Question

The tropospheric mixing ratios of carbon monoxide are higher in the northern hemisphere than in the southern hemisphere. However, it has been observed that there has been a general global decline in carbon monoxide concentration everywhere in the \(1990 \mathrm{~s}\). Two reasons have been suggested for this -one is the eruption of Mount Pinatubo and the other is the occurrence of several relatively dry years in the tropics. Comment on these two possibilities in terms of tropospheric and stratospheric processes.

Short answer

The Mount Pinatubo eruption and dry years in the tropics contributed to declining CO levels in the 1990s through stratospheric cooling and reduced biomass burning emissions.

Step-by-step solution

Understanding Tropospheric and Stratospheric Processes

The troposphere is the lowest layer of Earth's atmosphere where weather occurs, and it contains most of the atmospheric water vapor and particulate matter. The stratosphere is above the troposphere and is more stable, with a higher concentration of ozone. Carbon monoxide in the troposphere primarily originates from incomplete combustion processes and is affected by meteorological conditions.

Impact of Mount Pinatubo Eruption

The eruption of Mount Pinatubo in 1991 released large amounts of aerosols into the stratosphere which reduced the solar radiation reaching the Earth's surface. This cooling led to changes in atmospheric circulation patterns, potentially affecting the distribution and chemistry of carbon monoxide in the troposphere.

Dry Years in the Tropics and CO Levels

Relatively dry years can lead to reduced biomass burning in the tropics, which is a significant source of carbon monoxide production. Consequently, with less burning, less carbon monoxide would be emitted into the atmosphere, contributing to the observed decline in its global levels.

Synthesizing the Explanation

The decline in tropospheric carbon monoxide levels during the 1990s can be attributed to a combination of stratospheric cooling effects due to the Mount Pinatubo eruption and reduced tropospheric emissions from decreased biomass burning due to dry conditions in the tropics.

Key concepts

Tropospheric Chemistry

Tropospheric chemistry deals with the chemical reactions and processes that occur in the troposphere, the lowest layer of Earth's atmosphere. The troposphere is where most weather events take place and contains about 75% of the atmosphere's mass. It is characterized by the presence of clouds, wind, and rain. In this layer, various gases such as carbon monoxide (CO) are present, which primarily originate from sources like vehicle emissions, industrial processes, and wildfires.
Because the troposphere is dynamic, carbon monoxide and other pollutants are distributed by wind and weather systems. One of the key challenges is understanding how these pollutants are generated, transformed, and eventually removed. Chemical reactions with hydroxyl radicals (OH) play a crucial role in lowering CO concentrations in the atmosphere. These radicals are often referred to as the "detergent of the atmosphere" due to their ability to oxidize many pollutants, thus cleaning the air.

Stratospheric Processes

The stratosphere lies above the troposphere and is characterized by a more stable atmosphere with distinct chemical compositions. Unlike the troposphere, the stratosphere has a higher concentration of ozone, a molecule crucial for absorbing harmful ultraviolet radiation from the Sun.
The 1991 eruption of Mount Pinatubo had a significant impact on stratospheric processes. Huge amounts of sulfur dioxide were emitted into the stratosphere, forming sulfate aerosols that spread across the globe. These aerosols reflected sunlight away from the Earth's surface, causing a cooling effect. This cooling altered atmospheric circulation patterns, which can indirectly affect chemical reactions in the troposphere below.
Understanding these stratospheric influences helps scientists predict potential changes in environmental chemistry and their consequences on life and weather patterns on Earth.

Carbon Monoxide Distribution

Carbon monoxide distribution in the atmosphere varies greatly due to several factors. Many of these factors are influenced by human activities and natural occurrences. In the Northern Hemisphere, carbon monoxide levels are typically higher due to denser populations and more industrial activity, leading to increased emissions from vehicles and manufacturing.
Natural events, such as wildfires, can also contribute to variations in CO levels. During dry years in the tropics, there tends to be less biomass burning, which means fewer emissions of carbon monoxide. This was observed during the 1990s, where such dry periods coincided with reduced CO emissions, helping to contribute to a global decline in CO concentrations.
Monitoring the distribution of carbon monoxide is essential for environmental regulations and assessing air quality, as elevated levels of CO can be harmful to human health and contribute to other atmospheric chemical processes.

Atmospheric Circulation

Atmospheric circulation is the large-scale movement of air around the Earth, which helps distribute heat and moisture across the planet. It plays a critical role in weather patterns and climate. Circulation patterns are driven by the uneven heating of the Earth's surface by the Sun, leading to various pressure systems and wind patterns.
The eruption of Mount Pinatubo in 1991 disrupted normal atmospheric circulation by injecting aerosols into the stratosphere, which blocked sunlight and cooled the Earth's surface. These changes influence climate systems and could affect the transport of pollutants like carbon monoxide. For example, alterations in wind patterns can change the dispersion and dilution of pollutants in the troposphere.
Understanding these processes is crucial for predicting weather trends and assessing the impact of human activities and natural events on our climate. This knowledge helps in developing strategies for mitigating climate change and protecting the environment.