upper layers of the ozone layer.

Lower: very low UV-C, hence no atmospheric oxygen is present and no ozone forms.

Middle: most ozone is produced where O and $O_2$ combine, releasing heat and creating temperature inversions.

Upper: receives high UV causing dissociation.

How stable is the Ozone layer?

Very stable conditions; foreign materials introduced here persist for a long time

When was the complete phase out of CFCs committed to?

London Amendment (1990)

What did the Kigali agreement set out?

The phase out of HFCs to avoid +$0.5^{o}C$

What is a Halon?

Similar to HFCs but contain at least one Bromine atom. Used as an unreactive non-toxic fire suppressant

What are the First and Second Laws of Photochemistry?

The First Law (Grotthaus) states only absorbed light can produce photochemical change; the Second Law (Einstein) states one quantum of light is absorbed per molecule for the primary process.

State Planck's Equation for the energy of a photon.

The equation is $E = hv = \frac{hc}{\lambda} = hc\bar{\nu}$, where $h$ is Planck's Constant ($6.6256 \times 10^{-34}\,J\,s\,photon^{-1}$) and $c$ is the speed of light ($2.9979 \times 10^8\,m\,s^{-1}$).

Define 'Quantum Yield' in photochemical reactions.

The ratio of the number of molecules reacting to the number of photons absorbed.

What are the chemical equations for the light and dark reactions of photosynthesis?

Light: $12H_2O \rightarrow 6O_2 + 24H^+ + 24e^-$; Dark: $6CO_2 + 24H^+ + 24e^- \rightarrow C_6H_{12}O_6 + 6H_2O$.

What is the difference between a bonding orbital and an anti-bonding orbital?

A bonding orbital is a lower energy molecular orbital where shared electrons are paired in the ground state; an anti-bonding orbital is higher energy and typically unoccupied in the ground state.

Contrast Singlet and Triplet electronic excited states.

In a singlet state, unpaired electrons have opposite spin (zero net spin); in a triplet state, unpaired electrons have parallel spin (non-zero net spin).

What are the different types of EES?

1. Photosynthesis

2. Photosensitisation: indirect facilitation of a reaction

3. Photodissociation: splitting of the molecule

4. Photoionisation (photolysis): radiation removing electrons

5. Inter-molecular rearrangement: producing a new electronic state via collisions

6. Inter-molecular energy transfer: absorbed energy transfer to another molecule

Explain the Chapman mechanism for ozone formation in the Stratosphere.

It begins with photodissociation: $O_2 + hv \rightarrow O^{\bullet} + O^{\bullet}$ (for $\lambda < 240\,nm$), followed by combination: $O_2 + O^{\bullet} + M \rightarrow O_3 + M$.

Define 'Quenching' in the context of atmospheric photochemistry.

The process where an excited molecule ($AB^*$) transfers energy to a chemically unreactive molecule ($M$), returning to its ground state.

Define luminescence in terms of photochemical reactions.

Re-emission of energy at different wavelengths. Includes short lived fluorescence and longer-lived photoescence

Why is the hydroxyl radical ($^{\bullet}OH$) known as the 'cleanser' of the atmosphere?

It is the most important oxidant in the Troposphere and the main pathway for the removal of most oxidisable molecules.

Which allotrope of oxygen is accurately denoted as $O(^3P)$?

Atomic oxygen in its ground-level triplet state, which is highly reactive due to two unpaired electrons.

How is the hydroxyl radical primarily formed in the Troposphere from ozone?

Ozone photolysis produces excited oxygen ($O(^1D)$) at $\lambda < 320\,nm$, which reacts with water: $O(^1D) + H_2O \rightarrow 2HO^{\bullet}$.

What are the main requirements for the formation of photochemical smog?

Strong sunlight, stable meteorological conditions, nitrogen oxides ($NO_x$), and volatile organic compounds (VOCs).

State the general formula for peroxyacetyl nitrates (PAN).

The formula is $RC(O)OONO_2$.

How many ozone molecules can a single chlorine atom destroy over its lifetime in the Stratosphere?

As many as $100,000$ molecules of $O_3$.

What was the primary objective of the 1987 Montreal Protocol?

To phase out the production of human-made compounds responsible for stratospheric ozone destruction, such as CFCs.

What was the significance of the 2016 Kigali Amendment to the Montreal Protocol?

It added an agreement to phase out hydrofluorocarbons (HFCs), which are potent greenhouse gases.

What UK legislation mandates an $80\%$ reduction in net GHG emissions by 2050 compared to a 1990 baseline?

The Climate Change Act 2008.

What are the three most common ways to express the concentration of gaseous pollutants?

The mass of gaseous pollutant per unit volume of air ($\mu g\,m^{-3}$), the volume of gaseous pollutant per unit volume of air ($\mu l\,l^{-1}$), and the volume mixing ratio ($\text{ppm}$, $\text{ppb}$, or $\text{ppt}$).

Which unit of measurement is uniquely applicable to particulate pollutants?

Only the mass of pollutant per unit volume of air is applicable ($\mu g\,m^{-3}$).

What was the 78% decline in $NO_x$ emissions from 1970 to 2022.

Coal decline and public transportation

What does UV do to $NO_2$?

Decomposes it back to NO, this is accelerated by HCs and $O_3$.

Urban range, peak and indoor for $NO_x$

10:50 ppb, 800ppb, 500ppb

Urban range, peak and indoor conc. of $SO_x$.

3-20 ppb, 300 ppb, 20ppb

What are the characteristics of $SO_2$ gas?

Corrosive acidic gas produced by coal and crude oil combustion. Its anthropogenic and natural inputs are roughly equal, and is a key contributor to acid rain.

Characteristics of CO.

Colourless, odourless, tasteless and slightly lighter than air. Forms during the incomplete combustion of carbon containing fuels.

How does CO impact health?

Forms a strong coordination bond with the iron in haemoglobin to create carboxyhemoglobin, reducing oxygen capacity.

What is a VOC?

Umbrella term for organic chemicals that evaporate easily, including HCs, oxygenates, and halogen-containing species

NMVOC Emission trends

1970-2020: fell 67%

1990-2021: road transport sources fell from 33% to 3%

Scotch whiskey maturation emissions have been rising since 1990

Industrial has declined whilst household has grown with population growths

How is Peroxyacetyl formed? PAN

PAN is formed by HCs reacting with $NO_x$ a powerful eye irritant.

What are the background levels of $O_3$?

$$\frac{\mu g}{m^3}$$

10-80 $\frac{\mu g}{m^3}$. Anthropogenic sources have doubled background levels in the last century.

What is aerodynamic diameter?

How is the mixing ratio defined in atmospheric chemistry?

The ratio of the amount (mass or volume) of the substance of concern in a given volume to the amount (mass or volume) of all constituents of air in that volume.

What is the specific advantage of using the mixing ratio over concentration measurements?

The mixing ratio is unchanged by differences in pressure or temperature associated with altitude or meteorological variability.

Why is it preferable to refer to dry air when reporting mixing ratios of trace gases?

Because the presence of water vapor causes the mixing ratio to vary somewhat with humidity.

What assumption is made about the behavior of gaseous air pollutants for inter-conversion calculations?

It is assumed that air pollutants behave like a "perfect" or "ideal" gas.

What is the standard molar volume of one mole of gas at $1013\,mb$ and $273\,K$?

$22.4\,litres$.

According to the transcript, how is the molecular weight of $NO_2$ calculated?

By adding the atomic weights of nitrogen ($14$) and oxygen ($16$): $14 + 16 + 16 = 46$.

What is the definition of "averaging time" in pollution measurement?

The time period over which the measurement of atmospheric pollution concentration is carried out.

How are the "winter" and "summer" seasonal averaging periods defined?

"Winter" refers to the period from October to March, while "summer" refers to April to September.

What three processes underpin the health and environmental consequences of air pollutants?

The emission of pollutants from source, the transport of pollutants, and pollutant deposition.

How are primary pollutants defined?

Pollutants that are emitted directly into the atmosphere, such as $CO$ and $SO_2$.

What are secondary pollutants?

Pollutants formed in the air as a result of chemical reactions with other pollutants and atmospheric gases, such as ozone ($O_3$).

Why is nitrogen dioxide ($NO_2$) cited as a pollutant that can be both primary and secondary?

It is emitted directly from power stations and exhausts (primary) but also forms from the oxidation of $NO$ in the air (secondary).

What are the two major classifications for the sources of air pollutants?

Natural (e.g., volcanic eruptions, lightning) and man-made or anthropogenic (e.g., fossil fuel combustion).

How are "point sources" defined in geography-based anthropogenic pollution classification?

Large, geographically-concentrated emitters whose emission rates are significant by themselves, such as power stations or oil refineries.

What is the definition of "area sources"?

Collections of small geographically dispersed emitters that are not significant individually but are important collectively, such as residential areas.

What are "line sources"?

A collection of relatively small sources distributed roughly uniformly along a line, such as a motorway or a railway line.

What is an emissions inventory?

A schedule of the sources of an air pollutant or pollutants within a particular geographical area, including industrial sources and transport figures.

How is an emission factor defined?

A number representing the relationship between the mass of a given pollutant emitted from a particular source and the amount of raw material processed.