Abstract:
A system for sampling emission products from an emissions source, for example combustion engines including gasoline, diesel and natural gas engines, for subsequent measurement and analysis of the emission products. The system includes a dilution apparatus, a residence time chamber, a plurality of sampling probes within the residence time chamber, and a plurality of sampling trains connected to the sampling probes to take simultaneous representative emission samples for subsequent analysis. The system has particular use in quantifying chemical and toxic trace species from emissions sources. The results of the analysis can be used to formulate decisions on changes in engine design strategy, and can be used to determine the effectiveness of aftertreatment systems on the emissions source.

Description:
PRIORITY INFORMATION 
       [0001]    This application is a Divisional Application of U.S. application Ser. No. 11/530,728 filed Sep. 11, 2006 entitled SOURCE DILUTION SAMPLING SYSTEM FOR EMISSIONS ANALYSIS, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD 
       [0002]    A system for sampling emission products from an emissions source, for example combustion engines including gasoline engines, diesel engines, and natural gas engines for subsequent measurement and analysis of the emission products. 
       BACKGROUND 
       [0003]    Emissions of pollutant chemicals have increased orders of magnitudes in the past 100 years due primarily to anthropogenic releases associated with industrial, agricultural, domestic, and recreational activity. Current research indicates that there are very strong correlations between the increase in these emissions and an overall increase in atmospheric temperatures (i.e. global warming) and an increased number of Category 4 and 5 hurricanes per annum. Furthermore, it is believed that ambient particulate matter in aerosol phase may include potentially toxic components. Researchers also believe that particulate matter and gases from industrial activities and vehicles may cause various health problems, such as asthma. These correlations between emissions of pollutant chemicals and the negative impact on environment and human health has led to more stringent worldwide emission standards for automobiles and other vehicles, as well as power plants, mines, and other industries. 
         [0004]    In the United States, emission standards are set by the Environmental Protection Agency (EPA) as well as state governments (e.g. California Air Resource Board (CARB)). As of this writing, all new vehicles sold in the United States must meet the EPA&#39;s “Tier  1 ” emission standard. A more stringent standard, “Tier  2 ,” is being phased in for automobiles and should be completed by 2009. For diesel engines, on-road trucks and other vehicles will be required to meet more stringent standards by 2010 and off-road vehicles such as construction vehicles will be subject to Tier IV regulations. Accordingly, attaining ultra low emissions has become a top priority for combustion researchers as federal and state regulations continuously reduce the allowable levels of pollutants that can be discharged by engines, power plants, and other industrial processes. 
         [0005]    In order to meet the emission standards of today and the future, researchers have made, and are continually striving to make, improvements to combustion engines, for example heavy duty diesel engines, gas combustion engines, power-generating gas turbines, and the like, and other emission sources. In addition to these developments, researchers are endeavoring for better methods and devices of measuring smaller particulate matter and low level gases and quantifying the chemical compositions of emissions. 
         [0006]    Generally, chemical composition analysis of fine particulate matter, gases, and volatile and semi-volatile organic compounds from emissions sources consists of three major steps: (1) Representative conditioning and sampling; (2) Chemical analysis; and (3) Data analysis and explanation. The effective accuracies of Steps (2) and (3) are both dependent on step (1). For without an accurate and precise sampling procedure, no analysis of that sample could be said to represent valid data. Accordingly, without valid analysis, a full and complete explanation of the sample would not be available. 
         [0007]    A conventional system for assessing particle mass and quantifying chemical composition of emission gases mixes emission gas with filtered air in a mixing chamber. The conventional system is illustrated in  FIG. 1 , and includes a sampling port  2  that feeds exhaust gases to a diluter  4 , forming the mixing chamber, where the exhaust gases are diluted with the filtered air. The diluted gas mixture is then sampled by a sampling train  6 . However, this system has many well recognized disadvantages. First, the partial/full/partial dilution sampling system in this conventional system would introduce more errors than a full/partial/full system. Second, the conventional system allows only for assessment of single type of compound. Accordingly, multiple sample runs are required to detect each of the chemical compounds necessary for a full compound assessment (particulate matter, volatile organics, semi-volatile organics, and gases, etc.) Furthermore, these measurements are made with different samples each time, and may add to inherent errors that are unavoidable to this system. These errors may lead to inaccurate measurements and quantification of data. 
         [0008]    Work at the University of Wisconsin-Madison attempted to improve the conventional system. University of Wisconsin scientists used a device called an “augmented sampling system” to study the chemical composition and to assess particle size of diesel engine exhaust. See Chol-Bum Kweon, David E. Foster, James J. Schauer, and Shusuke Okada, “Detailed Chemical Composition and Particle Size Assessment of Diesel Engine Exhaust” SAE 2002-01-2670, Fall SAE Meeting 2002. The “augmented sampling system” disclosed by Kweon et al includes a secondary dilution tunnel for the diesel exhaust and a residence time chamber with radial sampling ports near the base of the residence time chamber. The secondary dilution tunnel of the augmented sampling system mixes filtered air with an emission gas sample without regard to temperature gradient between the surface of the dilution tunnel and the emission gas. This may lead to a high degree of particle loss and accordingly less accurate sampling due to thermophoresis. 
         [0009]    Thermophoresis, or Ludwig-Soret effect, is thought to be related to Brownian movement biased by a temperature gradient. The thermophoretic force is a force that arises from asymmetrical interactions of a particle with the surrounding gas molecules due to a temperature gradient. Generally, a particle is repelled from a hotter surface and attracted to a cooler surface. Thus, as emission particles travel through a sampling system, cooler surface temperature of the system as compared to the emission gas would lead to greater thermophoretic force on the emission particles. 
         [0010]    In the Kweon et al. augmented sampling system, the residence time chamber is heated to reduce thermophoresis. However, the heated residence time chamber is likely to fail in achieving realistic atmospheric conditions, as the addition of heat may underestimate the particulate matter emissions due to the reduced effects of nucleation and condensation and may also affect secondary reactions of volatile organic compounds and semi-volatile organic compounds and formation of secondary organic compounds. 
         [0011]    A system that allows more accurate and precise sampling of emission products is needed, thereby contributing to better measurement and analysis of the emission products. 
       SUMMARY 
       [0012]    A system is provided for sampling emission products from an emissions source, for example combustion engines including gasoline, diesel and natural gas engines, for subsequent measurement and analysis of the emission products. The system has particular use in quantifying particle size distributions and chemical species from low emissions sources. The results of the analysis can be used to formulate decisions on changes in engine design strategy, and can be used to determine the effectiveness of aftertreatment systems on the emissions source. 
         [0013]    The system uses a full/partial/full approach and includes an isokinetic sampling nozzle, a dilution apparatus, a residence time chamber, a plurality of sampling probes within the residence time chamber, and a plurality of sampling trains connected to the sampling probes to take simultaneous representative emission samples for subsequent analysis. 
         [0014]    The dilution apparatus is designed to be thermophoretic-resistant to reduce the thermophoretic force on emission particles, thereby reducing particulate matter losses. In addition, the dilution apparatus is designed to simulate atmospheric dilution, mixing and cooling processes, enabling the sampled gas and the dilution gas to thoroughly mix and cool to ambient temperature, allowing gas-phase organics in the sampled gas to nucleate and condense to their usual aerosol phase. 
         [0015]    The residence time chamber is designed to provide sufficient time for gas-to-particle conversion, which involves the diffusion limited transport of supersaturated vapor onto existing particles. Preferably, the residence time chamber is designed to provide at least 30 seconds of residence time. During this time, the sample flow and concentrations within the residence time chamber also become uniformly distributed before entering the sampling probes. 
         [0016]    The sampling probes are aligned coaxial to the flow direction within the residence time chamber (i.e. isoaxial) with the inlets of the probes facing into the direction of flow. This improves collection of the emission samples since the samples do not need to turn sharp corners to enter the probes. The plurality of sampling trains connected to the sampling probes permit the simultaneous sampling of different materials, including, but not limited to, volatile and semi-volatile organic, gas-phase, and particulate matter samples. 
         [0017]    A method of sampling emission products from an emissions source is also provided. The method includes directing a sample of a gas stream from the emissions source into a dilution apparatus. In the dilution apparatus, heat is exchanged between the gas stream sample and a dilution gas to cool the gas stream sample, and thereafter the dilution gas is introduced into the gas stream sample to mix with the gas stream sample. The gas mixture is then directed to a residence time chamber, and a sample of the gas mixture is taken from the residence time chamber through a sampling probe having an inlet that is substantially parallel to a direction of flow of the gas mixture within the residence time chamber. 
         [0018]    In one embodiment, a system for sampling emission products from an emissions source comprises a dilution apparatus connected to a sampling probe to receive a gas stream sample. The dilution apparatus includes an inlet through which the gas stream sample enters, a dilution gas inlet through which dilution gas enters the dilution apparatus, and an exit through which a mixture of dilution gas and the gas stream sample exits the dilution apparatus. A dilution gas source is connected to the dilution gas inlet of the dilution apparatus for supplying dilution gas. A residence time chamber is connected to the dilution apparatus and receives therefrom the gas mixture. The residence time chamber includes a plurality of isoaxial sampling probes disposed inside the chamber. Further, a sampling train is connected to each of the isoaxial sampling probes. 
         [0019]    In another embodiment, a system for sampling emission products from an emissions source comprises a dilution apparatus connected to a sampling probe to receive a gas stream sample. The dilution apparatus includes an inlet through which the gas stream sample enters, a dilution gas inlet through which dilution gas enters the dilution apparatus, an exit through which a mixture of dilution gas and the gas stream sample exits the dilution apparatus, and a plurality of holes axially spaced from the dilution gas inlet which allow introduction of the dilution gas into the gas stream sample. In addition, the dilution apparatus is configured so that the dilution gas exchanges heat with the gas stream sample prior to being mixed with the gas stream sample. A dilution gas source connected to the dilution gas inlet of the dilution apparatus, and a residence time chamber that is connected to the dilution apparatus receives therefrom the gas mixture. Further, a plurality of sampling trains are connected to the residence time chamber. 
         [0020]    In yet another embodiment, a system for sampling emission products from an emissions source comprises a dilution apparatus connected to a sampling probe to receive a gas stream sample. The dilution apparatus includes a longitudinal axis, an inlet through which the gas stream sample enters, a dilution gas inlet through which dilution gas enters the dilution apparatus, and an exit through which a mixture of dilution gas and the gas stream sample exits the dilution apparatus. A dilution gas source is connected to the dilution gas inlet of the dilution apparatus, and a residence time chamber is connected to the dilution apparatus and receives therefrom the gas mixture. The residence time chamber includes a longitudinal axis that is substantially perpendicular to the longitudinal axis of the dilution apparatus. Further, a plurality of sampling trains are connected to the residence time chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  illustrates a sampling system according to the invention connected to a dilution tunnel for sampling emissions from an engine. 
           [0022]      FIG. 2  illustrates the sampling system separate from the dilution tunnel. 
           [0023]      FIG. 3  is a cross-sectional view of the dilution apparatus taken along the longitudinal axis thereof. 
           [0024]      FIG. 4  is a detailed view of the perforations in the inner tube. 
           [0025]      FIG. 5  is a cross-sectional view of the residence time chamber taken along line  5 - 5  in  FIG. 2 . 
           [0026]      FIG. 6  is a cross-sectional view taken along line  6 - 6  in  FIG. 5 . 
           [0027]      FIG. 7  illustrates exemplary flow trains connected to the residence time chamber. 
           [0028]      FIG. 8  illustrates additional exemplary flow trains connected to the residence time chamber. 
           [0029]      FIG. 9  is a flow chart of an exemplary sampling method of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    With reference to  FIG. 1 , a system  10  for sampling emission products from an emissions source  12  is illustrated. The system  10  is constructed to simultaneously sample a number of different emissions products emitted from the emissions source  12 . The samples can then be analyzed to permit chemical characterization of the emissions products with respect to air toxics. 
         [0031]    The system  10  will be described herein as being applied to the sampling and chemical characterization of diesel emission exhaust from an emissions source  12  in the form of a diesel engine. However, the concepts described herein can be used to great advantage in sampling a number of other types of gases from a number of other types of emissions sources, both stationary and mobile. Examples of other types of gases includes, but is not limited to, gas combustion engine exhaust, turbine engine exhaust, and atmospheric gas. Examples of other types of emissions sources includes, but is not limited to, gas combustion engines, turbine engines, power plants, manufacturing plants, exhaust stacks, etc. 
         [0032]    As shown in  FIG. 1 , the entire exhaust from the engine  12  is ducted to a dilution tunnel  16  through suitable piping  18 . Filtered dilution air  20  is introduced into the tunnel upstream of the discharge for the engine exhaust, with the dilution air  20  then mixing with the engine exhaust in the tunnel  16  to dilute and cool the exhaust gas. 
       System 
       [0033]    With reference to  FIGS. 1-2 , the system  10  includes a dilution apparatus  22 , a residence time chamber  24 , a plurality of sampling probes  26   a, b, . . . n  ( FIGS. 7 and 8 ) within the residence time chamber, and a plurality of sampling trains  28   a, b, . . . n  ( FIGS. 9 and 10 ) connected to the sampling probes to take simultaneous representative emission samples for subsequent analysis. 
         [0034]    The dilution apparatus  22  is connected to a sampling probe  30  that extends into the dilution tunnel  16 . The probe  30  collects a gas stream sample from the engine  12  and directs the gas stream sample to the dilution apparatus  22 . The inlet of the probe  30  is preferably iso-kinetic and positioned proximate the center of the dilution tunnel  16  to minimize boundary effects caused by the walls of the tunnel  16 . In the dilution apparatus  22 , the sampled gas is diluted with dilution gas, cooled to ambient temperature, and thoroughly mixed with the dilution gas. 
         [0035]    The result is a full/partial/full dilution scheme, where the entire exhaust stream is initially diluted within the dilution tunnel  16 , a portion of the exhaust stream is sampled by the sampling probe  30 , and the entire portion of the gas sample is then diluted in the dilution apparatus. This full/partial/full dilution scheme is an improvement over conventional partial/full/partial dilution schemes, which direct only a portion of the exhaust stream into the dilution tunnel  16 . As a result, the number of particles seen in resulting samples is low compared to a full/partial/full dilution scheme. 
         [0036]    The gas mixture is then fed to the residence time chamber  24  which is designed to provide sufficient time for gas-to-particle conversion, which involves the diffusion limited transport of supersaturated vapor onto existing particles. The gas flow also becomes uniformly distributed before entering the sampling probes  26   a, b, . . . n . The sampling probes  26   a, b, . . . n  simultaneously collect multiple samples of the gas mixture and feed the samples to the sampling trains  28   a, b, . . . n  which are constructed to take various samples of the gas. Preferably, the sampling trains are configured to sample unregulated chemical species within the gas samples, for example volatile and semi-volatile organics, gas-phase compounds, and particulate matter. 
         [0037]    The components of the system  10  are preferably made of inert materials, including, but not limited to, stainless steel, plastic or polymer materials such as TEFLON, and plastic or polymer coated aluminum such as TEFLON-coated aluminum. In addition, the use of electrically non-chargeable materials, such as 304, 316 and 316L stainless steels, can also be used to reduce electrostatic deposition of charged particles that are typically polarized during combustion processes. In addition, the system  10  is preferably devoid of materials, for example oils, greases, rubbers and the like, that could outgas organics to avoid contamination of the gas stream and gas samples. 
         [0038]    Further, the system  10  is preferably configured to minimize vapor and particulate losses. For example, the system is designed to promote smooth flow transitions within the system  10 . 
       Dilution Apparatus 
       [0039]    With reference to  FIGS. 1-4 , the dilution apparatus  22  is designed to be thermophoretic-resistant to reduce the thermophoretic force on emission particles, thereby reducing particulate matter losses. In addition, the dilution apparatus  22  preferably simulates atmospheric dilution, mixing and cooling processes, enabling the sampled gas and the dilution gas to thoroughly mix and cool to ambient temperature, allowing gas-phase organics in the sampled gas to nucleate and condense to their usual aerosol phase. 
         [0040]    The sampled gas collected by the sampling probe  30  enters the dilution apparatus  22  through an inlet  32 . As shown in  FIGS. 3-4 , the dilution apparatus  22  has a cylindrical housing  34  with a first end  36  that includes the inlet  32 , a second end  38  and an interior space. An inner cylindrical wall  40  is located concentrically with the housing  34 , with the cylindrical wall  40  having a first end  42  adjacent the first end  36  of the housing and a second end  44  adjacent the second end  38  of the housing. The cylindrical wall  40  divides the interior space into a static pressure chamber  46  defined between the housing  34  and the wall  40  and that extends generally from the first end  42  of the wall to the second end  44  of the wall  40 , and a mixing chamber  48  that extends generally from the first end of the wall to the second end of the wall. 
         [0041]    The wall  40  has circumferentially and axially distributed perforations  50  near the first end  42  thereof that place the static pressure chamber  46  in communication with the mixing chamber  48 . In addition, the housing  34  has a plurality of evenly, circumferentially spaced inlet ports  52  near the second end  44  thereof that open into the static pressure chamber  46  for introducing a dilution gas into the static pressure chamber  46 . The inlets ports  52  communicate with a plenum  54  defined around the circumference of the housing  34 , and dilution gas is fed to the plenum  54  from a dilution gas source  56 . If desired, the dilution gas source  56  can be a source of over-pressure, such as a compressor, and a regulator  57 , such as a valve, can be used to regulate the flow of dilution gas into the dilution apparatus. The gas source  57  and/or regulator  57  can be used to control the amount of dilution gas that is fed to the dilution apparatus, thereby changing the dilution ratio of the gas stream sample and the dilution gas. 
         [0042]    In use, the sampled gas enters the mixing chamber  48  of the dilution apparatus through the inlet  32  as shown by the arrows in  FIG. 3 . In addition, dilution gas is introduced into the static pressure chamber  46  through the inlets ports  52 . As the dilution gas flows toward the first end  42  as shown by the arrows in  FIG. 3 , it exchanges heat with the sampled gas in the mixing chamber  48 . In an alternative embodiment, insulation material can be provided on the wall  40  to keep the inner part of the wall  40  the same temperature as the sample gas, thereby lowering the effect of thermophoresis. 
         [0043]    Once the dilution gas reaches the perforations  50 , it flows radially inward into the mixing chamber  48  to mix with the sampled gas.  FIG. 3  illustrates the flow of dilution air into the mixing chamber  48 . The perforation holes  50  create jets of dilution air that impinge upon the sampled gas to create turbulent mixing with the sampled gas. Preferably, the perforation holes  50  are configured to generally evenly distribute the dilution gas into the mixing chamber. In the illustrated embodiment, the holes  50  are circumferentially and axially evenly spaced about the wall  40 . Mixing of the dilution gas and the sampled gas also cools the sampled gas. 
         [0044]    The dilution gas is at a temperature lower than the sampled gas, so that the sampled gas is cooled through heat exchange with the static pressure chamber and as a result of mixing with the dilution gas, allowing gas-phase organics in the sampled gas to nucleate and condense to their usual aerosol phase in the atmosphere. Preferably, the sampled gas is cooled to a temperature that is at least within 5° C. of ambient temperature by the time the mixture of sampled gas and dilution gas reaches the exit of the dilution apparatus. More preferably, the sampled gas is cooled to ambient temperature by the time the mixture of sampled gas and dilution gas reaches the exit of the dilution apparatus. In certain embodiments, the sampled gas can be cooled to a temperature below ambient temperature. 
         [0045]    In addition, because the sampled gas is cooled to at or near ambient temperature, temperature differences between the exterior of the apparatus  22  and the gas mixture within the mixing chamber  48  is reduced, thereby reducing the thermophoretic force acting on particles in the flow. This reduces particle loss as the gas sample flows through the dilution apparatus  22 . 
         [0046]    The number and size of the perforation holes  50  is chosen based on the gas being sampled, the gas temperature, and the desired dilution rate. For diesel engine exhaust, the holes can provide between 20% to 80% porosity, have diameters ranging from about 0.125 inch to about 0.5 inch, and extend over a length L c  of the wall  40  ranging from about 0.06 inches to about 15 inches ( FIG. 3 ). In addition, the dimensions of the dilution apparatus  22  are chosen based on the temperature of the sampled gas and the flow rate. With reference to  FIG. 3 , for diesel engine exhaust, the length L c  of the mixing chamber  48  can vary between 18.0 inches to 63.0 inches, the housing can have a diameter D between 3.0 inches and 10.5 inches, and the gap G defining the static pressure chamber between the wall  40  and the housing  34  can vary between 0.2 inches and 2.0 inches. 
         [0047]    As shown in  FIG. 3 , a reducing cone  58  is connected to the end of the housing  34  and defines an exit  60  for the mixture of sampled gas and dilution gas from the dilution apparatus  22 . The reducing cone  58  includes a first constant diameter section  62  that connects to the housing  34 , a tapered section  64  that reduces in diameter to reduce the diameter of the flow path, and a second constant diameter section  66  that defines the exit  60  and which is directly connected to the residence time chamber  24 . The reducing cone  58  helps to provide a smooth flow transition of the gas mixture from the dilution apparatus  22  to the residence time chamber  24 . 
         [0048]    Further details on the dilution apparatus  22  can be found in copending U.S. patent application Ser. No. 11/530,758, filed on Sep. 11, 2006, and titled Thermophoretic-Resistant Gas Dilution Apparatus For Use in Emissions Analysis, which application is incorporated herein by reference. 
       Residence Time Chamber 
       [0049]    The residence time chamber  24  is best illustrated in  FIGS. 1 ,  2 ,  5  and  6 . The chamber  24  includes a housing  70  having a first end  72  and a second end  74 . In the illustrated embodiment, the housing  70  is oriented generally vertically so that the longitudinal axis of the housing  70  is oriented vertically and generally perpendicular to the longitudinal axis of the dilution apparatus  22  which is disposed generally horizontally. 
         [0050]    The housing  70  is connected to the reducing cone  58  of the dilution apparatus  22  at the first end  72 . Preferably, the first end  72  is in the form of a conical section, with the cone opening or facing downward. The gas mixture is received into the conical section  72 , with the conical section helping to promote a smooth flow transition of the gas mixture from the dilution apparatus to the residence time chamber. Likewise, the second end  74  is in the form of a conical section, with the cone opening or facing upward. The conical section  74  helps to promote a smooth flow transition from the residence time chamber to an exit port  76  located at the bottom of the conical section  74 . 
         [0051]    The housing  70 , except for the conical sections  72 ,  74 , is generally cylindrical and has a constant diameter from the conical section  72  to the conical section  74 . The housing  70  provides sufficient time for gas-to-particle conversion within the gas mixture, and allows the gas flow to become uniformly distributed. Preferably, the housing  70  provides at least 30 seconds of residence time for the gas flow from the time the gas flow enters the housing  70  to the time the gas flow reaches and enters one of the sampling probes. A residence time of 30 seconds can be provided by a housing  70  with a height of about 57 inches and a diameter of about 12.0 inches. 
         [0052]    As shown in  FIGS. 5 and 6 , the sampling probes  26   a, b, . . . n  are disposed inside of the housing  70  to simultaneously collect multiple samples of the gas mixture and feed each sample to the sampling train  28   a, b, . . . n . The sampling probes are aligned coaxial to the flow direction to achieve isoaxial and isokinetic sampling. In the illustrated embodiment, 8 sampling probes are provided, with each of the sampling probes  26   a, b , . . . n extending upward with the inlets to the probes facing upward toward the oncoming flow. To avoid boundary flow effects of the housing wall, the sampling probes are preferably spaced inwardly from the housing wall. Because the sampling probes are isoaxial and face upward toward the oncoming flow, sampling is improved because the sampled flow does not need to turn sharp corners to enter the probes. 
         [0053]    Further details on the residence time chamber and the sampling probes can be found in copending U.S. patent application Ser. No. 11/530,746, filed on Sep. 11, 2006, and titled Residence Time Chamber and Sampling Apparatus, which application is incorporated herein by reference. 
       Sampling Trains 
       [0054]    The gas samples entering the sampling probes  26   a, b, . . . n  are directed to the sampling trains  28   a, b, . . . n . The sampling trains can be configured to take samples of any kind of matter within the gas samples. Preferably, the sampling trains are configured to sample unregulated chemical species within the gas samples, for example volatile and semi-volatile organics, gas-phase compounds, and particulate matter. 
         [0055]    Examples of suitable sampling trains are illustrated in  FIGS. 7 and 8 . As shown in  FIG. 7 , sampling trains  28   a ,  28   b ,  28   c  each begin with a PM2.5 cyclone separator  80  that can be operated at a flow rate of 16.7 liters/min (1 μm) for removing particles that are about 2.5 microns and above. Flow through the trains  28   a ,  28   b ,  28   c  is controlled by downstream critical flow orifices  82  in series with a rotary vacuum pump  84  via a manifold  86 . A rotameter prior to each critical flow orifice can be used to monitor the flow rate. A filter  88 , for example a two stage TEFLON membrane filter, is disposed after the separator  80  to filter out material from the sampled gas. The filters  88  can then be analyzed for collected material. When the filters  88  are TEFLON membrane filters, analysis can be conducted for total mass, particulate matter sulfate ions, and particulate matter trace elemental composition. 
         [0056]    The sampling trains  28   d ,  28   e ,  28   f  illustrated in  FIG. 7  are similar to the sampling trains  28   a ,  28   b ,  28   c . However, the illustrated trains  28   d ,  28   e ,  28   f  utilize a filter  90 , preferably a two stage quartz fiber filter, in series with a polyurethane foam (PUF) cartridge  92 . An adsorption material substrate, such as an XAD™ substrate, could be used in place of the PUF cartridge in the case of higher flow rates. When the filters  90  are quartz filters, particle-phase organic compounds can be collected to analyze for particulate matter organics, nitro-PAH particulate matter, and particulate matter hydrocarbon distribution. In the case of PUF cartridges, semi-volatile organic compounds can be collected to analyze for semi-volatile organic compounds, semi-volatile nitro-PAH, and semivolatile hydrocarbon distribution. 
         [0057]    With reference to  FIG. 8 , a sampling train  28   g  that is designed for high flow samples is illustrated. The sampling train  28   g  is connected to the exit port  76  at the bottom of the conical section  74 . The sampling train  28   g  is not connected to a sampling probe. Instead, the sampling train samples the remainder of the gas flow that is not sampled by the sampling probes as the gas flow remainder exits through the bottom of the residence time chamber  24 . The train  28   g  includes a PM2.5 cyclone separator  100  that can be operated at a flow rate of 92 liters/min (lpm) for removing particles that are about 2.5 microns and above, followed in series by a filter  102 , for example a quartz filter, a PUF cartridge  104  (or XAD substrate), critical flow orifices  156 , and a rotary vacuum pump  106 . This kind of sampling train  28   g  is suitable for use in collecting samples for polycyclic aromatic hydrocarbon analysis from low emission sources. The sampling train  28   g  can also include a flow meter  154 , shown in  FIGS. 1 and 2 . 
         [0058]      FIG. 8  also illustrates a gaseous sampling train  28   h  which can be run in parallel to the sampling trains  28   a - f . The sampling train  28   h  includes two Dinitrophenyl-Hydrazine (DNPH) cartridges  110  arranged in parallel to collect samples which are subsequently analyzed for carbonyl species. The cartridges  110  can have different flow rates, for example about 1.5 lpm and about 0.3 lpm. The flow rates can be controlled by critical flow orifices  112  in series with the rotary vacuum pump  106 . In addition, the train  28   h  can include two volatile organic compound (VOC) tubes  114  arranged parallel to the DNPH cartridges to collect samples for hydrocarbon speciation. The flow rates through the VOC tubes  114  can range from about 10 standard cubic centimeter (SCCM) to 50 SCCM, controlled by two separate mass flow controllers  116  in series with the vacuum pump  106 . The mass flow controllers  116  can also be used to collect mass flow, volumetric flow, pressure, and temperature data. If desired, one or two filters prior to the DNPH cartridges  110  and VOC tubes  114  can be used to collect large particles. 
         [0059]    An additional sampling train  28   i  can be used to measure particle size distributions for steady-state and transient operations. In addition, a sampling train  28   j  can include temperature, humidity, and pressure sensors to monitor the residence time chamber conditions. 
         [0060]    Other types of sampling trains for collecting other types of materials within the sampled gas can be used. The sampling trains described herein are intended to be exemplary and not intended to be limiting. 
         [0061]    Further details on the sampling trains can be found in U.S. patent application Ser. No. 11/530,746, filed on Sep. 11, 2006, and titled Residence Time Chamber and Sampling Apparatus. 
         [0062]    The method of operation of the system  10  and of sampling exhaust gas from the engine  12  is apparent from the preceding description. With reference to  FIG. 9 , a sample of the exhaust gas from the engine is initially directed into the dilution apparatus  22 , through the sampling probe  30 . Next, in the dilution apparatus, the gas sample is cooled by the dilution gas and the dilution gas and the gas sample are mixed. The gas mixture is then directed to the residence time chamber, and a sample, preferably a plurality of simultaneous samples, of the gas mixture is taken from the residence time chamber through a sampling probe having an inlet substantially parallel to a direction of flow of the gas mixture within the residence time chamber. The sample is then directed to a sampling train which is configured to remove a desired material from the sample for subsequent analysis. 
         [0063]    The invention may be embodied in other forms without departing from the spirit or novel characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limitative. The scope of the invention is indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.