Time Resolved Characterization of Diesel Particulate Emissions. 1. Instruments for Particle Mass Measurements H. MOOSMU ¨ LLER,* W. P. ARNOTT, C. F. ROGERS, J. L. BOWEN, J. A. GILLIES, AND W. R. PIERSON † Desert Research Institute, University of Nevada System, 2215 Raggio Parkway, Reno, Nevada 89512 J. F. COLLINS, T. D. DURBIN, AND J. M. NORBECK Center for Environmental Research and Technology, University of California, Riverside, California 92521 The measurement of diesel vehicle exhaust particulate mass is currently accomplished using filter collection methods according to the Code of Federal Regulations (CFR). Such filter methods limit time resolution to a minimum of several minutes,making it impossible to study emissions during transient operating conditions. Extensive testing of five different measurement methods has demonstrated that fast response measurements of diesel exhaust particulate mass concentrations,consistent with CFRfilter measurements, are feasible using existing technology. The measurement principles of choice are the real time weighing of exhaust samples as implemented in the tapered element oscillating microbalance (TEOM) and the measurement of light scattering from exhaust particles as implemented in the DustTrak nephelometer. Each of these two instruments has distinctive strengths. The TEOM excels in the area of constant calibration, independent of vehicle. For the DustTrak, this calibration varies by vehicle. On the other hand, the DustTrak has an excellent signal-to-noise ratio, freedom from interference due to other exhaust sample properties, good time resolution, and simplicity. The strengths of the two measurement methods are complimentary, so an obvious suggestion is to integrate them. The nephelometer would obtain a fast response signal, with near real time calibration provided by the microbalance. Introduction The measurement ofdieselvehicle particulate massemissions iscommonlyaccomplished usingCode ofFederalRegulations (CFR) defined filter collection methods on exhaust sampled from a dilution tube (1, 2). To ensure reproducible measure- ment ofvolatile and semi-volatile components absorbed on the filter and on collected particles, the methods require that the filters equilibrate (usually 6-24 h) in a standard atmosphere at defined temperature and humidity levels before each mass determination (3). The time required for the CFRfilter sample collection can varyfrom severalminutes to a few hours per filter,resultingin poor temporalresolution duringtransient engine operatingconditions.Therefore,CFR filter methods are unable to determine particulate emission factorsasfunction ofspeed,acceleration,and operatingmode as needed for modal emission models. All together, the minimum time needed to obtain a mass emission value for a specific experiment is about 1 day. To study diesel vehicle particulate mass emissions as function of engine operating condition and driving mode, instruments with a time resolution on the order of 1 s are needed.Thiskind oftime resolution isparticularlyimportant as significantly greater particulate mass emissions occur during transient operating conditions such as hard accelera- tion. In addition, real time or near real time data availability is desirable for the timely evaluation of test results. Driving distance-based mass emission rates can be calculated from these concentrations if vehicle speed and tunnel flow rates are known. Evaluation ofsuch fast response instruments is nontrivial as neither reference measurement methods nor reference sources exist.Criteria that can be used for evaluation include (i) degree of correlation of time-integrated results with CFR filter measurements, (ii) reproducibility, (iii) noise and interference from nonparticulate mass sample properties, (iv) time resolution, and (v) simplicity and cost. Five promising candidate methods (four optical, one inertialmass)were identified,and instrumentsbased on these methods were developed or acquired. To test these fast response instruments, three diesel vehicles were operated on a chassis dynamometer using the Federal Testing Procedure (FTP)drivingcycle.Particulate data were collected during these driving cycles with CFR filter methods and by the fast response instruments. Experimental Section Real Time Instrumentation. Five different methods with potential for yielding a simple, accurate, and user-friendly instrument for the fast response measurement of particle mass in diesel exhaust have been identified. These methods include an inertial mass measurement method (tapered element oscillating microbalance, TEOM) and four optical methods: a light scatteringmethod (nephelometer,DT);two light absorption methods,one that measureslight absorption of aerosol deposited on a filter (aethalometer, AE) and one that measures light absorption in situ (photoacoustic instru- ment,PA);and a light extinction method (smoke meter,SM). Some ofthese methodshave previouslybeen used to measure diesel particulate mass emissions. The five instruments are described in the following sections. Tapered Element Oscillating Microbalance (TEOMSeries 1105). A continuous measurement of the inertial mass of aerosoldeposited on a filter substrate has been implemented using tapered element oscillating microbalance technology (4, 5). This technology utilizes a hollow tube with the wide end of the tapered tube fixed. The narrow end of the tube holds a filter cartridge, and a sample is passed through the filter and tube to a flow controller. The tube -filter unit acts as a simple harmonic oscillator with its oscillatingfrequency being a function of filter mass loading. The system can be calibrated by placing a calibration mass on the filter and recording the frequency change due to this mass (6). As the oscillatingfrequencyisa function oftemperature,the tapered tube, filter, and sampled air are temperature stabilized, typically at 50 °C. The heating prevents condensation and provides a standard sample conditioning in respect to the removalofsemi-volatile components.However,this aerosol *Correspondingauthor phone: (775)674-7063;fax: (775)674-7008; e-mail: hansm@dri.edu. † Deceased. Environ. Sci. Technol. 2001, 35, 781-787 10.1021/es0013935 CCC: $20.00  2001 American Chemical Society VOL. 35, NO. 4, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 781 Published on Web 01/18/2001