A large field wind tunnel was used to compare four types of CO2-baited mosquito traps. This study compared the plume structure and concentration of CO2 emitted by each trap, flow of suction into the trapping systems, flow of CO2 being released, trap shape and configuration, differences in visual appearance, and differences in temperature and humidity of emissions at the source of CO2 compared to ambient air. The structure of the CO2 plumes emitted by each trap differed considerably. All four plumes were turbulent, causing the concentration of CO2 within several metres of the source to attenuate to between 375 and 875 p.p.m. The Encephalitis Virus Surveillance (EVS) trap emitted concentrations of CO2 exceeding 20,000 p.p.m., the detection limits of our equipment, whereas the Mosquito Magnet Freedom (MMF), Mosquito Magnet Liberty (MML) and Mosquito Magnet X (MMX) traps released CO2 at peaks of about 3500, 7200 and 8700 p.p.m., respectively. The MMX trap produced the greatest air velocity at both the suction inlet and CO2 outlet, followed by the MMF, MML and the EVS traps, respectively.