Enhancing the Temporal Resolution and Sensitivity of Beta Attenuation Monitors via Concentrated Particle Deposition and Multi-Sensor Ensemble Averaging

Abstract. Beta attenuation monitors (BAMs) are widely used for continuous ambient particulate matter (PM) monitoring, yet their effectiveness for real-time, ultra-low-concentration detection is often limited by long response times and high detection limits. The variability of the beta source intensity is the main factor for the limitation of beta gauges. To enhance BAM performance under low-concentration conditions, two complementary approaches were investigated. A commercial BAM was modified to operate with different particle deposition areas (2.0, 1.0, 0.6, and 0.4 cm²), thereby increasing the areal particle loading on the filter tape, while a multi-sensor array was simulated through ensemble averaging of repeated measurements to reduce beta intensity variability. Different temporal smoothing windows were also evaluated to examine the trade-off between measurement stability and response time. All experiments were conducted in a controlled aerosol chamber to ensure stable testing conditions. The research evaluated the effects of particle deposition area, array size, and smoothing window on response time, signal stability, and limits of detection (LOD). Experimental results indicate that reducing the particle deposition area to 0.4 cm² effectively amplifies the mass change signal per unit area, thereby increasing measurement sensitivity for active monitoring. However, this reduction also shields the detector, lowering initial beta intensity and increasing relative statistical noise. Increasing the number of measurement units to six significantly improved measurement stability, reducing the coefficient of variation (CV) of the intensity from 1.02 % to 0.39 %. A cost-benefit analysis further indicated that the marginal improvements became negligible beyond six units, suggesting an optimal array size of 4–6 measurement units. The study demonstrates that the most effective performance is achieved through a synergistic integration of these approaches. The enhanced signal provided by the reduced particle deposition area, together with the improved counting stability of the six-unit ensemble, enabled the use of a shorter 30-minute smoothing window without sacrificing measurement precision. This integrated configuration reduced the instrumental response time from 60 to 22 minutes while maintaining a standard deviation of 2.67 μg m^-3 and a detection limit of 7.75 μg m^-3. These findings provide a technical foundation for the development of high-resolution, real-time array-type beta gauges capable of meeting increasingly stringent global air quality monitoring requirements.