On-road Emission Measurements Research Articles

On-road NOx and NH3 emissions measurements from in-use heavy-duty diesel and natural gas trucks in the South Coast air Basin of California

This study characterized nitrogen oxides (NOx) and ammonia (NH3) emissions from five in-use goods movement vehicles, including one diesel vehicle without selective catalytic reduction (SCR), two diesel vehicles with SCR, and two ultra-low NOx natural gas vehicles equipped with three-way catalysts (TWCs). Emissions testing was performed under real-world driving conditions in typical goods movement routes in the South Coast Air Basin (SCAB) of California. NOx emissions varied depending on the vehicle and the route. The no-SCR diesel vehicle showed the highest NOx emissions over all the routes. One of the SCR-equipped vehicles showed NOx emission rates that were two times higher than the other SCR-equipped vehicle with similar engine model year and mileage, which may be attributed to catalyst deterioration. The SCR-equipped diesel vehicles were within two to three times 0.2 g/bhp-hr certification standard over the different routes. The natural gas vehicles showed average NOx emissions around or below the optional Low NOx emissions standard of 0.02 g/bhp-hr. The three-bin moving average window (MAW) method was utilized to show the NOx emissions across various modes of operation, including idle, low load, and medium/high load. The highest fraction of vehicle operation was found in either the low load or medium/high load bins, depending on the route. NOx emissions for each of the bins varied between different vehicles, and are further analyzed in this paper. NH3 emissions formation was favored for the TWC-equipped natural gas vehicles, which produced about 20 times more NH3 emissions than the SCR-equipped diesel vehicles.

A new portable open-path instrument for ambient NH3 and on-road emission measurements

Increased attentions to vehicle emission of NH3 have been paid since it is generally regarded as an important source in urban areas. Here, we developed a movable instrument based on Differential Optical Absorption Spectroscopy (DOAS) principle for detecting on-road NH3, which can avoid the losses in the sampling process attributed to the non-sampling methods. For this mobile DOAS, the temporal resolution, detection limit and relative error for NH3 were 1 min, 2.29 ppbv and 4.57% ± 2.44%, respectively. By employed to the on-road measurements along the arterial highway in Shanghai, the spatial distributions of NH3 and NO were obtained, and their dependence of traffic and road conditions were studied. The slopes of linear regression between NH3 and NO were 0.40, 0.02 and 0.07 on the Middle Ring Road, Outer Ring Road and Chongming Island Ring Road. It indicates that light gasoline vehicles (LGVs) were found to be the main contributor to NH3 emissions, while heavy-duty diesel vehicles (HDVs) mainly emitted NO. Based on the measured NH3 in the tunnel, the mileage-based NH3 emission factor per vehicle was estimated to be 17.9 ± 6.3 mg/km. The reported open-path instrument can be broadly used in on-road pollutant monitoring or vehicle emissions, and the measurements can reveal the real situation of emission characteristics, even find the abnormal operations of vehicle catalyst system.

Gasoline and diesel passenger car emissions deterioration using on-road emission measurements and measured mileage

Modern gasoline and diesel vehicles are equipped with highly effective emission control systems that result in low emissions of pollutants such as nitrogen oxides (NO x ) when new. However, with increasing age or mileage, the emissions performance of vehicles can deteriorate over time, leading to increased emissions. In this work we use comprehensive vehicle emission remote sensing measurements collected over a wide range of conditions, together with individual vehicle measured mileage to quantify vehicle emissions deterioration. A quantile regression modelling approach is used to provide a more complete understanding of the distribution of deterioration effects that is not captured by considering mean changes over time. The approach accounts for factors such as driving conditions and ambient temperature, as well as determining whether deterioration affects whole populations of vehicles or a smaller subset of them. Accounting for these factors, we find that for most pollutants the rate of deterioration of emissions from pre-Euro 5 gasoline passenger cars is highly skewed. Between 5% and 10% of pre-Euro 5 gasoline passenger cars have emissions similar to a Euro 5 diesel car, suggesting that policies should be developed to accelerate their removal from the fleet. Furthermore, we find evidence that there are differences between vehicle manufacturers in the way emissions of NO x deteriorate. • 197,000 vehicle emission remote sensing measurements coupled with individual measured mileage data from vehicle inspections. • Quantile regression framework used to examine distribution of deterioration effects of passenger car NO x and PM emissions. • Rate of deterioration for gasoline NO x emission controlled by small proportion of vehicles. • Large fleet coverage of remote sensing is exploited to examine manufacturer-specific NO x emission deterioration.

On-Road NOx Emissions Evaluation of the Repair Effectiveness for Recalled Volkswagen Group Light-Duty Diesel Vehicles in the United States.

The admission by the Volkswagen Group in the fall of 2015 that they had duped the United States new vehicle emissions certification testing resulted in perhaps the most expensive violation of U.S. environmental vehicle emission regulations in its history. As part of the subsequent recall of more than 500 000 vehicles in the U.S., owners could sell their vehicles back to the companies or have them repaired. We have used a number of large on-road emission measurement data sets that were routinely collected before and after the recall to evaluate the fuel specific NOx emissions benefit for the vehicles that were repaired and remained in service. We found that on-road fuel specific NOx emissions were reduced by 83% from the prerepair group. The reductions increased to 91% if we only compare with vehicles that were fully repaired. NO2 emissions were dramatically reduced by an even larger percentage >95%. We find that the repairs resulted in fuel specific NOx emissions that are comparable or slightly lower than in-use light and medium-duty diesel trucks measured in Denver in 2020 indicating the repairs were a success.

A novel modal emission modelling approach and its application with on-road emission measurements

Modal emission models describe vehicular emissions with stratified vehicle kinetic conditions or engine parameters. They are widely adopted in regulatory applications in North America and Europe to estimate emissions and energy consumption from mobile sources. However, challenges exist in the development of modal bins, especially that previous approaches rely on manual adjustment and tuning, which increase the propensity to emission misclassification. This study proposes a new approach to generate modal bins, which overcomes the limitations of previous studies. It uses a Greedy algorithm to define optimal mode boundaries and improve model robustness. The model is calibrated with emission data from a portable emission monitoring system and validated against an independent dataset. Our modelling approach can effectively reflect carbon dioxide (CO2) emissions in steady and aggressive driving conditions with errors lower than 7% at the trip level. The introduction of engine parameters is found to improve model prediction for carbon monoxide (CO) and nitrogen oxides (NOx) by about 30% compared with the models relying on external variables.

Ultra-Low NOx Emissions with a Close-Coupled Emission Control System on a Heavy-Duty Truck Application

<div class="section abstract"><div class="htmlview paragraph">Heavy-duty vehicles represent a significant portion of road transport and they need to operate in a clean and efficient manner. Their emission control systems need to be enhanced to sustain the high conversion efficiencies seen during motorway conditions inother operating conditions. The European Commission is developing legislative proposals for Euro 7 emissions regulations for light- and heavy-duty vehicles. The new Euro 7regulation will likely focus on ensuring the emissions from heavy-duty vehicles are minimized over extensive on-road operating conditions and specifically on operating conditions such as urban driving and cold-start operation. These challenges are increased by the need to ensure low secondary emissions like NH<sub>3</sub> and N<sub>2</sub>O, as well as a low impact on CO<sub>2</sub> emissions. The paper outlines the low pollutant emissions achieved by a heavy-duty Diesel demonstrator vehicle. The vehicle is equipped with an innovative layout of state-of-the-art emission control technologies, combined with an advanced controlstrategy on an existing Euro VI long-haul truck. The new emissions control system integrates a close-coupled DOC, a catalyzed DPF, dual-SCR system -one in a close-coupled position-, with twin AdBlue® dosing controlled withFEV developed software. Both SCR catalysts contain an ammonia slip catalyst. The innovative system layout allows ultra-low NOx emissions and well controlled secondary emissions in even the most challenging conditions with minimal impact on CO<sub>2</sub> emissions. Pollutant emissions were evaluated over a broad range of operating conditions, including different payloads to show the emissions reduction potential. This paper also includes results obtained from the on-road Portable Emissions Measurement System (PEMS) testing.</div></div>

Emission Measurement of Buses Fueled with Biodiesel Blends during On-Road Testing

Increasing the biodiesel content of diesel fuels is encouraged because of its reduced carbon footprint. Pure rapeseed methyl ester (RME)and used cooking oil methyl ester (UCOME) are characterised by well-to-tank greenhouse gas (GHG) reductions of 54% and 88% compared to pure B0 petrodiesel, respectively. Captive fleets such as public transport buses could benefit from these GHG reductions by increasing the biodiesel content of their fuel because they have a consequent yearly fuel consumption. The aim of this paper is to compare on-road tailpipe emissions of a diesel bus when increasing the biodiesel concentration in the fuel. The tests were carried out on a standard city bus belonging to the Euro V EEV emission standard that was equipped with a portable emission measurement system measuring NO, NO2, PN, CO and CO2 at the tailpipe. The bus followed the SORT which is representative of urban bus driving. The heavy urban on-road measurements indicated increased NOx emissions (24–26%), decreased PN emissions (43–45%) and slightly decreasing CO emissions for B30 RME and UCOME compared to B7. A measurement uncertainty analysis showed that the CO emissions were less reliable. Similar conclusions were drawn for the easy urban on-road bus emission measurements with smaller differences between B7 and B30 RME and UCOME.

Statistical evaluation of on-road vehicle emissions measurement using a dual remote sensing technique

On-road remote sensing (RS) is a rapid, non-intrusive and economical tool to monitor and control the emissions of in-use vehicles, and currently is gaining popularity globally. However, a majority of studies used a single RS technique, which may bias the measurements since RS only captures a snapshot of vehicle emissions. This study aimed to use a unique dual RS technique to assess the characteristics of on-road vehicle emissions. The results show that instantaneous vehicle emissions are highly dynamic under real-world driving conditions. The two emission factors measured by the dual RS technique show little correlation, even under the same driving condition. This indicates that using the single RS technique may be insufficient to accurately represent the emission level of a vehicle based on one measurement. To increase the accuracy of identifying high-emitting vehicles, using the dual RS technique is essential. Despite little correlation, the dual RS technique measures the same average emission factors as the single RS technique does when a large number of measurements are available. Statistical analysis shows that both RS systems demonstrate the same Gamma distribution with ≥200 measurements, leading to converged mean emission factors for a given vehicle group. These findings point to the need for a minimum sample size of 200 RS measurements in order to generate reliable emission factors for on-road vehicles. In summary, this study suggests that using the single or dual RS technique will depend on the purpose of applications. Both techniques have the same accuracy in calculating average emission factors when sufficient measurements are available, while the dual RS technique is more accurate in identifying high-emitters based on one measurement only.

On-road emission measurements of reactive nitrogen compounds from heavy-duty diesel trucks in China

Emissions of major reactive nitrogen compounds, including nitric oxide (NO), nitrogen dioxide (NO2) and ammonia (NH3), from heavy-duty diesel vehicles (HDDVs) place substantial pressure on air quality for many large cities in China. To control nitrogen oxide (NOX) emissions from HDDVs, selective catalytic reduction (SCR) systems have been widely used since the China IV standards. To investigate the impacts of aftertreatment technologies and driving conditions on real-world emissions of reactive nitrogen compounds, a portable emissions measurement system was employed to test eighteen heavy-duty diesel trucks in China. The results showed that the China IV and China V HDDVs with appropriate SCR functionality could reduce NOX emissions by 36% and 53%, respectively, compared to the China III results, although their real-world emissions were still higher than the corresponding emission limits for regulatory engine tests. For these HDDVs, five samples were tested with NH3 emissions, ranging from 1.67 ppm to 51.49 ppm. The NH3 emission rates tended to significantly increase under high-speed driving conditions. The results indicate that the current SCR technology may have certain risks in exceeding the future China VI NH3 limit. However, five China IV/V HDDVs were found to have SCR temperature sensors that were intentionally tampered with, resulting in comparable or even higher NOX emissions and zero NH3 emissions. Increased NO2 emissions due to the adoption of diesel oxidation catalysts and diesel particulate filters were also found from our experiments. This study highlights the importance of enhancing in-use compliance requirements and eliminating aftertreatment tampering for China IV and China V HDDVs.

Three decades of on-road mobile source emissions reductions in South Los Angeles

ABSTRACT In May 2018, the University of Denver repeated on-road optical remote sensing measurements at two locations in Lynwood, CA. Lynwood area vehicle tailpipe emissions were first surveyed in 1989 and 1991 because the area suffered from a large number of carbon monoxide (CO) air quality violations. These new measurements allow for the estimation of fuel-specific CO and total hydrocarbon (HC) emissions reductions, changes in the longevity of emission-control components, and the prevalence of high emitters in the current fleet. Since 1989 CO emissions decreased approximately factors of 10 (120 ± 8 to 12.3 ± 0.2 gCO/kg of fuel) and 20 (210 ± 8 to 10.4 ± 0.4 gCO/kg of fuel) at our I-710/Imperial Highway and Long Beach Blvd. sites, respectively. These reductions are also reflected in the local ambient air measurements. Tailpipe HC emissions have decreased by a factor of 25 (50 ± 4 to 2.1 ± 0.3 gHC/kg of fuel) since 1991 at the Long Beach Blvd. location. The decreases are so dramatic that the vast majority of vehicles now have HC measurements that are indistinguishable from zero. The decreases have increased the skewedness of the emissions distribution with the 99th percentile now responsible for more than 37% (CO) and 28% (HC) of the totals. Ammonia emissions collected in 2018 at both Lynwood locations peak with 20-year-old vehicles (1998 models), indicating long lifetimes for catalytic converters. In 1989 and 1991, the on-road Lynwood fleets had significantly higher emissions than fleets observed in other locations within the South Coast Air Basin. The 2018 fleets now have means and emissions by model year that are consistent with those observed at other sites in Los Angeles and the U.S. This indicates that modern vehicle combustion management and after-treatment systems are achieving their goals regardless of community income levels. Implications: Recent on-road vehicle emission measurements at two locations in the Lynwood, CA area, first visited in 1989, found significant fuel specific CO and HC emission reductions. CO emissions have decreased by a factor of 10 and 20 at each location and HC emissions have declined by a factor of 25. This has increased the skewedness in both species emissions distribution. The 2018 fleets have means and emissions by model year that are now consistent with those observed at other U.S. sites indicating that modern vehicle emissions control advancements are achieving their goals regardless of community income levels.

The drivers behind differences between official and actual vehicle efficiency and CO2 emissions

Literature explaining the gap between official and actual vehicle efficiency and CO2 emissions focuses on descriptive analysis to calculate this gap without examining causality. In this paper, we explore this discrepancy in detail by drawing on a database from Emissions Analytics Ltd. that provides on-road emissions measurement on more than 650 vehicles in the period 2010–2017. The data reveal concerning results: firstly, the gap in data relates both to hybrid vehicles (that are supposedly ‘more fuel-efficient’) and to the biggest selling vehicles (medium-sized cars). Secondly, the average deviation rate increased prior to 2015, but decreased following ‘Dieselgate’. The Volkswagen scandal threw light on the discretionary behaviour of manufacturers on this question and highlighted how weak the official tests are: and this in turn points to a regulatory and compliance problem. In other words, the interpretation of the results suggests that after several years of adaptation to the protocol and the corresponding test (but no translated in real consumption), manufacturers have taken measures to reduce the divergence in real terms after the scandal.

First approach to exhaust emissions characterization of light vehicles in Montevideo, Uruguay

According to Act No. 17283 of November 28th, 2000, air quality protection is a general concern in Uruguay. Road transport is the main emitter of nitrogen oxides (NOx), as the National Inventory of Air Emissions 2006 stated. Actually, it is responsible for the emissions of 59.8% of NOx and 28% of carbon monoxide (CO). The number of households owning a car in Uruguay increased from 29% in 2005 to 39% in 2013, enhancing the importance of characterizing the vehicular emissions of the national fleet. In this paper, a first approach for this characterization is presented. It was carried out on a sample of 11 light vehicles currently in use in Montevideo city, Uruguay. On-road emissions measurements of nitrogen monoxide (NO), carbon monoxide (CO) and carbon dioxide (CO2) were carried out for calculating the emission factors. The fitness of the set of calculated emission factors values to different probability distributions was tested. When possible, the 95% confidence intervals were obtained for the mean emission factors (CO: 2.0g/km±0.3g/km; NO: 0.05g/km±0.01g/km). This procedure was useful to obtaining accurate confidence intervals from a relatively small sample size. Finally, the link between atmospheric emissions and some other parameters of the tested vehicles was studied using a multivariate statistical tool, highlighting the strong increase in carbon monoxide emissions observed for low vehicles speeds and fuel efficiencies.

Modal analysis of real-time, real world vehicular exhaust emissions under heterogeneous traffic conditions

This study presents the characteristics of real world, real time, on-road vehicular exhaust emission namely, carbon monoxide (CO), nitric oxide (NO), hydrocarbons (HC), and carbon dioxide (CO2) emitted under heterogeneous traffic conditions. Field experiments were performed on major category of vehicles in developing countries, i.e. two-wheelers, auto-rickshaws, cars and buses. The on-board monitoring was carried out on different corridors with varying road geometry. Results revealed that the driving cycle was dependent on the road geometry, with two lane mixed flow corridor having lot of short term events compared to that of arterial road. Vehicular emissions during idling and cruising were generally low compared to emissions during acceleration. It was also found that emissions were significantly dependent on short term events such as rapid acceleration and braking during a trip. Also, the standard emission models like COPERT and CMEM under predicted the real world emissions by 30–200% depending upon different driving modes. The on-road emissions measurements were able to capture the emission characteristics during the micro events of real world driving scenarios which were not represented by standard vehicle emission measured at laboratory conditions.

소형자동차 실제도로 주행 배출가스 측정을 위한 국내 주행경로 개발

소형자동차 실제도로 주행 배출가스 측정을 위한 국내 주행경로 개발

Investigating Real-World Emissions of China’s Heavy-Duty Diesel Trucks: Can SCR Effectively Mitigate NOx Emissions for Highway Trucks?

ABSTRACTThe Euro IV emission standards for heavy-duty diesel vehicles require a substantial reduction in NOx emissions, spurring the introduction of selective catalytic reduction (SCR) systems. However, previous studies have found unsatisfactory control of NOx emissions for SCR-equipped urban buses, which has raised concern among policy-makers and researchers regarding the capability of SCR adopted by heavy-duty diesel trucks (HDDTs) in real-world applications. This study tested sixteen HDDTs in China between 2010 and 2014, including six SCR-equipped Euro IV HDDTs using a portable emissions measurement system (PEMS). On-road emission measurement results show no significant difference in NOx emissions between Euro II and Euro III HDDTs. In contrast, we observed a substantial reduction in real-world NOx emissions, as low as 25.4 g kg-fuel–1, for six SCR-equipped HDDTs (Euro IV) compared with those without SCR systems (Euro II and Euro III), providing an overall reduction of ~50%. However, real-world brake-specific NOx emission factors for the SCR-equipped HDDTs were higher by ~45% than the lab test limit of 3.5 g kWh–1 due to off-cycle NOx emissions, indicating the importance of introducing real-world emission test requirements for HDDTs. Due to the introduction of SCR systems, distinctive impacts from real-world operating conditions are observed for HDDTs. For example, fuel-based NOx emission factors steadily decrease as the vehicle speed increases due to higher exhaust temperatures, which improve the efficiency of SCR systems.

Real-driving emissions of circulating Spanish car fleet in 2015 using RSD Technology

In this paper we present the results corresponding to on-road traffic emissions measurements obtained during two field campaigns developed in the Madrid region (Spain) during 2014 and 2015 in the framework of the CORETRA project. The experimental strategy was based on the use of a RSD 4600 remote sensor in interurban roads. These measurements have produced a global database of >190,000 vehicles with their associated emission data (NO/CO2, HC/CO2 and CO/CO2), which can be considered representative of the current Spanish circulating fleet. The results of M1 vehicles were analysed according to their distribution by Euro Standard and engine model. One of the relevant findings is that, despite the progressive introduction of increasingly stringent standards, no NO emission reduction is observed for diesel vehicles with time, although this behavior shows significative differences among brands and engine models.We have also investigated the presence of “high emitter” (HE) vehicles in the Spanish M1 circulating fleet and most of the HE detected corresponded to diesel vehicles with very high NO/CO2 values. With these results at hand, we strongly propose the future incorporation of the “high emitter vehicle” definition into the European environmental legislation, as well as the establishment of specific strategies in each country/region in order to identify these anomalous vehicles. Identification and repair of HE vehicles within the European circulating fleets, although are not easy tasks, should be considered very important for the improvement of air quality in the EU. The use of non-intrusive optical technologies (i.e. RSD) is an excellent option to provide instantaneous real emission data of each individual vehicle without disturbing traffic and for on-road fleet monitoring. In summary, it is a good strategy to obtain valuable information about the long term surveillance of real vehicle emission trends, specially after the introduction of new standard.

Integrated Traffic and Emission Simulation: a Model Calibration Approach Using Aggregate Information

Environmental impacts of road traffic have attracted increasing attention in project-level traffic planning and management. The conventional approach considers emission impact analysis as a separate process in addition to traffic modeling. This paper first introduces our research effort to integrate traffic, emission, and dispersion processes into a common distributed computational framework, which makes it efficient to quantify and analyze correlations among dynamic traffic conditions, emission impacts, and air quality consequences. A model calibration approach is particularly proposed when on-road or in-lab instantaneous emission measurements are not directly available. Microscopic traffic simulation is applied to generate dynamic vehicle states at the second-by-second level. Using aggregate emission estimation as standard reference, a numerical optimization scheme on the basis of a stochastic gradient approximation algorithm is applied to find optimal parameters for the dynamic emission model. The calibrated model has been validated on several road networks with traffic states generated by the same simulation model. The results show that with proper formulation of the optimization objective function, the estimated dynamic emission model can capture the trends of aggregate emission patterns of traffic fleets and predict local emission and air quality at higher temporal and spatial resolutions.

A PEMS study of the emissions of gaseous pollutants and ultrafine particles from gasoline- and diesel-fueled vehicles

On-road emission measurements of gasoline- and diesel-fueled vehicles were conducted by a portable emission measurement system (PEMS) in Shanghai, China. Horiba OBS 2200 and TSI EEPS 3090 were employed to detect gaseous and ultrafine particle emissions during the tests. The driving-based emission factors of gaseous pollutants and particle mass and number were obtained on various road types. The average NOx emission factors of the diesel bus, diesel car, and gasoline car were 8.86, 0.68, and 0.17 g km−1, all of which were in excess of their emission limits. The particle number emission factors were 7.06 × 1014, 6.08 × 1014, and 1.57 × 1014 km−1, generally higher than the results for similar vehicle types reported in the previous studies. The size distributions of the particles emitted from the diesel vehicles were mainly concentrated in the accumulation mode, while those emitted from the gasoline car were mainly distributed in the nucleation mode. Both gaseous and particle emission rates exhibit significant correlations with the change in vehicle speed and power demand. The lowest emission rates for each vehicle type were produced during idling. The highest emission rates for each vehicle type were generally found in high-VSP bins. The particle number emission rates of the gasoline car show the strongest growth trend with increasing VSP and speed. The particle number emission for the gasoline car increased by 3 orders of magnitude from idling to the highest VSP and driving speed conditions. High engine power caused by aggressive driving or heavy loads is the main contributor to high emissions for these vehicles in real-world situations.

Emission Changes Resulting from the San Pedro Bay, California Ports Truck Retirement Program

Recent U.S. Environmental Protection Agency emissions regulations have resulted in lower emissions of particulate matter and oxides of nitrogen from heavy-duty diesel trucks. To accelerate fleet turnover the State of California in 2008 along with the Ports of Los Angeles and Long Beach (San Pedro Bay Ports) in 2006 passed regulations establishing timelines forcing the retirement of older diesel trucks. On-road emissions measurements of heavy-duty diesel trucks were collected over a three-year period, beginning in 2008, at a Port of Los Angeles location and an inland weigh station on the Riverside freeway (CA SR91). At the Port location the mean fleet age decreased from 12.7 years in April of 2008 to 2.5 years in May of 2010 with significant reductions in carbon monoxide (30%), oxides of nitrogen (48%) and infrared opacity (a measure of particulate matter, 54%). We also observed a 20-fold increase in ammonia emissions as a result of new, stoichiometrically combusted, liquefied natural gas powered trucks. These results compare with changes at our inland site where the average ages were 7.9 years in April of 2008 and 8.3 years in April of 2010, with only small reductions in oxides of nitrogen (10%) being statistically significant. Both locations have experienced significant increases in nitrogen dioxide emissions from new trucks equipped with diesel particle filters; raising the mean nitrogen dioxide to oxides of nitrogen ratios from less than 10% to more than 30% at the Riverside freeway location.

Real-world operation conditions and on-road emissions of Beijing diesel buses measured by using portable emission measurement system and electric low-pressure impactor

On-road measurement is an effective method to investigate real-world emissions generated from vehicles and estimate the difference between engine certification cycles and real-world operating conditions. This study presents the results of on-road measurements collected from urban buses which propelled by diesel engine in Beijing city. Two widely used Euro III emission level buses and two Euro IV emission level buses were chosen to perform on-road emission measurements using portable emission measurement system (PEMS) for gaseous pollutant and Electric Low Pressure Impactor (ELPI) for particulate matter (PM) number emissions. The results indicate that considerable discrepancies of engine operating conditions between real-world driving cycles and engine certification cycles have been observed. Under real-world operating conditions, carbon monoxide (CO) and hydrocarbon (HC) emissions can easily meet their respective regulations limits, while brake specification nitrogen oxide (bsNOx) emissions present a significant deviation from its corresponding limit. Compared with standard limits, the real-world bsNOx emission of the two Euro III emission level buses approximately increased by 60% and 120% respectively, and bsNOx of two Euro IV buses nearly twice standard limits because Selective Catalytic Reduction (SCR) system not active under low exhaust temperature. Particle mass were estimated via particle size distribution with the assumption that particle density and diameter is liner. The results demonstrate that nanometer size particulate matter make significant contribution to total particle number but play a minor role to total particle mass. It is suggested that specific certified cycle should be developed to regulate bus engines emissions on the test bench or use PEMS to control the bus emissions under real-world operating conditions.