Relative Humidity Effect on PM 2.5 Readings Recorded by Collocated Beta Attenuation Monitors

Abstract

This study examined how relative humidity affects the hourly PM_2.5 readings of two collocated beta attenuation monitors in the field. Relationships between PM_2.5 readings by a stationary beta gauge A and a portable beta gauge B were evaluated at various relative humidity (RH) setpoints and ambient RHs. The PM_2.5 concentrations of two beta attenuation monitors were also compared with those of a collocated manual sampler. This work demonstrated that the mean PM_2.5 concentrations by the beta gauge monitors with RH_A and RH_B (RH setpoint in beta gauge A and B) of 35% result in good agreement with those by the manual sampler. The PM_2.5 readings of RH_B of 35% correlated well with those 35% of RHA , whenever the inlet air of beta gauge B was conditioned with water vapor or the beta gauge was operated normally under ambient conditions. However, the PM_2.5 readings of the beta gauge obtained at RH B=65% exceeded those obtained at RH_A=35%. The mean PM_2.5 readings for RH B65% to RH_A = 35% were 1.91 and 1.26, with (ambient RH = 78.2–93.1%) and without (ambient RH = 61.3–77.1%) added water vapor in the inlet of the beta gauge, respectively. Experimental results demonstrated that the relative humidity markedly affected the beta gauge readings, especially at a high ambient RH. Accordingly, maintaining the RH setpoint of the beta attenuation monitor in an appropriate range for field monitoring is important.