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Sulfate aerosols are formed from sulfur dioxide (SO₂) through a series of chemical reactions in the atmosphere to form sulfuric acid (H₂SO₄) droplets along with other compounds such as ammonium sulfate ((NH₄)₂SO₄)

Summary​

Sulfate aerosols, particularly sulfuric acid (H₂SO₄), are considered for stratospheric aerosol injection due to their proven albedo-enhancing effects and well-understood atmospheric behavior. Formed through oxidation of sulfur dioxide (SO₂), these aerosols scatter solar radiation effectively, potentially reducing global temperatures. Their formation involves multiple chemical reactions: SO₂ reacts with hydroxyl radicals to form intermediates that ultimately produce sulfuric acid (H₂SO₄) droplets. While effective for climate intervention, sulfate aerosols pose risks including ozone depletion through interactions with chlorine and bromine compounds. Natural analogs like volcanic eruptions (e.g., Mt. Pinatubo) demonstrate their temperature-lowering potential. Established research shows sulfate aerosols can persist in the stratosphere for 1–3 years, with climate models indicating significant radiative forcing impacts. However, challenges remain in balancing cooling effects with ozone layer protection and managing atmospheric chemistry interactions.

Sulfate aerosols are often proposed for stratospheric aerosol injection (SAI) because they are practical and well-understood. The key reasons include their inspiration from natural phenomena, high albedo effect, and established scientific understanding.

The physical mechanism of sulfate aerosols​

Aerosols are tiny particles or droplets in solid or liquid phase that are suspended in a gas, such as air. Therefore, sulfate aerosols in the atmosphere can be defined as any small solid or liquid particle containing sulfate (SO₄) that is suspended in air. Some common types of sulfate aerosols include:

  • Sulfuric Acid (H₂SO₄)
  • Ammonium Sulfate ((NH₄)₂SO₄)
  • Sodium Sulfate (Na₂SO₄)
  • Calcium Sulfate (CaSO₄)
  • Magnesium Sulfate (MgSO₄)

In the context of SAI research and development, the most common type of sulfate aerosol is Sulfuric Acid (H₂SO₄). Sulfuric acid is formed in the atmosphere via the injection of sulfur dioxide (SO₂), which is produced by volcanic eruptions and industrial processes. When in the stratosphere, SO₂ reacts with other compounds, such as hydroxyl radicals (OH), through a process called oxidation through the following simplified sequence 1.

Step 1: Initial Oxidation:
Sulfur dioxide reacts with atmospheric chemicals (OH radicals) to form an intermediate compound called a hydroxysulfonyl radical (HOSO₂):

SO2+OH→HOSO2\begin{align*} \text{SO}_2 + \text{OH} &\rightarrow \text{HOSO}_2 \end{align*}

Step 2: Sulfur trioxide Formation:
The HOSO₂ intermediate reacts with oxygen (O₂) to form sulfur trioxide (SO₃) and a peroxy radical (HO₂):

HOSO2+O2→SO3+HO2\begin{align*} \text{HOSO}_2 + \text{O}_2 &\rightarrow \text{SO}_3 + \text{HO}_2 \end{align*}

Step 3: Sulfuric Acid Formation:
Sulfur trioxide (SO₃) reacts with water vapor (H₂O) in the stratosphere to produce gaseous sulfuric acid (H₂SO₄):

SO3+H2O→H2SO4\begin{align*} \text{SO}_3 + \text{H}_2\text{O} &\rightarrow \text{H}_2\text{SO}_4 \end{align*}

Step 4: Condensation:
The sulfuric acid (H₂SO₄) then condenses (changes from the gas to liquid phase), becoming an aerosol suspended in the atmosphere

Step 5: Further reactions
The sulfuric acid (H₂SO₄) aerosol can then react with other chemicals in the atmosphere such as ammonia (NH₃) to form Ammonium Sulfate ((NH₄)₂SO₄)

2NH3+H2SO4→(NH4)2SO4\begin{align*} 2\text{NH}_3 + \text{H}_2\text{SO}_4 &\rightarrow (\text{NH}_4)_2\text{SO}_4 \end{align*}

The Albedo effect of sulfate aerosols​

Sulfate aerosols are highly effective at scattering incoming solar radiation which could increase the earth's albedo and help in reducing global temperatures. There are various measurements of the efficiency of an aerosol to scatter light known as optical properties including: single scattering albedo, scattering efficiency, optical depth, and refractive index, some of which have a dependency on the thickness and density of a sulfate aerosol layer. Sulfate aerosols exhibit desirable optical properties for the deployment of SAI as they are reflective, optically dense, and can persist in the atmosphere for extended periods 2 3.

Sulfate aerosols and ozone depletion​

Sulfate aerosols are highly reactive with other particles in the atmosphere that lead to the depletion of ozone (O₃). Sulfate aerosols such as sulfuric acid (H₂SO₄) do not react chemically with ozone (O₃), but they provide surfaces for reactive particles, such as chlorine (Cl) and bromine (Br) which are present in the atmosphere, to attach to and react with ozone (O₃) 2. The process in which sulfate aerosols assist in ozone depletion is complex and involves a number of chemical reactions, but it revolves around surface area for reactions to occur. Chlorine and bromine compounds are highly reactive, but in their gaseous form, they are not very effective at reacting with ozone (O₃). However, in the presence of sulfate aerosols, these chlorine and bromine compounds adhere to the aerosols' surfaces where they are close enough and in high enough concentrations to react with other atmospheric particles. Here, they undergo another series of reactions to form chlorine radicals (Cl) and bromine radicals (Br) which are highly reactive with ozone through a catalytic cycle which can allow a single chlorine atom to destroy many ozone molecules 4.

Established scientific understanding of sulfate aerosols​

The mechanisms of how sulfate aerosols operate in the atmosphere are well-established and understood as well as their potential benefits for SAI. The main reason for this being that sulfate aerosols are released into the atmosphere through natural and anthropogenic processes such as volcanic eruptions of Mt Pinatubo in 1991 and human activities such as the presence of sulfur in maritime shipping fuel. In many of these cases, atmospheric sulfate aerosols have led to measurable reduction in global temperatures 2 3. Because of this, sulfate aerosols have also been extensively studied and modeled in climate simulations.

Sources​

Footnotes​

  1. McKeen, S. A., Liu, S. C., & Kiang, C. S. (1984). On the chemistry of stratospheric SO₂ from volcanic eruptions. Journal of Geophysical Research: Atmospheres, 89(D3), 4873–4881. https://doi.org/10.1029/JD089iD03p04873 ↩

  2. Huynh, K. A., & McNeill, V. F. (2024). The potential environmental and climate impacts of stratospheric aerosol injection: A review. Environmental Science: Atmospheres, 4(1), 114–143. https://doi.org/10.1039/D3EA00134B ↩ ↩2 ↩3

  3. Crutzen, P. J. (2006). Albedo enhancement by stratospheric sulfur injections: A contribution to resolve a policy dilemma? Climatic Change, 77(3-4), 211–220. https://doi.org/10.1007/s10584-006-9101-y ↩ ↩2

  4. World Meteorological Organization. (2018). Scientific assessment of ozone depletion: 2018 (Global Ozone Research and Monitoring Project Report No. 58). https://doi.org/10.1787/9789264302072-en ↩