Direct field observation of the relative humidity effect on the β-gauge readings

The effect of ambient relative humidity (RH) on hourly particulate matter (PM₁₀) readings of β-gauge monitors has been studied using two collocated monitors in the field. The inlet air of monitor 1 was conditioned with water vapor to increase its RH, whereas monitor 2 operated normally in ambient conditions. Experimental data showed that PM₁₀readings of monitor 1 were nearly the same as monitor 2, as long as the RH of its conditioned incoming air did not exceed -80–85%. However, when the RH exceeded -80–85%, PM₁₀readings of monitor 1 became higher than monitor 2, and the difference increased with increasing RH. The measurement of pressure drop across the filter was also conducted, and the data revealed that the increase of pressure drop per unit of PM₁₀concentration decreased when RH was higher than -80–85%, as compared with the case when RH was lower than 80–85%. This is perhaps because of more porous structure of deposited particles in the β-gauge monitor when RH is greater than -80–85%. The theoretical calculation using an evaporation model and a thermodynamic model has been conducted to simulate the β-gauge readings. The results show that the theoretical PM₁₀concentrations using the evaporation model are in better agreement with the actual β-gauge readings than those using the thermodynamic equilibrium model.