Nitrous oxide emissions from light duty vehicles

Atmospheric Environment 43 (2009) 2031–2044

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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Nitrous oxide emissions from light duty vehicles Lisa A. Graham a,1, *, Sheri L. Belisle a, Paul Rieger b a b

Environment Canada, Emissions Research and Measurement Division, 335 River Road, Ottawa, Ontario, Canada K1A 0H3 California Air Resources Board, Monitoring & Laboratory Division, 9528 Telstar Ave, El Monte, CA 91731, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 October 2008 Received in revised form 31 December 2008 Accepted 1 January 2009

Nitrous oxide (N2O) emissions measurements were made on light duty gasoline and light duty diesel vehicles during chassis dynamometer testing conducted at the Environment Canada and California Air Resources Board vehicle emissions laboratories between 2001 and 2007. Per phase and composite FTP emission rates were measured. A subset of vehicles was also tested using other driving cycles to characterize emissions as a function of different driving conditions. Vehicles were both new (<6500 km) and in-use (6500–160,000 km) and were tested on low sulfur gasoline (<30 ppm) or low sulfur diesel (<300 ppm). Measurements from selected published studies were combined with these new measurements to give a test fleet of 467 vehicles meeting both US EPA and California criteria pollutant emissions standards between Tier 0 and Tier 2 Bin 3 or SULEV. Aggregate distance-based and fuel-based emission factors for N2O are reported for each emission standard and for each of the different test cycles. Results show that the distinction between light duty automobile and light duty truck is not significant for any of the emission standards represented by the test fleet and the distinction between new and aged catalyst is significant for vehicles meeting all emission standards but Tier 2. This is likely due to the relatively low mileage accumulated by the Tier 2 vehicles in this study as compared to the durability requirement of the standard. The FTP composite N2O emission factors for gasoline vehicles meeting emission standards more stringent than Tier 1 are substantially lower than those currently used by both Canada and the US for the 2005 inventories. N2O emission factors from test cycles other than the FTP illustrate the variability of emission factors as a function of driving conditions. N2O emission factors are shown to strongly correlate with NMHC/NMOG emission standards and less strongly with NOX and CO emission standards. A review of several published reports on the effect of gasoline sulfur content on N2O emissions suggests that additional research is needed to adequately quantify the increase in N2O emissions as a function of fuel sulfur. Ó 2009 Published by Elsevier Ltd.

Keywords: Nitrous oxide emissions Light duty vehicles Gasoline Diesel

1. Introduction Emissions from transportation sources are an important contributor to national greenhouse gas (GHG) inventories accounting for 27% of both the Canadian 2005 national inventory (Environment Canada, 2007) and the US 2005 national inventory (US EPA, 2007). While CO2 emissions dominate the transportation contribution to the total GHG inventories, transportation is the second largest source (after agriculture) of N2O emissions in both countries, accounting for 18% of N2O emissions in Canada and 8% of N2O emissions in the US. On-road gasoline fueled vehicles account for 88% of the N2O emissions attributed to road transportation sources in both countries.

* Corresponding author. Tel.: þ64 021 0226 2133. E-mail address: [email protected] (L.A. Graham). 1 Present address: Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. 1352-2310/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.atmosenv.2009.01.002

Nitrous oxide emission inventories are more difficult to develop than CO2 inventories because N2O emissions depend more on driving cycle, fuel properties and catalyst technology than on fuel consumption. In contrast, CO2 emissions are directly related to fuel consumption. Transportation emissions in the Canadian national GHG inventory are estimated using fuel usage as the measure of activity and fuel-based emission factors for 33 different vehicle/fuel categories. The CO2 and N2O emission factors used in the 2005 inventory for light duty vehicles are summarized in Table 1. The U.S. inventory applies distance-based emission factors by vehicle size, fuel type, and control technology to estimates of annual vehicle miles traveled (VMT) for these vehicle categories. A recent update of methane and nitrous oxide emission factors (US EPA, 2004) for on-highway vehicles recommends the N2O emission factors summarized in Table 1. The emission factors used in the two inventories are for the most part similar. The major differences are with the Tier 0 gasoline vehicles and with the diesel vehicles. The Canadian Tier 0 emission

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L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044

Table 1 Comparison of N2O emission factors used in the 2005 Canadian and US GHG Inventories.

Gasoline

Diesel

2005 Canadian Inventory

2005 US Inventory

g L1 fuel

From EPA 420-P-04– 016 Nov 2004 Table 28

From EPA 420-P-04–016 Nov 2004 Table 15 CO2 emission rate (g mile1)

Fuel Consumption (L mile1)

g mile1 (calc)a

g mile1

mg km1

Autos

LEV Tier 1 Tier 0 Aged Oxidation catalyst Non-catalyst

– 0.16 0.66 0.20 0.028

– 459 480 616 855

– 0.203 0.212 0.272 0.378

– 0.032 0.140 0.054 0.011

0.012 0.030 0.054 0.042 0.017

7.47 18.7 33.6 26.1 10.6

Trucks

LEV Tier 1 Tier 0 Aged Oxidation catalyst Non-catalyst

– 0.25 0.66 0.20 0.028

– 637 801 801 967

– 0.282 0.354 0.354 0.427

– 0.070 0.234 0.071 0.012

0.009 0.067 0.090 0.054 0.019

5.60 41.7 56.0 33.6 11.8

Autos

Advanced Control Moderate Control Uncontrolled

0.22 0.21 0.16

381 399 513

0.138 0.145 0.186

0.030 0.030 0.030

0.001 0.001 0.001

0.62 0.62 0.62

Trucks

Advanced Control Moderate Control Uncontrolled

0.22 0.21 0.16

531 533 668

0.193 0.193 0.242

0.042 0.041 0.039

0.002 0.002 0.002

1.24 1.24 1.24

Gasoline density 737 g L1, diesel density 864 g L1, Gasoline FFC 0.838, Diesel FFC 0.871. a Calculated from g L1 fuel emission factors assuming the fuel consumption rates given in the table and typical fuel density and fuel fraction carbon values.

factors are higher as they represent vehicles aged and operated on high sulfur gasoline (>300 ppm) typically found in Canada prior to 2002. The US Tier 0 emission factors appear to be based on test results from newer vehicles operating on low sulfur gasoline. The Canadian N2O emission factors for light duty diesel vehicles came from Lipman and Delucchi (2002). Lipman and Delucchi commented that there was a lack of N2O data for light duty and heavy duty diesel vehicles, so they decided it was reasonable to assume that light duty diesel engines emit roughly the same amount of N2O as do gasoline engines of a similar size and emission control technologies (which seems counterintuitive since diesel vehicles aren’t equipped with three-way catalysts as found on gasoline vehicles). The midpoint of the proposed range was used for all three technologies. The Lipman and Delucchi emission factors (distancebased) were converted to a fuel basis using the default fuel consumption rates given in the 1996 IPCC Guidelines. The US N2O emission factors for light duty diesel vehicles were estimated from heavy duty diesel vehicle testing by scaling the measured emission factors with the CO2 emission rate for light duty diesel vehicles (US EPA, 2004). While N2O is not routinely measured as a requirement of vehicle emission certification or compliance surveillance, there have been several studies recently reported in the literature (e.g. Durbin et al., 2003; Huai et al., 2003, 2004; Karlson, 2004; Behrentz et al., 2004; and Winer and Behrentz, 2005) where N2O was measured that add to the body of data reported in the US EPA review (US EPA, 2004). It is well known that N2O emissions from light duty gasoline vehicles are very dependent on the type of pollution control technology and the age of this technology. Emissions also vary with the vehicle in question and on operating conditions such as fuel sulfur level, driving cycle, ambient temperature and catalyst operating temperature (Jobson et al., 1994; Laurikko and Aakko, 1995; Odaka et al., 1998, 2000, 2002; Koike et al., 1999; Baronick et al., 2000; Meffert et al., 2000). Nitrous oxide forms under certain conditions over the three-way catalysts present on all vehicles certified to Tier 0 and more stringent emission standards. Thorough reviews of the formation mechanisms are given in other sources (e.g. Lipman and Delucchi, 2002; Huai et al., 2003) and will not be further discussed. Vehicle technology and fuels, especially for light duty vehicles, have changed dramatically since Tier 1 emission standards were

first implemented in 1994. In 1997, the US EPA finalized the regulations for the National Low Emission Vehicle (NLEV) program. This was a voluntary program that came into effect starting in the northeastern states in model year 1999 and nationally in model year 2001. The emission standards were based on those in effect in California at the same time. With the NLEV program agreement these standards were enforceable in the same manner as any other federal new motor vehicle program. In 2004, Tier 2 standards were implemented for the US and Canada while California implemented LEV II standards. Canadian emission standards have been harmonized with the US EPA standards since 1997. Table 2 compares the different light duty gasoline vehicle criteria pollutant emission standards that were in force in different jurisdictions from Tier 0 to present, as applicable to the vehicles included in the fleet considered by this study. As can be seen, the numerical emission rate standards are identical for a given emission standard for Canada, the US and California, except for Tier 0. There are, however differences in implementation timing and

Table 2 Comparison of emission standards (g km1). Jurisdiction

Emission Standard

Vehicle Type

NMHC

US/CDN California US/CA/CDN CA CA US/CA/CDN US/CA/CDN CA CA US/CA/CDN CA CA CA US/CDN US/CDN US/CDN US/CDN US/CDN US/CDN US/CDN

Tier 0 Tier 0 Tier 1 Tier 1 Tier 1 TLEV LEV LEV LEV ULEV ULEV ULEV SULEV T2.B10 T2.B9 T2.B8 T2.B6 T2.B5 T2.B4 T2.B3

LDV LDV LDV MDV MDV LDV LDV MDV MDV LDV MDV MDV MDV LDV LDV LDV LDV LDV LDV LDV

0.21 0.24 0.16 1 2

1 2 1 2 2

NMOG

CO

NOx

0.156 0.199 0.078 0.047 0.078 0.100 0.025 0.047 0.062 0.031 0.078 0.047 0.062 0.047 0.047 0.044 0.034

2.1 4.4 2.1 2.1 2.7 2.1 2.1 2.1 2.7 1.1 1.1 2.7 1.4 2.1 2.1 2.1 2.1 2.1 1.3 1.3

0.62 0.25 0.25 0.25 0.44 0.25 0.12 0.25 0.25 0.12 0.12 0.25 0.12 0.25 0.12 0.09 0.05 0.03 0.02 0.02

L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044 Table 3 Test fuel characteristics.

Specific Gravity (kg m3) Sulfur (mg L1) Research Octane No. RVP (psi)

California Phase 2 RFG

California Summer

California Winter

743 30 97 5.7

740 20 N/A 6.8

740 13.3 N/A 10.5

possibly in on-board diagnostics (OBD) requirements among the jurisdictions, but the numerical standards are the same. This means, for example, that a vehicle meeting the Tier 1 LDV standard will be required to meet the same numerical emission rate standards regardless of the jurisdiction in which it is sold. It is the catalyst performance with respect to NOX and NMHC or NMOG which is thought to be most influential to N2O emissions. In order to meet the most stringent NMOG emissions, cold start emissions must be reduced which is achieved by reducing the time it takes for the catalyst to reach light-off. It is during this period of catalyst operation, when the catalyst has warmed up sufficiently to start NMOG and NOX conversion but has not yet reached full operating temperature, when the highest production rate of N2O occurs. At the same time, regulated fuel specifications were changing to permit the proper functioning of these advanced emission control systems. The largest change in fuel specifications was the lowering of sulfur content to less than 30 ppm from levels upwards of 300 ppm. These fuel specification changes occurred in both Canada and the US on approximately the same timeline. Starting in January 2005, low sulfur gasoline (<30 ppm) was required throughout Canada. As an interim step, gasoline with an average sulfur level of not more than 150 ppm was required starting in 2002. The change in fuel sulfur occurred much earlier in California with the adoption of Phase 2 gasoline regulations in March 1996 when the fuel sulfur content decreased from approximately 300 ppm to a cap limit of 80 ppm, however the typical sulfur level seen by inspectors was around 30 ppm. Given these major changes in both vehicle technologies and fuel specifications, N2O emission factors need to be updated. This report brings together data from a number of published and unpublished sources, including results of recent testing conducted at Environment Canada’s Emissions Research and Measurement Division and the California Air Resources Board

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Haagen-Smit Laboratory, to develop emission factors for light duty gasoline vehicles meeting current EPA Tier 2 emission standards and current California emission standards. The combined studies increase the test fleet size to 467 vehicles. The vehicle fleet spans the range emission standards from Tier 0 to EPA Tier 2 Bin 3 and California SULEV. This report also endeavors to bring together more recent emissions measurements from a variety of sources to update the fuel sulfur effect discussion.

2. Experimental methodology 2.1. Testing in Canada Two hundred light duty gasoline and diesel vehicles were tested at Environment Canada Emissions Research and Measurement Division between April 2004 and June 2007. The majority of the vehicles were new with less than 6500 km accumulated prior to testing. This accumulation was done on low sulfur gasoline (<30 ppm). Seventy-two of the vehicles had accumulated up to 40,000 km in on-road driving on low sulfur fuel. The vehicles were tested on California Phase 2 RFG or certification diesel fuel over the Federal Test Procedure. Some of the vehicles were tested over other driving cycles (e.g. US06, HWFET, NYCC). CO2, CO and THC concentrations in the dilute exhaust and dilution air were determined with nondispersive IR analyzers (CO2 and CO) and a flame ionization detector (THC). NOX emissions were determined with a chemiluminescence analyzer. Fuel consumption (FC) was calculated following the standard carbon balance procedures. All tests were conducted following standard chassis dynamometer test procedures as required by the Canadian Environmental Protection Act (CEPA) 1999, Part 7, Division 5. These procedures and requirements are identical to those found in the US EPA Code of Federal Regulations (40 CFR 86). Tests were conducted at standard temperature (20  C). Typical test fuel properties can be found in Table 3. Each vehicle was tested at least twice and the average emission rates were entered into the database. Separate samples of dilute exhaust and dilution air for determining N2O were collected over each test. Nitrous oxide was determined by gas chromatography with electron capture detection as described in Graham et al. (2008). The limit of detection for the N2O analysis is 4 ppb in the dilute exhaust, which corresponds to an emission rate of approximately 0.5 mg km1.

Table 4 Summary of published studies incorporated into the current study. Title

Reference

Nitrous oxide emission factors for mobile sources

Michaels et al. (1998) Becker et al. (1999) Durbin et al. (2003) Huai et al. (2004)

Nitrous oxide (N2O) emissions from vehicles The effect of fuel sulfur on NH3 and other emissions from 2000 to 2001 model year vehicles Estimates of the emission rates of nitrous oxide from light duty vehicles using different chassis dynamometer test cycles Analysis of nitrous oxide and ammonia emissions from motor vehicles Measurements of nitrous oxide emissions from light duty motor vehicles: a pilot study Estimates of nitrous oxide emissions from motor vehicles and the effects of catalyst composition and aging

Huai et al. (2003) Behrentz et al. (2004) Winer and Behrentz (2005)

No. of N2O Analysis Method Vehicles 23

Nicolet Magna-IR 560.Samples collected in Tedlar bags for analysis.

22

Mattson Nova-Cygni 120 operated at 0.25 cm1 with a 21.75 m cell.Diluted exhaust scanned in real-time at 3-s intervals.Tunnel studies in Germany utilized GC/ECD. Pierburg AMA/Mattson FTIR.Real-time 3-s interval measurements and collection in bags for analysis.The complete data set of 83 vehicles was provided to ERMD by the Center for Environmental Research and Technology, University of California, Riverside and includes information not appearing in the published references. Each of the three published references reports on a different subset of the complete data set.

60

102

Nicolet Magna 560 (0.5 cm1 with a 10 m path cell) Tedlar bag samples of diluted exhaust analyzed according to SOP MLD #136 (http://www.arb.ca.gov/testmeth/slb/ slb136_rev_2.pdf) The complete data set of 178 vehicles was provided to ERMD by the California Air Resources Board, Monitoring and Laboratory Division and includes additional N2O emissions data and other information not appearing in the published references. Each of the two published references reports on a different subset of the entire data set.

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L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044

Table 5 Description of vehicle fleet for the statistical analysis in this study. Rows highlighted in bold indicate aggregation of LDV and LDT vehicle types and aggregation of catalyst age. Emission Standard

Vehicle Type

Catalyst Age

ERMD

CARB

CECERT

Michaels Total et al. (1998)

Tier 0 Tier 0 Tier 0

LDT LDV

Aged Aged Aged

0 0 0

13 29 42

0 0 0

0 0 0

42

Tier 1 Tier 1 Tier 1 Tier 1

LDT LDT2 LDV

Aged Aged Aged Aged

0 1 7 8

25 0 15 40

8 0 6 14

0 0 2 2

64

TLEV TLEV TLEV

LDT LDV

Aged Aged Aged

0 0 0

10 14 24

5 3 8

0 0 0

32

TLEV TLEV TLEV NLEV NLEV LEV LEV LEV LEV

LDT1 LDV LDT LDV LDT LDT1 LDT2 LDV

New New New Aged New Aged Aged Aged Aged

1 1 2 0 1 0 2 7 12

0 0 0 0 0 19 0 0 36

0 0 0 1 0 6 0 0 8

0 0 0 0 0 2 0 0 2

LEV LEV LEV LEV LEV

Aged New New New New

21 0 1 1 10

55 0 0 0 0

14 4 0 0 4

4 0 0 0 0

94

LDT LDT1 LDT2 LDV

LEV ULEV ULEV ULEV

New Aged Aged Aged

12 0 1 7

0 2 0 7

8 1 0 17

0 0 0 0

20

LDT LDT2 LDV

ULEV ULEV SULEV SULEV Tier 2 Bin 10 Tier 2 Bin 9 Tier 2 Bin 9

LDV LDV LDV LDV LDV LDV

Aged New Aged New New Aged New

8 3 0 0 2 5 11

9 0 8 0 0 0 0

18 12 4 4 0 0 0

0 0 0 0 0 0 0

35 15 12 4 2

Tier 2 Bin 9 Tier 2 Bin 8 Tier 2 Bin 8

New Aged New

16 4 24

0 0 0

0 0 0

0 0 0

16

LDV LDV

Tier 2 Bin 8 Tier 2 Bin 6 Tier 2 Bin 6

New Aged New

28 5 2

0 0 0

0 0 0

0 0 0

28

LDV LDV

Tier 2 Bin 6 Tier 2 Bin 5 Tier 2 Bin 5

New Aged New

7 21 64

0 0 0

0 0 0

0 0 0

7

LDV LDV

Tier 2 Bin 5 Tier 2 Bin 4 Tier 2 Bin 4

New Aged New

85 1 2

0 0 0

0 0 0

0 0 0

85

LDV LDV

Tier 2 Bin 4 Tier 2 Bin 3

LDV

New New

3 4

0 0

0 0

0 0

3 4

2 1 1

2.2. Testing in California One hundred and seventy-eight light duty gasoline vehicles were tested at the California Air Resources Board Haagen-Smit Laboratory between 2001 and 2004. The vehicles tested during the study represented a subset of the much larger number of vehicles tested under ARB’s 16th and 17th Vehicle Surveillance Program in which in-use vehicles were recruited for various emissions testing requirements. This sample of vehicles was comprised of gasoline powered passenger cars and light duty trucks, defined as those with a gross vehicle weight of less than 2614 kg (5750 lb).

Table 6 Fuel-based emission factors (g L1 fuel) for gasoline fuelled vehicles. Min

1st Qu.

Median

3rd Qu.

Max

N

FTP Composite Tier0 Aged Tier1 Aged TLEV New TLEV Aged NLEV New LEV New LEV Aged ULEV New ULEV Aged SULEV New SULEV Aged T2.B10 New T2.B9 New T2.B8 New T2.B6 New T2.B5 New T2.B4 New T2.B3 New

0.026 0.025 0.031 0.065 0.009 0.009 0.007 0.005 0.005 0.006 0.000 0.014 0.009 0.008 0.011 0.003 0.006 0.005

0.124 0.070 0.031 0.100 0.009 0.022 0.044 0.010 0.021 0.007 0.007 0.018 0.019 0.014 0.012 0.011 0.007 0.009

0.242 0.135 0.032 0.154 0.009 0.033 0.072 0.013 0.033 0.007 0.009 0.023 0.028 0.021 0.014 0.015 0.008 0.015

0.435 0.286 0.032 0.224 0.009 0.050 0.150 0.026 0.044 0.008 0.015 0.027 0.036 0.030 0.016 0.021 0.008 0.023

1.454 0.922 0.032 0.638 0.009 0.122 0.665 0.080 0.173 0.013 0.017 0.032 0.060 0.061 0.024 0.438 0.008 0.030

42 48 2 24 1 20 83 15 29 4 11 2 16 28 7 85 3 4

FTP Phase Tier0 Tier1 TLEV TLEV NLEV LEV LEV ULEV ULEV SULEV SULEV T2.B10 T2.B9 T2.B8 T2.B6 T2.B5 T2.B4 T2.B3

1 Aged Aged New Aged New New Aged New Aged New Aged New New New New New New New

0.031 0.048 0.075 0.097 0.030 0.012 0.026 0.010 0.012 0.024 0.012 0.000 0.017 0.011 0.032 0.000 0.014 0.006

0.158 0.126 0.083 0.220 0.030 0.065 0.127 0.023 0.038 0.024 0.018 0.018 0.077 0.040 0.036 0.022 0.015 0.009

0.271 0.241 0.090 0.319 0.030 0.100 0.177 0.038 0.086 0.027 0.026 0.036 0.091 0.060 0.040 0.036 0.016 0.009

0.432 0.315 0.098 0.407 0.030 0.143 0.269 0.096 0.137 0.032 0.050 0.054 0.103 0.076 0.044 0.054 0.017 0.026

0.736 1.028 0.106 0.524 0.030 0.275 0.544 0.184 0.404 0.037 0.062 0.072 0.198 0.181 0.048 2.039 0.017 0.074

42 48 2 24 1 20 83 15 29 4 11 2 16 28 7 85 3 4

FTP Phase Tier0 Tier1 TLEV TLEV NLEV LEV LEV ULEV ULEV SULEV SULEV T2.B10 T2.B9 T2.B8 T2.B6 T2.B5 T2.B4 T2.B3

2 Aged Aged New Aged New New Aged New Aged New Aged New New New New New New New

0.008 0.002 0.003 0.002 0.003 0.000 0.000 0.000 0.000 0.000 0.000 0.004 0.000 0.000 0.000 0.000 0.002 0.005

0.054 0.033 0.007 0.024 0.003 0.000 0.008 0.000 0.000 0.000 0.000 0.007 0.003 0.004 0.003 0.003 0.003 0.006

0.105 0.056 0.010 0.045 0.003 0.005 0.019 0.000 0.007 0.000 0.002 0.009 0.004 0.005 0.004 0.005 0.004 0.009

0.268 0.176 0.014 0.111 0.003 0.007 0.061 0.003 0.013 0.000 0.006 0.011 0.010 0.015 0.004 0.009 0.005 0.017

2.074 0.771 0.017 0.546 0.003 0.071 0.359 0.015 0.073 0.000 0.006 0.013 0.015 0.027 0.006 0.042 0.005 0.032

42 48 2 24 1 20 83 15 29 4 11 2 16 28 7 85 3 4

FTP Phase Tier0 Tier1 TLEV TLEV NLEV LEV LEV ULEV ULEV SULEV SULEV T2.B10 T2.B9 T2.B8 T2.B6

3 Aged Aged New Aged New New Aged New Aged New Aged New New New New

0.063 0.028 0.029 0.042 0.005 0.000 0.000 0.000 0.000 0.000 0.000 0.026 0.004 0.004 0.008

0.187 0.078 0.029 0.138 0.005 0.013 0.039 0.005 0.008 0.000 0.003 0.033 0.009 0.010 0.010

0.400 0.192 0.030 0.227 0.005 0.035 0.085 0.021 0.026 0.004 0.003 0.040 0.021 0.016 0.012

0.650 0.427 0.031 0.367 0.005 0.058 0.245 0.054 0.054 0.013 0.004 0.047 0.047 0.022 0.017

1.005 1.182 0.031 0.948 0.005 0.237 1.560 0.155 0.373 0.029 0.022 0.054 0.089 0.078 0.042

42 48 2 24 1 20 83 15 29 4 11 2 16 28 7

L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044 Table 6 (continued ) Min

1st Qu.

Median

3rd Qu.

Max

N

T2.B5 T2.B4 T2.B3

New New New

0.001 0.006 0.003

0.006 0.007 0.003

0.011 0.007 0.010

0.021 0.008 0.017

0.114 0.009 0.019

85 3 4

NYCC Tier1 LEV LEV ULEV T2.B9 T2.B8 T2.B5

Aged New Aged Aged New New New

0.050 0.127 0.155 0.039 0.009 0.006 0.010

0.050 0.127 0.172 0.049 0.035 0.007 0.018

0.050 0.127 0.189 0.083 0.062 0.009 0.027

0.050 0.127 0.206 0.140 0.147 0.010 0.031

0.050 0.127 0.223 0.219 0.231 0.012 0.036

1 1 2 4 3 2 3

US06 Tier1 LEV LEV ULEV ULEV T2.B9 T2.B8 T2.B6 T2.B5 T2.B4

Aged New Aged New Aged New New New New New

0.033 0.004 0.008 0.004 0.002 0.002 0.002 0.005 0.000 0.004

0.033 0.006 0.009 0.004 0.005 0.003 0.006 0.005 0.004 0.004

0.033 0.007 0.011 0.004 0.007 0.006 0.008 0.006 0.006 0.004

0.033 0.008 0.013 0.004 0.010 0.011 0.011 0.006 0.011 0.004

0.033 0.009 0.016 0.004 0.013 0.017 0.021 0.006 0.025 0.004

1 2 3 1 2 6 10 4 20 1

HWFET Tier1 LEV LEV ULEV T2.B9 T2.B8 T2.B5

Aged New Aged Aged New New New

0.071 0.016 0.002 0.001 0.004 0.003 0.000

0.071 0.016 0.006 0.003 0.004 0.004 0.006

0.071 0.017 0.008 0.005 0.005 0.005 0.008

0.071 0.017 0.022 0.008 0.007 0.041 0.011

0.071 0.018 0.062 0.011 0.010 0.077 0.020

1 2 4 4 5 3 11

LA92 Phase 1 Tier0 Aged Tier1 Aged TLEV Aged LEV Aged ULEV Aged

0.054 0.103 0.145 0.037 0.015

0.144 0.285 0.332 0.245 0.140

0.265 0.346 0.478 0.343 0.304

0.378 0.419 0.628 0.486 0.505

0.991 0.772 1.337 0.787 0.651

31 40 20 58 11

LA92 Phase 2 Tier0 Aged Tier1 Aged TLEV Aged LEV Aged ULEV Aged

0.024 0.013 0.007 0.001 0.002

0.075 0.051 0.061 0.018 0.009

0.118 0.062 0.075 0.035 0.021

0.201 0.130 0.115 0.072 0.024

0.465 0.702 0.345 0.266 0.055

31 40 20 58 11

LA92 Phase 3 Tier0 Aged Tier1 Aged TLEV Aged LEV Aged ULEV Aged

0.007 0.035 0.057 0.001 0.006

0.246 0.161 0.163 0.061 0.031

0.490 0.315 0.293 0.148 0.039

0.763 0.526 0.523 0.369 0.057

1.181 1.797 0.895 1.188 0.115

31 40 20 58 11

For most vehicles, testing was performed with the fuel already present when the vehicles were recruited, which was likely to be from a local gas station. For those vehicles brought in with insufficient fuel, the fuel tank was drained and commercial California Phase 2 gasoline was added, with the characteristics summarized in Table 3. High resolution (0.5 cm1) FTIR spectroscopy was used to quantify nitrous oxide concentrations of CVS diluted exhaust collected in Tedlar bags (see Table 4).

2.3. Prior studies Results for vehicles tested on low sulfur fuels that were reported in the studies listed in Table 4 were also incorporated into the database developed in this study. N2O was consistently measured using Fourier Transform Infrared Spectroscopy (FTIR), in various arrangements as summarized in Table 4.

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Table 7 Distance-based emission factors (mg km1) for gasoline fuelled vehicles. Min FTP Composite Tier0 Aged Tier1 Aged TLEV New TLEV Aged NLEV New NLEV Aged LEV New LEV Aged ULEV New ULEV Aged SULEV New SULEV Aged T2.B10 New T2.B9 New T2.B8 New T2.B6 New T2.B5 New T2.B4 New T2.B3 New

1st Qu.

Median

2.91 2.54 2.93 1.86 0.62 4.97 0.83 0.62 0.62 0.55 0.62 0.00 2.16 1.36 0.71 1.12 0.36 0.65 0.41

14.29 8.24 3.15 9.57 0.62 4.97 1.79 4.77 0.76 1.94 0.62 0.62 2.74 2.48 1.47 1.25 1.09 0.72 0.50

24.13 14.00 3.37 14.77 0.62 4.97 3.75 7.02 1.24 3.15 0.62 0.81 3.31 3.12 2.48 1.47 1.47 0.80 0.74

3rd Qu.

Max

N

39.81 24.32 3.59 23.14 0.62 4.97 6.48 16.68 3.42 5.28 0.78 1.32 3.89 3.55 3.63 1.67 2.22 0.91 1.02

160.02 123.31 3.81 73.33 0.62 4.97 19.88 75.28 8.08 15.24 1.24 1.53 4.47 7.23 7.97 2.48 40.29 1.02 1.25

42 64 2 32 1 1 20 94 15 35 4 12 2 16 28 7 85 3 4

FTP Phase Tier0 Tier1 TLEV TLEV NLEV LEV LEV ULEV ULEV SULEV SULEV T2.B10 T2.B9 T2.B8 T2.B6 T2.B5 T2.B4 T2.B3

1 Aged Aged New Aged New New Aged New Aged New Aged New New New New New New New

3.83 5.22 8.92 12.65 2.13 1.04 2.75 1.24 1.36 2.49 1.24 0.00 2.66 1.52 3.71 0.00 1.49 0.58

17.59 16.87 9.12 24.29 2.13 5.13 13.42 2.14 4.35 2.49 1.86 2.65 7.74 4.90 3.82 2.19 1.85 0.63

30.40 27.63 9.33 31.60 2.13 13.88 20.22 3.73 8.08 2.80 2.81 5.29 10.05 7.57 4.19 3.84 2.22 0.67

48.73 39.00 9.53 45.70 2.13 15.91 28.18 10.08 15.79 3.26 4.58 7.94 12.72 9.65 5.00 5.79 2.24 1.47

169.69 139.07 9.74 64.88 2.13 30.13 68.13 19.88 40.09 3.73 5.84 10.59 22.68 18.64 5.57 190.33 2.27 3.79

42 48 2 24 1 20 84 15 29 4 12 2 16 28 7 85 3 4

FTP Phase Tier0 Tier1 TLEV TLEV NLEV LEV LEV ULEV ULEV SULEV SULEV T2.B10 T2.B9 T2.B8 T2.B6 T2.B5 T2.B4 T2.B3

2 Aged Aged New Aged New New Aged New Aged New Aged New New New New New New New

0.91 0.24 0.37 0.28 0.18 0.00 0.00 0.00 0.00 0.00 0.00 0.62 0.00 0.00 0.00 0.00 0.21 0.21

6.31 3.98 0.80 2.60 0.18 0.00 0.73 0.00 0.00 0.00 0.00 1.02 0.32 0.41 0.32 0.30 0.23 0.32

12.79 6.41 1.23 4.66 0.18 0.49 2.00 0.00 0.50 0.00 0.16 1.42 0.44 0.71 0.34 0.55 0.25 0.38

26.19 19.14 1.66 8.51 0.18 0.77 5.98 0.22 1.45 0.00 0.52 1.81 0.85 1.68 0.45 1.00 0.48 0.69

196.74 110.93 2.09 62.45 0.18 12.43 41.55 1.16 6.84 0.00 0.55 2.21 2.39 3.83 0.58 3.22 0.70 1.54

42 48 2 24 1 20 84 15 29 4 12 2 16 28 7 85 3 4

FTP Phase Tier0 Tier1 TLEV TLEV NLEV LEV LEV ULEV ULEV SULEV SULEV T2.B10 T2.B9

3 Aged Aged New Aged New New Aged New Aged New Aged New New

6.05 2.62 2.53 4.47 0.30 0.00 0.01 0.00 0.00 0.00 0.00 3.69 0.39

19.48 8.14 2.68 12.36 0.30 0.96 3.06 0.29 0.59 0.00 0.18 4.51 0.95

35.18 19.63 2.84 22.20 0.30 2.56 7.93 2.49 2.87 0.31 0.21 5.33 2.28

60.98 42.45 2.99 33.80 0.30 6.84 20.15 5.05 4.97 1.09 0.28 6.16 3.84

124.96 137.81 3.15 100.50 0.30 34.18 154.45 13.67 29.01 2.49 1.86 6.98 6.23

42 48 2 24 1 20 84 15 29 4 12 2 16

(continued on next page)

2036

L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044

Table 7 (continued) 1st Qu.

Median

T2.B8 T2.B6 T2.B5 T2.B4 T2.B3

New New New New New

Min 0.30 0.69 0.10 0.72 0.14

0.94 0.93 0.57 0.74 0.28

1.64 1.12 1.13 0.75 0.62

3rd Qu. 2.40 1.59 1.96 0.77 0.93

Max 9.08 4.41 11.87 0.78 1.01

N 28 7 85 3 4

NYCC Tier1 LEV LEV ULEV T2.B9 T2.B8 T2.B5

Aged New Aged Aged New New New

14.15 30.64 39.12 5.36 3.15 0.87 2.00

14.15 30.64 39.63 6.15 9.91 1.26 4.05

14.15 30.64 40.13 9.86 16.67 1.64 6.10

14.15 30.64 40.64 16.77 29.27 2.03 6.88

14.15 30.64 41.14 27.16 41.87 2.41 7.66

1 1 2 4 3 2 3

US06 Tier1 LEV LEV ULEV ULEV SULEV SULEV T2.B9 T2.B8 T2.B6 T2.B5 T2.B4

Aged New Aged New Aged New Aged New New New New New

1.86 0.00 0.00 0.00 0.00 0.00 0.00 0.37 0.17 0.48 0.00 0.53

2.80 0.00 0.62 0.00 0.00 0.00 0.00 0.49 0.58 0.55 0.42 0.53

4.35 0.48 1.24 0.00 0.96 0.00 0.00 0.54 0.83 0.57 0.60 0.53

5.82 0.67 1.60 1.24 1.86 0.00 0.00 1.28 1.23 0.58 1.16 0.53

6.48 9.32 10.56 2.49 6.84 0.00 0.00 1.86 1.97 0.59 2.32 0.53

4 10 15 13 17 4 4 6 10 4 20 1

HWFET Tier1 LEV LEV ULEV T2.B9 T2.B8 T2.B5

Aged New Aged Aged New New New

5.45 0.88 0.18 0.04 0.28 0.20 0.00

5.45 1.00 0.44 0.14 0.34 0.22 0.35

5.45 1.12 0.55 0.25 0.45 0.24 0.54

5.45 1.24 1.49 0.38 0.46 3.13 0.81

5.45 1.37 4.24 0.52 0.95 6.02 1.37

1 2 4 4 5 3 11

LA92 Phase 1 Tier0 Aged Tier1 Aged TLEV Aged LEV Aged ULEV Aged

11.68 19.56 40.94 7.42 3.19

31.67 50.16 59.26 38.66 19.99

49.73 62.76 75.57 66.05 54.26

76.98 86.64 114.03 92.22 74.33

155.82 211.33 254.65 162.95 121.57

31 40 20 58 11

LA92 Phase 2 Tier0 Aged Tier1 Aged TLEV Aged LEV Aged ULEV Aged

1.99 1.00 0.99 0.06 0.21

7.91 5.04 5.63 1.81 0.95

11.82 7.63 7.25 3.08 1.86

23.37 15.14 10.58 8.35 2.49

108.83 63.64 42.72 19.26 4.76

31 40 20 58 11

LA92 Phase 3 Tier0 Aged Tier1 Aged TLEV Aged LEV Aged ULEV Aged

0.81 3.28 9.37 0.11 0.80

37.14 20.88 25.90 6.98 4.08

62.22 40.07 39.39 22.74 6.23

107.20 64.68 69.85 42.69 9.01

184.15 349.03 132.84 145.59 14.37

31 40 20 58 11

Michaels et al. (1998) presented N2O emission rates for 23 vehicles with all but one of the vehicles tested on high sulfur fuel (285 ppm). Seven of the 23 vehicles were tested on low sulfur gasoline (Tier 1 certification fuel, 24 ppm). The vehicles were tested with air conditioning on and off. Both emission rates are included in the database for this study when available. Neither CO2 emission rates, nor fuel economy were presented, therefore emission rates in g L1 could not be calculated for these vehicles. The details of fuel exchange and preconditioning given in the study did not suggest an extensive catalyst conditioning procedure as used in the Durbin and Huai studies described below, so it is possible that the results of tests on low sulfur fuel may reflect some extent of catalyst deactivation due to residual sulfur in the catalyst present from testing first on high sulfur fuel. This issue will be discussed further when the effect of fuel sulfur on N2O emissions is considered later in the paper.

Table 8 Fuel-based emission factors (g L1 fuel) for diesel-fueled vehicles. Min

1st Qu.

Median

3rd Qu.

Max

N

FTP Composite T2.B10 New Euro3 New Euro4 New

0.049 0.087 0.104

0.053 0.106 0.121

0.065 0.125 0.137

0.076 0.134 0.196

0.106 0.143 0.252

9 3 5

FTP Phase T2.B10 Euro3 Euro4

1 New New New

0.063 0.086 0.107

0.072 0.099 0.123

0.083 0.112 0.142

0.094 0.125 0.172

0.119 0.137 0.291

9 3 5

FTP Phase T2.B10 Euro3 Euro4

2 New New New

0.035 0.090 0.107

0.043 0.109 0.135

0.063 0.129 0.150

0.082 0.146 0.226

0.109 0.164 0.258

9 3 5

FTP Phase T2.B10 Euro3 Euro4

3 New New New

0.040 0.082 0.085

0.059 0.095 0.106

0.070 0.108 0.108

0.076 0.117 0.151

0.091 0.125 0.206

9 3 5

New New New

0.036 0.058 0.036

0.038 0.058 0.050

0.040 0.058 0.064

0.046 0.058 0.076

0.052 0.058 0.089

3 2 3

US06 T2.B10 Euro3 Euro4

Becker et al. (1999) presented N2O emission rates for 9 gasoline fuelled passenger cars and 8 light duty trucks. The vehicles were both production and prototype and met emission standards between Tier 1 and California ULEV, though specific details on each vehicle were not presented. These vehicles were assigned to the Tier 1 emission standard in the database for this study. The vehicles were tested on low sulfur gasoline (Tier 1 certification fuel or California Phase 1 Reformulated). Two light duty diesel vehicles were also tested on low sulfur diesel fuel (approximately 300 ppm). Emission rates in g L1 could not be calculated because CO2 emission rates or fuel consumption data were not available. Durbin et al. (2003) presented N2O emission rates for 12 vehicles meeting California LEV, ULEV and SULEV standards. The vehicles were tested as received with mileage accumulation ranging from 10,000 to 50,000 km and again with catalysts bench aged to approximately 200,000 km. The vehicles were tested on fuels with sulfur levels of 5, 30 and 150 ppm. Prior to testing, the vehicle

Table 9 Distance-based emission factors (mg km1) for diesel-fueled vehicles. Min

1st Qu.

Median

3rd Qu.

Max

N

FTP Composite T2.B10 New Euro3 New Euro4 New

4.02 7.78 6.57

4.20 7.89 8.79

4.25 8.00 12.14

4.77 8.63 12.39

8.30 9.25 15.72

9 3 5

FTP Phase 1 T2.B10 New Euro3 New Euro4 New

4.18 7.54 8.22

5.28 8.00 9.38

5.81 8.46 11.32

7.10 8.96 12.01

7.71 9.46 19.48

9 3 5

FTP Phase 2 T2.B10 New Euro3 New Euro4 New

3.13 7.96 6.65

3.58 8.21 9.90

4.20 8.46 13.96

4.98 9.76 14.69

9.08 11.06 16.22

9 3 5

FTP Phase 3 T2.B10 New Euro3 New Euro4 New

2.87 6.17 5.19

3.33 6.49 6.27

3.76 6.81 8.05

4.14 7.00 9.76

7.31 7.19 11.96

9 3 5

US06 T2.B10 Euro 3 Euro 4

1.72 3.82 2.61

1.88 3.85 4.10

2.03 3.89 5.58

2.70 3.93 6.01

3.36 3.97 6.43

3 2 3

New New New

L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044

2037

0.70 This Study

Emission Factor (g/L fuel)

0.60

Cdn Inventory - LDV 0.50

Cdn Inventory - LDT

0.40 0.30 0.20 0.10 0.00 Aged Aged New Aged New Aged New Aged New Aged New Aged New New New New New New New Tier0

Tier1

TLEV

TLEV

NLEV

NLEV

LEV

LEV

ULEV

ULEV

SULEV SULEV T2.B10

T2.B9

T2.B8

T2.B6

T2.B5

T2.B4

T2.B3

60 This Study

Emission Factor (mg/km)

50

US Inventory - LDV 40

US Inventory - LDT

30

20

10

0 Aged Aged New Aged New Aged New Aged New Aged New Aged New New New New New New New Tier0

Tier1

TLEV

TLEV

NLEV

NLEV

LEV

LEV

ULEV

ULEV SULEV SULEV T2.B10 T2.B9

T2.B8

T2.B6

T2.B5

T2.B4

T2.B3

Fig. 1. Comparison of fuel-based and distance-based N2O emission factors for light duty gasoline vehicles by emission standard. Also shown are the Canadian and US GHG inventory emission factors used for each category of vehicle. Column height is median value, error bars show first and third quartiles.

catalysts were purged of sulfur by a special conditioning cycle and were conditioned on each test fuel for a significant distance before testing. CO2 emission rates for each vehicle were obtained from the authors to estimate fuel consumption and subsequently the N2O emission rates in g L1 fuel for this study. Huai et al. (2003) is a review that included the results from Durbin et al. (2003) and previously unpublished results from this research group. N2O emission rates from a total of 61 vehicles were reported: 12 vehicles tested on 5, 30 and 150 ppm sulfur gasoline from Durbin et al. (2003); 10 vehicles tested on 30 and 330 ppm sulfur gasoline and 39 vehicles tested on California Reformulated gasoline only (<30 ppm). Huai et al. (2004) is the literature citation for Huai et al. (2003). Behrentz et al. (2004) presented the results of the pilot study included in Winer and Behrentz (2005). The 37 vehicles reported in Behrentz et al. (2004) are included in the data set of 134 vehicles reported in Winer and Behrentz (2005). The vehicles met emission standards between California Tier 0 and California ULEV and were tested on California Phase 2 Reformulated gasoline (<30 ppm).

3. Measured emission rates Emission rates measured in this study were combined with emission rates taken from the literature and were aggregated by fuel type, emission standard and vehicle type (e.g. LDV, LDT) and catalyst age. Each aggregated data set was examined for skewness in order to decide whether the mean or median value would best represent the distribution of measured emission rates. All distributions with greater than 5 measurements were found to be highly skewed (0.4 < jskewnessj < 8), so the median, minimum, maximum, first and third quartile values are presented as representative of the measured values. Tests for significance were conducted to determine (1) if there was a statistically significant difference between light duty automobiles and light duty trucks by fuel, catalyst age and emission standard and (2) if there was a statistically significant difference between new and aged catalyst vehicles by fuel, vehicle type (LDV vs. LDT) and emission standard. Tests for statistical significance were conducted using the Wilcoxon rank-sum test as the data were not normally distributed. The significance level a ¼ 0.05 was used for all tests.

Emission Factor (mg/km)

Emission Factor (g/L fuel)

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L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044

0.25 0.20 0.15

This Study Cdn Inventory LDV&LDT

0.10 0.05 0.00 New

New

New

Tier 2 Bin 10

Euro 3

Euro 4

7 5

This Study US Inventory - LDV

4

US Inventory - LDT

6

in durability requirements of the standard and the relatively low mileage of the aged fleet. Explicit mileage information was not available for all ULEV and SULEV vehicles but was available for all Tier 2 vehicles. The highest mileage accumulated on Tier 2 vehicles was 30,000 km. Most of the aged Tier 2 vehicles had accumulated less than 10,000 km. The distinction between new and aged was retained for all emission standards except Tier 2. The Tier 2 vehicles that were considered aged were clearly not that old in comparison to the 160,000 km durability requirement. Table 5 describes the vehicle fleet used for the statistical analysis, including the aggregation of the various data sources, removing the distinction between LDV and LDT for all emission standards and removing the distinction between new and aged catalyst for Tier 2 vehicles. It should be noted that the Tier 0 vehicles present in this fleet met only the California Tier 0 standard. The complete statistical descriptions of the distance- and fuelbased emission rate data sets are given in Table 6–9.

3 2

3.1. Fuel-based emission rates

1 0 New

New

New

Tier 2 Bin 10

Euro 3

Euro 4

Fig. 2. Comparison of fuel-based and distance-based N2O emission factors for light duty diesel vehicles by emission standard. Also shown are the Canadian and US GHG inventory emission factors used for each category of vehicle. Column height is median value, error bars show first and third quartiles.

There was no statistically significant difference between LDV and LDT emission rates for any emission standard except for vehicles with new catalysts meeting the LEV standard. This difference could be a function of the small population size for LEV LDT compared to LEV LDV (6 LDT vs. 14 LDV). Therefore, the distinction between LDV and LDT was dropped for all emission standards. There was no statistically significant difference between new and aged catalysts except for LEV LDV. Vehicles were considered new if they had accumulated less than 6500 km at the time of testing and aged if they had accumulated more than this at the time of testing. The lack of difference for LEV LDT could again be due to the small sample size for new LEV LDT (6 new vs. 36 aged). The difference between new and aged for ULEV vehicles is only marginally insignificant (i.e. would be significant if a ¼ 0.1 instead of 0.05). These are the two oldest emission standards for which the comparison could be made. For vehicles meeting SULEV and Tier 2 standards, there was no difference. The lack of difference between new and aged for newer emission standards could be the difference

3.1.1. Light duty gasoline vehicles Fig. 1 shows a comparison of N2O fuel-based FTP composite emission factors for light duty gasoline vehicles for the range of emission standards represented in this study. A comparison to the emission factors used in the Canadian Emission Inventory is also shown. These results show that the fuel-based emission factors decrease as more stringent emission standards are met and that the fuel-based emission factors used in the Canadian emission inventory are higher than those measured for Tier 0 vehicles but agree fairly well with those obtained for Tier 1 vehicles. The reason for this is most likely the effect of fuel sulfur since the vehicles reported in this study were tested on low sulfur fuel (w30 ppm) while the emission factor chosen for emission inventory purposes reflects the use of much higher sulfur fuel (>500 ppm). The decisions made in estimating the current inventory Tier 0 N2O emission factors may need to be revisited as fuel sulfur effects appear to be reversible. Operating on low sulfur fuel after operation on high sulfur fuel appears to return much of the catalyst performance and reduces N2O emissions. This will be discussed in more detail later in the paper. 3.1.2. Light duty diesel vehicles Fig. 2 shows a comparison of N2O fuel-based FTP composite emission factors for light duty diesel vehicles for the range of emission standards represented in this study. A comparison to the emission factors used in the Canadian Emission Inventory is also shown. Only a few light duty diesel vehicles were tested. None of the vehicles were light duty trucks. Half of the vehicles tested met

Fig. 3. Effect of increasing fuel sulfur on test fleet average N2O emissions.

Durbin & Huai, 2003, 2004 Tier 1 30 y = 0.0422x + 6.4095 R2 = 1

25 20 15 10 5 0 0

100

200

300

400

Emission Rate (mg/km)

Emission Rate (mg/km)

L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044

LEV 50 40 30 20 10

y = 0.0745x + 6.0309 R2 = 0.9625

0

-10 0

100

ULEV y = 0.0434x + 2.1478 R2 = 0.9821

25 20 15 10 5 0 0

100

200

Emission Rate (mg/km)

70 60 50 40 30 20 10 0

300

400

300

400

SULEV

2.0 1.5 1.0

y = 0.0030x + 0.6434 R2 = 0.9709

0.5 0.0 0

100

Fuel Sulfur (ppm) Baronick, 2000

200

Fuel Sulfur (ppm)

Emission Rate (mg/km)

Emission Rate (mg/km)

Fuel Sulfur (ppm)

30

2039

200

300

400

300

400

Fuel Sulfur (ppm)

LEV

y = 0.0995x + 4.1568 R2 = 0.9716

0

100

200

300

400

Emission Rate (mg/km)

Michaels et al., 1998

Tier 1

100 y = 0.0012x + 44.982 R2 = 1

80 60 40 20 0 0

100

200

300

400

Fuel Sulfur (ppm)

Emission Rate (mg/km)

Fuel Sulfur (ppm)

LEV 70 60 50 40 30 20 10 0

y = 0.0952x + 14.33 R2 = 1

0

100

200

Fuel Sulfur (ppm)

Fig. 4. Effect of increasing fuel sulfur content on N2O emissions by emission standard. Error bars show standard deviation on average (blue diamond). Red open square indicates median value. Linear regression uses average values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

European emission standards, not North American standards, but all vehicles were tested using North American test cycles and fuels. The mileage accumulation of the aged vehicles was just above the 6500 km decision point for new and aged, so the distinction between aged and new vehicles cannot be made. These results show that the fuel-based emission factors clearly increase as more stringent emission standards are met and that the fuel-based emission factor for advanced technology diesel vehicles used in the Canadian emission inventory is again higher than measured for the vehicles in this study. 3.2. Distance-based emission rates 3.2.1. Light duty gasoline vehicles Fig. 1 shows a comparison of N2O distance-based FTP composite emission factors for light duty gasoline vehicles for the range of

emission standards represented in this study. A comparison to the emission factors used in the US Emission Inventory is also shown. Since the US inventory methodology requires emission factors for a number of different operating conditions, the FTP emission factors are shown for comparison to the FTP composite data from this study. The same observations as made with fuel-based emission factors are seen with distance-based emission factors. 3.2.2. Light duty diesel vehicles Fig. 2 shows a comparison of N2O distance-based FTP composite emission factors for light duty diesel vehicles for the range of emission standards represented in this study. A comparison to the emission factors used in the US Emission Inventory is also shown. These results show that the distance-based emission factors clearly increase as more stringent emission standards are met and

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L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044

Fig. 5. Comparison of fuel-based N2O emission rates for the FTP and LA92 test cycles.

that the distance-based emission factors for advanced technology diesel vehicles used in the US emission inventory are much lower than measured for the vehicles in this study. 3.3. Fuel sulfur effect on light duty gasoline vehicles Four reports were found (Michaels et al., 1998; Baronick et al., 2000; Durbin et al., 2003; Huai et al., 2004) that showed the effect of increasing fuel sulfur content on N2O emissions from light duty gasoline vehicles. These results are summarized in Fig. 3. All of the studies show a trend of increasing N2O emissions with increasing fuel sulfur, but the magnitude of the increase is different for each study. Fig. 4 separates the fuel sulfur effect for each study by vehicle emission standard. Linear regression lines are shown for each data set and are based on average emission rates for each vehicle fleet and sulfur level. As can be seen in Fig. 4, the median value (open square) is often quite different from the average value (solid diamond), indicating the distributions of measurements at a given fuel sulfur content are skewed. The median value gives a nonlinear trend to which an exponential curve could be fit (data not shown) with a slightly higher correlation coefficient (R2) than the linear model. However, since number of measurements for each fuel sulfur level was quite small (less than 15), the R2 values for the linear models were greater than 0.94 and the improvement in fit obtained with the exponential fit was marginal, it was decided to use the linear model fit to the average value for simplicity in comparison. The slope of the linear regression, therefore, represents the rate of increase of N2O emissions due to increasing fuel

sulfur content (mg km1 ppm1 S) while the intercept represents the base N2O emission rate of the vehicle technology (mg km1) – the emission rate of N2O that would be observed with no sulfur present in the fuel. The reader is cautioned that prediction of N2O emission rate outside the range of fuel sulfur content range represented by the experimental data should be carefully considered as the two models (exponential vs. linear) will give quite different results, especially for higher fuel sulfur levels. From the Durbin and Huai studies, the base emission rates (6.4 and 6.0 mg km1) are similar for Tier 1 and LEV vehicles, however the rate of increase of emission rate with fuel sulfur is greater for the LEV vehicles than the Tier 1 vehicles (0.042 vs. 0.075 mg1 km1 ppm S). The rate of increase of emission rate for ULEV vehicles is similar to that seen for Tier 1 vehicles (0.043 mg1 km1 ppm S) but the intercept is smaller (2.1 mg km1), reflecting lower base emissions from this technology compared to the earlier technology. The fuel sulfur effect is very small for vehicles meeting SULEV standards. The rate of increase of emission rate with fuel sulfur and base emission rates are approximately 10% of that seen for the Tier 1 vehicles (0.003 mg1 km1 ppm S and 0.64 mg1 km). The Baronick study is based on two identical vehicles but with two different catalyst formulations, one production and one prototype. The vehicles met LEV standards. The base emission rate observed in this study is lower than that observed in the Durbin and Huai studies for LEV vehicles (4.1 mg1 km), while the rate of increase of emission rate with fuel sulfur is slightly higher (0.99 mg1 km1 ppm S). This could be a result of different catalyst technology.

L.A. Graham et al. / Atmospheric Environment 43 (2009) 2031–2044

2041

Fig. 6. Comparison of distance-based N2O emission rates for the FTP and LA92 test cycles.

The results from the Michaels study do not agree with the other studies. For the Tier 1 vehicles, this could be a result of sample size bias in the Michaels data as only three Tier 1 vehicles were tested on low sulfur fuel and 31 Tier 1 vehicles were tested on high sulfur fuel. For LEV vehicles, the Michaels study gives a rate of increase in N2O emissions with fuel sulfur higher than that of the Durbin and Huai studies and very similar to that of the Baronick study (0.095 vs. 0.075 and 0.99 mg1 km1 ppm S, respectively). The base emission rate is also twice that of the Durbin and Huai study and three times that of the Baronick study (14.3 vs. 6.0 mg1 km and 4.1 mg1 km respectively). This higher base emission rate could be a result of the vehicle conditioning procedures and the possibility of having residual sulfur present in the catalyst after testing on high sulfur fuel which could decrease the catalyst conversion efficiency and increase N2O production. It appears that each of the studies gives a different rate of increase of N2O emissions with fuel sulfur which may result from differences in both testing procedures (condition of the catalyst with respect to residual sulfur) and catalyst formulation. Additional research is needed to provide an unambiguous evaluation of the fuel sulfur effect on N2O emissions. 3.4. Effect of vehicle operating conditions 3.4.1. Aggressive driving – LA92 Aggressive driving is represented by two different driving cycles – the US06 and the LA92 or unified driving cycle. The LA92 cycle is a three phase cold start test that is modeled after the FTP in that it represents urban and suburban driving but has more aggressive accelerations and decelerations, less idling and higher speeds than

the FTP. It is currently used as a research test cycle. A subset of the test vehicles were tested over the US06 and/or the LA92 cycles. The per phase FTP and LA92 fuel-based emission rates are compared in Fig. 5 and the distance-based emission rates are compared in Fig. 6. For simplicity, only median values are shown in the figures. The ratios of LA92 to FTP emission rates are also shown in each figure. This ratio is almost always greater than 1, is highest for cold engine start (phase 1) and increases as emission standards become more stringent; indicating that aggressive driving increases N2O emissions under cold start, hot start and stabilized urban and suburban driving conditions and has a larger influence on emissions of vehicles meeting more stringent emission standards. 3.4.2. Aggressive driving – US06 The US06 is part of the Supplemental FTP and is used for regulatory compliance. It is a single phase hot start test that has aggressive accelerations and decelerations and highway speeds. The US06 emission rates for gasoline fueled vehicles are compared to the FTP composite emission rates in Fig. 7. As a result of the high catalyst temperatures experienced during the US06 test, the N2O emission rates (both distance- and fuel-based) are much lower than the FTP composite emission rate. For diesel-fueled vehicles, the US06 emission rates are compared to the FTP composite emission rates in Fig. 8. Again, N2O emission rates are lower for the US06 as compared to the FTP composite. 3.4.3. Highway driving The HWFET is used to determine highway fuel economy and is a hot-running test, meaning that engine start is not part of the

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Fig. 7. Comparison of FTP Composite, US06, HWFET and NYCC N2O emission rates for gasoline fuelled vehicles.

test. It is conducted as two repeats of the test cycle driven back to back, with no stop, and with emissions measured only on the second repeat. It represents fairly steady highway driving speeds. As observed with the US06, the high catalyst temperatures combined with fairly steady driving speeds lead to very low N2O emission rates. The measured emission rates are shown in Fig. 7. 3.4.4. Congested urban driving The NYCC simulates congested urban driving with approximately 40% of the test cycle spent at idle. The relatively cool exhaust temperatures experienced during this test result in increased N2O emissions as compared to the FTP composite, as shown in Fig. 7.

3.4.5. Cold engine start As a measure of the effect of cold engine start on N2O emission rates, a cold start enrichment ratio (defined as the ratio of phase 1 to phase 3 emission rates) was calculated for the FTP and LA92 tests. FTP distance-based cold start enrichment ratios ranged from 0.65 to 13.8 with a median value of 2.9. The LA92 cold start enrichment ratios ranged from 0.67 to 7.6 with a median value of 2.3. The highest ratios were generally seen for the vehicles meeting the most stringent emission standards. For both tests, the fuel-based enrichment ratios followed the same trend but were slightly lower in magnitude due to the increased fuel consumption on cold start as compared to hot start while the distance traveled in both cases was the same.

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70 60 50 40 30 20 10 0 -10 0.00

y = 93.0x + 1.7 2 R = 0.85 y = 49.1 x -0.4 2 R = 0.51 New Aged

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Fig. 8. Comparison of FTP Composite and US06 N2O emission rates for diesel-fueled vehicles.

30 25 20 15 10 5 0 0.00

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Fig. 9. Correlation of N2O emission rates with emission standards.

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3.5. Correlation of N2O emissions with emission standard

Acknowledgements

Fig. 9 shows the correlation of the numerical emission standards given in Table 2 with the distance-based and fuel-based FTP composite N2O emission rates for new light duty gasoline vehicles given in Table 7. Due to the fact that NOX and CO emission standards remain constant for several sets of decreasing NMHC or NMOG emission standards, the NMHC or NMOG emission standards appear to be a better predictor of N2O emission rates than either NOX or CO, and also better than the product of the NMOG and NOX emission standards or the product of NMOG, NOX and CO emission standards (not shown). The linear model shown was found to best describe the correlation. Fig. 9 also shows that the aged catalyst vehicles tend to have higher N2O emission rates than new vehicles and also that the N2O emissions for aged catalyst vehicles increase at a higher rate with increasing emission standard NMOG or NMHC emission rate than do the new vehicles.

The authors would like to acknowledge the contributions of the staff at both emissions laboratories for their efforts in conducting the vehicle tests and sample analyses. Funding for the measurements conducted at Environment Canada was received from the Climate Change Technology and Innovation Initiative – Transportation program.

4. Conclusions Measurements of N2O emissions from 467 vehicles have been combined into a single database to produce updated distance- and fuel-based N2O emission factors for light duty gasoline and dieselfueled vehicles operating on low sulfur fuels. The analysis has shown that the distinction between light duty automobiles and light duty trucks within a given emission standard is not significant whereas the distinction between new and aged catalysts remains. For the newest vehicles tested, those meeting Tier 2 emission standards, accumulation of up to 30,000 km is insufficient to categorize the vehicle as aged. As shown in Fig. 1, the light duty gasoline vehicle emission factors determined in this study are generally much lower than the emission factors currently used in both the Canadian and US emission inventories. The light duty diesel vehicle emission factors determined in this study are based on both North American and European certified vehicles, but all were tested on North American fuels and driving cycles. As seen in Fig. 2 the fuel-based emission factors determined from these vehicles are lower than those used in the Canadian inventory and the distance-based emission factors are higher than those used in the US emission inventory. Results also show how driving conditions (test cycle and cold start) affect N2O emission rates as compared to FTP composite emission rates. Test cycles that produce high exhaust temperatures such as the US06 and HWFET result in decreased fuel-based and distance-based N2O emission rates as compared to the FTP composite emission rate. Cold engine start and congested urban driving, which have cooler exhaust temperatures result in increased N2O emission rates as compared to the FTP composite. Aggressive urban driving as represented by the LA92 test also results in increased N2O emission rates when the exhaust temperatures are lower (phases 1 and 3) and only marginal differences when the catalyst is at normal operating temperatures (phase 2). The N2O emission rates appear to be well correlated with the numerical NMHC or NMOG emission standard to which the vehicles were certified, and less so with the NOX or CO emission standards. For the vehicles of this fleet, those with aged catalysts show a greater rate of increase of N2O emissions with NMHC or NMOG emission standard than do new vehicles.

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