A look at the bright side to air pollution

Air pollution
Iron in air pollution may end up feeding plankton.
Credit: Wikimedia Commons

The long-held hypothesis that adding iron particles increases the ocean’s ability to absorb carbon has been backed by some promising evidence uncovered by scientists from the University of Birmingham in the UK, and Shandong University in China.

A team of researchers led by Birmingham environmental scientist Zongbo Shi collected three types of iron-rich particles from above the Yellow Sea between mainland China and Korea. The finds were then examined on a nanoscale level to ascertain their exact molecular composition.

Shi’s team reports in Science Advances that the particles – including fly-ash and mineral dust – had originated as emissions from east Asian coal burning operations and been propelled above the open sea by wind currents.

The deliberate or accidental addition of iron to ocean waters – a process dubbed iron fertilisation – is known to be a useful method of stimulating phytoplankton growth. The plankton, in turn, absorbs carbon dioxide during photosynthesis.

While large land-based plants such as trees do the same thing and then store the CO2 more or less permanently, phytoplankton mainly release it back into the atmosphere within a year or so. However, a significant proportion of it – embodied in dead plants, fragments, and zooplankton faeces – sinks to the ocean floor, where it remains.

A study published earlier this year, led by Galen McKinley from the University of Wisconsin-Madison, estimated that “since preindustrial times, the ocean has removed from the atmosphere 41% of the carbon emitted by human industrial activities.”

McKinley’s team noted that although “significant uncertainties” existed, available evidence suggested that the ocean’s ability to absorb carbon was increasing in line with humanity’s ability to wantonly pump it out.

Yet while the chemistry is straightforward and the ocean-plankton-carbon cycles well understood, there has been no field evidence to account for one crucial step – the dissolution of airborne iron particles by acids (in the form of natural or anthropogenic emissions), thus transforming them into oceanic bioavailable nutrients.

Shi’s researchers found evidence that seems to confirm the process. They found that iron particles, after one or two days in the atmosphere, gather a coating of acidic sulfate. This in turn transforms the insoluble iron at the core of the particle into soluble iron sulfate, making it available when it enters the water.

The team calls this evidence the “smoking gun” for acid dissolution, “because iron sulfate was not detected in the freshly emitted particles and there is no other source or mechanism of iron sulfate formation in the atmosphere.”

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