Question

It was estimated that the eruption of the Mount Pinatubo volcano resulted in the injection of 20 million metric tons of \(\mathrm{SO}_{2}\) into the atmosphere. Most of this \(\mathrm{SO}_{2}\) underwent oxidation to \(\mathrm{SO}_{2}\), which reacts with atmospheric water to form an aerosol. (a) Write chemical equations for the processes leading to formation of the aerosol. (b) The aerosols caused a \(0.5-0.6^{\circ} \mathrm{C}\) drop in surface temperature in the northern hemisphere. What is the mechanism by which this occurs? (c) The sulfate aerosols, as they are called, also cause loss of ozone from the stratosphere. How might this occur?

Short answer

(a) The chemical equations for aerosol formation are: 1. \(2\,\mathrm{SO}_2 + O_2 \longrightarrow 2\,\mathrm{SO}_3\) 2. \(\mathrm{SO}_3 + H_2O \longrightarrow \mathrm{H}_2\mathrm{SO}_4\) (b) Sulfate aerosols cause a drop in surface temperature by increasing Earth's albedo, reflecting sunlight back into space and allowing for less energy to reach the surface. (c) Sulfate aerosols contribute to loss of ozone in the stratosphere by acting as surfaces for heterogeneous reactions involving ozone-depleting species and promoting the formation of polar stratospheric clouds (PSCs), which enhance ozone-destroying reactions.

Step-by-step solution

(a) Chemical equations for aerosol formation

To determine the chemical equations for aerosol formation, first, recall that the majority of \(\mathrm{SO}_2\) injected into the atmosphere underwent oxidation to \(\mathrm{SO}_{3}\). The reaction is: \[2\,\mathrm{SO}_2 + O_2 \longrightarrow 2\,\mathrm{SO}_3\] Next, \(\mathrm{SO}_3\) reacts with the atmospheric water to form sulfuric acid, or \(\mathrm{H}_2\mathrm{SO}_4\). The equation for this reaction is: \[\mathrm{SO}_3 + H_2O \longrightarrow \mathrm{H}_2\mathrm{SO}_4\] The sulfuric acid formed forms an aerosol by condensing with other atmospheric molecules and particles.

(b) Mechanism of temperature drop

The mechanism by which sulfate aerosols cause a drop in surface temperature is by increasing Earth's albedo - the reflection of sunlight back into space. Aerosols scatter the incoming solar radiation, allowing for less energy to reach Earth's surface. This results in a cooling effect on the surface temperature. This phenomenon explains the observed \(0.5-0.6^{\circ} \mathrm{C}\) drop in surface temperature in the northern hemisphere.

(c) Loss of ozone from the stratosphere

Sulfate aerosols can contribute to loss of ozone in the stratosphere through several mechanisms. One possible mechanism is by acting as surfaces on which heterogeneous reactions can take place. These reactions involve species like chlorine, which can break down ozone in a catalytic cycle. Additionally, the presence of sulfuric acid can lead to the formation of polar stratospheric clouds (PSCs), which also provide surfaces for ozone-destroying reactions to occur. As a result, these sulfate aerosols enhance the potential for ozone-depleting reactions to take place, leading to a decrease in stratospheric ozone levels.

Key concepts

Sulfate Aerosols

Sulfate aerosols are tiny particles formed primarily from the oxidation of sulfur dioxide (\(\mathrm{SO}_2\)) in the atmosphere. When a volcanic eruption occurs, like the eruption of Mount Pinatubo, large amounts of \(\mathrm{SO}_2\) are released into the atmosphere. This \(\mathrm{SO}_2\) undergoes a chemical transformation process to form \(\mathrm{SO}_3\), as depicted in the equation:
\[2\,\mathrm{SO}_2 + O_2 \rightarrow 2\,\mathrm{SO}_3\]
Following this transformation, \(\mathrm{SO}_3\) readily reacts with water vapor present in the atmosphere, resulting in the formation of sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)), which can be represented by the following equation:
\[\mathrm{SO}_3 + H_2O \rightarrow \mathrm{H}_2\mathrm{SO}_4\]
This sulfuric acid can condense or combine with other particles to form sulfate aerosols. These aerosols have significant environmental impacts, affecting both climate and atmospheric chemistry.

Ozone Depletion

Ozone depletion refers to the thinning of the Earth's ozone layer, particularly in the stratosphere. Sulfate aerosols, like those formed after volcanic eruptions, can contribute to this process. These aerosols act as surfaces for chemical reactions involving ozone-depleting substances, such as chlorine radicals. For instance:

• Sulfate aerosols provide a surface for heterogeneous reactions.
• These reactions can involve chlorine compounds from human-made chemicals like chlorofluorocarbons (CFCs).
• The result is the release of active chlorine species that can break down ozone molecules.

Moreover, sulfuric acid in the aerosols can enhance the formation of polar stratospheric clouds (PSCs), which further facilitate ozone-depleting reactions under certain conditions. The degradation of ozone is concerning because it plays a crucial role in blocking harmful ultraviolet (UV) radiation from reaching the Earth's surface.

Earth's Albedo

Earth's albedo is a measure of how much sunlight is reflected back into space without being absorbed by the Earth's surface. Sulfate aerosols influence this by increasing the planet’s reflectivity or albedo. When these aerosols are present in the atmosphere:

• They scatter and reflect incoming solar radiation.
• This reduces the amount of solar energy that reaches the Earth's surface.
• Consequently, there is a cooling effect on the global or regional climate.

This was evidenced by the Mt. Pinatubo eruption, where the resulting sulfate aerosols temporarily increased Earth's albedo, leading to a noticeable drop in temperatures. The effect emphasizes how impactful aerosols can be in climate modulation and underscores the importance of understanding aerosol-related processes in climate science.

Sulfur Dioxide Oxidation

Sulfur dioxide oxidation is a crucial process in the formation of sulfate aerosols. After a volcanic eruption injects massive quantities of \(\mathrm{SO}_2\) into the atmosphere, these molecules undergo a series of reactions. The primary transformation process involves:

• The reaction of \(\mathrm{SO}_2\) with atmospheric oxygen to form sulfur trioxide (\(\mathrm{SO}_3\)).
• This can be accelerated in the presence of catalysts such as hydrocarbons or nitrogen oxides.

The chemical reaction is as follows:
\[2\,\mathrm{SO}_2 + O_2 \rightarrow 2\,\mathrm{SO}_3\]
Subsequently, \(\mathrm{SO}_3\) reacts with water vapor to form sulfuric acid \(\mathrm{H}_2\mathrm{SO}_4\), which contributes to the creation of sulfate aerosols. Understanding this oxidation process is vital because it links volcanic activity directly with atmospheric chemistry alterations and climate effects, highlighting the intricate interplay between natural events and environmental changes.