In order to investigate episodic changes in atmospheric corrosivity with a high time resolution, a new sensor system for in situ field studies has been developed. Atmospheric corrosivity probes, based on the quartz crystal microbalance technique, are used together with probes for temperature and relative humidity and analyzers for H2S, SO2, NO2, and O-3. To demonstrate the capability of the sensor system, an indoor field characterization has been conducted in an electrical control room within a pulp and paper factory. The characterization included simultaneous exposures of gold, silver, and copper together with gas analysis during normal operation and simulated malfunctions of the air filtration system. The precision of in situ mass response measurements has a standard deviation of only +/-1 ng cm(-2), corresponding to a mass sensitivity of less than one equivalent monolayer of corrosion products, e.g., Cu2O. The mass sensitivity enabled differences in mass increase rates of copper, silver, and gold to be detected within hours after exposure start in normal operating conditions and on silver within minutes after simulated malfunctions of the air filtration system.