CN and radon concentrations have been measured at coastal and island sites in programs measuring baseline concentrations of atmospheric trace constituents. It is shown that addition of CN and radon concentrations to meteorological criteria for characterisation of an air mass, substantially improves the objectivity of baseline selection. Wind direction and CN concentration are both useful for indicating likelihood of pollution from sources within a few km. Beyond that the fetch over land is enough for radon to be useful. For sources more than a thousand km away, wind direction and CN become poor indicators, whereas radon concentrations still yield reliable indications of land contact.