An open flow reactor was developed and used to study the pH dependency of atmospheric aerosol weathering. Under ultraclean conditions, this reactor enables experiments below the saturation of hydroxy salts and over the short time span (2 hours) that is typical for weathering by rain and cloud water. The weathering simulations show a two-step process of dissolution rates: First, the rate increases quickly during the hydration of the solid particle surface, then after a maximum, it progressively decreases. In general, there is an increase in the dissolution rate of dissolved elements as the pH is lowered. However, between pH 3.80 and 5.30, the dissolution rate for Fe and Cu depends on both H+ and OH- concentrations and exhibits a minimum as a function of pH. This minimum can be related to pHpzc of the mineral containing these elements. The affinity of minerals for H+ was also determined by correlating hydration constants and pH. ¿ 1999 American Geophysical Union