The reactivity and hydration properties of amorphous carbon were studied in a low-pressure Knudsen cell reactor at room temperature (298 K). Reactions of NO2 (&ggr;=0.11¿0.04) and HNO3 (&ggr;=0.038¿0.008) were observed and may be important for nitrogen partitioning in the atmosphere. Water uptake was measured before and after exposure to various gases. Treating the amorphous carbon with NO2 and O3 does not alter the H2O uptake, while treatment with SO2, HNO3, and H2SO4 significantly increases the H2O uptake. The experimental results support current assumptions in jet aircraft plume models that sulfuric acid condensation is involved in the activation of soot particles as condensation nuclei.¿ 1997 American Geophysical Union