Evaluation and comparison of continuous fine particulate matter monitors for measurement of ambient aerosols.

ABSTRACT

To provide a scientific basis for the selection and use of continuous monitors for exposure and/or health effects studies, and for compliance and episode measurements at strategic locations in the State of New Jersey, we evaluated the performance of seven continuous fine particulate matter ([PM.sub.2.5]) monitors in the present study. Gravimetric samplers, as reference methods, were collocated with real-time instruments in both laboratory and field tests. The results of intercomparison of real-time monitors showed that the two nephelometers used in this study correlated extremely well ([r.sup.2] ~0.97), and two tapered element oscillating monitors (TEOM 1400 and TEOM filter dynamics measurement system [FDMS]) correlated well ([r.sup.2] > 0.85), whereas two beta gauges displayed a weaker correlation ([r.sup.2] < 0.6). During a summertime controlled (laboratory) evaluation, the measurements made with the gravimetric method correlated well with the 24-hr integrated measurements made with the real-time monitors. The SidePak nephelometer overestimated the particle concentration by a factor of approximately 3.4 compared with the gravimetric method. During a summertime field evaluation, the TEOM FDMS monitor reported approximately 30% higher mass concentration than the Federal Reference Method (FRM); and the difference could be explained by the loss of semi-volatile materials from the FRM sampler. Results also demonstrated that 24-hr average [PM.sub.2.5] mass concentrations measured by beta gauges and TEOM (50 [degrees]C) in winter correlated well with the integrated gravimetric method. Seasonal differences were observed in the performance of the TEOM (50 [degrees]C) monitor in measuring the particle mass attributed to the higher semi-volatile material loss in the winter weather. In applying the real-time particulate matter monitoring data into Air Quality Index (AQI) reporting, the Conroy method and the 8-hr end-hour average method were both found to be suitable.

INTRODUCTION

Numerous epidemiological studies have yielded results about human exposure to particulate matter (PM) and associated health responses. These have shown strong associations between the elevated levels of PM and increases in mortality and morbidity (e.g., exacerbation of asthma and other respiratory diseases, lung function decrements, and cardiovascular disease). (1-7) Fine particles smaller than 2.5 [micro]m in aerodynamic diameter ([PM.sub.2.5]) are of the greatest concern owing to their size and transportability in the human body. The U.S. Environmental Protection Agency (EPA) estimates that tens of thousands of premature deaths yearly are associated with exposure to excess levels of [PM.sub.2.5]. (8) On the basis of the conclusions of many studies, the new National Ambient Air Quality Standards (NAAQS) [PM.sub.2.5] standard was promulgated by EPA in 1997 and upheld by the U.S. Supreme Court in 2002. (9) However, the current NAAQS [PM.sub.2.5] standard only regulates to achieve 24-hr and annual average mass concentrations. These do not address transient elevations in fine particle concentrations. Several studies have found that acute exposure to particulate air pollution may trigger or exacerbate cardiovascular events such as myocardial ischemia, peripheral thrombosis, acute arterial vasoconstriction, and progression of atherosclerosis in sensitive populations. (10,11) In addition to having the advantage of real-time monitoring, continuous instruments may have potentially higher accuracy than a discrete filter-based gravimetric method due to a decreased loss of semi-volatile material. (12) For example, the Federal Reference Method (FRM) in Houston, TX and Seattle, WA was observed to underestimate ambient PM mass when compared with continuous methods. (13)

The goal of this paper is to provide an evaluation and recommendations associated with the operation of six continuous [PM.sub.2.5] monitors under a variety of conditions. This evaluation will help improve the regulatory compliance measurements using these devices. Ambient aerosols of New Jersey were used for testing in a summer and winter season in a controlled environment facility (CEF) and in real-world settings, respectively.

EXPERIMENTAL METHODS

[PM.sub.2.5] Monitors under Evaluation

The continuous monitors being evaluated were selected on an as-available basis, i.e., the models that have been commonly used for research and compliance purposes and that were available to us. The instrument models and their operating principles are summarized in Table 1. The nephelometer, SidePak, was factory calibrated against Arizona road dust.

Evaluation in the CEF

The CEF used was a 25-[m.sup.3] stainless steel chamber in which temperature, humidity, and air-exchange rate were controlled, and the air supplied was treated by a series of conditioning processes including cooling/heating and humidification/dehumidification processes. Activated carbon and high efficiency particulate arrestance (HEPA) filters were used to remove ambient PM when testing the effects of relative humidity (RH) on the monitor performances. Eight small brushless fans were installed in the CEF to completely mix the chamber.

To test the relationship between a continuous monitor and the Harvard Impactor (HI) gravimetric method, the CEF incoming air filters (HEPA and activated carbon) were removed from the pathway of sampling air to the CEF before another set of experiments that sampled ambient air. Six continuous particle mass monitors and a HI sampler were collocated in the CEF. The two sampling campaigns were conducted from January to March 2004, and from July to August 2004, respectively. The temperature and RH of the sampled outdoor air entering the CEF were monitored by the beta gauge (Metone) and validated by the sensors installed within the CEF.

The HI sampler used 37-mm diameter Teflon filters that were conditioned in the weighing room at a temperature of 20 [degrees]C and RH of approximately 30-40% and weighed three times before and after sampling. A MT5 microbalance (Mettler Toldeo Inc.) was used for weighing the filters. The balance had a sensitivity of [+ or -]1 [micro]g. The weighing procedures followed guidelines in EPA's Quality Assurance Program. (14) The tolerance for triplicate weighing of filters was in agreement within [+ or -]3 [micro]g. The mean net mass of the field blank and weighing room blank were examined and observed to have minimal changes (less than [+ or -]5 [micro]g), thus, they were not subtracted from the net mass of sample filters. The sample volume was calculated as the product of the sample duration and the mean of pre- and post sampling flow rates measured by an electronic flow meter (DryCal DC-Lite, Bios Inc.). The operation of continuous monitors strictly followed the manufacturer's manuals.

Evaluation in the Field

The field sampling site (40[degrees]17' N and 74[degrees]49' W, elevation 59 m) was located at the Technical Center of the New Jersey Department of Environmental Protection, Ewing, NJ. The trailer used for sampling was set up at a street with a perpendicular distance of 100 m from a heavily trafficked freeway. The site also lies 500 m north of the Trenton Mercer Airport that accommodates patrol jets. Parking lots, office buildings, roads, and woods surrounded the site. Construction work was being completed in an office building about 50 m away from the trailer during two sampling periods. The monitoring data we collected during September 2004 and December 2004 to February 2005.

The tapered element oscillating monitors filter dynamics measurement system (TEOM FDMS), TEOM (50C), beta gauge (Anderson), and beta gauge (Metone) were collocated inside the trailer with their sample inlets located approximately 1 m above the trailer roof. The FRM (Partisol Model 2025, R & P) was located outside the trailer at a distance of 3 m. The major difference between the field tests in Ewing and the CEF test was that the ambient aerosol was directly drawn from the ambient environment into the continuous monitors during the field test, whereas the air sampled for the CEF test was conditioned for specific variables (e.g., RH). Another difference was that a manufacturer (Anderson Inc.) provided heating devices for the beta gauge (Anderson) used in the winter field campaign.

The distance between two inlets of any continuous monitors was greater than 1 m to avoid potential interferences. A solar powered weather station (Davis Inc.) was also located on the trailer roof. Inside the trailer, temperature was controlled at a range of 0-30 [degrees]C to maintain suitable operation conditions for the electronic units in real-time monitors. For practical considerations, the SidePak and Radiance nephelometers were not tested during this comparison because they are designed for personal monitoring and did not have weather-protected inlets. The filter samples were transported in a cooler.

Air Quality Index Reporting