PM2.5 is a sub-set of PM10.
PM10 includes all particles up to 10 micro-meter size. Therefore, there is no question of PM2.5 becoming more than PM10. If you are using sensor, then please check the quality and calibration for the appropriateness of output.
You are correct about the definition, so the problem is with your sensor. Many low cost sensors which are available commercially give very poor quality data.
Your definition is correct, the problem is the sensor that you are using.
As the others have mentioned, this is a measurement problem. Please tell us more about the sensor you are using. Is it a single instrument or are you comparing the measurements from 2 separate instruments? Are they optical instruments? gravimetric? When were they last calibrated?
Theoretically, the PM10 concentration is always greater than PM2.5. Is the measuring principle of your device sensorized? Or is it a dust device that Beta radiates into the dust filter? If the second, the possible reasons are the following:
PM2.5 may read higher if the filter paper is of poor quality or humidified. Or PM2.5 may read higher if it is raining a lot outside.
If your sensor is a particle counter, this is likely the reason for the confusion. The assumption that the PM-10 fraction should be greater than the PM-2.5 fraction is based upon a theoretical standard breakdown of TSP (total suspended particulate). Standard means that the cut-size breakdown follows a specific assumption. If the density of the particulate matter is uniform (weight/mass), then smaller-sized particulate matter would weigh less (PM-10 > PM-2.5). As mentioned in other responses, the precision and accuracy of monitors are not uniform. Desiccation of the filter paper (if using that type of device) is critical to obtaining accurate readings. Reference level monitors have greater accuracy of their readings, and off-the-shelf monitors have a relative accuracy based upon cost. Some measurement devices have limitations based on the volume collected and the mass/volume of air. Make sure the speciation (PM filter) is cleaned and, of course, follow the directions for using the device. I have used many devices since I entered the profession in 1972.
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