Airborne Pb-212 has been measured at and near the Mauna Loa Observatory (MLO) to evaluate its performance as a tracer to detect periods when air samples have been influenced by passage close to Hawaiian land. This complements radon measurements which do not distinguish between local and distant land contact, and measurements of isoprene which is present in air samples which have contacted flora in daylight. One source of Pb-212 is decay of thoron from barren recent lava. This source is sufficient to indicate when an air sample from the free troposphere has been close to lava on its way to the observatory. At a height of 3 m above the ground, samples in the clean air wind sector sometimes contained measurable amounts of Pb-212, particularly when strong temperature gradients and low wind speeds trapped Pb-212 near the ground. In combination with Rn-222 and isoprene measurements, Pb-212 can be very useful for establishing the presence of baseline air which has not interacted with land for several weeks, as well as air which has contacted distant land but not Hawaii. ¿ American Geophysical Union 1996