Would Stratospheric Aerosol Injection Add to Acid Rain?

Deploying SAI to cool the planet would involve adding millions of tonnes of tiny particles – known as aerosols – to the stratosphere. The most studied idea is to release SO₂ into the stratosphere, which would react to form tiny sulphate aerosol particles.¹ However, SO₂ emissions are a major contributor to acid rain.² 

How significant is the threat from acid rain, and could sulphate-based SAI affect it? 

Acid rain formation and impacts 

Acid rain refers to any form of precipitation that is more acidic³ than unpolluted rainwater. However, unpolluted rainwater is slightly acidic (pH around 5.6) because water and carbon dioxide in the atmosphere naturally react to form weak carbonic acid.⁴

When water, oxygen, and other chemicals react with certain pollutants in the atmosphere, primarily SO₂ and nitrogen oxides, acid rain is formed. The main source of SO₂ emissions is the burning of fossil fuels with sulphur impurities.

Acid rain harms ecosystems in several ways. It can change the acidity of soils and inland water systems making them no longer suitable for some organisms. It can also dissolve nutrients in soils, weakening trees and other plants, and release aluminium from soil, which can be carried to lakes and streams, harming plants and aquatic animals.

The worst impacts of acid rain are concentrated near heavily industrialised regions due to the intense burning of fossil fuels, though air pollution regulations can help to mitigate this. While acid rain itself is not particularly harmful to humans, the SO₂ and other pollutants emitted also contribute to poor air quality, posing serious risks to human health.

The effort to clean up acid rain

As the consequences of air pollution on health and the environment became clearer,⁵ governments began to enact regulations to limit certain pollutants, such as the 1990 Clean Air Act in the United States. As a result, emissions of SO₂ and other pollutants have decreased substantially in Europe and North America since the 1970s. Efforts to decrease these emissions in Asia are also starting to yield results.

These efforts have decreased acid rain substantially and more progress is expected in the future. SO₂ emissions are projected to decrease by more than 75% globally by the end of this century relative to 2020 even under a high emissions scenario.​²​ 

Acid rain from SAI 

SAI would cool the planet by adding aerosol particles in the stratosphere to reflect a small amount sunlight back to space. Studies often focus on using SO₂, which reacts to form sulphate aerosols.¹

The aerosol particles have a lifetime of around one to two years in the stratosphere,⁶ and would have to be released continuously to maintain the cooling effect. All the material released for SAI would eventually fall back to the Earth’s surface.​²​

Under a moderate emissions scenario, global sulphate deposition would fall by roughly 60% by the end of this century according to one study.² If SAI were used to keep temperatures at 2020 levels under this scenario, offsetting more than 2°C of warming, global sulphate deposition would only fall by roughly 40%.² 

While the total amount of acid rain in this SAI scenario would be lower than current levels, its distribution would be different. The aerosols from SAI would eventually settle at high latitudes, so SAI would add very little to acid rain in the tropics and sub-tropics.²  

If SAI were deployed, most areas currently and historically affected by acid rain would still see less in the future, but acid rain could increase in regions that have seen little acid rain to date, such as the Pacific Northwest and Southern Greenland.² However, for most ecosystems, these increases are unlikely to be substantial enough to cause harm.⁷ 

Although most SAI research focuses on sulphate particles, some scientists have suggested using other particles such as calcite or alumina, which would avoid some side effects like acid rain.⁸ However, much less is known about the effects of these alternative aerosols in the stratosphere compared to sulphates.