The heterogeneous reaction of NO2 with different carbon aerosol particles was investigated in situ. The NO2 was labeled with the beta(+)-emitter N-13 (half-life 10.0 min) which allowed application of NO2, at very low concentrations. The carbon aerosol was either produced by a spark discharge generator using graphite electrodes or by a brush generator resuspending commercial soot material. Monodisperse size cuts between 50- and 490-nm diameter were selected and mixed with the (NO2)-N-13. After a defined reaction time, the different reaction products were separated by means of selective traps and detected on-line by gamma-spectrometry. A sticking coefficient for chemisorption of NO2 between 0.3 x 10(-4) and 4.0 x 10(-4) and a rate constant for the reduction of adsorbed NO2 to NO(g) between 4.0 x 10(-4) and 9.4 x 10(-4) s(-1) were determined for both aerosols. The sticking coefficient obtained in this study in situ with aerosol particles is 2 orders of magnitudes smaller than the uptake coefficient recently reported with bulk carbon material.