Evaluation of Diesel Exhaust Continuous Monitors in Controlled Environmental Conditions

Abstract

Diesel exhaust (DE) contains a variety of toxic air pollutants, including diesel particulate matter (DPM) and gaseous contaminants (e.g., carbon monoxide (CO)). DPM is dominated by fine (PM2.5) and ultrafine particles (UFP), and can be representatively determined by its thermal-optical refractory as elemental carbon (EC) or light-absorbing characteristics as black carbon (BC). The currently accepted reference method for sampling and analysis of occupational exposure to DPM is the National Institute for Occupational Safety and Health (NIOSH) Method 5040. However, this method cannot provide in-situ short-term measurements of DPM. Thus, real-time monitors are gaining attention to better examine DE exposures in occupational settings. However, real-time monitors are subject to changing environmental conditions. Field measurements have reported interferences in optical sensors and subsequent real-time readings, under conditions of high humidity and abrupt temperature changes. To begin dealing with these issues, we completed a controlled study to evaluate five real-time monitors: Airtec real-time DPM/EC Monitor, TSI SidePak Personal Aerosol Monitor AM510 (PM2.5), TSI Condensation Particle Counter 3007, microAeth AE51 BC Aethalometer, and Langan T15n CO Measurer. Tests were conducted under different temperatures (55, 70, and 80°F), relative humidity (10, 40, and 80%), and DPM concentrations (50 and 200 μg/m(3)) in a controlled exposure facility. The 2-hr averaged EC measurements from the Airtec instrument showed relatively good agreement with NIOSH Method 5040 (R(2) = 0.84; slope = 1.17±0.06; N = 27) and reported ∼17% higher EC concentrations than the NIOSH reference method. Temperature, relative humidity, and DPM levels did not significantly affect relative differences in 2-hr averaged EC concentrations obtained by the Airtec instrument vs. the NIOSH method (p < 0.05). Multiple linear regression analyses, based on 1-min averaged data, suggested combined effects of up to 5% from relative humidity and temperature on real-time measurements. The overall deviations of these real-time monitors from the NIOSH method results were ≤20%. However, simultaneous monitoring of temperature and relative humidity is recommended in field investigations to understand and correct for environmental impacts on real-time monitoring data.

Keywords: diesel exhaust; diesel particle matter; environmental conditions; real-time monitor.

FIGURE 1

The scatter plot for the EC_realtime and EC_NIOSH measurements (N=27) *The 1:1 dotted line was added in the scatter plot for the comparison.

FIGURE 2

Box plots for calculated relative difference (RD) between EC_realtime and EC_NIOSH measurements in different a) temperature (Temp) (H = 80 °F, L = 55 °F, M = 70 °F), b) relative humidity (RH) (H = 80%, L = 10%, M = 40%), c) diesel particulate matter (DPM) levels (H = ~200 µg/m³, L = ~50 µg/m³), and an interaction between Temp and RH (HH = 80 °F & 80%, HL = 80 °F & 10%, HM = 80 °F & 40%, LH = 55 °F & 80%, LL = 55 °F & 10%, LM = 55 °F & 40%, MH = 70 °F & 80%, ML = 80 °F & 10%, MM = 70 °F & 40%).

FIGURE 3

The comparision of real-time monitors running with low DPM (50 µg/m³), low RH (10%), and low temperature (55 °F) in a session (#8) on 1/8/2014 *CO concentrations are divided by 100 for presentation purpose only.