Atmospheric Chemistry Notes

Atmospheric Chemistry

Introduction

This section introduces Earth's atmosphere and chemical processes, focusing on atmospheric structure, temperature, pressure distribution, smog formation (industrial and photochemical), the Chapman cycle of ozone formation and destruction, and the Antarctic ozone hole. Key topics include the catalytic removal of ozone by CFCs. Reading material includes van Loon and Duffy chapters 2, 3, and 4, covering the Earth’s atmosphere, stratospheric chemistry, and tropospheric chemistry, respectively.

Learning Outcomes

  • Describe atmospheric structure, temperature, and pressure distribution.

  • Explain the processes dictating changes in atmospheric temperature with height.

  • Discuss conditions for industrial and photochemical smog formation and their health effects.

  • Describe the Chapman cycle and account for the vertical distribution of O3.

  • Discuss the formation of the Antarctic ozone hole and catalytic ozone removal by CFCs.

Importance of Atmospheric Chemistry

Clean air is essential for human health and wellbeing. Air pollution poses a significant global health threat, contributing to up to a million premature deaths annually. The atmosphere's composition directly influences the quality of life through climate, temperature, human and ecosystem health, and damage to infrastructure. The atmosphere is relatively small compared to Earth's total storage, making its composition easily changeable.

Structure of the Earth's Atmosphere

Atmospheric gases are held close to Earth by gravity. Pressure and density decrease with increasing distance from the surface. The troposphere and stratosphere contain 99.9% of the atmosphere, with almost half within 6 km of the Earth's surface.

Atmospheric Layers

The atmosphere is divided into four layers based on temperature profiles relative to altitude. Varying solar radiation penetrates each layer, creating unique properties and compositions that drive different chemical reactions and temperature changes.

  • Troposphere: Contains 85% of the atmosphere. Major gases include H2O, O2, N2, Ar, and CO2. Temperature decreases with altitude from 14°C to -60°C. The Earth's surface is warmed by absorbing incoming solar radiation. Temperature decrease drives condensation, keeping most water near the surface. Convective mixing occurs.

  • Stratosphere: Temperature increases with altitude from -60°C to -2°C due to heating by UV radiation. Lacks convective mixing, forming layers. Contains the ozone layer (O3) that absorbs high-energy/short-wavelength radiation.

  • Mesosphere: Temperature decreases with altitude from -2°C to -90°C.

  • Thermosphere: Temperature increases with altitude from -90°C to 1200°C. O2 absorbs high-energy/short-wavelength radiation. The breakup of N2 and O2 leads to the formation of NO, N, and O species.

Key Variables in Atmospheric Properties

Variable incoming and absorbed solar radiation is responsible for physical properties. Pressure, density, temperature, mixing, and composition are all influenced by solar radiation.

Cooling by Expansion

Most gases cool upon expansion at room temperature, except H, He, and Ne, which cool at lower temperatures. The adiabatic lapse rate (ΔT/Δh) describes this, where adiabatic means no heat exchange with the outside.

Lapse Rates

Lapse rates depend on the amount of water in the atmosphere. Dry air cools at approximately 10^\circ C/km, while moist air cools at less than 6^\circ C/km. Water vapor in rising air condenses to liquid, releasing heat and offsetting cooling. The lapse rate is influenced by radiative distribution, cooling by expansion, and moist convection.

Mixing of Gases in the Troposphere

The troposphere has intense air movement, contributing to convective mixing, which results in well-mixed gases vertically and laterally. Vertical mixing is quick with gases reaching the top of the troposphere in about 2 days. Lateral mixing is slower, taking about a month within each hemisphere. Gases with long residence times, such as N2, O2, and Ar, are well-mixed across both hemispheres.

Mixing Ratios

Mixing ratios are the number of molecules of a gas relative to the total number of molecules present:

ri = \frac{Ni}{N_a}

Where:

  • r_i = mixing ratio (units = mol/mol or v/v; or ppmv/ppbv)

  • N_i = number of molecules of gas

  • N_a = number of molecules of air

Mixing ratios of major gases remain relatively constant up to ~80 km. Kinetic energy overcomes gravitational settling, preventing stratification. Trace gas concentrations vary considerably due to localized sources, reactivity (low residence time), and stability (high residence time, e.g., CFCs).

The main gases in the troposphere are N2 (78%), O2 (21%), and Ar (0.09%).

Molar Mass of Air

The molar mass of dry air can be calculated as follows:

M{air} = (M{N2} \times f{N2}) + (M{O2} \times f{O2}) + (M{Ar} \times f_{Ar})

M_{air} = (28 \text{ g mol}^{-1} \times 0.78) + (32 \text{ g mol}^{-1} \times 0.21) + (40 \text{ g mol}^{-1} \times 0.01) = 28.96 \text{ g mol}^{-1}

This calculation reveals that the mass of air is closest to that of N2 because it's the most abundant.

Mixing Ratios for Trace Gases

Mixing ratios for trace gases are expressed in ppm, ppb, or ppt.

To convert a mixing ratio from mole/mole to mass/mass, the following formula is used:

\xi = \xi \times \frac{M{CO2}}{M_{AIR}}

Example: For CO2 at 420 ppm, with M{CO2} = 44 \text{ g mol}^{-1}:

\xi = 420 \text{ ppm} \times \frac{44}{28.96} = 638 \text{ ppm}

Gas Phase Reactions

For a reaction to occur, an energy barrier must be overcome. Collisions must have the correct geometry and sufficient energy. Reaction rate depends on absolute concentration (pressure-dependent).