Low-cost optical particle counters are increasingly used for long-term particulate matter (PM) monitoring, but their long-term performance remains poorly understood. This study evaluates the field performance of six Alphasense OPC-N3 sensors colocated with Federal Equivalent Method (FEM) instruments in Salt Lake City, Utah, over deployment periods ranging from 5 to 24 months. It develops seasonal PM₁₀ correction models. However, the performance of the correction models for the three longest-deployed sensors exhibits progressive degradation after approximately 10–12 months, with correction model slopes increasing by up to sevenfold. In contrast, colocated Plantower PMS5003T sensors show no comparable long-term degradation in PM_2.5 measurements. Comparison of OPC-N3s deployed for more than 12 months in the field with those for less than 3 months in the field reveals substantial differences in PM₁₀ response, with old-versus-new comparisons yielding an average slope of 0.18, compared with 0.72 for old-versus-old and new-versus-new comparisons. Post-deployment laboratory evaluations identify irreversible corrosion of the OPC-N3’s internal elliptical mirror in the most degraded OPC-N3 sensor, with corresponding laboratory regression slope increases of more than an order of magnitude compared to pre-deployment evaluation. Mirror replacement substantially improves laboratory performance, suggesting that mirror replacement is a potential pathway to recover performance. Additionally, we observe a 56.3% decrease in the OPC-N3 PM_2.5-to-PMS5003T PM_2.5 ratio within the first year for the three longest-deployed sensors, suggesting that this ratio may provide a practical indicator of OPC-N3 sensor degradation when FEM measurements are unavailable. These results provide the first evidence linking long-term OPC-N3 performance degradation to internal mirror corrosion and demonstrate that sensor performance can decline substantially after approximately one year of deployment.