Accumulation rates of deposited aerosol particles on an inert gold surface and the resulting humidity interactions were investigated in situ outdoors using an atmospheric corrosivity probe based on the quartz crystal microbalance technique. In six exposures in one rural and two urban sites the common behavior is that the gold surfaces undergo both irreversible mass increase and reversible mass changes. Irreversible mass increase is interpreted as a deposit mainly consisting of aerosol particles from the accumulation mode. The irreversible mass increase could be correlated to aerosol particle concentrations measured at the exposure sites. The reversible mass changes are mainly due to water sorption, which is influenced by deposited aerosol particles and varies with relative humidity. Upon closer analysis of the reversible mass changes, typical episodic cases are identified. An analogy between the humidity interaction of wetted metal surfaces and of aerosol particles explains the wetting characteristics of the surface. Time of wetness, defined by predetermined levels of temperature and relative humidity is shown to severely underestimate the actual time period when a surface with aerosol particle deposits is wetted.