Mod1_AirIntro-Composition-and-UV

Module 1: Atmospheric Structure and Composition

  • Introduction

    • Understanding ozone (O3) depletion and smog.

Overview and Objectives

  • Purpose of the Module:

    • Introduces the structure and composition of Earth's atmosphere.

    • Examines changes in temperature and pressure with height in the atmospheric column.

    • Discusses absorption of high-energy ultraviolet photons from the sun.

    • Analyzes changes in ozone (O3) concentration with height.

  • Module Objectives:

    1. Explain temperature and pressure changes with height.

    2. Describe main gases present in Earth's atmosphere.

    3. Identify ultraviolet spectrum regions absorbed by O2 and O3 and their importance.

    4. Calculate concentrations of atmospheric gases using the ideal gas law.

Evolution of Oxygen in Earth’s Atmosphere

  • Critical for Life:

    • Oxygen (O2) is essential, not just for breathing but also has geochemical importance.

    • Early Earth: Oxygen was locked in minerals.

    • Great Oxidation Event: Cyanobacteria evolved, performing photosynthesis and releasing free oxygen (O2).

    • Associated with the Great Dying: A period of massive volcanic eruptions and mass extinction.

Effects of UV Radiation

  • Types of UV Radiation:

    • UV-A: 320 – 400 nm

    • UV-B: 280 – 320 nm (harmful effects)

    • UV-C: 100 – 280 nm (most harmful, absorbed by atmosphere)

  • Negative Impacts of UV Radiation:

    • Severe sunburns, skin cancer, cataracts.

    • Damaging impacts on DNA and ecosystems, including plants and phytoplankton.

Temperature and Pressure in the Atmosphere

  • Atmospheric Layers:

    • Troposphere: Temperature decreases with height; pressure decreases exponentially.

    • Stratosphere: Temperature increases with height.

    • Mesosphere: Temperature decreases again with height.

    • Observations indicate non-zero pressure in the stratosphere and above.

Chemical Composition of the Atmosphere

  • Major Components:

    • Nitrogen (78.0%)

    • Oxygen (21.0%)

    • Argon (0.9%)

    • Trace gases (including Methane, Helium, Nitrous Oxide, Neon, Ozone, and Carbon Dioxide)

Trace Gases and Their Importance

  • Trace gases are crucial for atmospheric chemistry, although present in smaller quantities.

  • Ozone concentration is relatively low at 0.010%.

Selective Absorption of Light by Gases

  • Key Concept: Gases in the atmosphere can selectively absorb certain wavelengths of light.

    • Ozone (O3) absorbs UV light between 200-320 nm; acts as a "sunscreen" in the stratosphere.

Understanding Ozone Concentration

  • Ozone Layer:

    • Not composed entirely of O3; represents less than 10 out of 1,000,000 air molecules in the stratosphere.

  • Calculating Gas Concentration:

    • Concentration calculations involve PV = nRT.

    • Mixing ratios ( Χ𝑌 ) relate to mole fractions of gases:

      • Mole fraction = Number of moles of Y / Number of moles of air.

      • Expressed in parts-per-million (ppmv), parts-per-billion (ppbv), and parts-per-trillion (pptv).