Heterogeneous conversion of NO2 on acid surfaces is discussed as an important source of HONO in the atmosphere. The overall NO2 conversion rate depends not only on the liquid phase reaction rate but also on liquid phase diffusion Dl and solubility H. We have adopted a gas chromatographic technique to study heterogeneous interactions at stratospheric temperatures: a thin sulfuric acid film (1 - 2 mym film thickness, 41 - 83 wt-% sulfuric acid) is deposited as the stationary phase in a fused silica column of 530 mym i.d. and 2.8 m length. The column is immersed in a cryogenic liquid. Solubilities and diffusion coefficients of stable trace gases can be determined independently from their retention times and peak widths as function of the carrier gas flow rate, using the well established theory of capillary column chromatography. The technique worked well for SO2 in the temperature range 193 - 243 K. The same experimental technique was employed to study interactions of NO2 with cold sulfuric acid films. In contrast to the (nearly) Gaussian-shaped SO2 peaks, peaks of NO2 (which was detected with a chemiluminescent NO analyzer after catalytic reduction) exhibited pronounced asymmetry, indicating that the effective solubility of NO2 does not obey Henry's law. This is consistent with the formation, particularly at low temperatures, of the dimer N2O4 which is significantly more soluble than the monomer. Solubilities were deduced by simulating peak shapes with a simple cell model, which includes the known temperature dependence of the equilibrium 2 NO2 <=> N2O4. However, while the peak contour was well reproduced, extensive peak tailing occurred at all temperatures, and a broad secondary peak emerged above 233 K. We speculate that this is due to the reversible reaction N2O4 + H2O <=> HONO + HNO3, which may thus be studied under stratospheric conditions.