Longevity of Air Pollution Mitigating Photo-Catalytic Coatings on Transportation Infrastructure

It is known that in large urban areas, a high concentration of air pollutants at the street level can harm severely sensitive populations and affect the general public health with cumulative exposure. The sensitive population groups include children and elderly as well as some with weakened immune systems. When these people are exposed to high air-pollutant concentrations, they present higher risk to be diagnosed of respiratory diseases like asthma and emphysema. As reported by the United States Environmental Protection Agency (EPA) the most commonly found harmful air-polluting chemicals in urban areas include Nitrogen Oxides (NOₓ), Ground Level Ozone (O3), Particulate Matter (PM10 and PM2.5), Carbon Monoxide (CO), Sulfur Dioxide (SO₂), and Lead (Pb). The Clean Air Technology Center (CATC), defines NOₓ as one of the main man-made air polluting chemicals, of which can be found in seven forms (N₂O, NO, N₂O₂, N₂O₃, NO₂, N₂O₄, N₂O₅) (1). Even when all mentioned nitrogen oxide forms impose a threat to human health, the EPA only regulates Nitrogen Dioxide (NO₂) levels, because the NO₂ form is the most common in the atmosphere. This form is directly generated from the combustion of fossil fuels by humans who use vehicle transportation and heating furnaces, as well as through power generation and industrial production plants exhaust. Moreover, when in the presence of Ultra Violet (UV) light during the day, NO₂ actively reacts in the atmosphere with any other synthetic or naturally produced Volatile Organic Compounds (VOCs) to produce ground level tropospheric ozone O3, acid rain, and PM2.5. NOₓ plays a significant role in air pollution as it can create O3 in the presence of UV, and it contributes to the formation of PM2.5. It has been reported that breathing O3 and inhaling PM2.5 can trigger a variety of health problems that include chest pain, coughing, throat irritation, congestion, and can also reduce lung function and produce inflammation of the lining of the lungs (2). It has been observed that all regions of the United States have been in-compliance with the current National Ambient Air Quality Standards (NAAQS) (1) for NO₂ (defined as 53 ppb averaged annually, or 100 ppb averaged over one hour). In addition, the country has reduced concentrations of PM2.5 below of the national standard in recent years due to tighter restrictions on vehicle and industrial exhaust (3). However, most regions of the US do not meet the O3 standards as shown in Figure 1 (4). It is anticipated that although the limits of the NOₓ are lower than the standard, they will contribute to this heightened O3 concentration. Consequently, large portions of the population are still being exposed to hazardous levels of ozone. However, because NOₓ is the main pollutant that is created by human activities, this research project will be targeting it in an attempt to reduce it further. It is imperative to find new and better ways to reduce the amount air pollutants on behalf of the health and wellbeing of people living in urban areas. Over the past ten years, a significant number of studies have been focused on understanding photocatalytic properties of several materials for air and water purification. Amongst these photocatalytic materials, titanium dioxide (TiO₂) is a naturally occurring compound found in four stable crystalline forms: ilmenite, brookite, rutile, and anatase. Because it has no absorption in the visible region, TiO₂ appears to be white to the human eye, and it has been widely used as a white pigment for centuries (5). It is also similarly used in common household products such as toothpaste, food coloring, sunscreen, paint, plastics, and cosmetics. Photocatalytic TiO₂ has been studied because of its ability (while in the presence of UV light) to break down water molecules into hydroxyl radicals without consuming itself. These hydroxyl radicals are highly reactive and can further combine with nearby molecules in air or water. In the presence of harmful pollutants such as NOₓ’s or VOC’s in the air, the hydroxyl radicals generated by photocatalysis will combine with these molecules breaking them up to form other non-toxic compounds. Additionally, photocatalytic TiO₂ activated by UV light can decompose other non-volatile organic materials like dirt, grime, oil, and particulates, which gives materials coated with TiO₂ self-cleaning characteristics. Some commercial building materials have been designed including TiO₂ in their formulations and are reported to reduce NOₓ significantly (up to 97.92%) from the surrounding air (5). These photocatalytic construction materials often are highly expensive to produce, causing most contractors not to utilize them in their bids in order to stay economically competitive, unless environmental-based goals are specified. More importantly, it has been recently observed that the photocatalytic efficiency of concrete containing embedded TiO₂ is drastically reduced by 77% to 86% within less than a year, and is specifically associated with the acceleration of a chemical reaction and limestone by-product formed on the surface of the concrete (6–8). Researchers have identified that Relative Humidity (RH) of the surrounding air, in combination with UV irradiance levels, do affect the TiO₂ photocatalytic reaction rates (6, 9–12). Previous research at the University of Utah focused on rejuvenation methods to restore the reduced photocatalytic properties of concrete containing TiO₂ (6). A separate study focusing on the TiO₂ efficiency for removing toluene instead of NOₓ, found that the photocatalytic efficiency was blocked after low RH conditions, but then completely rejuvenated after a short exposure of high RH along with UV light (9). It is hypothesized that TiO₂ products such as spray-on coatings may be more effective than embedded TiO₂ , particularly if concrete is used as the constructed surface material. This research will specifically investigate using TiO₂ coatings on various transportation infrastructure materials in order to investigate if not only the coating can effectively remove NOₓ, but more importantly to understand an appropriate method to rejuvenate or reactivate TiO₂ surface treatments should they become blocked or present reduced NOₓ removal efficiency.


  • English


  • Status: Active
  • Contract Numbers:


  • Sponsor Organizations:

    Research and Innovative Technology Administration

    University Transportation Centers Program
    1200 New Jersey Avenue
    Washington, DC  United States  20590
  • Project Managers:

    Kline, Robin

  • Performing Organizations:

    University of Utah, Salt Lake City

    College of Engineering, Department of Civil Engineering
    Salt Lake City, UT  United States  84112-0561
  • Principal Investigators:

    Bordelon, Amanda

  • Start Date: 20150731
  • Expected Completion Date: 20180731
  • Actual Completion Date: 0
  • Source Data: MPC-490

Subject/Index Terms

Filing Info

  • Accession Number: 01579927
  • Record Type: Research project
  • Source Agency: Mountain-Plains Consortium
  • Contract Numbers: DTRT13-G-UTC38
  • Files: UTC, RiP
  • Created Date: Oct 27 2015 4:07PM