Group 3: Vehicle control (Low Nicotine, Low NNTA Group). E-cigarette cartridges will contain less than 1/10 of the nicotine in the other two groups, and NNTA levels will be correspondingly low. This group will control for changes in cigarette consumption that may occur due to study participation or to the non-nicotine behavioral/sensory factors entailed in e-cigarette use, which could influence cigarette consumption.