VOC from Vehicular Evaporation Emissions: Status and Control Strategy

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VOC emissions from the vehicle evaporation process: status and control strategy Huan Liu, Hanyang Man, Michael Tschantz, Ye Wu, Kebin He, and Jiming Hao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04064 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on November 25, 2015

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Environmental Science & Technology

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VOC emissions from the vehicle evaporation process: status and control strategy

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Huan Liu1,2,3,*, Hanyang Man1, Michael Tschantz4, Ye Wu 1,2,3,, Kebin He1,2,3, Jiming Hao1,2,3

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1. State Key Joint Laboratory of ESPC, School of Environment, Tsinghua University, Beijing

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10084, China

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2. State Environmental Protection Key Laboratory of Sources and Control of Air Pollution

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Complex, Beijing 100084, China

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3. Collaborative Innovation Centre for Regional Environmental Quality, Beijing, China

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4. Specialty Chemicals Division, MeadWestvaco Corporation, North Charleston, SC, USA

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*

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Phone (fax): 86-10-62771679; E-mail: [email protected].

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Keywords: VOC emissions; Vehicle; Evaporative emission; Control strategy

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TOC Art

Corresponding author: School of Environment, Tsinghua University, Beijing 10084, China.

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ACS Paragon Plus Environment


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Abstract

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Vehicular evaporative emissions is an important source of volatile organic carbon (VOC),

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however, accurate estimation on emission amounts and scientific evaluation on control

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strategy for these emissions have been neglected outside of the United States. This study

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provides four kinds of basic emission factors: diurnal, hot soak, permeation and refueling.

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Evaporative emissions from the Euro 4 vehicles (1.6 kg/year/car) are about four times

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compared with those of the US vehicles (0.4 kg/year/car). Closing this emissions gap would

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have a larger impact than the progression from Euro 3 to Euro 6 tailpipe HC emission

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controls. Even in the first 24 h of parking, China’s current reliance upon the European 24 h

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diurnal standard results in 508g/vehicle/year emissions, higher than 32 g/vehicle/year from

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Tier 2 vehicles. The US driving cycle matches Beijing real-world condition much better on

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both typical trip length and average speed than current European driving cycles. At least two

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requirements should be added to the Chinese emissions standards: an onboard refueling vapor

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recovery to force the canister to be sized sufficiently large, and a 48 h evaporation test

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requirement to ensure that adequate purging occurs over a shorter drive sequence.

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Introduction

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Tailpipe emissions have been well-controlled across the globe, but vehicular evaporative

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emissions have been largely neglected outside the United States. Evaporative emissions

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represent a major source of atmospheric VOCs, as well as fuel loss, in most tropical and

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temperate zones (1-4). With the control of tailpipe exhaust emissions, evaporation loss

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constitutes an increasing share of the VOC emissions inventory and impacts both air quality

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and energy. From an energy perspective, evaporation from refueling alone will lead to 0.26%

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fuel loss every year in China (5). A further detailed estimation on the full magnitude of

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evaporative emissions from diurnal, running loss, hot soak, and permeation is not available

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because the emission factors for evaporative loss based on local fuel and vehicles are lacking

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in China. From an environmental perspective, vehicular evaporative emissions comprise a

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major source of ambient VOC concentrations (4, 6-7), which are the most important

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precursors of O3 and PM2.5 (8-10). Some researchers use the ratio of the toluene and benzene

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concentrations (T/B) to evaluate the contribution from vehicle sources to ambient VOCs. The

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typical T/B ratio is 2.0 for tailpipe exhaust, whereas this ratio is higher for evaporation due to

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the high toluene concentration in gasoline in Asia. The T/B ratio in most urbanized areas is

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high e.g., 37 in Hong Kong, 10 in Manila and Bangkok, and 6 in Seoul, indicating the major

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contribution is from evaporative emissions (11-14). Researchers in Taiwan found that

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isopentane was the most abundant VOC component in tunnels, which is also a primary

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indicator of evaporative emissions (15).

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European strategies for vehicle emissions control are predominantly followed around the

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world over US strategies (Figure S1). When developing countries choose control roadmaps,

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the first priority to be considered is tailpipe emissions (16). For example, China has

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successfully reduced the threshold of tailpipe hydrocarbon emissions from 2.685 g/km to

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0.075 g/km, dropping 97.2%. There have been previous studies comparing the tailpipe

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emission controls between the US and European strategies (17-18). However, few

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comparisons exist relative to evaporative emissions, which are now becoming dominant

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sources of vehicular VOC emissions. The disparity between evaporative emissions control for

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US and European regulations is even greater than with tailpipe emissions control. For

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evaporative emissions, the distinctions between the two sets of standards include the test

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driving cycle, duration and number of heat builds of the test, inclusion of a running loss test,

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inclusion of a refueling test, and emission limits (19-21). In all aspects, the US regulations are

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more stringent than those of Europe, and cover more critical factors affecting evaporative

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emissions. The US Tier 2 regulations include passenger car standards of 0.65 g/test for 48 h

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diurnal plus hot soak and 0.50 g/test for 72 h diurnal plus hot soak (calculated by adding the 1

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h hot soak result to the worst diurnal day result) (22). Comparatively, the emissions limit for

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Euro 3, 4, and 5 is 2.0 g/test for a 24 h diurnal plus a hot soak (23). As a result, vehicles sold

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in US and Europe are equipped with different technologies to comply with the emissions

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standard. In the US, the technology is called on-board refueling vapor recovery (ORVR),

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which consists of an approximately 2-3 L carbon canister (24). In Europe, the Light Duty

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vehicle fleet is mainly dominated by diesel vehicles, accounting for 55% of all new

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registrations (25). This situation differs notably from China, US, or other major car markets.

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Thus, smaller canisters of 0.5-1 liter without ORVR are widely used in Europe. In China, the

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national V emission control standards, which equals to Euro 5, has been published and will

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be implemented since January 1, 2018. Both the national and Beijing environmental

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protection agencies are discussing the next step on vehicle emission standards, either Euro 6

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or Tier 2/3 in the future.

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Although there are already two types of regulations established, the lack in understanding of

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evaporative emissions is an obstacle for most developing countries to build their own

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roadmaps for implementing emissions standards. First, emission factors and profiles of

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evaporative VOCs are extremely limited. Vehicular evaporative emissions can be generally

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grouped into hot soak, diurnal, permeation, running loss and refueling processes (20). A set

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of Sealed Housing for Evaporative Determination (SHED) tests is required to obtain the

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emission factors, but these tests are complex and expensive. Pang et al. (26) reported

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evaporative emission factors for the US from 49 in-use vehicles between the 1999 and 2003

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fleets. Mellios et al. (27) tested four vehicles in Europe to validate the evaporative emission

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results. Second, real-world environmental and vehicle design conditions influence the

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emissions and require modeling to tie laboratory testing together with fleet inventory

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estimates. These conditions include: (a) gasoline volatility; (b) fuel system components; (c)

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absolute ambient temperature and temperature variation; (d) driving conditions (e.g., average

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trip length, speed, and acceleration); and (e) vehicle use behavior (vehicle use frequency,

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parking time) (21, 28-29). Thus, a systematic study on evaporative emissions must be

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conducted.

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In this paper, we aim to answer the following questions: 1) What are the emission amounts

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for now and for future under different control policies, and 2) what is the recommended

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control strategy for developing countries to further control evaporative VOCs? A set of

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cross-over experiments are designed to build emission factors, to quantify difference between

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Tier 2 and Euro 4 vehicles, and to identify the performance variance of a vehicle under

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different test procedures. Real-world investigations are conducted to compare the regulations

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with actual conditions. We evaluate US and European evaporation control strategies from the

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following factors: emission factors for each emission process, effectiveness of the driving

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cycle on canister purge, relevance between driving cycle and real-world traffic conditions,

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and vehicle parking time versus duration of diurnal test in the standards.

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Methods

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Laboratory tests: emission factors and canister purge

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In total, thirty cross-over tests were conducted in a gas tight Imtech variable-temperature

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SHED (VT-SHED) chamber in China to evaluate the total amount of evaporative emissions

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and emission rate. These cross-over tests covered five vehicles of two technical categories,

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four evaporation processes, and two regulatory authorities’ testing procedures. Five vehicles,

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including two US Tier 2 vehicles and three Euro 4 vehicles (equivalent to China 4) were

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recruited (Table S1). The three Euro 4 vehicles represent a cross-section of vehicles for China,

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including a compact car, a MPV, and a middle car. All these three models dominate the

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league table for car sales in China 2013. The selection of Tier 2 vehicles is somewhat limited

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to what vehicles we will have access to in China. In current Chinese fleet, only a small

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portion of luxury imported cars are Tier 2. The Tier 2 and Euro 4 vehicles were mainly 6


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distinguished by the vapor control technology: ORVR or conventional diurnal control. The

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conventional carbon canister of the Euro 4 vehicles is widely used in China. More

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information on the test facility can be found in a previous publication (5). The fuel used in

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this study was non-oxygenated (E0) gasoline from Sinopec. Sinopec is the largest supplier of

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petroleum products in China, having more than 60% Chinese market share. Table S2

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provides detailed parameters for fuel quality. The Reid vapor pressure (RVP) is 8.4 psi. In

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Europe, fuel RVP was required to between 8.1 to 8.7 psi, while in US this value was required

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to between 8.7 to 9.2 psi. The diurnal, refueling, hot soak and permeation evaporative

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processes were considered in this study. Running loss was not included in this study because

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no facility in China could accommodate the test procedures or track-based tank temperature

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profile generation. The test procedures included US 48 and 72 h diurnal plus hot soak,

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permeation, US refueling tests, and Euro 24 h diurnal plus hot soak tests. Details on the test

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procedures are provided in Table S3 (22, 23). For the diurnal, hot soak and refueling tests,

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every vehicle was subjected to each test in the VT-SHED, and a flame ionization detector

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(FID) analyzer was employed to determine the total emissions. For the permeation tests, the

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canister was vented of the VT-SHED and a constant 72°F was used. During the vehicle

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preconditioning and conditioning driving of each test, the carbon canister purge rate was

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monitored by a flowmeter taking an instantaneous measurement each second. The canister

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was also weighed immediately before and after the purge. During the weighing procedure,

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there is no significant weight loss of the canister without air flowing through. ORVR

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efficiency was generated based on canister weight gain versus total emissions (5).

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Field test: driving cycle and parking duration. 7



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The real-world vehicle activity, including on-road driving cycles and vehicle parking

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durations, were collected from 127 passenger cars in Beijing from April 25 to September 7,

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2013. These vehicles were randomly selected, and 80% were private cars and 20% were

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business cars. This percentage is comparable with the private car percent statistic (85.5%) in

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Beijing. All the vehicles had GPS units installed (Multi-Function Columbus GPS data logger

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V-990, produced by GPS WebShop Inc., in Markham, ON, Canada), and drivers maintained

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their business-as-usual during the test time period. A sensor was embedded to the GPS unit to

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capture the voltage change of the cigarette-lighter. The GPS data logger was set to

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automatically to turn on or off when the engine of the investigated car is turned on or off.

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Thus, both the second-by-second driving activity and parking activity could be monitored. In

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total, 26,620,403 valid seconds of driving were collected. The driving cycle successfully

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covered all types of roads (e.g., highway, arterial, sub-arterial and residential) in Beijing on

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both weekdays and weekends.

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To account for start-stop (i-stop) technology, a minimum duration of 180 s was set for normal

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parking events. Overall, 8,352 parking events from 127 passenger vehicles were collected in

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Beijing and is considered statistically representative. (31)

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Estimation of emissions.

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Diurnal, hot soak, permeation and refueling evaporative emissions were calculated for

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vehicles in China. The average emission factor of each type vehicle was used to calculate

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corresponding emissions. As the vehicle technology category Euro 4 is for all of China, the

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evaporative emissions were calculated for single car in China using Beijing vehicle activity. 8


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Parking events were classified into three categories: shorter than 24 h, between 24 and 48 h,

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and longer than 48 h. The emission categories are further defined as: emissions happen in the

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first 24 h, in 24-48 h, in 48-72h, and after 72 h. Different equations and emission factors were

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used for each category. The detailed equations and parameters were discussed in Table S4

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and Table S5.

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It should be noted here, all the emission factors for calculation are using the corresponding

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test results for corresponding vehicle categories, e.g., Tier 2 vehicle's diurnal emissions on

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the first, second, third day were used for Tier 2 vehicles only. There is one exception, that the

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EFdiur>72 is get assigned the value of emission factors of Euro 4 vehicles during the third day

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diurnal for both Tier 2 and Euro 4 vehicles, as emissions without control. The combined

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efficiency of the Stage-II vapor control system considered the coverage percent, e.g., 70% in

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Beijing (5).

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Results

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Emission factors for vehicles certified to US and European standards from laboratory

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testing.

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Table 1 lists emission factors on two US Tier 2 vehicles, three Euro 4 vehicles and the

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average. In general, the emissions from Tier 2 vehicles are considerably lower than those

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from Euro 4 vehicles. Based on the European testing procedures (24 h diurnal), all vehicles

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tested below the certification limit of 2 g/test independent from whether an ORVR or small

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canister was utilized. However, significant differences between the Euro and US vehicles

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were found when following the US 48 h and 72 h test procedures. The emissions from Euro 4 9



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vehicles exceeded the 2 g cap in the first 24 h. Over time, the emissions from Euro 4 vehicles

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increased to 6.369±1.033 g/day on the second day and 8.144±1.989 g/day on the third day.

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The combined control efficiency of the Euro 4 vehicles was reduced to approximately 65%,

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based upon the ratio of canister weight gain and total vapor generated. US Tier 2 vehicles

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showed reliability on vapor control, where the control efficiency is higher than 98% and

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emissions remained below 0.3 g/day across all tests. As a result, parking emissions from Euro

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4 vehicles are approximately 23 and 37 times higher than those of US Tier 2 vehicles on the

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second and third days, respectively.

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Another major difference is from refueling vapors. The Euro 4 vehicles failed to control the

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refueling emissions, because there are no control requirements on European vehicles. Since

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the US Tier 2 vehicles are certified with ORVR, European vehicle refueling emissions are 71

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times higher than US Tier 2 vehicles. This effect is relevant only if refueling station Stage II

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controls are not considered. For the other two processes, i.e. hot soak and permeation, the

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differences are considerably smaller because the magnitude of the base uncontrolled

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emissions are small and controlled emissions are not influenced by the canister. The

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permeation rate is estimated from the total daily SHED measurement with canister emissions

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vented outside the SHED. Here, permeation emissions could include gasoline emissions from

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small leaks and from the engine as well as for non-fuel based hydrocarbon emissions (e.g.,

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tire and upholstery). A series of tests focused specifically on the fuel tank and liquid filled

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hoses could provide more accurate permeation-only estimates.

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All the results reveal that, US and European regulations have significant differences in

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evaporative losses. For the US Tier 2 vehicles, the control efficiencies exceeded 98% in all

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tests. Although the Euro 4 vehicles meet the 24 h diurnal certification limits required in

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Europe, they performed poorly under other test conditions outside the certification tests.

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Impact of the driving cycle on emission control performance.

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To determine the underlying reason for the different performances of the Euro 4/Tier 2

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vehicles under different test procedures, we analyzed the conditioning driving cycle and

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found that it was a key factor influencing the final emissions, particularly for the first

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24-hours of diurnal control, since the canisters were designed with different capacities based

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upon the vapor load of the certifying procedures.

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The impact of driving cycles is primarily through the purge process of the canister. Assuming

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the same amount of vapor is generated under similar vehicle usage and temperature

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conditions, then the emission released would only be influenced by the canister control.

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The impact from the driving cycle on the canister purge was analyzed. The second-by-second

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purge rates of canisters were measured during multiple driving cycles based on dynamometer

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tests. The second-by-second speed profiles for several driving cycles used in this study are

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provided in Figure S2 (23, 32). Figure 1 shows purge maps based on vehicle running speeds

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and vehicle-specific power (VSP) (33-35). The Euro 4 vehicles show a moderate purge effect

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during the NEDC driving cycles; the average purge rate was 3.3±1.8 L/min. However, the

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performance is poor during the other driving cycles, such as during UDDS and NYCC. The

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purge rate for Euro4 vehicles averages only 1.1±0.7 L/min across the FTP driving cycle. The

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high purge rates could only be achieved in the high-speed zones (>60 km/h), which occur 11



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only in the EUDC driving cycle. The US Tier 2 vehicles have larger purge rates (17.36±0.97

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over the NEDC and 7.17±3.20 over the FTP), which are approximately 6.8 times higher than

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those of the Euro 4 vehicles. In more than 80% of the driving time, the Tier 2 vehicles purge

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greater than 3 L/min.

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Real-world vehicle usage and parking behavior.

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Multiple factors influence real-world vehicular evaporative emissions. In this section, we

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analyzed the real-world vehicle activity, including trip length, driving cycle and parking

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durations, and then compared real-world conditions with multiple regulated driving cycles.

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In total, data on 127 cars, including 26,620,403 valid seconds and 8,352 continuous parking

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events, were collected in Beijing, China. Figure 2 summarizes the activity specifics of vehicle

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usage. The trip length distribution was compared with data for an Italian province, Modena

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(30) (Figure 2a). The distribution of trip length in Beijing and Modena are highly similar.

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More than 50% of trips in both cities are less than 5 km, whereas 90% trips are shorter than

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20 km. The average distance of all trips was only 11.2 km in Beijing. This indicates the purge

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time would be extremely limited for real-world driving, which would reduce the canister

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capacity significantly.

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Figure 2b provides the average speed for each trip length category. The average speed for all

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trips is 23.57 km/h. The NEDC driving cycle used for regulation in the European evaporative

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test procedure (driving cycle before 24 h test) runs up to 59 minutes and 32.8 km, which is

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nearly three times longer than the average trip distance in real world. The average speed is

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33.3 Km/hour for NEDC driving cycle, which is 41% higher than the average speed in the 12


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actual conditions. The NYCC, UDC, UDDS cycles and EPA-75 (driving cycles before the 48

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h test) have speeds comparable with the real-world city speeds, and EUDC, UDDS and

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EPA-75 are all representative for trip distance. In the US, a single EPA-75 is used in the EPA

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48 h evaporation test procedure to purge the canister (UDDS + 10 min soak + 505; 17.77 km

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total distance). This indicates that a test procedure similar to the EPA 48 h evaporation test is

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needed in China to ensure adequate purge occurs over a short and moderate driving

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sequences. The EPA 72 h evaporation and ORVR test procedures provide additional driving

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time and distance (EPA-75 + UDDS + NYCC + NYCC + UDDS; 45.54 km total distance to

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purge the canister), but these procedures are more important in their use to establish canister

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capacity, control running loss, and control refueling.

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Beijing limits by law the days in which vehicles may be driven within the Fifth Ring Road.

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Based on the vehicle parking monitoring results, the average number of days without vehicle

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usage is 2.3 days per week in Beijing. On the days with driving, the average number of daily

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parking events was 3.6, which is similar to the result in Florence, Italy (3.5

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events/vehicle-use-day) (30). Considering the days without vehicle usage, the average value

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for parking events was 2.4 events/day, and the average duration of parking event was 9.4 h.

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The parking duration for vehicle usage is analyzed in Figure 2c. The percentage of parking

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events with durations shorter than 2 h was more than 45% in number but only 2.6% in total

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duration. Parking events with durations under 24 h have percentages of 86.6% in number and

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37.6% in total duration. The parking data were further used to calculate real-world diurnal

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emissions.

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The on-road driving cycles in Beijing were analyzed by purge maps and then compared to

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different regulated driving cycles: ECE-15, EUDC, NYCC, UDDS and WLTC (Worldwide

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harmonized Light vehicle Test Procedure (36) (Figure S3). Only 20.4% of the bins, when the

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Euro 4 vehicles could effectively purge, mimic real-world driving. Compared with the actual

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driving bin distribution, the US UDDS or NYCC driving cycle is closer to on-road conditions

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than the Euro regulations. The Low3 and Medium3-2 phases of the WLTP driving cycle are

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also similar to the actual driving cycle. However, both the High3-2 and Extra High3 phases of

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the WLTC and EUDC phase of NEDC differ considerably from actual urban driving. The

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statistical purge rate of the Euro 4 vehicles in real-world driving patterns is only 1.3 L/min,

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which is lower than the purge rate for any regulated driving cycle, indicating that the actual

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final emissions in real-world would be worse than those in laboratory tests.

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In summary, the EPA-75 represents Beijing conditions best based on typical trip length and

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average speed. The incorporation of the FTP75 in combination with high canister capacity

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resulting from ORVR would generate purge maps very well suited for Beijing. The current

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regulation for 24 h control permits a driving cycle that is too long, contains too much high

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speed driving, which results in significant purge rates occurring during only high speed

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driving. Because of the characteristics of Euro 4 purge calibrations, it is expected that

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emissions factors for Euro 4 vehicles in Beijing are higher than estimated in this study.

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Real-world evaporative emissions by US or Euro control strategies.

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The laboratory test-based emission factors, canister performance and real-world vehicle

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usage were combined to calculate the on-road evaporative emissions (Figure 3). For the Euro 14


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4 vehicles used in a Stage II fully covered region, the total evaporative emissions are 1,557 g/

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vehicle/year, including 1,169, 73, 27, and 288 g/vehicle/year for diurnal, hot soak,

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permeation, and refueling emissions, respectively. For Euro 4 vehicles in the region without

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Stage II, the refueling emissions factor is 959 g/vehicle/year, and the total evaporative

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emissions are 2,228 g/vehicle/year. For the Tier 2 vehicles, regardless of whether Stage II is

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in the service stations, the total evaporative emissions under the same conditions are only 418

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g/vehicle/year. This indicates the real-world percent reduction of evaporative emissions could

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reach 70% based on US regulations compared with Euro regulations.

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Among all four evaporation processes examined in this study, the diurnal emissions represent

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the largest portion. This result is also observed in other countries (3-4, 37). Even in the first

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24 h of parking, China’s current reliance upon the European 24 h diurnal standard results in

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508g/vehicle/year emissions, higher than 32 g/vehicle/year from Tier 2 vehicles. This

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indicates that the canister capacity and purge calibration resulting from Euro 24 h diurnal

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standards do not result in suitable control, particularly in large cities. For Euro 4 vehicles, the

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diurnal emissions happen in the first 24 h parking contribute 43.5% of the total diurnal

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emissions. For Tier 2 vehicles, the diurnal emissions in the first 24 h parking only contribute

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19.7% of the total diurnal emissions. The majority of diurnal emissions for Tier 2 vehicle is

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from long-term parking, e.g. 75.9% from parking longer than 72 hours. If the US 48 h

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regulation could be implemented, 65.8% of the total diurnal emissions would be under

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control. If ORVR is implemented along with a 48 h evaporation test requirement, the ORVR

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test will force the canister to be sized sufficiently large to control three diurnal days of

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hydrocarbon emissions. Thus, a combination of ORVR with a 48 h evaporation test could 15



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provide long-duration control and ensure short-trip/low-speed purge performance.

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The distance-based emission factors for real-world evaporative emissions were calculated

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from the total emissions and total VKT and then compared to the tailpipe emissions in

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Beijing (Figure 4). Since more than 95% of the fleet are certified to European standards (the

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remaining are designed for US certification), the evaporative emission factors for the Euro 4

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vehicles were used in this comparison. Considering a fuel efficiency of 10.14 L/100 km and a

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30.5 km/day VKT as the fleet average, the distance-based evaporative emission factor is 0.14

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g/km. This calculation includes the 70% refueling vapor control efficiency from the Stage-II

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vapor recovery systems installed in Beijing. For areas without the Stage-II system, the

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evaporative emission factor is 0.21 g/km. The reported tailpipe NMHC emission factors are

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0.66, 0.31, 0.19, 0.08 and 0.06 for Euro1 to Euro5 vehicles in China (38-39) and 0.25, 0.12,

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and 0.07 for in-use Euro 2, Euro 3 and Euro 4 cars in the Copert 4 model (40), respectively.

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The evaporative emissions are higher than the Euro3 tailpipe emissions. Thus, it is important

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to enhance the vehicular evaporative emission control to reduce this major contribution of

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VOCs emissions.

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Additionally, the potential to reduce evaporative emissions with improved controls resulting

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from regulatory progress was analyzed. With stricter evaporation control regulations, such as

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the US strategies, the evaporative emissions could be reduced by 81%, from 2,228 to 418

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g/vehicle/year. Thus, the reductions would provide nearly the same benefits as achieved with

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reducing tailpipe emissions from Euro 3 to Euro 6. On the other hand, the effect from Stage II

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system installed in service stations on the total evaporation reduction is smaller. When the

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Stage II cover rate changes from 0 to 100%, the evaporative emissions change from 2,228 to

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1,557 g/vehicle/year. The effect is smaller because the Stage II controls refueling emissions

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only.

332

It should be noted here, all these estimates do not include running losses. To the best of our

333

knowledge, no running loss emission factor data of Euro 4 vehicles has been given by

334

previous studies. Thus, running losses could only be evaluated using the emission factors

335

provided by MOVEs2010 (20). The emissions factors of model year group 1978-1995, when

336

ORVR were not carried out, were used for Euro 4 vehicles. The emission factors of Model

337

year group 2004 and later were used for Tier 2 vehicles. According to our real-world vehicle

338

activity data, the average running length of vehicles in Beijing was 1.30 hours per day. Then

339

the running losses of Tier 2 vehicles were estimated to be 111 g/vehicle/year, while those of

340

Euro 4 vehicles reached 5486 g/vehicle/year. However, it should be noted that, although

341

running losses were not considered in the European standard, advanced vehicle technologies

342

and materials have been used since 1995. Thus, there would be a relatively large uncertainty

343

using the emission factors of Model year group 1978-1995 in MOVES2010 to represent those

344

of the current Euro 4 vehicles.

345

Discussion

346

A comparison was made between our results with other studies in different countries. Several

347

test programs (41-43) in the US evaluated the evaporative emissions from Tier 0 to Tier 2

348

vehicles. Figure S4 displays the comparison between the results of our study and those of

349

these test programs. For diurnal emissions, when Euro Type IV tests were conducted, the 17



350

average emission from Euro 3/4 vehicles in the first day was 0.834 g/day, lower than that

351

from Tier 1 vehicles (0.378 to 0.747 g/day) but similar to that from normal Tier 0 vehicles

352

(1.564 to 2.053 g/day). However, when US tests were conducted, the average emission from

353

Euro 3/4 vehicles changed to 2.962 g/day, higher than that from normal Tier 0 vehicles. For

354

permeation emissions, the average emission from Tier 0 vehicles estimated by CRC E-9

355

program (violet bar) was 0.554 g/day, similar to that from Euro 3/4 vehicles in this study

356

when RVP and temperature in these tests were both similar. Overall, these results indicated

357

that evaporative emission from Euro 3/4 vehicles was between those from Tier 0 and Tier 1

358

vehicles.

359

A car in Europe emits 602-1,147 g/year VOCs through diurnal, hot soak and permeation (29).

360

The difference between European studies (the high end) with our study is approximately

361

9.5%. A car in Japan emits 526 g/year VOCs through diurnal (using the total vehicular

362

evaporative emissions 37,476 tons/year divided by the number of gasoline vehicles of

363

71,244,174) (4), which is approximately 50% of the estimation in our study. A car in South

364

Africa emits 0.97% of its fuel use through diurnal, hot soak and running loss (3), which is

365

converted to 7,647 g/year VOCs (considering VKT as 12000 km/year, fuel economy as 0.09

366

L/km, gasoline density as 730 g/L). The estimation in South Africa is higher than our

367

estimation and due to the large value of running loss. The difference could also be explained

368

by different environment temperature, fuel RVP, vehicle technology and driving activity.

369

US regulations fit China better compared with European regulations. In general, the results

370

show that a combination of ORVR with a 48 h evaporation test could improve emissions

18


Page 18 of 31

Page 19 of 31


371

from both short and long-term parking events as well as ensure short-driving/low-speed purge

372

performance. It’s also important to make sure the new regulations are representative for

373

real-world condition, especially the purge driving cycle. Evaporative emissions are a major

374

source of VOC emissions in China and reductions are necessary. Control technologies and

375

regulatory approaches that lead to effective controls are well-established and can serve as a

376

basis for China to develop a set of evaporative control regulations optimized and best-suited

377

for its local vehicle fleet, climate, and driving patterns.

378

Acknowledgement

379

This work is supported by the National Natural Science Foundation of China (41571447), the

380

National Environmental Protection Public Welfare Research Fund (201409021), the National

381

Program on Key Basic Research Project (2014CB441301) and MeadWestvaco Corporation.

382

We thank Mr. Ken Middleton, Dr. Wei Shen, Dr. Timothy Wallington, and Mr. Weijian Han

383

from Ford Motor Company for helpful discussions. We thank Mr. Giorgos Mellios (EMISIA

384

Co.) for reviewing the draft and providing helpful comments.

385

Supporting Information Available

386

The supporting information section contains 4 figures and 2 tables. The Supporting

387

Information is available free of charge via the Internet at http://pubs.acs.org/.

388

19



389

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530 531

Figure 1. Canister purge rate of (a) Euro 4 vehicles under NEDC cycle, (b) Euro 4 vehicles

532

under FTP cycle, (c) Tier 2 vehicles under NEDC cycle, and (d) Tier 2 vehicles under FTP

533

cycle

27



534

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Figure 2. Characteristics of real-world vehicle usage: (a) trip length distribution for each trip

536

category; (b) average speed for each trip category; (b) parking duration distribution

28


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537

538

Figure 3. Evaporative emissions in real world

29



539 540

Figure 4. Comparison between tailpipe emissions and evaporative emissions

30


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541


Table 1. Emission factors for five evaporative processes of Euro 4 and Tier 2 vehicles Vehicle

Vehicle

Vehicle

Vehicle

Vehicle

1

2

3

4

5

Tier 2 24-hour diurnal (g/day)

Euro 4

Average Emission factors of Tier 2 vehicles

Average Emission factors of Euro 4 vehicles

0.323

0.270

0.876

1.010

0.616

0.297±0.026

0.834±0.164

Day1

0.316

0.300

4.229

1.586

3.663

0.308±0.008

3.160±1.136

Day2

0.274

0.288

7.715

6.190

5.202

0.281±0.007

6.369±1.033

Day1

0.344

0.227

5.867

1.804

0.620

0.286±0.058

2.764±2.247

Day2

0.263

0.199

8.917

6.192

1.302

0.231±0.032

5.470±3.151

Day3

0.254

0.187

9.970

9.085

5.378

0.220±0.034

8.144±1.989

Refuelling (g/L)

0.010

0.014

0.950

0.760

0.835

0.012±0.002

0.848±0.078

24 hour

0.068

0.041

0.071

0.103

0.075

48 hour

0.065±0.015

0.066±0.026

0.070

0.081

0.049

0.0791

0.020

0.212

0.214

0.375

0.709

0.668

0.213±0.001

0.584±0.179

48-hour diurnal (g/day)

72-hour diurnal (g/day)

Hot soak (g/hour)

Permeation (g/day) 542 543

31


VOC from Vehicular Evaporation Emissions: Status and Control Strategy

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