We use trace gas measurements (N₂O₅, ClNO₂, NO₃) and particle properties (surface area, nitrate content etc.) to derive uptake coefficients (the probability of removal from the gas-phase on a per-collision basis) for the interaction of N₂O₅ with ambient aerosol and also the efficiency of formation of ClNO₂. The uptake coefficients show high variability but are reasonably well captured by parameterisations based on laboratory measurements.