Our paper discusses a mobile monitoring system for measuring levels of the main traffic air pollutants (C₆H₆, NO₂, NO_x, CO, and CO₂) in cities. The proposed monitoring device can be mounted on any vehicle (such as a car, bus, or truck) rather than being attached permanently to a dedicated van, as most systems of this kind described in the literature are. The analyzers used in this system are small portable structures containing an electronic instrument to measure, record, and transmit relevant data on pollutant levels to a website. Sample results from measurement campaigns in Gdańsk are presented.