Avalanches in gas-based detectors operating at atmospheric pressure and using Ar-CO2 or Ne-CO2 as drift medium produce in a first instance mainly Ar+, Ne+ and CO2+ ions. The noble gas ions transfer their charge to CO2 in a few ns. A few ns later, the CO2+ ions have picked up CO2 molecules, forming cluster ions, in particular CO2+ center dot (CO2)(n). Since the cluster ions are slower than the initial ions, the signals induced by ion motion are altered. The effect is shown to be present in constant-field detectors and TPC readout chambers, and is expected to affect devices such as Micromegas and drift tubes.