The effect of the surface concentration of submicrometer ammonium sulfate (NH4)(2)SO4 dust particles on the mechanism of atmospheric corrosion of copper has been investigated at 298 K and 93% relative humidity (RH). For surface concentrations from 1 mug/cm(2) to 10 mg/cm(2), the same corrosion products are formed at relative humidities well above the critical relative humidity of (NH4)(2)SO4. Initially, a Cu(NH3)(2+) complex is formed. The basic copper sulfate posnjakite precipitates when its solubility product is exceeded while NH3 evaporates. Posnjakite may be converted to antlerite or brochantite if higher amounts of (NH4) SO4 particles were deposited. The basic copper sulfates then decompose at the inner interface to form Cu2O. With low amounts of (NH4)(2)SO4 particles, the conversion is 100%. With high amounts of particle deposits, residual copper sulfate remains on top of a Cu2O-layer. With increasing amounts of deposited (NH4)(2)SO4 particles on copper, the sequence of corrosion products that are formed takes more time. The mechanism proposed in earlier work for the corrosion of copper with (NH4)(2)SO4 particles in humid air is improved to explain the new experimental findings. (C) 2003 The Electrochemical Society.