From the EU’s National Emission Ceilings Directive to the Clean Air Act in the United States, several major global powers have taken bold steps to reduce pollutants in the air around us. Why, then, is the second suggested Google search term related to acid rain “is acid rain still a problem”? Here, Dr. Alexander Krajete, founder and CEO of emissions revalorization specialist Krajete GmbH, discusses how acid rain continues to threaten agricultural productivity, soil health, and crop quality, emphasizing the urgent need for further emissions mitigation efforts to protect our food systems and the environment.

Acid rain refers to any precipitation with dissolved nitric and sulfuric acid, though it may also contain carbonic acid and alkaline traces, with a pH below 5.6. Emissions of nitrogen and sulphur oxides (NOx and SOx) from manufacturing processes react with atmospheric moisture, creating the acidic precipitation that falls to Earth, where it directly affects soil, crops, and livestock.

The more obvious consequences of acid rain, such as direct harm to land-based and aquatic animal populations and damage to crops, have drastically reduced in regions where serious efforts have been made to address pollution. This has bred the idea that the problem itself has disappeared. Unfortunately, that is not true.

The clouds of acid rain still linger

While NOx emissions in Europe fell by 48 per cent and SOx emissions by 80 per cent between 2005 and 2020, this trend is not consistent, either globally or within Europe.

For example, in 2019, the Republic of Ireland's Environmental Protection Agency issued a pollution warning due to dangerously high nitrogen dioxide levels from traffic in Dublin. Meanwhile, countries like China and India continue to suffer from the effects of acidic atmospheric pollution.

The more obvious impacts of acid rain manifest in infrastructure, where the corrosion of statues and historical monuments becomes increasingly apparent. In more severe cases, acid rain can compromise the structural integrity of bridges, leading to catastrophic collapses that threaten public safety. These visible damages serve as a stark reminder of the dangers posed by acidic precipitation.

However, the consequences extend beyond human-made structures. The non-human world may face the greatest risks from acid rain. Its effects on soil chemistry can be subtle yet profoundly harmful. When acidic precipitation infiltrates the ground, it alters soil pH levels, diminishing the earth’s natural ability to buffer against acidity and releasing toxic metals like aluminium.

These changes create a hostile environment for crops, leading to lower yields and poor quality. Acidic precipitation hampers plant growth and nutrient absorption, resulting in less nutritious feed for livestock. Consequently, this affects the quality of meat, dairy and produce, impacting the entire food chain from farm to table.

Aquatic eco-systems suffer greatly from acid rain — atmospheric deposition and terrestrial run-off of nitrogenous material into lake and marine habitats results in eutrophication (over-enrichment) of plants and algae.

Overgrown during life, these organisms die and decompose, leaching dissolved oxygen from the water around them and starving aquatic life in the area of it. This leads to so-called ‘dead zones’ where aquatic life struggles to survive.

Why is acid rain still a problem?

One reason for the persistence of the acid rain phenomenon is the global community’s fixation on carbon dioxide emissions. The warming effects caused by increased atmospheric CO₂ levels are enormous, with 37.1 million tonnes released in 2022 alone, and great progress has been made to reduce emissions there.

However, this has led to some neglect by fossil fuel processors and combustion engine manufacturers to mitigate other emissions, such as NO_X and SO_X. This is mostly true in countries where regulations are less stringent.

A second factor is the destructive approach usually taken to treating captured NO_X and SO_X emissions. Sacrificial reactions involving NO_X, such as selective catalytic reduction (SCR), can release other species such as NH_3­. These both have their own harmful effects and SCR has a lower particle capture limit of ten ppm, meaning it cannot eradicate emissions entirely.

How should we deal with small-particle emissions?

Milder physisorption techniques are increasingly being used to capture NO_X and SO_X emissions without unwanted reactions and side products. The emissions gas abatement approach uses an adsorptive surface to bind pollutant particles without a chemical reaction, relying on weak surface bonding.

These weak intermolecular forces enable simple, low-energy release of the pollutants from the saturated adsorber, either through rinsing or mild heating. Collecting small-particle atmospheric contaminants in this way confers two large benefits.

Firstly, the amassed emissions are purified as part of the process. This means they can be repurposed within a company’s production cycle or sold onto a third-party with great financial efficiency. This could be NO_X as a pre-cursor for industrial nitric acid supply or production of nitrogen-rich fertiliser for agriculture.

On a grander scale, however, this method for reducing air pollution closes the emissions loop. Rather than converting NO_X and SO_X into “less harmful” gases to release into the atmosphere, physisorption is capable of meeting stringent pollutant legislation by capturing 99 per cent of small-particle emissions.

Adsorption processing of emissions, in which Krajete is a world leader, is suitable in offices, industrial facilities, in the average commuter car and on huge freight ships. To find out how Krajete can help you close the emissions loop and revalorise your waste gases, visit https://www.krajete.com/about-us/our-vision.