Experimental studies of the dilution of vehicle exhaust pollutants by environment-protecting pervious pavement

Abstract

This study determines whether environment-protecting pervious pavement can dilute pollutants immediately after emissions from vehicle. The turbulence-driven dry-deposition process is too slow to be considered in this aspect. The pavement used is the JW pavement (according to its inventor's name), a high-load-bearing water-permeable pavement with patents in over 100 countries, which has already been used for more than 8 years in Taiwan and is well suited to replacing conventional road pavement, making the potential implementation of the study results feasible. The design of this study included two sets of experiments. Variation of the air pollutant concentrations within a fenced area over the JW pavement with one vehicle discharging emissions into was monitored and compared with results over a non-JW pavement. The ambient wind speed was low during the first experiment, and the results obtained were highly credible. It was found that the JW pavement diluted vehicle pollutant emissions near the ground surface by 40%–87% within 5 min of emission; whereas the data at 2 m height suggested that about 58%∼97% of pollutants were trapped underneath the pavement 20 min after emission. Those quantitative estimations may be off by ±10%, if errors in emissions and measurements were considered. SO₂ and CO₂ underwent the most significant reduction. Very likely, pollutants were forced to move underneath due to the special design of the pavement. During the second experiment, ambient wind speeds were high and the results obtained had less credibility, but they did not disprove the pollutant dilution capacity of the JW pavement. In order to track the fate of pollutants, parts of the pavement were removed to reveal a micro version of wetland underneath, which could possibly hold the responsibility of absorbing and decomposing pollutants to forms harmless to the environment and human health.

Implications:

The potentials of pervious pavements in improving the urban environment are advancing every day. In this paper, the authors present evidences of automobile exhaust dilution by a special-designed high-load-bearing pervious pavement. This study outlines an effective approach on evaluating such dilution effect; still more studies are needed.

Supplemental Materials: Supplemental materials are available for this article. Go to the publisher's online edition of the Journal of the Air & Waste Management Association for experimental studies of the dilution of vehicle exhaust pollutants by environment-protecting pervious pavement.

Introduction

Theoretically, if every motor vehicle were to meet the local emissions standards, the air pollutants emitted from vehicles, including CO₂, would directly enter the atmosphere and could not be further reduced. These emissions contribute to the formation of fine particles and photochemical ozone pollution in metropolitan areas as well as to global warming (Seinfeld and Pandis, 2006). This study explores whether water-permeable pavement (Ferguson, 2005) can dilute motor vehicle pollutant emissions and, if so, how effective this technology is.

The removal of pollutants from air by pavement considered here has to be much more efficient than the turbulence-driven dry-deposition process, which is to transport species from the atmosphere onto surfaces in the absence of precipitation (Seinfeld and Pandis, 2006). The rate of species settled through deposition is governed by the level of atmospheric turbulence, the chemical properties of the target species, and the nature of the surface. The estimated deposition velocity (Arya, 1999) usually ranges from 0.05 to 10 cm sec⁻¹. Therefore, pollutants from an idled car exhaust with temperature (at least 380 ° F) (Ning et al., 2005) higher than the ambient air have to travel a long distance before finally settled on the surface. What if the pavement can trap pollutants right after they are released, before photochemical reactions among them are fully active?

Mitigation of urban heat island effects by increasing coverage of urban trees and high-albedo surfaces are considered potentially useful to reduce urban energy use and improve local air quality (Akbari et al., 2001). Therefore, the Leadership in Energy and Environmental Design (LEED), an internationally recognized green building certification system, gives credit to pavements with a solar reflectance index (SRI) (Boriboonsomsin and Reza, 2007) of at least 29 (U.S. Green Building Council (USGBC), 2010). In the meantime, pervious pavements are effective in reducing runoff and permitting treatment of pollution from stormwater prior to release into the urban drainage system (Tennis et al., 2004). Otherwise, submicrometer fine-particle pollutants are constantly produced from the road pavement-tire interface (Dahl et al., 2006).

Road pavement that can swiftly dilute air pollutants emitted from motor vehicles can be referred to as environment-protecting pavement. Since dry deposition can't be the responsible mechanism, the chemical compositions of the pavement materials are hence unimportant, but rather the structure of the specific pavement is critical. The pervious pavement used in this study is not made of conventional porous concrete, porous asphalt, grass permeable paver, or commercially available brick or block paver, but rather it is an environment-protecting concrete pavement with high-load-bearing properties and high-permeability characteristics. This pavement construction method has obtained patents in more than 100 countries and is referred to as the “JW ecological method” or “JW pavement,” in reference to the inventor's English initials.

This pavement was installed in the campus of the Taipei National University of Technology in 2003, along with several other commercially available permeable pavements (National Taipei University of Technology (NTUT), 2003). These permeable pavements were primarily used as campus roads, including pedestrian walkways and roads for automobile traffic on campus. In the past 5 years, several buildings located next to permeable pavement surfaces have been renovated due to their old age (they were all built around 1970). As a result, the vehicles passing over these pavements included trucks carrying construction materials and cranes lifting heavy weights to the rooftops of nearby 4- and 10-floor buildings. After experiencing considerable heavy vehicle traffics from these renovations, the JW pavement used in this experiment was free of cracks and did not require any repairs. However, all other pervious pavements installed at the same time required repairs at least twice (Tsai, 2010).

In addition, these pervious pavements have experienced several heavy rainfall events since installation and the penetration of the collected water through the JW pavement remains fastest in the area (Tsai, 2010). Due to the accumulation of dust and fallen leaves, most permeable pavements eventually lose their water permeability due to the occlusion of water-permeable pores (Scholz and Grabowiecki, 2007). The Taipei National University of Technology has no funding for regular cleaning of these surfaces. Therefore, most permeable pavements lost their water permeability after few years of installation or repair, except for the JW pavement due to its special design (Li, 2004; Chen, 2005).

For the studies described in this paper, pavements were swept clean before experiments were performed. Part of the campus was closed, with automobile pollutant emissions only over the selected JW pavement and a neighboring brick pavement (which is referred to as the non-JW pavement), so as to determine whether the JW pavement has the diluting functions on automobile gas pollutants.

JW Pavement

In recent years, construction of green buildings is popular (Kibert, 2005). Water-permeable pavements are highly recommended by LEED (U.S. Green Building Council (USGBC), 2010). The structure of the selected JW pavement is illustrated in Figure 1. An air-cycle aqueduct frame made of polypropylene (PP), as shown in Figure 2a and marked in Figure 1, acts as conventional steel rods for reinforcing concrete. Once installed, adhesive concrete are poured over the frame so that the frame will be embedded into the dried concrete later. Only tiny holes can be revealed as shown in Figure 2b. Tests (Li, 2004) revealed that 7.5-cm-thick pavement could withstand a vertical point load of about 20–30 kg/cm² (equivalent to 300–450 psi). Normally, truck produces approximately 90 psi. If the thickness is extended to 10 or 15 cm, the pavement can withstand 30–40 and 40–60 kg/cm², respectively.

Figure 1. Basic structure of the environment-protecting pervious pavement (or named as the JW pavement).

Figure 2. (a) The JW air-cycle aqueduct frame. (b) The finished pavement surface. (c and d) What was observed below the pavement that was used for the experiments in this paper.

Through aqueducts, rainwater falls directly into the lower layers. Tests (Li, 2004) showed that, when all the aqueduct entrances were not blocked by dusts and leafs, the pavement had an average infiltration rate of approximately 12,557 mm/hr; if blocked, it still maintained an average infiltration rate of 1487 mm/hr. It is because of the interior smooth circular surface of each aqueduct that water can slip through downward along the interior edge. Furthermore, the benefit of open-ended aqueducts guarantees nonstagnant airflow and the exchange of materials above and below the pavement, especially under the influence of moving vehicles.

The molecular formula of polypropylene is (C₃H₆)_n with a melting point of 171 ° C. Under normal condition, air pollutants won't react with the solid PP. PP is only liable to chain degradation from exposure to heat and ultraviolet (UV) radiation. Figure 2b shows that after construction, the frame is enveloped by concrete and only small holes can be seen from above. The influence of UV irradiation on degrading and aging the polypropylene (Kaczmarek et al., 2005) is avoided. For this research, part of the pavement has been damaged to reveal the condition below the surface. Figure 2c, d show that the frame and the surrounding concrete are intact after 8 years, whereas the gravel and soil layers below are either covered with water (Figure 2c) or being wetter than the upper layer (Figure 2d). For the condition in Figure 2d, the water content is about 4.2% and 21.5%, respectively, in the gravel and soil layer. The pH level measured in the water shown in Figure 2c is about 11.7. For comparison, the yearly averaged pH level of rainfall in Taipei is 5.3, 5.2, 5.2, 5.1, 4.9, 5.0, 5.3, and 5.2, respectively, from 2003 to 2010 (Central Weather Bureau, Taiwan, 2011). Clearly, with sufficient moisture, urban air pollutants deposited into the lower layers will be either dissolving into water or attaching to gravels or soils (Schmel, 1980; Arya, 1999; Seinfeld and Pandis, 2006).

The interior smooth circular surface of each aqueduct can't act as an adsorbent to accumulate materials. Still, each aqueduct can be blocked by accumulated dusts and leafs. If vacuumed and washed regularly, blocking can be avoided. But with no regular maintenance at all at the experimental site, it is noted that the JW pavement still holds a notable rainwater infiltration condition during the past 8 years (Tsai, 2010), which suggests that the rainwater can flush clean each aqueduct quite efficiently and supports the fact that nothing can accumulate stubbornly along the inner surface of aqueduct.

The concrete is a mixture of cement and sand (fine or coarse). Minerals such as tricalcium silicate (Ca₃SiO₅), dicalcium silicate (Ca₂SiO₄), tricalcium aluminate (Ca₃Al₂O₅), calcium aluminoferrite (Ca₄Al_nFe_2-nO₇), and silica (silicon dioxide, SiO₂) are detected. The solid concrete pavement won't react with air pollutants and is commonly named as the hard pavement in contrast to the soft asphalt pavement for remaining intact after many years (Papagiannakis and Masad, 2008). Still, the surface is not smooth; pollutants can be captured into tiny holes on the surface. However, such opportunity-based deposition process is too slow to be considered in this study.

Certainly, over the surface of the road pavement, many different kinds of materials could fall every day, such as leafs, dusts, oil greases, etc. They could possibly play certain roles on affecting the fate of pollutants in the air, but it is impossible to trace them constantly and must be ignored.

In the meantime, with sufficient air and water, tree roots are growing widely under the pavement (Figure 2c, d). To conventional rigid pavement, tree roots are not allowed to penetrate under the pavement (Papagiannakis and Masad, 2008), for the structure will crack due to upward forcing from roots. But in this case, the JW pavement has shown no sign of crack or any other kind of damage caused by tree roots. It is due to the fact that the loosened gravel and soft soil layers let the tree roots to expand freely. Furthermore, the existence of meiofauna such as chironomid larvae and desmoscolex and the detection of a considerable amount of microorganisms in the lower layers are astonishing (Chao, 2011; Chen, 2011). This phenomenon suggests that the decomposition of grease and organic compounds, the fixation of nitrogen for tree roots, etc., are underway steadily in the lower layers. It is a micro version of wetland below the JW pavement. Pollutants entering the lower layers will be trapped and decomposed as nutrients to bacteria, then to be flushed out in raining days with forms harmless to the environment or human health. The micro wetland acts as a biological filter to clean the urban environment. The wide-spreading tree roots under the pavement also show a sign of a healthy livable environment underneath.

The additional pervious surface layer shown in Figure 1 can be installed if needed. For instance, this additional layer can be water-permeable asphalt for roads with specific desired friction and smoothness. In this study, no additional pavement layer was added.

Experimental Design

In theory, if automobile pollutants are traveling directly into layers below the pavement from the vehicle's exhaust pipe, the pavement is said to have an effective direct diluting function. If pollutants penetrate through the pavement after some time in the air, the process is named as continuous dilution. In reality, it is difficult to measure these dilution effects with complicated airflow caused by moving vehicles and atmospheric turbulence. However, if there exist two intersecting roads each with a considerable length of the desired pavement, then the dilution effects can be detected by analyzing the differences of the air quality at the intersection point with respect to those measured over conventional pavements. Currently, the selected JW pavement is used mainly in the parking lots, squares, pedestrian walkways, and basketball fields in Taiwan. The chosen 30-m-length campus driveway is the closest to what we desire. For avoiding confusion, the designed experiments in this study are for detecting the dilution effects on exhausts from one single idling vehicle, with constraints that no damages are allowed to the pavement.

Experiment I

In this experiment, two mobile air quality monitoring vehicles were used to measure air pollutant concentrations at 0.5 and 2 m heights above a normal pervious brick pavement (described subsequently as non-JW) and the JW pavement. The air pollutants measured including carbon monoxide (CO), carbon dioxide (CO₂), sulfur dioxide (SO₂), nitric oxide (NO), nitrogen dioxide (NO₂), ozone (O₃), and nitrogen oxides (NO_x = NO + NO₂). The specifications of all monitoring instruments were in accord with the equipment used at the ambient air quality monitoring stations of the Taiwan Environmental Protection Agency (TEPA, 2011) and met the quality assurance and quality control specifications set by TEPA (2011). Table 1 provides the information of these instruments. Prior to sampling, the instruments were calibrated using multipoint calibration. Zero/span calibration and drift inspection were conducted.

Table 1. Instruments used for the air quality monitoring

Items	Manufacturer	Method	Range	Detection Limit	Accuracy
CO	Horiba APMA-360	Nondispersion cross-modulation infrared analysis method	0∼50 ppm	0.05 ppm	±1
SO2	Horiba APSA-360	UV fluorescence	0∼0.5 ppm	0.5 ppb	±1%
NOx	Horiba APNA-360	Chemiluminscence	0∼0.5 ppm	0.5 ppb	±1%
O3	Horiba APOA-360	UV absorption method	0∼20 ppm	0.5 ppb	±1 ppb
CO2	PPM SAS	Nondispersive infrared sensing	0%–20%	1 ppm	<3%

The experimental method employed herein is illustrated in Figure 3. On the JW and non-JW pavements, a cubic fence, 3 m on each side, was constructed, and the bottom of the cube was closed whereas the top was open. The fence was covered with plastic canvas so that its internal air volume could undergo vertical exchange with the atmosphere at the top of the cube, but no horizontal transmission or mixing with external environment. At the center of the fence, air sampling tubes were installed at 0.5 and 2 m above the ground. These tubes collected samples at different heights to reveal the vertical pollutant distribution. Because the experimental site was situated between buildings, the typical wind speed was low, and it is unlikely that the prevailing winds on the top can affect the dilution of air at the bottom.

Figure 3. Design of Experiment I on the JW pavement. Air quality was monitored at 0.5 and 2 m above the ground in the center of the fenced cube. Similar exercises were done over a neighboring non-JW pavement for comparison.

The pollution source was vehicle exhaust that was introduced into the fenced cube by a connection tube, with the opening of the tube located 0.5 m above the ground. The experimental procedure entailed discharging exhaust for 5 min and continuing to collect air samples for an additional 30 min. The car used was a 2006 Toyota Wish 2.0, a 2.0-L sports utility vehicle (SUV) sold in Taiwan. The maximum horsepower (hp) was 141 hp/5600 rpm, and the maximum torque was 19.3 kg·m/4400 rpm, with a total distance traveled of 91,114 km. During the experiment, the SUV was turned on, allowed to idle for about 5 min with a fixed speed of 800 rpm and an error of approximately ±50 rpm. It was estimated that the variation in pollutant discharge rates during each experiment was within ±7%.

The test date was February 2, 2010; the weather on that day was partly cloudy, and the anemometer was set on the monitoring vehicle at approximately 5 m above the ground. The wind speed was not high and was found to be between 0.5 and 2.5 m sec⁻¹, with an average of 0.87 m sec⁻¹. Because there were only two mobile air quality monitoring vehicles available, the first experiment started around 12:00 p.m., and two trials were conducted on the JW pavement, whereas another two trials were done later on the non-JW pavement. The averaged air temperature was about 20.5 and 20.8 ° C, respectively, above the JW and the non-JW pavement. The difference was only about 1.5% and was not likely to cause significant differences in the vertical diffusion capacity.

Experiment II

The second experiment was performed on March 31, 2010. The primary difference between this and the first experiment was an extension of the enclosed region to 10 m, but the height and the width remained 3 m, with the bottom closed and the top open. With limited budget, the air quality measurement at a height of 2 m was ignored; instead, the air was collected and measured at a height of 0.5 m above the ground at 1 m away from the exhaust-entering fence (which is named as the front site) and at 1 m away from the far-away fence (which is named as the back site) (Figure 4). The goal was to understand the dilution of the pollutant beyond a certain distance.

Figure 4. Design of Experiment II on the JW pavement. Air quality was monitored at 0.5 m above the ground at 1m away from the exhaust-entering fence (which is named as the front site) and at 1 m away from the far-away fence (which is named as the back site). Similar exercises were done over a neighboring non-JW pavement for comparison.

The vehicle used in this experiment was a four-door Ford sedan sold in Taiwan, 2003 Tierra, 1.6 L; the maximum horsepower was 106 hp/5500 rpm, the maximum torque was 14.8 kg·m/4000 rpm, and it had only run for 29,998 km. During the experiment, the car was turned on and allowed to idle. The fixed speed was 800 rpm with an error of about ±50 rpm. It was estimated that the difference in pollutant discharge during each experiment was within ±7%. In order to increase the pollutant discharge amount of the vehicle, it was allowed to discharge for 30 min. In addition, because there were only two monitoring vehicles available, this sedan was used to perform two trials on the JW pavement followed by two trials on the non-JW pavement for comparison.

On the day of the experiment, the weather was clear with a strong southwesterly wind. The wind speed at 5 m above ground at the experiment site was between 1.5 and 5.5 m sec⁻¹ with an average speed of 2.9 m sec⁻¹, which was significantly higher than normal. Meanwhile, the length of the fence at the experimental site was extended; therefore, strong winds did possibly affect the dilution of air pollutants inside the fence as the plastic canvas was vibrating strongly. The air temperature was about 26.7 and 26.3 ° C, respectively, above the JW and the non-JW pavement. The difference was about 1.5%, which was not likely to cause significant difference in the vertical diffusion capacity.

Experimental Results

Experiment I

Figure 5a and b show the temporal variation of CO concentrations measured at different heights above the JW and non-JW pavement. There are four sets of curves in the graphs, representing the data from each of the two trials on each type of pavement. From the initial time to 6 min, all curves correspond to background concentrations of CO (Table 2a); from 6 to 11 min, the exhaust discharge from the vehicle is being characterized, whereas from 11 min onward, the discharge was stopped.

Table 2. Lists of (a) averaged background concentration, (b) averaged peak-increased-concentration within 5 min after discharging, (c) averaged peak-increased-concentration within 6–10 min after discharging, and (d) time needed to return back to the background level after discharging, at 0.5 m above the ground over the JW and non-JW pavements in Experiment I

	CO (ppm)	CO2 (ppm)	SO2 (ppb)	O3 (ppb)	NO (ppb)	NO2 (ppb)	NOx (ppb)
	(a) Averaged Background Concentration
JW	0.26	458	1.48	20.5	7.37	12.8	20.1
Non-JW	0.29	467	2.69	10.8	13.2	31.6	44.7
Ratio (JW/non-JW)	0.90	0.98	0.55	1.89	0.56	0.40	0.45
	(b) Averaged Peak-Increased-Concentration within 5 min after Discharging
JW	13.1	1365	9.55	0.32	698	46	733
Non-JW	46.0	10,577	26.3	1.13	1168	229	1355
Ratio (JW/non-JW)	0.28	0.13	0.36	0.28	0.60	0.20	0.53
	(c) Averaged Peak-Increased-Concentration within 6–10 min after Discharging
JW	6.44	264	3.68	0.02	541	44	584
Non-JW	29.2	1365	23.8	0.81	895	155	1051
Ratio (JW/non-JW)	0.22	0.19	0.16	0.02	0.60	0.28	0.56
	(d) Time Needed to Return Back to the Background Level after Discharging (min)
JW	3.5	4.5	4.0	3.5	4.0	4.0	3.5
Non-JW	5.0	6.0	6.5	6.0	6.0	6.0	5.0
Notes: Here, the averaged value represents the mean data obtained in two trials over each kind of pavement. The increased-concentration is obtained after subtracting the background level from the value measured.

Because the vehicle exhaust was confined within the experimental space, it tended to gather at the bottom first, then gradually diffused upwards, and became diluted. Therefore, within 5 min after discharging, the experimental chamber had a concentration peak at a height of 0.5 m (Figure 5a), but when the discharge was stopped, the high concentration gradually lowered while moving upwards. From Figure 5b, it can be seen that the concentration at a height of 2 m was relatively low, but that a peak did appear after discharging.

Figure 5. Temporal variation of CO over the JW and non-JW pavement at (a) 0.5 m and (b) 2 m above the ground in Experiment I. Two trials over each pavement are being marked as 1 and 2.

Figure 5a shows that the CO concentration at 0.5 m is lower on the JW pavement than that on the non-JW pavement. After subtracting the background level, an averaged peak increased-concentration of 13.1 ppm within 5 min after discharging was measured over the former, which is about 72% lower than the average of 46.0 ppm on the latter surface (Table 2b). After the discharge was stopped, the highest mean concentration increased over the former was 6.44 ppm, which is about 78% lower than the latter average of 29.2 ppm. Very similar amounts of time were required for the near-surface concentration to decrease to the background level. The former required about 3.5 min, and the latter took about 5 min (Table 2).

At 2 m above the ground (Figure 5b), after subtracting the background level, the averaged peak increased-concentration measured during the 5-min discharging period was 0.30 and 0.21 ppm, respectively, over the JW and non-JW pavement (Table 3b). After the discharging stopped, the concentration above the JW pavement began to decrease, whereas above the non-JW pavement, the concentration gradually increased, followed by a subsequent decrease. Overall, more CO were diffused upwards over the non-JW surface, hence longer time (about 17 min) was needed to return to the background level (Table 3). To get a quantitative estimate, the averaged increased-concentration within 20-min periods after discharging was calculated and listed in Table 3c. It was 0.065 and 0.16 ppm, respectively, over the JW and non-JW pavement. The difference suggests that about 61% of CO emitted into the experimental cube was trapped below the JW pavement without coming out the only exit. The fluctuations in airflow, named as the air turbulence (Schmel, 1980), from above have caused a periodic change of concentration with time in Figure 5b.

Table 3. Lists of (a) averaged background concentration, (b) averaged peak-increased-concentration within 5 min after discharging, (c) averaged increased-concentration within 20 min after discharging, and (d) time needed to return back to the background level after discharging, at 2 m above the ground over the JW and non-JW pavements in Experiment I

	CO (ppm)	CO2 (ppm)	SO2 (ppb)	O3 (ppb)	NO (ppb)	NO2 (ppb)	NOx (ppb)
	(a) Averaged Background Concentration
JW	0.39	494	2.1	22.5	3.29	22.6	26.0
Non-JW	0.54	517	2.0	12.3	5.19	33.8	39.0
Ratio (JW/non-JW)	0.72	0.96	1.05	1.83	0.63	0.67	0.67
	(b) Averaged Peak-Increased-Concentration within 5 min after Discharging
JW	0.30	368	1.73	−7.24	5.18	16.2	21.1
Non-JW	0.21	1463	4.35	−4.93	3.85	16.1	19.6
Ratio (JW/non-JW)	1.45	0.25	0.40	1.47	1.34	1.00	1.08
	(c) Averaged Peak-Increased-Concentration within 20 min after Discharging
JW	0.065	106	0.034	−1.75	0.89	4.60	5.46
Non-JW	0.16	445	1.31	−4.13	2.60	12.4	15.0
Ratio (JW/non-JW)	0.39	0.24	0.026	0.42	0.34	0.37	0.36
	(d) Time Needed to Return Back to the Background Level after Discharging (min)
JW	7	5	7.5	7	5	5	5
Non-JW	17	19	13	15	17	18	18
Notes: Here, the averaged value represents the mean data obtained in two trials over each kind of pavement. The increased-concentration is obtained after subtracting the background level from the value measured.
^+For O3, the value indicates the amount that lost through titration.

Table 4. List of averaged background concentration and averaged peak-increased-concentration within 20 min after discharging at the front and back sites in Experiment II

		JW-Front	JW-Back	Back/Front	Non-JW Front	Non-JW Back	Back/Front	JW/Non-JW Front
Averaged background concentration	CO (ppm)	0.68	0.50	0.74	0.94	0.47	0.50	0.73
	CO2 (ppm)	474	465	0.98	822	515	0.63	0.58
	SO2 (ppb)	2.68	1.92	0.72	1.40	1.61	1.15	1.92
	O3 (ppb)	4.26	5.57	1.31	4.05	5.66	1.40	1.05
	NO (ppb)	5.71	8.83	1.55	28.0	27.8	0.99	0.20
	NO2 (ppb)	10.9	13.6	1.25	15.0	28.8	1.92	0.73
	NOx (ppb)	16.6	22.4	1.35	43.0	56.6	1.32	0.39
Averaged peak-increased- concentration within 20 min after discharging	CO (ppm)	0.36	0.32	0.89	0.40	2.36	5.90	0.91
	CO2 (ppm)	3,946	51.2	0.01	4,054	17.2	0.00	0.97
	SO2 (ppb)	2.71	0.49	0.18	4.22	0.62	0.15	0.64
	O3 (ppb)	0.37	0.33	0.89	0.97	0.31	0.32	0.37
	NO (ppb)	148	8.48	0.06	299	9.76	0.03	0.49
	NO2 (ppb)	24.8	8.84	0.36	39.8	16.9	0.42	0.62
	NOx (ppb)	153	16.7	0.11	335	26.2	0.08	0.46
Notes: Also listed are ratios between values over the JW and non-JW pavements at the front and back sites. Here, the averaged value represents the mean data obtained in two trials over each kind of pavement. The increased-concentration is obtained after subtracting the background level from the value measured.

Since measurements at 0.5 and 2 m heights both showed that the CO concentrations were returning to the background levels 20 min after discharging, the lesser amount of CO measured over the JW pavement indicated that CO molecules ought to be trapped down below the pavement, as they were not escaping out the closed cube through the only exit at the top. In the meantime, pollutants emitted out of the idled car exhaust were with much higher temperature (at least 380 ° F with 640–720-rpm idle speed) (Ning et al., 2005) than the ambient air (about 20.5 ° C), hence the turbulence-driven dry-deposition process could not play any role in removing pollutants from air to surface within a short period of time. We suspect that air rushing out of the automobile exhaust (with exit velocity about 4.8 m sec⁻¹) (Ning et al., 2005) would push air into and out of those aqueducts and generate automatic air circulations around all aqueducts and force pollutants into layers underneath. The efficient vertical advection process was the likely mechanism responsible for moving pollutants downward along the aqueducts, instead of the slow vertical eddy motions.

Figure 6a depicts CO₂ concentrations at 0.5 m above the ground. Within 5 min after discharging, only one peak concentration could be detected above the JW pavement, whereas there were two concentration peaks above the non-JW pavement, with the second peak being significantly larger than the first peak. A similar double peak phenomenon was also observed at 2 m above the ground (Figure 6b). In other graphs of pollutants (SO₂, NO, NO₂, and NO_x), which are included in the Supplemental Material, similar patterns also appear. The reason for the double-peak phenomenon could be that the discharged exhaust flew past the sample collection site and then migrated back to this site within the confined cube. When it was above the JW pavement, the second peak concentration was significantly lower than the first (Figure 5a), whereas above the non-JW pavement, the second peak concentration was, in most cases, in a close range with the first, except for CO₂ (Figure 6a). Obviously, the near-surface CO₂ included not only the directly discharged CO₂, but also the CO₂ converted from CO and hydrocarbons (HC), so that the second peak was significantly larger than the first. This may also explain why the CO₂ concentrations at 2 m over the non-JW pavement were significantly higher than the background concentrations after the 5-min vehicle discharge period was complete (Figure 6b). In contrast, the CO concentration at 2 m was not far away from the background level (Figure 5b).

Figure 6. Temporal variation of CO₂ over the JW and non-JW pavement at (a) 0.5 m and (b) 2 m above the ground in Experiment I. Two trials over each pavement are being marked as 1 and 2.

As for SO₂, NO, NO₂, and NO_x, their temporal variations were quite similar to CO, i.e., one or two peak values after exhaust discharging, followed by subsequent decrease. At 0.5 m height, the level of peak increase was considerably higher than the background concentration. The level of increase within 5 min after discharging was about 50, 3, 6, 95, 4, and 36 times that of the background level, respectively, for CO, CO₂, SO₂, NO, NO₂, and NO_x over the JW pavement, whereas it was 159, 29, 10, 88, 7, and 30 times that of the background level, respectively, over the non-JW pavement (Table 2). The difference between these two kinds of pavements was clearly due to the fact that a considerable amount of pollutants were transported into the lower layers of the JW pavement.

At 2 m height, the temporal fluctuation of pollutant concentration was more pronounced than that appeared at lower altitude due to air turbulence from above. Also, the level of increase was close to the background concentration, except for CO₂, and the peak increase was not necessary within the 5-min discharging period. The ratio of the averaged increased-concentration within 20 min after discharging between that over JW to that over non-JW was about 0.39, 0.24, 0.026, 0.34, 0.37, and 0.36, respectively, for CO, CO₂, SO₂, NO, NO₂, and NO_x (Table 3c). Clearly, over the JW pavement, fewer amounts of pollutants were diffusing upward, since a lot of pollutants were trapped below the surface.

These results show that JW pavement has an excellent ability to dilute automobile exhaust. First, when the exhaust is delivered into the control cube, some of the gas molecules move downwards to penetrate below the pavement as a direct dilution effect. Over time, additional gas molecules can still penetrate beneath the pavement through the air aqueducts; this can be named as the continuous dilution effect. These effects can be seen during the discharge process, because the exhaust circulates within the control cube, and they result in the second pollutant concentration peak being lower than the first peak. However, this experiment cannot distinguish the difference between these two effects, so the air pollutant dilution effect of the JW pavement is measured by examining the difference between the highest increased-concentration above these two pavements during the 5-min discharge period and those measured within 5 min after discharging at 0.5 m above the ground (Table 2). Furthermore, since all pollutants diffusing upwards eventually, the measurement at 2 m above the ground provides another objective examination (Table 3).

Table 2 summarizes the background concentrations at 0.5 m above the ground before the discharge, as well as the averaged peak increased-concentrations within 5 min and within 6–10 min after discharging. The results are obvious for most of the pollutants (except for ozone). During the 5-min discharging period, the pollutant peak increased-concentrations above the JW pavement were only 13%–60% of those found above the non-JW pavement (Table 2b), which imply that the vehicle exhaust was diluted and decreased by a factor of 40%–87%, whereas CO₂ was the pollutant with the most significant reduction. Within 6–10 min after discharging (Table 2c), the averaged peak increased-concentrations of pollutants above the JW pavement ranged from 16% to 60% of those above the non-JW pavement; these values are equivalent to diluting the exhaust by a factor of 40%–84%, and SO₂ and CO₂ were the pollutant undergoing the most significant reduction. Furthermore, the time required (Table 2d) for the pollutant levels to return to the background concentrations were all less than that above the non-JW pavement, although the difference was not significant.

Table 3 lists the analysis results at 2 m above the ground. For all pollutants, the background concentration varied insignificantly in the vertical direction. During the discharge, the concentration of pollutants increased, whereas after the discharge, the concentrations gradually decreased. Hence, the concentrations measured at higher altitude were lower than those recorded near the ground. It should be noted that, after the discharging stopped, significantly longer amounts of time were required for the air pollutant levels to return to background concentrations above the non-JW pavement. Considering that all pollutants had to diffuse upwards to the top of the experimental area, the ratio between the averaged increased-concentration within 20 min after discharging measured over those two different kinds of pavement suggests that about 58%∼97% of pollutants were trapped below the JW pavement, with SO₂ being the most significantly reduced. These results adequately demonstrate the ability of the JW pavement in diluting near-ground vehicle exhaust.

The two JW pavement experiments were conducted at noon, and the two non-JW pavement experiments were done after 3:30 p.m. in the afternoon; the experiment sites were located on campus between university buildings and trees. The near-ground ozone background concentrations for the JW and non-JW pavement were 20.5 and 10.8 ppb, respectively (Table 2a). At 2 m above the ground, the JW pavement O₃ background concentration was 22.5 ppb, and the non-JW concentration was 12.3 ppb (Table 3a). The vertical difference in these cases was not significant. During the exhaust discharge, the ozone concentration near the ground changed negligibly, regardless of the type of pavement used, because no ozone was directly released as part of the discharge. The noted minor increase of ozone in Table 2 may be resulted from the photochemical production of ozone when the exhaust pollutants were circling in the confined cube near surface.

At 2 m height, the ozone concentrations decreased after discharging of the exhaust. Above the JW pavement, the peak decreased-level was about 7.24 ppb, whereas above the non-JW pavement, it was about 4.93 ppb (Table 3). This phenomenon indicates that ozone was destroyed through titration with NO diffusing upwards from surface and the destruction rate was positively correlated with the level of ozone concentration (Seinfeld and Pandis, 2006). After the emissions were discontinued, ozone concentrations gradually returned to background levels. For the averaged decreased-concentration within 20 min after discharging, it was 1.75 and 4.13 ppb, respectively, over the JW and non-JW pavement. The ratio is about 0.42 and indicates that about 58% more of ozone lost over the non-JW pavement due to more amounts of NO molecules reaching at this higher altitude.

Experiment II

The main difference between the first and the second experiment is that the length of the fence used in the latter trials was more than tripled. During the experiment, the vehicle was allowed to discharge continuously for 30 min to increase the emissions levels. However, on the day of experiment, the ambient wind speed was high, and although the wind did not blow directly into the fence, the plastic canvas swung back and forth and affected the vehicle exhaust diffusion inside the fence. Furthermore, the vehicle used in the second trial had a smaller cylinder volume as compared with the vehicle used in the first experiment, and as a result, the highest measured pollutant concentrations in the second trial are lower than those measured in the first experiment. Because the ambient wind speed was much higher than usual, all of the results can only be used as qualitative references and are not suitable for use as quantitative evidence for drawing conclusions.

Figure 7 shows the concentrations of each pollutant measured at the front and back site (Figure 4) at a height of 0.5 m during the first trial over the JW pavement. Figure 8 shows the first trial results over the non-JW pavement. In general, the data during the first 5 min correspond to the background concentration, and because exhaust is delivered into the fence, within 10 min after discharging, all pollutant concentrations increase rapidly at the front site with either a single- or a double-peak pattern. Subsequently, the concentration decreases rapidly, except for the CO₂ concentration increases again after the decrease. One can speculate that this later increase in CO₂ may occur as the result of CO and HC being converted to CO₂ and accumulating at the ground. Similar characteristics were observed over both pavements (Figures 7a and 8a), and most of the peak values over the non-JW pavement are higher than those over the JW pavement.

Figure 7. Temporal variation of CO, SO₂, O₃, NO, NO₂, NO_x, and CO₂ at the (a) front and (b) back site during the first trial over the JW pavement in the Experiment II.

Figure 8. Temporal variation of CO, SO₂, O₃, NO, NO₂, NO_x, and CO₂ at the (a) front and (b) back site during the first trail over the non-JW pavement in the Experiment II.

At the back site, the pollutant concentrations were lower, still a peak appeared 10 min after the emissions began; this peak is significantly lower than the value detected at the front site. Additionally, the characteristics of the concentration change for each pollutant are very similar, although most of the concentration values over the non-JW pavement are higher than those for the JW pavement (Figures 7b and 8b). In the meantime, turbulence embedded in the airflow made the surrounding plastic canvas to vibrate periodically and hence causing a periodic change of concentration with time in both Figures 7 and 8.

These trials were conducted twice on each of two different pavements. Table 4 lists the results at the front and back sites, including the average background concentrations, the average peak increased-concentrations measured within 20 min after discharging, the concentration ratio of the front versus the back, and the ratio of the detected values at the front over the two pavements.

Although the two detection sites are only 8 m apart and both are inside the fence, the detected background concentrations are not the same, possibly due to the high ambient wind speed condition. For the same reason, ozone molecules did not accumulate, resulting in very low concentrations. After the vehicle emissions, the ozone concentrations did not change significantly either. For all of the other pollutants, their concentrations increased significantly after the vehicle started to discharge.

Over the JW pavement, within 20 min after the emission, the pollutant peak increased-concentration in the back was lower than that in the front. Over the non-JW pavement, with the exception of CO, similar phenomena were also noted. As for the back-to-front ratio, with the exception of CO and ozone, there were no significant differences between the different pavement types. The level of increase of CO was all much lower than those measured in Experiment I. It is speculated that the distance between the front and the back was great enough and that the environmental wind speeds were high enough that the pollutants diffused and became rapidly diluted, whereas different pavements influenced little on the results.

Regarding the averaged peak increased-concentrations measured at the front site within 20 min after discharging, the values on the JW pavement were lower. The ratio between pavements is about 0.49–0.97 (ozone was not considered), corresponding to a decrease of 3%–51%. This fact can be attributed to the special design of the JW pavement, but with the high ambient wind speed, those results can only be considered as qualitative evidences.

Conclusion

This study included two different experiments in order to understand whether the environment-protecting pervious pavement can dilute the air pollutants discharged by automobiles. Because the pavement used is a high-load-bearing pervious pavement, it can be used as a general road pavement and has been used successfully for more than eight years in Taiwan. Therefore, this is a study of a robust material that is ready for practical application. The results from this study can be considered as a first step toward more thorough evaluation of the potential air-cleaning effects that may be achieved if this type of pavement were to be widely used.

Due to limited funding for this study, only two experiments were conducted. For each test, both JW pavement and non-JW pavement were selected to perform a comparative experiment. Due to differences in the experimental design and in ambient wind speeds, the results of the first experiment can be used to conclude that the JW pavement efficiently dilutes vehicle exhaust, whereas the results of the second experiment can be used to qualitatively confirm these findings.

Because the pavement includes air-cycle aqueducts, the exhaust can directly penetrate under the pavement after it is discharged, resulting in direct and continuous dilution of pollutants, although these two mechanisms were not distinguishable in the current study. In Experiment I, it was determined that, within a 3 m × 3 m × 3 m space, the JW pavement can dilute vehicle emissions at a height of 0.5 m by approximately 40%–87% to the peak increased-concentration within 5 min after exhaust discharging, and CO₂ was reduced by the greatest amount; then, by 40%–84% to the peak increased-level within 6–10 min after discharging, with SO₂ and CO₂ undergoing the most significant reduction. As all pollutants were diffusing upwards, analyses at 2 m height suggest that about 58%∼97% of pollutants were trapped below the JW pavement, with SO₂ being the most significantly reduced. Those quantitative estimations may be off by 10%, if errors in emissions and measurements were considered. Still, the notable decrease of ozone molecule concentration resulting from titration, and a longer period of time needed for all pollutants returning to the background concentration, at higher altitude over the non-JW pavement, supports the above-stated findings. In the future, more repeatable experiments must be made to give out useful numbers to rely on.

There are two interesting subjects remained to be explored in the future. First, how the pollutants are transported downward into layers below the JW pavement? During the first experiment, measurements at both 0.5 and 2 m heights showed that all concentrations were returning to the background levels 20 min after discharging, which means that about 58%∼97% of pollutants were not escaping out the closed cube through the only exit at the top over the JW pavement, hence they must be trapped underneath the pavement. As pollutants emitted out of the idled car exhaust were with much higher temperature (at least 380 ° F with 640–720-rpm idle speed) (Ning et al., 2005) than the ambient air (about 20.5 ° C), hence they could be floating upward with enough of buoyancy rather than moving downward. However, at the same time, air rushing out of the automobile exhaust was with an exit velocity (Ning et al., 2005) about 4.8 m sec⁻¹. Very likely, pollutants were pushing forward and downward into those aqueducts in the JW pavement. Since air was circulating inward and outward, automatic air circulations were generated around all aqueducts and force pollutants into layers underneath through advection instead of diffusion. The efficient vertical advection process was the likely mechanism responsible for moving pollutants downward along the aqueducts, instead of the slow vertical eddy motions. Still, such speculation is worthy to be explored and confirmed through either experiments or simulations in the future.

Another subject of interest is the fate of pollutants trapped underneath. How could they stay below with the hypothesized air circulation being active around all aqueducts? Theoretically, the circulation could take them downward and could also carry them upward. The measurement data shown here strongly indicate that pollutants were captured by gravels and soils underneath quite efficiently, for they were not detected above the pavement 20 min after discharging. Furthermore, will pollutants staying below be released into the atmosphere after a certain period of time? There is no direct measurement data available to answer specifically the above-raised questions. However, pictures shown in Figure 2c and d and the physical and microorganic measurement data (Chao, 2011; Chen, 2011) in the lower layers after the surface pavement was destroyed suggest that a micro version of wetland exists underneath. The wide-spreading tree roots indicate an ecologically healthy environment. Hence, it is proposed that pollutants were either dissolving into water as shown in Figure 2c or adhering to the moist gravels or soils, once entering the lower layers. Later, they became nutrients to bacteria and decomposed. Materials that could be flushed out by rainwater in other days would no longer bear the original or any other forms that would be damaging the environment and human health (Mander and Mitsch, 2009).

This research is successful in presenting the experimental procedure for evaluating the dilution effect of environment-protecting pervious pavement on emissions from automobile. More studies are needed to explore the proposed air circulations around aqueducts and the efficiency of the micro wetland underneath in treating pollutants. Still, even without data from these suggested studies, it is no denial that the JW pavement did dilute pollutants from car exhaust in a very short period of time.

If the JW pavement is to be used widely in the future, then the vehicle exhaust pipe opening may need to angle towards the ground so as to take advantage of the pervious pavement function, namely, to send the pollutants directly into the pavement and effectively clean the air and reduce carbon emissions. At the same time, many designs can be considered for layers below the pavement to improve the carbon fixation effect.

Acknowledgments

The authors thank the National Taipei University of Technology for providing experimental studying opportunities.