Sea salt in the urban atmosphere

Is the salt in sea spray a form of air pollution? And how far does sea spray travel? These are questions being tackled by NIWA’s Healthy Urban Atmosphere team, not least because over half of New Zealanders live within 20 kilometres of the coast.

NIWA is leading field research, under a campaign called COAST, to measure the contribution of particles carried in sea spray to urban pollution. Sea spray generates airborne salt particles, and our aim is to find out how, where, and when this influences PM10 (the mass of particulate matter smaller than 10 µm per cubic metre of air). PM10 is subject to a National Environmental Standard.

Our field campaign

So far we have undertaken pilot studies at a range of beaches – Piha, Awatoto near Napier, and Long Bay, Auckland. These are different types of beach, with different surf and weather conditions.

We used a range of particle spectrometers to rapidly count particles and identify their sizes – the size helps identify how and where the particles were formed. We also used meteorological instruments to gather data about wind speed and direction, which also provides clues about where the particles come from.

NIWA’s air-quality monitoring trailer at South Piha beach. The particle spectrometer is seen on the trailer roof. A six-metre high meteorological mast is alongside the trailer. (Photo: Ian Longley)

Early results

Results from Piha indicated that when the wind came off the sea, most of the PM10 in the air was coming from surf breaking on the beach, with smaller amounts coming from further offshore. Contrary to our expectations, higher levels of PM10 occurred when winds were light, even though visible mist can be seen on windy days. This surprise result will be one focus of our next stage of research. Results from Long Beach confirmed that, in calm conditions, it was surf on the beach, rather than on rocks, that was generating most PM10.

At Awatoto, NIWA worked with Landcare Reseach and Environet Ltd to combine physical measurements with filter sampling for chemical analysis. We will have results from the analysis later this year.

In future we may move our measurement sites to different distances from the coast to see how far inland the effect of sea spray penetrates.

Airborne sea salt particles also alter the chemistry and radiative properties of the troposphere, so our results in the COAST project will feed into research that other NIWA scientists are undertaking – for example looking at the importance of airborne sea salt in atmospheric composition, climate science, and global biogeochemical cycles.

Contact: Ian Longley

Particle spectrometer, Awatoto. [NIWA]
Research subject: Air Quality