Development and Evaluation of a Continuous Ambient PM_2.5Mass Monitor

Abstract

A Continuous Ambient Mass Monitor (CAMM) for fine particle mass (PM2.5) has recently been developed at the Harvard School of Public Health. The principle of this method is based on the measurement of the increase in pressure drop across a membrane filter (FluoroporeTM) during particle sampling. The monitor consists of a conventional impactor inlet to remove particles larger than 2.5 mu m, a diffusion dryer to remove particle-bound water, a filter tape to collect particles, a filter tape transportation system to allow unassisted sampling, and a data acquisition and control unit. For each sampling period (typically 30- 60 min), a new segment of the filter tape is exposed so that particles remain close to equilibrium with the sample air during their collection. This results in mini mization of volatilization and adsorption artifacts during sampling. Furthermore, since the required flow rate for the fine particle mass monitoring channel is only 0.3 L / min, the relative humidity of the air sample can be easily reduced to 40% or less using a NafionTM diffusion dryer to remove particle-bound water. The CAMM has a detection limit of > 5 mu g / m3 for PM2.5 concentrations averaged over 1 h. The performance of the newly developed monitor was investigated through laboratory and field studies. Laboratory tests included a calibration of the CAMM using polystyrene latex (PSL) and silica particles. A series of field studies were conducted in 7 cities with presumably different PM2.5 chemical composition. The 24 1-h CAMM measurements were averaged and compared to Harvard Impactor (HI) 24 h PM2.5 integrated measurements. Based on 211 valid sampling days, the measurements obtained from the Harvard Impactor and the CAMM were highly correlated (r2 = 0.90). The average CAMM-to-HI concentration ratio was 1.07 (+- 0.18).