A model for the relative humidity effect on the readings of the PM ₁₀ beta-gauge monitor

In the previous paper (Atmos. Environ. 15 (1981) 1087), we found that the PM₁₀ concentrations detected by the Wedding beta-gauge PM ₁₀ monitor and those measured by the manual hi-vol PM₁₀ sampler were quite close when the ambient relative humidity (RH) was lower than the deliquescence RH (DRH) of aerosols. However, when the deliquescent point was exceeded, PM₁₀ concentrations of the beta-gauge were found to be higher and differences increased with an increasing ambient RH. In addition, theoretical water mass calculated by a thermodynamic model (ISORROPIA model, (Aquat. Geochem. 4 (1998) 123)) was found to be much higher than the actual values. In this study, models were developed to determine water evaporation loss from collected particles on the filter tape of the beta-gauge during sampling and in the monitoring room. Simulated results show that all absorbed water will evaporate completely at RH lower than about 85%. However, absorbed water does not evaporate completely at RH higher than about 85%, and remaining water in particles accounts for higher beta-gauge readings than the hi-vol concentrations. The simulated daily beta-gauge PM₁₀ concentrations are close to the actual beta-gauge readings obtained previously.