The location of the sensor that it is opposite the direction of the wind is reasonable and is needed to identify the plant's emissions. However, there might be a bias to that if it is located a mile from the plant's smokestack. The distance may be quite far. Considerations in identifying the location of the sensors such as changes in wind direction, wind speed, atmospheric stability, physical stack height, diameter, and other important factors are needed to have an accurate reading of the pollutants.
