Grantee Research Project Results

Final Report: Remote Sensing Instrument for Particulates and NOx From Heavy-Duty Diesel Vehicles

EPA Contract Number: 68D02046
Title: Remote Sensing Instrument for Particulates and NOx From Heavy-Duty Diesel Vehicles
Investigators: McManus, J. Barry
Small Business: Aerodyne Research Inc.
EPA Contact: Richards, April
Phase: II
Project Period: June 1, 2002 through June 1, 2004
Project Amount: $224,647
RFA: Small Business Innovation Research (SBIR) - Phase II (2002) Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)

Description:

Emissions of carbonaceous particles (soot) from motor vehicles, especially heavy-duty diesel vehicles (HDDVs), are a concern because of impacts on human health and on the radiative balance of the atmosphere. There are numerous techniques for measuring particulates from HDDVs, but most require direct access to the vehicle or the plume. A relatively small effort has gone into techniques to remotely measure particulate emissions from motor vehicles. The goal of this project was to develop a multiwavelength remote-sensing instrument for soot emissions. An instrument to remotely measure soot and carbon dioxide (CO₂) could have commercial application for screening high emitters of soot, especially in locations where diesel soot pollution is a serious problem.

Summary/Accomplishments (Outputs/Outcomes):

Aerodyne Research Inc. has successfully completed a prototype instrument for soot, indexed to CO₂ and measured in an open path. The soot measurement is based on four coaligned lasers, from the blue to the near infrared (405, 635, 690 and 980 nm). CO₂ is measured on the same path with a 2,000-nm laser. In these remote-sensing measurements, a probe beam is sent across a roadway, where it encounters a (pre-positioned) retroreflector, which returns light to the instrument for detection and analysis. Aerodyne Research calls this the Rainbow Soot instrument, after the range of wavelengths employed. The remote-sensing configuration permits the rapid measurement of soot emission index by observing optical extinction due to the exhaust plume left by vehicles passing on roadways. By simultaneously measuring CO₂, the emissions can be indexed to fuel combustion rate. By using lasers, the measurement range and sensitivity is extended over what is possible with lamp light sources. Aerodyne Research has used its instrument to measure soot up to a range of 50 meters.

The final form of the open-path particulate opacity (Rainbow Soot) instrument is shown in Figure 1. The multiple-laser optical module is mounted above electronics in a small cart. The computer monitor and keyboard shown on top of the instrument is placed on a small separate table during use, to avoid jostling the instrument. The mounted instrument measures 0.9 meters by 0.45 meters by 1 meter, and can be positioned and set up by a single person. An aiming mirror attached to the cart directs a probe beam to a remote retroreflector. The retroreflector returns the beam in the reverse direction, with a lateral displacement, to the (custom-designed) collection telescope and detectors. The beam traverses the intervening exhaust plume on both outgoing and return paths.

Figure 1. The Final Form of the Rainbow Soot Instrument Configured for Open-Path Measurements

The completed instrument measures opacity at four wavelengths (405, 635, 690 and 980 nm) with a precision of 10^-3 (typical) at 10 Hz. That translates into soot column densities of 10 to 100 µg/m² at 10 Hz. At the same time, CO₂ is measured, with a column noise typically 30 to 50 ppm-m at 10 Hz. The measurement increases weakly with distance of up to at least 50 meters (instrument to retroreflector).

Open-Path Measurements of Diesel Vehicle Exhaust:

The Rainbow Soot instrument worked well enough in tests to observe plume CO₂ and opacity for a diesel vehicle with relatively low particulate emissions. Despite the fact that the truck plume was not visually smoky, Aerodyne Research observed correlated multiwavelength plume opacity and CO₂ column for a diesel vehicle while stationary and when driven through the sensing beam. In multiple traverses of the sensing beam, Aerodyne Research observed CO₂ in the exhaust plume (approximately 2,000 to 3,000 ppm-m) and associated opacities of approximately 0.02 to 0.04, with a data rate of 10 Hz.

A sample of data taken with a stationary diesel truck is shown in Figure 2, with CO₂ column (red trace) and opacities divided into two parts corresponding to extinction by soot (black trace) and Rayleigh scattering (blue trace) particles. The green bars indicate the truck’s revolutions per minute (RPM). The soot contribution to opacity is seen only at high RPM.

Figure 2. Stationary Truck Data, With CO₂ Column and Opacities Separated into Soot and Rayleigh Parts. The green bars indicate RPM.

The pattern of soot emission as a function of truck RPM, which was indicated in Figure 2, is more apparent if soot is plotted as a function of CO₂ (see Figure 3). A strong soot signature only appears at 3,000 RPM.

Soot and CO₂ were also measured for a diesel truck in motion, as it drove through the multiwavelength probe beam. A sample of mobile measurement data is shown in Figure 4. The figure shows the result of 10 traverses of the probe beam; as in Figure 2, the opacities are separated into soot-like and Rayleigh-like parts.

Figure 3. Soot Column as a Function of CO₂ Column, for the Data Set in Figure 2. Points are colored according to RPM.

Figure 4. Optical Densities and Change in CO₂ Column for Repeated Traverses of The Truck Through the Probe Beam

Closed-Path (Extractive) Measurements of Diesel Vehicle Exhaust:

The Rainbow Soot instrument can easily be reconfigured to measure soot and CO₂ in an extractive mode, by drawing air into a multipass cell. As a result, that measurement capacity can be added to a measurement suite for numerous pollutants that are based on extractive sampling. With further development, this type of instrument could have higher sensitivity and faster response than currently available black-carbon instruments (e.g., multi-angle absorption photometer, MAAP).

Aerodyne Research has tested a closed-path (extractive sampling) version of the opacity instrument, and compared that to a MAAP instrument for total carbon. The results are shown in Figure 5, where the Rainbow Soot instrument (purple trace) is in good agreement with the MAAP data (black trace). Under the particular circumstances of that test, electromagnetic interference was encountered and this increased the noise level by 30 times over the normal level. Thus, Aerodyne Research believes that much better correspondence with the MAAP instrument is possible, and much faster data rates than allowed by the MAAP instrument are achievable.

Figure 5. Data From the Rainbow Soot Multiwavelength Instrument (Colored Traces) and the MAAP Instrument (Black Trace). The overlapping colored traces are optical densities, and the purple trace is the mass density from the Rainbow Soot instrument.

Conclusions:

Aerodyne Research has successfully completed a prototype instrument for soot, indexed to CO₂ and measured in an open path. The soot measurement is based on four coaligned lasers, from the blue to the near infrared (405, 635, 690, and 980 nm). CO₂ is measured on the same path with a 2,000-nm laser. The firm has used its instrument to measure soot up to a range of 50 meters. The Rainbow Soot instrument worked well enough in tests to observe plume CO₂ and opacity for a diesel vehicle with relatively low particulate emissions. Despite the fact that the truck plume was not visually smoky, Aerodyne Research observed correlated multiwavelength plume opacity and CO₂ column for a diesel vehicle while stationary and when driven through the sensing beam. In multiple traverses of the sensing beam, Aerodyne Research observed CO₂ in the exhaust plume (approximately 2,000 to 3,000 ppm-m) and associated opacities of approximately 0.02 to 0.04, with a data rate of 10 Hz. Additional work is needed to produce user interface and automatic data processing software, and to compare the instrument to others.

Supplemental Keywords:

remote sensing, diesel emissions, carbonaceous particles, soot, heavy-duty diesel vehicles, carbon dioxide, exhaust plume, laser, Rainbow Soot instrument, multiwavelength, retroreflector, Rayleigh scattering, multi-angle absorption photometer, EPA, small business, SBIR,, RFA, Air, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, particulate matter, mobile sources, Environmental Chemistry, Engineering, Chemistry, & Physics, Analytical Chemistry, Monitoring/Modeling, Atmospheric Sciences, air toxics, Environmental Engineering, Environmental Monitoring, air quality, diesel exhaust, vehicle emissions, aerosol particles, diesel exhaust particulates, ambient aerosol, emissions, air sampling, Nox, Nitrogen Oxides, motor vehicle emissions, automotive emissions, airborne particulate matter, aersol particles, ambient particle properties, atmospheric particles, PM, remote sensing, NOx control, atmospheric aerosol particles, diesel particulates, ambient air quality, atmospheric measurements

SBIR Phase I:

Remote Sensing Instrument for On-Road Heavy-Duty Diesel NOx and PM Emissions