In order to perform in situ monitoring of minute corrosion attacks during outdoor field exposures, an atmospheric corrosivity probe has been developed based on the quartz crystal microbalance. A remotely controlled computerized mobile field station was also constructed. The whole setup allows simultaneous in situ measurements of mass changes with a resolution of +/- 10 ng cm(-1), and of climatic parameters. The observed mass changes during introductory exposures of gold and copper in an urban atmosphere are both reversible and irreversible. Aided by ex situ x-ray photoelectron spectroscopy analysis, the results can be interpreted in terms of salt deposition, water adsorption, water desorption, and atmospheric corrosion effects.