摘要: |
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. |