Field evaluation of a modified DataRAM MIE scattering monitor for real-time PM2.5 mass concentration measurements

In this paper, we investigated the feasibility of using a modified DataRAM nephelometer (RAM-1, MIE Inc., Billerica, MA) as a continuous PM2.5 monitor to measure concentrations of ambient and concentrated aerosols in real time. The DataRAM operated with a diffusion dryer tube in its inlet in order to reduce the relative humidity of the sampled air to less than 50%. A total of 39 field tests were conducted in which the average dry DataRAM concentration was compared to the gravimetrically determined mass concentration, corrected for nitrate losses. Tests were conducted over one calendar year (from January to December 1999) in order to capture maximum seasonal variations in the levels of relative humidity, PM size distribution and chemical composition in the Los Angeles Basin. Our experimental results indicated that the aerosol mass median diameter (MMD) is the single, most important parameter in affecting the response of the DataRAM. As the MMD increases from 0.3 to 1.1 mu m, the DataRAM-to-MOUDI ratio increases from approximately 0.7 to about 1.6. The DataRAM-to-MOUDI ratio subsequently decreases to about 1.0, as the MMD further increases to 1.5 mu m. For MMD values in the range of 0.4-0.7 mu m (i.e., the MMD size range that is most commonly associated with urban aerosols), the DataRAM and gravimetrically measured mass concentrations (corrected for nitrate losses) agree within +/- 20%. Based only on ambient data, the average DataRAM-to-gravimetric concentration ratio was 0.93 (+/- 0.17), whereas the average DataRAM-to-gravimetric concentration ratio for concentrated PM2.5 aerosols was 1.23 (+/- 0.20). Our field evaluation also indicated that the effect of particle chemical composition on the DataRAM-to-gravimetric concentration ratio is much less important than that of particle size distribution. (C) 2000 Elsevier Science Ltd. All rights reserved.