Sea breezes can increase concentrations of air pollutants at the coast by around 20%, according to research based on a decade of atmospheric measurements in southwest England.
Scientists led by Plymouth Marine Laboratory (PML) analysed ten years of observations from the Penlee Point Atmospheric Observatory, at the mouth of Plymouth Sound, identifying 428 individual sea-breeze events.
The researchers found that weak offshore winds and limited atmospheric mixing can allow pollutants to accumulate over the sea overnight. As the land warms the following day, the resulting sea breeze can carry this polluted air back towards the coast.
Daytime concentrations of gases and airborne particles at Penlee Point were generally around 20% higher during sea-breeze events than during comparable periods of onshore flow.
Lead author Dr Mingxi Yang said: “We often think of sea air as something that’s very healthy, but our observations show that the reality can be much more complicated. During sea breeze events, pollutants can build up and be recirculated between the land and sea, leading to higher concentrations at the coast.”
The effect was particularly strong for sulphur dioxide, associated with shipping, with daytime concentrations more than twice as high during sea-breeze events.
Higher concentrations were also observed in Plymouth city centre. Yang said: “During the day, all pollutant concentrations during sea breeze events are higher than during periods of onshore and even offshore wind flow.”
The findings could also have implications for atmospheric monitoring. Sea-breeze circulation can cause air to move repeatedly between land and sea, meaning apparently Atlantic air sampled at coastal stations may retain terrestrial influences.
Researchers found that excluding sea-breeze events altered observed daily and seasonal patterns in ozone and methane. Such events were also not always represented accurately by atmospheric models or air-mass back-trajectory calculations.
The study, “Diurnal sea breeze worsens coastal air quality and complicates monitoring of remote North Atlantic air”, was published in Atmospheric Chemistry and Physics on 5 August.
