Atmospheric Chemistry Flashcards
Why did our atmosphere go from weakly reducing to an oxidising atmosphere?
Photosynthetic organisms evolved, reducing the concentration of carbon dioxide and producing oxygen
Why did it take so long for life to migrate from the ocean to land?
The build-up of oxygen produced ozone which is required for UV protection
What are the three natural cycles that influence Earth’s climate?
The Obliquity Cycle (Axial Tilt)
The Eccentricity (Elliptical) Cycle
The Precession Cycle (Wobble)
What is the obliquity cycle?
The axial tilt is usually around 23.5 degrees, but can vary between 22 and 24.5 degrees over a period of 41,000 years.
When obliquity increases, the Earth gets hotter
What is the eccentricity cycle?
The Earth’s orbit is elliptical, but the exact orbit path depends on the gravitational pull from other planets.
This is the longest cycle, about every 100,000 and 400,000 years
What is the precession cycle?
Precession is the trend in the direction of the Earth’s axis of rotation relative to the fixed stars, and this occurs every 26,000 years.
Driven by tidal forces (position of sun and moon)
When does warming occur?
Warming occurs when our northern hemisphere (landmass) is pointed towards the sun (increased obliquity) and our orbit is more elliptical.
When does cooling occur?
Cooling occurs when we attain a more circular orbit and tilt decreases (decreased obliquity)
Why is the current climate not solely due to these natural cycles?
The effects we are seeing exceed the cause
What is the probable sequence of events for the heating of the climate?
1) Changes in the obliquity and eccentricity causes the Earth to warm
2) Warming oceans cause emissions of CO₂ to rise about 200-500 years later
3) CO₂ further warms the whole planet, leading to even further CO₂ release e.g. melting of permafrost
What role does the ocean play in the carbon cycle?
It absorbs CO₂ - 26% of the CO₂ released between 2002 and 2011 was absorbed
During the pre-industrial era, this value is estimated to be around 98%.
What is the chemical equation for the uptake of CO₂ by the ocean?
CO₂ + CO₃²⁻ + H₂O ⇌ 2HCO₃⁻
Does the ocean have an infinite capacity for CO₂ uptake?
1) As water temperature increases, its ability to dissolve CO₂ decreases
2) As surface water warms, the harder it is for winds to mix the surface layers with the deeper layers, limiting infusion of fresh carbonate-rich water from below
3) The stagnant water also supports fewer phytoplankton, therefore carbon dioxide uptake from photosynthesis slows
What is radiative forcing?
It is the change in net irradiance (solar plus long-wave) at the tropopause.
It is normally calculated as the change since the industrial revolution.
What are the advantages of showing the effects of emissions compared to concentrations?
It is emissions that can be directly controlled
It allows for many of the indirect effects to be seen e.g. reaction with other gases
It shows that air quality (CO, VOCS, NOₓ) affects climate via indirect effects on ozone
What does the radiative efficiency depend on?
1) Strength and spectral location of absorption of IR radiation
2) Atmospheric lifetimes
3) The time period over which the radiative forcing (RF) will be calculated
What is the equilibrium climate sensitivity (ECS)?
The equilibrium change in global temperature that occurs in response to doubled CO₂ since the pre-industrial era.
What is the equation for equilibrium climate sensitivity (ECS)?
ECS: ΔT / ΔF
Why does temperature decrease with height in the troposphere?
Most heat is absorbed by the planet surface. The warmed air at the surface rise and cools.
Why does the temperature increase with height in the stratosphere?
The absorption of solar radiation by ozone reaches a maximum in this region
Energy is released from the photodissociation of O₃ and O₂
What is the formula to calculate the air mass?
Air mass = (Length of the path of direct solar radiation) / (Length of the vertical path through the atmosphere)
What does the amount of scattering that takes place dependent on?
The wavelength of the incoming radiation
The size of the scattering particle or gas molecule
What are the major pollutants of concern today?
Particulate matter (PM), oxides of nitrogen (NOₓ) and ground-level ozone
PM₂.₅ is more harmful than PM₁₀
Ozone limits are regularly exceeding in urban areas in the summer
What is a primary pollutant?
A primary pollutant is defined as those emitted directly into the air e.g. hydrocarbons, SO₂, NO, CO, Pb, PM etc…
What is a secondary pollutant?
A secondary pollutant are the result of the chemical transformations of primary pollutants e.g. ozone, NO₂, PM etc…
How has the global trend in [VOC]s changed over time?
There is little global trend in the [VOC]s as emissions are dominated by biogenic sources
What is the cause of substantial reductions in PM in Europe/USA?
Decreasing SO₂ levels (mainly due to shutting down coal-fired powerplants)
Why isn’t PM reduction happening in Asia?
The [SO₂] is still increasing
What are other sources of emissions of PM?
Power plants
Domestic biomass combustion and cooking
What is wet deposition?
The pollutants are dissolved in clouds, fog, rain or snow and deposited when these water droplets impact the Earth’s surface
What is dry deposition?
The pollutants are transported to ground level and absorbed by materials there without first being dissolved
What is the equation for deposition velocity?
1 / Rₜ
where Rₜ = sum of resistances
How is ozone formed in the troposphere?
NO₂ + hv → NO + O
O + O₂ + M → O₃ + M
How is ozone destroyed in the atmosphere?
NO + O₃ → NO₂ + O₂
What do peroxy radicals do?
Convert NO to NO₂ whilst preserving active OH radicals capable of oxidising further molecules of CO and CH₄ or converting more NO to NO₂.
What is the rate-limiting step for O₃ production?
The production of NO₂ from a peroxy radical and NO
What is the lifecycle for OH radicals at low NOₓ concentrations?
OH radical forms from photolysis of O₃ reacts with VOCs or CO to form peroxy radicals which react together to form peroxides
What is the lifecycle for OH radicals at mid/high NOₓ concentrations?
OH radicals form from the photolysis of O₃ that reacts with VOCs or CO to form peroxy radicals. These then react with NO to form NO₂ and OH radicals, with NO₂ being the key contributor to the formation of O₃.
What is the lifecycle of OH radicals at very high NOₓ concentrations?
It starts to act as a sink of radicals
OH + NO₂ + M → HNO₃ + M
Why do we have a self-cleansing atmosphere?
OH radicals react with many organic and inorganic compounds, converting them to non-toxic CO₂ or transforming them to more soluble species easily removed from the atmosphere
Without the ability to self-regulate the atmosphere would simply get more and more polluted with trace chemicals
What is the two-step formation of the hydroxyl radical?
O₃ + hv → O(¹D) + O₂
O(¹D) + H₂O → OH + OH
What other process produces OH (in polluted air)?
HO₂ + NO → OH + NO₂
What is the equation for the lifetime of a first-order reaction (e.g. photolysis)?
Lifetime = 1/k
What is the equation for the lifetime of a second-order reaction (e.g. [A] + [B])?
Lifetime of A = 1/k[B]
What is the equation for 1/total lifetime for a species undergoing multiple reactions?
(1 / lifetime of [a]) + (1 / lifetime of [b]) etc…
What is the equation for the lifetime of a species undergoing multiple reactions?
k’ = k₁ + k₂ + k₃, lifetime = (1/k’)
What controls photon flux at the surface?
- Absorption of shortwave radiation by ozone and oxygen
- Scattering and absorption of radiation by gases and particles affected by path length through the atmosphere
- Scattering and absorption by gases and by an atmospheric aerosol layer
What is the total light intensity reaching a given volume of gas made up of?
- Direct solar radiation
- Light reflected from the earth’s surface (albedos)
- Diffuse solar radiation (scattering by gases and particles)
How does the wavelength and intensity change when scattering occurs?
Scattering leads to no change in wavelength or intensity, but does reduce the amount of incoming radiation reaching the Earth’s surface
