Two main phenomena cause on dispersion of pollutants in the air. One is diffusion and the other is turbulence. The turbulence effect is minimal near the walls, due to the boundary layers. This effect can be seen here as well for the wall sensor. The pollutants that can reach the sensor at the wall are less than the pollutants at the center and the device. After several minutes all sensors show similar numbers since the mixing happens during this time. The air purifier fan adds turbulence to the room which helps to have mixing. More CFD simulation studies can be done to scrutinize this condition.