248 Figure 3 – Sampling protocol using sensor mounted on an UAV in surface coal mine 2.3 Sensor calibration Performance of the Low-cost PM sensors needs to be assessed before deploying them for real-time pollution monitoring. The standard protocol is to collocate the sensor with a regulatory instrument at the actual field of installation (surface mines in this case) for few days and expose them to the varying PM concentration levels and prevailing meteorological conditions. At the end of the colocation experiment, a pairwise linear regression model is fitted to the sensor and the reference data to derive a calibration equation or model. Colocation experiments were conducted by exposing the sensor and the reference instrument (PAS) to iron ore and coal dust particles. For this purpose, the experiment was conducted near the pit top of the coal and iron ore surface mine. This location is generally considered to be the representative location of overall PM emissions from surface mines and is less exposed to dynamic and hazardous conditions prevailing inside the pit. The experiment was conducted for at least 12 hours continuously without any control over the PM emissions and conditions prevailing at the site. The sampling inlet of the sensor was positioned unidirectionally and close to the reference instrument (within 0.5 m) to minimize the sampling error due to spatial differences. The instrument setup (OPC N3, PAS and Kestrel) was placed 1 m above the ground level. Higher linear performance was observed in coal mine (R2 = 0.97) compared to the iron ore mine (R2 = 0.77). The obtained values in both the surface mine were well above the recommended criterion (R2 = 0.70). Sensor exhibited higher linearity and precision values in coal and iron ore mines (Table 1). Results from the calibration experiment show that the sensor performs reasonably good in measuring mining-generated airborne PM concentration levels. Overall performance of the OPC N3 sensor is satisfactory considering the high PM concentration and harsh mining environments. The linear equation derived from the colocation was applied to the raw sensor data to obtain reference equivalent PM concentration.
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