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Atmospheric Environment 42 (2008) 4699–4707

www.elsevier.com/locate/atmosenv

Evidence of increased mass fraction of NO2 within real-worldNOx emissions of modern light vehicles — derived from

a reliable online measuring method

Robert Alvarez�, Martin Weilenmann, Jean-Yves Favez

Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Internal Combustion Engines,

Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland

Received 14 November 2007; received in revised form 18 January 2008; accepted 25 January 2008

Abstract

Ambient roadside concentrations of nitrogen dioxide (NO2) have stabilized in recent years while concentrations of

nitrogen oxides (NOx) decline. Oxidation catalytic converters of modern vehicles facilitating the formation of NO2 in the

exhaust line, especially in diesel cars equipped with original equipment manufacturer (OEM) particle filters, are assumed to

be responsible. NO2 is toxic and increased proportions of NO2 in total NOx in the atmosphere cause higher ambient ozone

concentrations. These observations lead to a need for reliable emission factors for NO and NO2 for road vehicles, while

only NOx is recorded in standard emission measurements. In this regard, it was recently shown that NO2 needs to be

detected by an adequate online measuring method.

The present work provides novel insight into these topics gained from an experimental campaign carried out with

modern gasoline and diesel vehicles of certification categories Euro 3 and Euro 4. Reliable emission factors for NO and

NO2 are presented for different driving situations, such as real-world driving, cold start and statutory tests, together with

corresponding particle emission data. Highest emissions of NOx are recorded for diesel cars equipped with OEM particle

filters with mass ratios of NO2 within NOx of up to 70%. The NOx emissions exceed the statutory emission limit and real-

world emissions are even more pronounced, especially in urban driving conditions. Their particle emissions are greatly

reduced, but the contribution of NO2 to soot oxidation is thought to be minor.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: NO2; Emission; Vehicle; Particle filter; Real-world

1. Introduction

Emissions of nitrogen oxides (NOx) have againbecome topical in recent years in relation to airquality monitoring. NOx are inevitable combustion

e front matter r 2008 Elsevier Ltd. All rights reserved

mosenv.2008.01.046

ing author. Tel.: +4144 823 48 69;

40 44.

ess: robert.alvarez@empa.ch (R. Alvarez).

products originating not only from gasoline anddiesel engines, but also from industrial furnaces,heating installations and agricultural activities.Although levels of emitted NOx have fallen sharply,roadside measurements indicate that concentrationsof nitrogen dioxide (NO2) appear to be stabilizingclose to the future limit levels in Europe or even tobe rising (Carslaw, 2005; Carslaw and Beevers,2004a, b). Besides its toxicity to humans, NO2 also

.

ARTICLE IN PRESSR. Alvarez et al. / Atmospheric Environment 42 (2008) 4699–47074700

impacts on atmospheric ozone-forming chemistry.Higher source ratios of NO2 to NO direct theatmospheric ozone chemistry to equilibria withhigher ozone levels (Zielinska, 2005).

Formerly, it was assumed that NOx emitted byvehicles typically consist of 95% nitrogen oxide(NO) and 5% NO2 (Soltic and Weilenmann, 2003).In the last few years, however, the increasing ratiosof NO2 to NOx monitored near roadsides indicatethat the proportion of NO2 emitted by traffic haschanged (Hueglin et al., 2006). This rise in the ratioof NO2 to NOx parallels the implementation ofoxidation catalytic converters in diesel vehicles.These converters oxidize carbon monoxide andhydrocarbons originating from imperfect combus-tion in the engine, but may also convert NO to NO2

in certain temperature conditions. In addition, moreand more diesel cars are equipped with originalequipment manufacturer (OEM) particle filter (PF)systems that employ NO2 to oxidize trapped soot atlower temperatures. This NO2 is intentionallygenerated from engine-out NO in the catalyticconverter preceding the trap, but not controlled.Excess NO2 may thus escape from the system astailpipe emissions. Consequently, precise measure-ment of vehicle emissions of NO and NO2 hasbecome necessary.

Measuring NO and NO2 accurately in emissionlaboratories is not, however, a common exercise(Gense et al., 2006; Weilenmann et al., 2005). Thestandard procedure for vehicle emission measure-ments, where a sample of diluted exhaust gas isstored in bags during the test run and subsequentlyanalyzed, is inappropriate. The bag’s surface areaallows NO to oxidize within minutes to NO2 atroom temperature and thus prevents accuratemeasurements. Consequently, measurements needto be executed online. A small sample of exhaust gasis analyzed immediately during the test run,typically at 1 or even 10Hz, and the resulting signalpatterns are integrated over time to gain meaningful

Table 1

Main characteristics of the single car samples

Sample Size Type Category Mass (k

G4 17 Gasoline Euro 4 1334

D3 5 Diesel Euro 3 1604

D4 9 Diesel Euro 4 1514

D4 PF 6 Diesel Euro 4 1584

Physical specifications are average values. Displ.: displacement. Details

values. The sample lines have to be heated becauseNO2 is hydrophilic and may be dissolved in thecondensed steam of the exhaust. In this regard,exhaust gas dryers positioned upstream of theanalyzer represent another possible source of error.Also no standard methods exist for the analysis ofNO2. In regulated NOx emission measurements onlyNO is measured directly using chemiluminescencedetection (CLD). Converters placed in front of theCLD convert NO2 to NO in order to permit itsdetection. Thus, NO2 can be estimated by taking thedifference of the measured signal traces from twocomparable CLD devices, one of which omits theconversion of NO2. However, this is only an indirectdetermination of NO2 and needs to be validated.

In this paper, emission values of NO and NO2 ofcurrent car fleets are presented for real-worlddriving situations such as urban, rural and motor-way driving, for cold start and for statutory tests.The vehicles are grouped according to theircombustion principle and certification category:gasoline Euro 4 (G4), diesel Euro 3 (D3) and Euro4 (D4). Diesel cars with OEM PFs are discussed as aseparate group (D4 PF), since NO2 is intentionallyformed in the precatalytic converter of some ofthese systems to oxidize particles in the trap. NO2

has been measured according to the techniquementioned above and cross-checked with accuratechemical ionization mass spectrometry (CI-MS)running in parallel.

2. Methodology

2.1. Vehicle samples

Three samples of passenger cars for differentcombustion principles and certification categorieshave been composed for the experimental investiga-tion, see Table 1. The diesel cars of certificationcategory Euro 4 that are equipped with an OEM PF

g) Displ. (cm3) Power (kW) Mileage (km)

2039 102 54519

2175 96 57014

1893 86 64146

2236 112 46988

of each car are continued in Table S1.

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are grouped separately. The car samples have beenselected in order to match the Swiss fleet distribu-tions regarding engine size, weight, manufacturerand chassis type. All the vehicles included wereowned by private customers and had an averagemileage of around 60,000 km (see Table S1 fordetailed description). The cars had not beenspecially maintained before testing.

2.2. Experimental program

For each car, a test series was conducted on achassis dynamometer test bench using appropriateroller settings for the driving resistance of each car.A vehicle payload of 100 kg was applied. The testenvironment was controlled to 23 1C ambienttemperature and 50% air humidity. Other settingsand procedures were applied according to theEuropean Council Directive 70/220/EEC for pas-senger cars. The test series involved measurementswith adequate driving cycles to simulate differentdriving situations: the warm-started CADC cycleincludes representative urban, rural and motorwaydriving patterns derived within the Europeanresearch program ARTEMIS (Andre, 2004). TheIUFC15 cycle (Andre et al., 1999) is the mostsuitable for investigating the cold-start effect onvehicle emissions, as it consists of 15 repetitions of areal-world urban driving pattern equally dividedinto three sections that allows engine warm-up tobe distinguished from the hot stabilized phase.The cold-started statutory cycle NEDC wasalso included, consisting of both the syntheticurban cycle section ECE and the extra-urban cyclesection EUDC.

2.3. Sampling and analyzing

Accurate measurement of NO2 involves bothadequate sampling and analyzing methods. Reg-ulatory procedures for vehicle emission measure-ments stipulate that the entire vehicle exhaust issufficiently diluted with ambient air in order toprevent condensation of steam contained in theexhaust when cooling down to ambient tempera-tures. A sample of the diluted exhaust is thencollected in a tedlar bag during the single-cyclesections and analyzed after the test run is finished.It has been shown that NO oxidizes to NO2 inthese bag conditions before the sample is ana-lyzed (Gense et al., 2006; Weilenmann et al., 2005),which eliminates this procedure for the desired

investigation. Online measuring of the undilutedexhaust is considered as a way out. There, theexhaust sample needs to be kept above thecondensation temperature (typically 190 1C) or tobe dehumidified before analysis as NO2 is hydro-philic. When dehumidifying, the loss of water mustbe taken into account for the total exhaust volumeflow calculation needed to process concentrations toabsolute values. Loss of NO2 dissolved in liquidwater also has to be avoided, and dry dehumidifica-tion thus has to be performed, i.e. without havingliquid water in contact with the exhaust gas.

Various options are available to analyze NO2.CLD are the standard systems used to measure NO.Regulated NOx emissions are collected when thedetector is preceded by a catalytic converter unitthat converts NO2 to NO. NO2 concentrations cantherefore be determined by taking the differencebetween two CLD signal traces, one of which omitsNO2 conversion. But ammonia in the exhaust, asappears for vehicles with selective catalytic reduc-tion (SCR) after-treatment systems or gasolineengines in rich combustion conditions, may alterthe functioning of the NO2 converter. MeasuringNO2 with CLD analyzers therefore requires valida-tion by an independent method. The CI-MSrepresents a convenient measuring method, thanksto its accuracy and lack of cross-sensitivities of NOand NO2. It is usually not employed, however,because it requires precise calibration at shortintervals and has a fairly low time resolution.Fourier transform infrared spectroscopy (FTIR)methods often reach their detection limit in testbench operation (short tubes, fast sampling) andadditionally show interference by water. Dispersiveultra-violet (DUV) analysis appears to be a promis-ing measuring method, but no operating experiencefor the desired investigation is known to the authorsat present.

The experimental setup created for the presentexperimental campaign is depicted in Fig. 1. Heatedraw exhaust online sampling has been implementedand NO2 is detected with the difference methodof two CLD signal traces presented above using aCLD analyzer with a catalytic NO2 converter(CLA-750A, HORIBA). Dry dehumidification ofthe CLD probe sample is executed before analyzing.CI-MS detection (Airsense 2000, V&F) of NO andNO2 is carried out in parallel for validationpurposes. Particle mass emissions (PMm) are quan-tified according to the statutory measurement pro-cedure (European Council Directive 70/220/EEC)

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Dilution TunnelBlower

Dilution Air Inlet

ChassisDynamometer

Heated Conduit

HFM

Sample Line 2

PMm

CVS-System

UF

c(NO)

c(NO+NO2)

Sample Line 1

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PMc

Fig. 1. Schematic of the experimental setup, including sampling points.

R. Alvarez et al. / Atmospheric Environment 42 (2008) 4699–47074702

and particle number emissions (PMc) are detectedwith a condensation particle counter (CPC, Model3022A, TSI; lower detectable limit: 50% of 7 nmparticles) preceded by an evaporation tube. Thissampling setup has been created according to thedraft version (ECE/TRANS/WP.29/GRPE/2007-05) of the Particle Measurement Program (PMP).An ultrasonic flow (UF) measuring device isemployed to determine the raw exhaust volumeflow for the calculation of absolute emission values.The exhaust is then diluted with a constant volumesampling (CVS) system in order to quantifyregulated particle mass emissions. The amount ofthis mixing air is measured with a hot-film air-massflow meter (HFM). All components of the experi-mental setup are periodically revised to ensureproper functioning and the measuring devices arecalibrated before each test run if possible.

2.4. Data processing

Regulated NOx emission calculations in gramsper kilometer are based on the molar mass of NO2.Thus, comparisons of NO to NO2 or their ratios toNOx need to be made on a concentration basis or bypreviously carrying out a mass correction. The truevalues of NOx in grams per kilometer resulting fromthe respective sum of calculated NO and NO2 massemissions are in any case lower than the values forregulated NOx. The concentration profiles of NOand NO2 obtained in the present experimentalcampaign are computed to absolute values in gramsper kilometer using the measured total volume flowof the exhaust, see Fig. 1, and the molar masses ofthe single substances. The volume flow is corrected

to take account of losses caused by sampling. Thesignal traces recorded are corrected in a post-process regarding time and mixing delay of theanalysis setup (Ajtay, 2006).

3. Results

3.1. Validation of measuring method

The measuring method employed for NO2 bytaking the difference between two CLD signal tracesthat continuously analyze a sample extracted fromthe exhaust generally provides reliable data accord-ing to the respective CI-MS measurements, seeFig. 2(a). Note that NO2 is slightly overestimated insome CI-MS measurements. There, the rather slowtime resolution of this measuring device (around0.7Hz) results in coarse reproduction of peakemissions of NO2 that leads to exceeded integratedvalues. Therefore, the faster CLD measuringmethod (10Hz) has been preferred for the subse-quent analysis. But the experimental campaign alsoreveals its performance limit. The noise of the twoCLD detectors is dominant for concentrations ofNO2 below 20 ppm, see Fig. 2(b). Around 45% ofthe measurements with gasoline cars were affectedby this circumstance, as their NO2 emissions onlyconsist of sporadic, very low peak events, incontrast to diesel cars. In accordance with therespective CI-MS measuring data giving massfractions of NO2 within NOx of less than 1%,NO2 emissions have been set to zero in these cases.

Note that nitrous acid (HONO), formed byheterogeneous reaction of NO2 and steam onthe particle’s surface, may also appear in vehicle

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emis

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Fig. 3. Averages and standard deviations of NO and NO2

emissions of the different vehicle samples in the statutory cold-

start cycle NEDC.

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Fig. 2. (a) CLD vs. CI-MS measurements of NO and NO2 (dashed lines represent 20% deviation from equity). (b) Course of NO2

emissions of a gasoline vehicle in a NEDC cycle recorded with CLD and CI-MS analyzers.

R. Alvarez et al. / Atmospheric Environment 42 (2008) 4699–4707 4703

exhaust and is detected by CLD analyzers. Itsprimary emission can thus not be explicitly quanti-fied from the given data set. But its proportionwithin total NOx is assumed to be minor as NO2 isnot overestimated by CLD measurements in com-parison to CI-MS measurements, see Fig. 2(a). Thisassumption complies with the results of otherexperimental investigations on primary vehicleemissions of HONO (Gutzwiller et al., 2002;Kurtenbach et al., 2001).

3.2. Emission performance

Figs. 3–5 show the main results of the experi-mental investigation, where the averages andstandard deviations of absolute NO and NO2

emissions recorded with CLD measurements aredisplayed for each vehicle sample and drivingcycle. The sometimes rather high standard devia-tions are to be attributed to the selection ofthe car samples, which were to be representative ofthe Swiss car fleet and thus feature a wide rangeof makes and mechanizations. Table 2 summarizesthe true emissions of NOx calculated from the sumof NO and NO2 emissions, in the following referredto as NOx, together with their respective massratios.

The emission performance of the vehicle samplesin the statutory cycle NEDC, see Fig. 3, shows thatNOx emissions of the gasoline car sample G4 are

very low and almost entirely consist of NO.Regarding diesel cars, the discharge of NOx isreduced by a factor of about two from vehiclesample D3 to D4, keeping the proportion of NO2 inNOx at around 25%. However, the vehiclesequipped with particle filter (D4 PF) have clearlymore pronounced emissions of NOx compared tothe sample D4 of the same certification class. Thisrise is caused by an increased discharge of NO2,which also leads to higher ratios of NO2 in the totalNOx of up to 60%. The mean emission level of thesum of NO and NO2 obtained for D4 PF surpassesthe respective statutory emission limit by about 4%,

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IUFC15 cold IUFC15 warm IUFC15 hot

emis

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NO NO2

Fig. 4. Averages and standard deviations of NO and NO2

emissions of the different vehicle samples in the real-world cold-

start cycle IUFC15.

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Fig. 5. Averages and standard deviations of NO and NO2

emissions of the different vehicle samples in the real-world warm-

start cycle CADC.

R. Alvarez et al. / Atmospheric Environment 42 (2008) 4699–47074704

and NOx emissions calculated by the statutorymethod even exceed its limit by almost 32%.Besides, a considerable shift to higher ratios ofNO2 to NOx is visible from the cold-started firstsection ECE to the subsequent section EUDC.This observation indicates that the formation ofNO2 is enhanced when the light-off temperatureof the installed oxidation catalytic converter isreached.

The same behavior can be established from theexperimental results obtained with the cycleIUFC15, see Fig. 4, because the characteristics ofthis cycle clearly show the influence of the warmingup of the engine on vehicle emissions. The ratioof NO2 to NOx of the diesel samples rises byabout 40–60% from the first to the second cyclesection. In addition, NOx emissions of the gasolinesample are reduced when the light-off temperature

of the three-way catalytic converter is reached.The higher absolute emission levels with regard tothe emission performance in the statutory cyclerepresent a notable difference, which is, however,usually achieved in real-world cycles (Soltic et al.,2004).

This finding also reflects on the emission perfor-mance in the CADC cycle, see Fig. 5. There,however, no shift to higher shares of NO2 is visible,as it features a warm engine start. But the increasingengine load over the single-cycle sections showsanother effect on emissions of NOx. A broad rise isshown from rural to motorway driving. Highercombustion temperatures at these operating condi-tions are thought to be responsible. Additionally,the ratio of NO2 to NOx drops slightly for all dieselcar samples, which indicates that the conversion ofNO to NO2 in the oxidation catalytic convertersbecomes somewhat less efficient at higher exhaustflows. Note that the rather high discharge of NOx inurban driving conditions is to be attributed to thevarious stop times within this cycle section, whichaffect emissions values gained per unit of grams perkilometer.

Regarding particle mass emissions of the vehiclesamples, a slight increase is visible from sample D3to D4 in all sections of the selected driving cycles,see Fig. 6. Efforts in lowering NOx emissions mayhave led to this rise. However, the mean sampleemission level for D4 exceeds the particle massemission limit by almost 22%, with only 2 out of 10cars meeting the limit value. It appears that onlythe adoption of PF systems allows a substantialreduction in particle emissions in order to satisfy thestatutory requirements. In fact, particle mass emis-sions of D4 PF are the lowest for the diesel carsamples and at about the same level as the gasolinevehicle sample G4.

The measured discharge of particle numberemissions confirms these findings, see Fig. 7.The emission levels of D3 and D4 are about thesame, whereas they typically fall about two ordersof magnitude for G4 and even around threeorders of magnitude for D4 PF. Interestingly,both the particle number emissions of sample G4and D4 PF drop from the first to the secondsection of the cycle IUFC15. A cold-start effecton particle number emissions can thus bestated, which is to be attributed to the initialinsufficient fuel-mixture generation and the appear-ance of more condensation particles in the coldexhaust gas.

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Fig. 6. Averages and standard deviations of particle mass emissions of the different car samples in the selected driving cycles.

1.E+09

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IUFC15 hot

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Fig. 7. Averages and standard deviations of particle number emissions of the different car samples in the selected driving cycles.

Table 2

Absolute emissions of NOx (NO+NO2) and mass ratios of NO and NO2 for each vehicle sample and driving cycle

NOx (g km�1) NO (%) NO2 (%) NOx (g km�1) NO (%) NO2 (%) NOx (g km�1) NO (%) NO2 (%)

ECE EUDC NEDC

G4 0.057 99.2 0.8 0.015 499 o1 0.031 99.8 0.2

D3 0.389 87.3 12.7 0.310 70.1 29.9 0.339 77.4 22.6

D4 0.199 81.9 18.1 0.145 57.3 42.7 0.165 68.3 31.7

D4 PF 0.312 70.1 29.9 0.229 35.6 64.4 0.260 51.0 49.0

IUFC15 cold IUFC15 warm IUFC15 hot

G4 0.108 499 o1 0.026 97.8 2.2 0.034 99.4 0.6

D3 0.681 77.8 22.2 0.688 64.2 35.8 0.730 61.1 38.9

D4 0.438 63.4 36.6 0.466 47.8 52.2 0.504 46.0 54.0

D4 PF 0.566 57.4 42.6 0.666 35.8 64.2 0.797 37.4 62.6

CADC urban CADC rural CADC mway

G4 0.054 499 o1 0.034 499 o1 0.022 499 o1

D3 0.787 62.9 37.1 0.419 63.4 36.6 0.766 65.6 34.4

D4 0.468 49.4 50.6 0.270 53.1 46.9 0.596 56.7 43.3

D4 PF 0.785 32.9 67.1 0.432 37.5 62.5 0.716 46.8 53.2

R. Alvarez et al. / Atmospheric Environment 42 (2008) 4699–4707 4705

4. Summary and conclusions

The present experimental investigation demon-strates the increase in the primary NO2 emissions of

modern light diesel cars. Improvements in absoluteemissions of NOx are observed from vehicle sampleD3 to D4; however, with already notable ratiosof NO2 of 15–50%. But the vehicles equipped with

ARTICLE IN PRESSR. Alvarez et al. / Atmospheric Environment 42 (2008) 4699–47074706

a PF show again higher emissions of NOx thatexceed the statutory emission level and are mainlycomposed of NO2, reaching maximum proportionsof NO2 of 35–70%. NOx emission levels of thediesel samples are mostly more pronounced inreal-world driving conditions, especially in urbanoperation, than in the statutory cycle. In contrast,the gasoline car sample G4 emits by far thelowest amounts of NOx, which almost entirelyconsist of NO.

The formation of NO2 detected in diesel cars isclearly attributable to the use of oxidation catalyticconverters, as the proportion of emitted NO2 inNOx increases when the light-off temperature of thementioned converters is reached. The increase in theamount of emitted NO2 from the sample D4 to D4PF results from its excessive formation to supportthe oxidation of trapped soot in the PF, whichoccurs intentionally, but not in a controlled manner.In fact, the emission level for NOx recorded withinthe experimental campaign surpasses the respectivelimit value. However, particle emission data deter-mined with both the gravimetric and countingdetection method unequivocally demonstrates thatOEM PFs represent an indispensable component indiesel exhaust after-treatment systems in order tomeet the required emission performance for thispollutant.

Considerable environmental concerns arise whenaccount is taken of the ascertained emissionbehavior of such vehicles regarding the dischargeof NOx. The increase in source emissions of NO2

arguably affects air quality near roadsides, espe-cially in urban areas. Both the toxicity and theozone-forming potential of NO2 may consequentlycause substantial damage to human health. Inaddition, model calculations predict that futureambient concentration limits for NO2 are likely tobe exceeded for higher vehicle emission ratios ofNO2 to NOx (Carslaw et al., 2007). These findingsimply that action should be taken to reduce theNOx and NO2 emissions. A first step would beto design oxidation catalytic converters that convertNO to NO2 only in temperature ranges whereNO2-supported soot oxidation in the PF can occur.Exhaust after-treatment of NOx based on selectivecatalytic reduction (SCR) or NOx storage reduction(NSR) represent further feasible approaches forlean-burn diesel passenger cars (Jobson, 2004;Klingstedt et al., 2006) and first implementationsare entering the market (Breitbach et al., 2007,Tsuzuki et al., 2003).

In any case, the evolution of NO2 emissions forfuture vehicle classes should be observed. Themeasuring method applied in the present experi-mental campaign, where a sample of raw exhaust isanalyzed online during the test run, providesreliable data. However, the analyzing methodemployed of taking the difference of two CLDsignal traces, where one analyzer is preceded by acatalytic NO2 converter to include its detection, isnot adequate for very low concentrations of NO2 inthe exhaust. Photolytic NO2 converters generallyshow better conversion efficiency, but are typicallyslower and thus not adequate for online analysis ofvehicle exhaust. A direct analyzing method for NO2

should therefore be considered, where dispersiveultra-violet (DUV) detection seems to be the mostpromising approach.

Acknowledgment

The authors thank the Swiss Federal Office forthe Environment (FOEN) for principally fundingthe study.

Appendix A. Supplementary materials

Supplementary data associated with this articlecan be found in the online version at doi:10.1016/j.atmosenv.2008.01.046

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