Performance evaluation of the active-flow personal DataRAM PM_2.5 mass monitor (Thermo Anderson pDR-1200) designed for continuous personal exposure measurements

Abstract

The need for continuous personal monitoring for exposure to particulate matter has been demonstrated by recent health studies showing effects of PM exposure on time scales of less than a few hours. Filter-based methods cannot measure this short-term variation of PM levels, which can be quite significant considering human activity patterns. The goal of this study was to evaluate the active-flow personal DataRAM for PM_2.5 (MIE pDR-1200; Thermo Electron Corp., Franklin, MA) designed as a wearable monitor to continuously measure particle exposure. The instrument precision was found to be good (2.1%) and significantly higher than the passive pDR configuration tested previously. A comparison to other proven continuous monitors resulted in good agreement at low relative humidities. Results at higher humidity followed predictable trends and provided a correction scheme that improved the accuracy of pDR readings. The pDR response to particle size also corresponded to previously observed and theoretical errors. The active flow feature of the pDR allows collection of the sampled particles on a back-up filter. The 24-h mass measured on this filter was found to compare very well with a Federal Reference Method for PM_2.5 mass.

Introduction

Exposure to ambient particulate matter (PM) has recently received considerable attention as the result of epidemiological findings showing associations between ambient PM, PM₁₀ and PM_2.5 concentrations (particles with diameters less than 10 and 2.5 μm, respectively) and mortality (Ozkaynak and Thurston, 1987; Schwartz and Dockery, 1992; Dockery et al., 1993; Pope III et al., 1991; Katsouyanni et al., 1995; Pope et al., 1995; Schwartz et al., 2002). Findings from these health studies have raised concerns about the sufficiency of the National Ambient Air Quality Standard (NAAQS) for PM, in terms of the regulated level, the upper size cut of the particles, the adequacy of measuring only particle mass, and the applicability of fixed-site monitoring to personal exposure. These concerns have been difficult to address, since little is known about the relationship between outdoor particle concentrations and actual human exposure. Several studies have indicated that outdoor and even indoor concentrations may be poor estimators of personal exposures to PM₁₀ or PM_2.5 and its components. For example, daytime personal PM₁₀ exposures were found to be approximately 50% higher than corresponding indoor and outdoor levels (Thomas et al., 1993), while personal SO₄²⁻ and H⁺ exposures were found to be higher than indoor, but lower than outdoor concentrations (Suh et al., 1994). Understanding patterns of individual exposures to PM can be significantly improved by the use of personal monitors, as these samplers incorporate the effects of factors such as indoor pollutant sources and human activity patterns.

A variety of personal samplers have been developed to-date. Amongst those commercially available are: the PM_2.5 Personal Exposure Monitor (PEM Model 200 MSP Corp., Minneapolis, MN, 4 liters per minute (l/min⁻¹)) (Buckley et al., 1991); a personal PM_2.5 sampler with cyclone (GK2.05 KTL Cyclone, BGI Inc., Waltham, MA, 4 l/min ⁻¹) used in the European 6-city EXPOLIS study and others (Koistinen et al., 1999); the Institute of Occupational Medicine sampler (IOM Personal Inhalable Dust Sampler, SKC inc., Eighty Four, PA, PM₁₀) (Mark et al., 1986); a High Flow Personal Sampler (HFPS) for PM_2.5 (Adams et al., 2001) with a flow rate of 16 l/min⁻¹ that allows for shorter sampling periods; and a Personal Cascade Impactor Sampler (PCIS) that provides size-fractionated PM personal data at 9 l/min⁻¹ (Misra et al., 2002). In all of the aforementioned personal samplers, particle mass concentrations are determined based on gravimetric analysis of filter and/or impaction substrates on which PM are collected over periods of several hours or an entire day. Although federal air quality standards for PM in the United States are based on integrated 24-h and annual mass concentrations, recent studies have provided evidence of adverse health effects from short-term exposures to PM (US EPA, 1996; Schwartz and Neas, 2000; Delfino et al., 1998; Peters et al., 2001). Nevertheless, the values of key metrics influencing personal exposure to PM, such as the emission strengths of outdoor and indoor particle sources, “personal cloud” generation, and other micro-environmental factors, fluctuate on time scales that are substantially shorter than several hours. Individual activity patterns influencing exposure, such as amount of time spent indoors, outdoors and in commute, also vary on time periods considerably shorter than several hours.

Several recent articles have hypothesized about the health significance of brief airborne particle excursions on a time scale of a few minutes to several hours, suggesting that these excursions might explain some of the excess mortality (Michaels (1996), Michaels (1997); Michaels and Kleinman, 2000). This hypothesis is based on a number of acute human exposure studies (∼25) cited in Michaels and Kleinman (2000) and recent animal studies (Godleski et al., 1999; Clarke et al., 2000) that show evidence of adverse health effects due to such brief excursions. Two recent asthma panel studies of 22–24 asthmatic children examined effects of hourly PM10 data and found maximum 1-h PM10 showed the strongest associations with asthma symptoms (Delfino et al (1998), Delfino et al. (2002)). The need for developing personal monitors that measure particle concentration in shorter time intervals (on the order of 1–2 h, or less) is therefore of paramount importance to environmental health, as it leads to substantial improvements in exposure assessment to ambient particulates.

The objective of the study presented in this paper was to evaluate the performance of the active-flow, personal DataRAM (MIE pDR-1200; Thermo Electron Corp., Franklin, MA) for providing real-time mass concentration measurements and to assess the effect of particle size and humidity on the relationship between the response of the pDR and the actual aerosol mass concentration. The active pDR, like previous DataRAM models (MIE pDR-1000AN; Thermo Electron Corp., Franklin, MA) (Liu et al., 2002), is an integrating nephelometer that measures continuously the amount of light (with wavelength, λ, equal to 880 nm) scattered by particles drawn though a sensing zone at a flow rate of 4 l/min⁻¹. The amount of light scattered is converted to particle concentration readings using well-established light scattering theory (Kerker, 1969) and factory calibrations. In this paper, we present results from a field evaluation of this instrument conducted in Southern California from 6 December, 2002 to 19 February, 2003. The pDRs were tested while stationary (not worn by subjects for personal monitoring) and thus, the current study addresses the accuracy and precision of the instrument's measurement technique, but does not address reliability or durability issues that may be encountered in actual field use. The performance of the active pDR was compared to other collocated continuous and time-integrated PM_2.5 samplers, which is not practical for actual personal sampling.