A case study on the effect of contaminated inlet tubes on the accuracy of mid-cost optical particle counters for the ambient air monitoring of fine particles

This study investigates the impact of the long-term use of inlet-heated tubes on the performance of mid-cost optical particle counters (OPCs) for ambient air monitoring of fine particles (PM_2.5). Two OPCs, equipped with inlet-heated tubes, were deployed over a 6-month period, with a beta attenuation monitor (BAM) serving as the reference device. The performance of the OPCs using the same inlet tubes for the first 3 months was compared to their performance after the frequent replacement of clean tubes during the final 3 months. The correlation coefficients (r²) for the 1 h and 24 h average PM_2.5 concentrations between the OPCs and the BAM were lower with long-term contaminated tubes (0.82 < r² < 0.93) compared to clean tubes (r² > 0.93). The relative mean errors and biases significantly increased over time with contaminated tubes. Temperature, humidity, precipitation, and wind speed were found to have an insignificant effect (r² < 0.1) on the performance of the two OPCs with inlet-heated tubes over the 6-month period. The relative average PM_2.5 error when using clean tubes was less than 4%. These findings highlight the importance of inlet-heated tubes in improving OPC performance, particularly for mitigating humidity effects.