The reactivity of siderite (FeCO3) was investigated under ambient atmosphere and acidic aerosols using an atomic force microscope (AFM). Scans of freshly cleaved siderite crystals showed a relatively irregular microtopography, consisting of high-density kink-steps, compared to other isostructural carbonates (e.g. Iceland-spar calcite). Under ambient conditions siderite is inert with no spontaneous surface reconstruction as is reported for calcite. Under controlled conditions of pH and humidity, siderite was exposed to mists of HC1 and H2CO3 and the reaction was imaged with submicron resolution. The kinetics of the reaction varied with pH, humidity levels, and also as a function of the initial microtopography. Preferential growth and dissolution were observed as a function of initial topography and due to crystallographic anisotropies. Although extreme reaction conditions were excluded by certain aspects of AFM (high capillary forces, tip corrosion, charged surface), this technique has allowed real-time, in situ observations of surface reactions.