evsc week 6 - Stratospheric Ozone Depletion

Where is Ozone Found?

in the thin layer of gases that surrounds Earth. Composed of several sub-layers that differ in density, gas composition, and temperature

different layers of the atmosphere

1. Troposphere

2. Stratosphere 

3. Mesosphere

4. Thermosphere

5. Exosphere 

what are the three conditions needed to enable ozone destruction

1. Stable Polar Vortex in winter

2. Polar Stratospheric Clouds (PCSs) 

3. Chemical Reactions → Cl- and Br

1. Stable Polar Vortex in winter

In the Antarctic winter, winds generally circle the pole from west to east (counter-clockwise). This isolates the air in the middle of the “vortex”, helping to lower air temperatures and generally prevent the introduction of “new” air into the vortex. This keeps temperatures low and concentrates ozone-depleting chemicals.

2. Polar Stratospheric Clouds (PCSs) 

• the abundance of the chemicals which destroy ozone correlates with the abundance of PCSs.

3. Chemical Reactions → Cl- and Br

When the polar vortex is strong and temperatures are very cold during winter, certain chemicals build up in the atmosphere. These chemicals contain chlorine (Cl) and bromine (Br), but they stay “trapped” until sunlight returns in spring. Once the sunlight comes back, it helps release free Cl and Br atoms, which can then take part in chemical reactions — like those that destroy ozone.

what is the key with ozone holes over Antarctica

generally appear in spring because the winter polar vortex isolates the stratosphere and cools it to extremely low temperatures

what is the role of stratospheric ozone in the atmosphere

Stratospheric ozone plays a critical role in protecting life on Earth by absorbing the Sun’s harmful ultraviolet (UV) radiation.

• blocks all of the most energetic UV-C radiation, most UV-B radiation, and about half of UV-A radiation, preventing these rays from reaching the surface.

• Without this ozone layer, UV radiation would damage plants, animals, and humans, causing increased skin cancers, eye damage, and harming ecosystems. Essentially, stratospheric ozone acts as a natural sunscreen for the planet.

How the ‘hole’ in the ozone layer was first discovered,

• The ozone “hole” was first noticed in the early 1970s and officially reported in 1985, when scientists observed unusually low concentrations of stratospheric ozone over Antarctica.

how ongoing research monitors ozone levels

• using instruments that measure the total ozone in a column of air from the Earth’s surface to the top of the atmosphere, expressed in Dobson Units (DU).

• Areas with values below 220 DU are considered to have experienced severe ozone depletion, allowing scientists to track changes and recovery in the ozone layer over time.

What is the control variable for stratospheric ozone depletion 

The control variable is the concentration of ozone (O₃) in the stratosphere, measured in Dobson Units (DU).

what is the planetary boundary for stratospheric ozone depletion 

The planetary boundary is set at 275 DU, meaning that ozone levels below this threshold indicate serious risks to human health and the environment, such as increased UV radiation exposure.

Main goal of the Montreal Protocol:

Phase out the production and use of ozone-depleting substances (ODSs), including CFCs, to protect the stratospheric ozone layer.

Why the Montreal Protocol was amended over time:

• To address emerging issues such as the use of HFCs (which are potent greenhouse gases) as CFC replacements.

• To respond to increasing global demand for cooling technologies and new scientific findings on ozone depletion.

Outcomes of the Montreal Protocol

1. Successfully reduced the production and use of ODSs worldwide

2. Led to recovery of the ozone layer; Antarctic ozone is expected to return to 1960 levels by 2100.

3. Demonstrated a global environmental policy success, being signed by every country

4. Encouraged the development of alternative chemicals and technologies with less environmental impact.