New study: Coastal waves create more aerosols than expected – may affect climate and air quality
A new discovery about how sea spray forms on coastlines may change our understanding of the oceans’ impact on climate. In coastal areas, aerosol concentrations have been greatly underestimated — but globally, they may have been overestimated. The findings, from a study that included a researcher at Stockholm University, could also affect air quality in coastal cities. And it all began during a car journey on a windless day in the Azores.
The small island of Graciosa in the Azores is home to one of the US Department of Energy's many atmospheric measurement stations. The continuous measurements taken at these stations form a large part of the basis for what we know today about climate change. They capture not only the effects of greenhouse gases but also the role of cloud-forming aerosols from land and sea.
Matthew Salter, a researcher at Stockholm University, had been studying aerosols and their effects for a long time when he visited the Azores in the summer of 2022 to see how a research campaign led by his PhD students, together with American collaborators, was progressing. On a beautiful, almost windless summer's day, he and some colleagues drove across Graciosa to reach the observatory at its northernmost point. Although it was almost windless on the island, high waves were crashing against the coast and the air was filled with sea spray.
“We drove through a dense mist of spray that was so thick we had to use the windscreen wipers. Yet it was such a calm day, with almost no wind,” explains Matthew Salter.
Researcher Matthew Salter. Photo: Lisa Bergqvist
This raised questions. Previously, researchers had linked the occurrence of sea spray mainly to high wind speeds. When the wind is strong, large waves form, and when they break, many small droplets and particles – aerosols – are released into the air as sea spray. In the global models used to calculate the amount of aerosols released from the sea and their impact on, for example, cloud formation, wind speed is therefore a crucial parameter. But if the Azores station showed high waves and sea spray without wind, perhaps the calculations weren’t accurate. Waves from distant storms might travel across the ocean and break at the coast, creating sea spray in calm conditions — a process the models may have overlooked.
“The measurements at coastal stations might not represent the open ocean as well as we thought. I turned to my American colleague and said: we need to look into this,” says Matthew Salter.
A systematic analysis
The collaboration quickly got underway. Working from Washington University in St. Louis, Jian Wang and PhD student Shengqian Zhou led the systematic analysis of the coastal data. Zhou studied how aerosol concentrations varied depending on wave height versus wind speed, and also compared them with measurements from other coastal stations. The results of the study were published in an article in the journal Science Advances.
“When it is very windy, large waves form out at sea that break and emit sea spray. But swell waves from a windy area can also travel long distances and not break until they reach the coast, where there may be no wind at all. Sea spray then forms at the coast, but not in the area offshore,” explains Matthew Salter.
More bubbles are also formed when a wave hits the coast than when it breaks in open water, resulting in more sea spray and amplifying the effect.
“Our study shows that the number of cloud-forming particles can increase threefold in coastal areas during high-wave periods,” says Matthew Salter.
Overestimation in global models
Because long-term open-ocean measurements are difficult, measurements at coastal stations often form the basis for models and global estimates. The new knowledge therefore implies that the total amount of aerosols in these calculations is overestimated.
“When we base our global calculations on measurements from coastal areas, we overestimate how much aerosol the oceans actually produce. Since seventy per cent of the Earth's surface is ocean, this can significantly affect our climate models,” explains Matthew Salter.
However, it is still too early to say what the new discovery will ultimately mean for climate change assessments.
“There have been areas where it is difficult to measure and understand the impact of aerosols, for example over the Southern Ocean, where there have been gaps between what we have measured and what we have calculated. This research may help explain some of these discrepancies,” says Matthew Salter.
Sea spray and aerosols
Aerosols are small particles – solid or liquid – that are finely dispersed in a gas. They can be, for example, fog (condensed water vapour) or various air pollutants.
When ocean waves break and small bubbles burst, aerosols are released into the air through what is known as sea spray. They can contain salts and organic material, but also environmental contaminants. The concentrations of these substances can therefore be elevated along coastlines.
Aerosols can also affect the climate by scattering solar radiation and by contributing to cloud formation, both of which influence how much sunlight reaches Earth's surface.
Out on the open sea, the amount of aerosols released from the water is mainly affected by wind speed. However, according to the new study, wave height is more important along coastlines.
Underestimation of health risks
Aerosols in sea spray can affect not only cloud formation and climate, but also human health by spreading harmful substances. And while aerosol amounts have been overestimated globally, the opposite is true for coastal areas. There, the study suggests that aerosol concentrations may be underestimated when calculations are based primarily on wind speed.
“In some places, high waves crashing against the coast can increase the airborne transport of pollutants from contaminated seawater,” says Matthew Salter.
One example is the Tijuana River between the United States and Mexico, where large amounts of untreated sewage are discharged into the sea. When sea spray forms from this polluted water during high waves, pollution levels in the air can become high locally.
But even natural substances, such as chloride, from seawater can be problematic. When chloride combines with air pollution from traffic and industry in coastal cities, it can exacerbate the negative health effects, explains Matthew Salter.
“To get an accurate picture of the health effects of air quality in coastal areas, we need to take into account the contribution from sea spray,” he says.
Reduced emissions required
Matthew Salter and his colleagues have previously found that PFAS — the so-called 'forever chemicals' — become concentrated in sea spray formed from contaminated seawater. When this sea spray reaches land, PFAS levels in soil and groundwater can become elevated in coastal areas, even if the area is far from the original source of contamination. In some areas of the Danish coast, for instance, cows are prohibited from grazing near the sea due to PFAS contamination that is likely linked to sea spray deposition.
“The chemical problem is global, and emissions in one place can affect a completely different area. That is why we need global regulations and to stop releasing substances that we know are harmful, or that we do not know enough about. The sea itself is not toxic, but it can contribute to concentrating and spreading toxins — not just diluting them, as was previously believed," says Matthew Salter.
Together, the results show that coastal wave-driven sea spray has implications not only for climate and air quality, but also for the spread of pollutants and harmful chemicals.
Text: Lisa Bergqvist
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Last updated: 2026-05-11
Source: Baltic Sea Centre