If you do like to be beside the seaside, it might be best to avoid beaches near major ports. The mix of sea salt, ship fumes and city smoke leads to a chemical reaction that encourages the formation of ozone smog, adding to the pollution that forms in cities. James Roberts from the National Oceanic and Atmospheric Administration Earth System Research Laboratory in Boulder, Colorado, and his colleagues have developed a mass spectrometer that can detect nitryl chloride (NO2Cl) - a chemical that aids the formation of ozone.