Lecture 24 ESPM 15 Stratospheric Ozone Depletion Study Notes

ESPM 15: Stratospheric Ozone Depletion - Guest Lecture by Professor Allen Goldstein

Departments of Environmental Science, Policy and Management, Civil and Environmental Engineering, University of California, Berkeley

Introduction

  • Focus on stratospheric ozone depletion.

  • Importance of ozone layer for life on Earth and implications of its depletion.

Vertical Structure of Earth’s Atmosphere

  • Temperature vs. Altitude:

    • Temperature increases with altitude in the stratosphere, with key altitudes including:

    • Stratopause

    • Tropopause

    • Stratosphere

    • Troposphere

    • Mesosphere

    • Note: Pressure is measured in hectopascals (hPa) on a logarithmic scale.

    • Temperature ranges from about 200K at higher altitudes to 280K closer to the troposphere.

Global Transport Time Scales

  • Reference to typical time scales for global horizontal transport (Jacob, 1999).

  • Distinct time scales for vertical transport are also outlined to understand movement within stratospheric layers.

World Meteorological Organization Guidelines

  • Guidelines for Ozone:

    • Stratospheric Ozone (O3): Avoid destruction.

    • Tropospheric Ozone (O3): Avoid increasing levels.

    • Smog Ozone (O3): Aim to reduce levels.

Chapman Cycle: Ozone Formation & Destruction

  • Chemical Mechanism Proposed by Sydney Chapman (1930):

    1. O$_2$ + hν → O + O (slow)

    2. O + O$2$ + M → O$3$ + M (fast)

    3. O$3$ + hν → O$2$ + O (fast)

    4. O$3$ + O → 2O$2$ (slow)

  • Odd Oxygen (Ox): Defined as Ox = O + O$_3$.

  • Question: Why is there ozone present in the stratosphere?

Steady State Solution

  • The steady state solution for O$_3$ involves:

    • Balancing the production and loss rates.

    • Relative rates of reactions 2 and 3 (fast steady state) determine the ratio of O to O$_3$.

    • Destruction of O$_3$ contingent upon destroying Ox (reaction 4, slow process).

Limitations of the Chapman Cycle

  • Chapman’s model displaying qualitative agreement but predicts higher O$_3$ levels than observed.

  • Issues exist with not accounting for Catalytic Cycles for Ozone Loss:

    • General idea: O$3$ + X → XO + O$2$

    • X as a catalyst is neither created nor destroyed in reactions, highlighting that the rate of the reaction depends on catalyst concentrations.

Nitrogen Oxide (NOx) Radicals

  • Composition of NOx: NO + NO$_2$.

  • Initiation Mechanism:

    • N$_2$O + O → 2NO

    • N$_2$O persists in atmosphere for ~150 years, primarily degraded in the stratosphere.

  • Propagation Mechanisms:

    • NO + O$3$ → NO$2$ + O$_2$

    • NO$2$ + O → NO + O$2$

    • Net Reaction: O + O$3$ → 2O$2$

  • NOx serves as an ozone loss catalyst but not the sole catalyst.

Anthropogenic Contributions to Ozone Depletion

  • Human activities have led to increased concentrations of:

    • Nitrous oxide (N$_2$O)

    • Methane (CH$_4$)

    • Chlorofluorocarbons (CFCs), which worsen stratospheric O$_3$ depletion.

Stratospheric Distribution of CFC-12

  • Visual representation showing CFC-12 concentrations across various pressures and latitudes, indicating significant presence in the stratosphere.

Catalytic Cycles for Ozone Loss: Chlorine Radicals

  • Chlorine Radical Chemistry:

    • CFC-12 (CF$2$Cl$2$) undergoes photodissociation:

    • CF$2$Cl$2$ + hν → CF$_2$Cl + Cl

    • CFCs have lifetimes of ~100 years, destroyed only in the stratosphere.

  • Propagation and Net Reactions:

    • Cl + O$3$ → ClO + O$2$

    • ClO + O → Cl + O$_2$

    • Net: O$3$ + O → 2O$2$

  • Termination Process:

    • Cl + CH$4$ → HCl + CH$3$

    • HCl + OH → Cl + H$_2$O

    • ClO + NO$2$ + M → ClNO$3$ + M

    • ClNO$3$ + hν → ClO + NO$2$

Antarctic Ozone Hole

  • First reported in 1985 by Joe Farman, British Antarctic Survey.

  • Total Ozone Measurements: Over Halley Bay indicate severe depletion.

Observations of the Ozone Hole

  • Early 1980s observations raised critical questions about significant ozone loss:

    • Why was there massive ozone loss exclusively over Antarctica?

    • Why was this phenomenon limited to the spring?

    • Which catalytic cycles were primarily responsible?

  • Notable flights into the ozone hole recorded on September 16, 1987, measuring O$_3$ and reactive chlorine (ClO).

    • Discovery of chlorine catalytic cycles as dominant ozone loss processes.

Polar Stratospheric Clouds (PSCs)

  • Role in releasing chlorine in an active form, notably contributing to ozone depletion.

Temperature Metrics for Polar Regions

  • Recorded temperature data indicates significant cold in the polar lower stratosphere, affecting PSC formation and ozone depletion processes.

Chronology of Antarctic Ozone Hole Development

  • Vortex and PSC formation timeline leading to significant ozone depletion events during springtime.

Remote Sensing of Stratospheric Ozone

  • Use of UV solar backscatter for measuring stratospheric ozone from space.

  • Example of a global ozone image taken on October 21, 2024, revealing the Antarctic ozone hole.

Increase in UV Radiation

  • Ozone depletion leads to increased erythema and other UV-related health effects, notably observed over the United States and mid-latitude areas, though not as severely as in Antarctica.

Mitigation Efforts

  • Montreal Protocol:

    • An international treaty aimed at phasing out substances responsible for ozone depletion.

    • Signed in 1987 and amended based on scientific developments and political consensus.

    • Chart showing projected changes in stratospheric chlorine concentrations as a result of the protocol's success and timelines for recovery.

Conclusion: The Efficacy of the Montreal Protocol

  • Evidence suggests that the Montreal Protocol has been effective in reducing ozone-depleting substances, leading to anticipated recovery of the ozone layer in the coming decades.

  • References to visual materials illustrating the progress made due to these international efforts, notably the "World Avoided" video series by NASA.