The Gyeongbu Expressway is used as an automobile-only road spanning 7km from Yangjae-dong to the southern end of Hannam Bridge. Currently, Seoul City is pursuing a project in earnest to relocate the Gyeongbu Expressway underground and turn the above-ground space into a park. The Gangnam area is lacking in green space, and the Seoul Linear Park project is expected to provide green space and bring new vitality to the area. It is expected that the creation of a linear park will be able to overcome the disconnect between regions and improve urban competitiveness. However, the change to underground can cause various problems such as fire risk or difficulty in handling accidents. It amounts to 160,000 to 240,000 units per day. It emits pollutants such as VOCs and PM2.5 that are harmful to the human body and destroy the environment. Handling pollutants from highways is an important task.

Seoul’s air quality (AQI), especially fine dust (PM2.5/10) concentration, is at a very poor level globally. The transportation sector accounts for 39% of Seoul’s air pollutants. Areas with concentrated traffic (Olympic Expressway, Gangbyeonbuk-ro, Gyeongbu Expressway) show serious air quality and fine dust.

Using photocatalysts, it is possible to build an eco-friendly and sustainable ventilation system. Photocatalysts decompose organic substances by generating hydroxide ions (OH radicals) when exposed to ultraviolet rays. When photocatalysts receive ultraviolet light, electrons and holes move through the band gap and then generate hydroxide ions. This ion has a strong oxidizing power similar to ozone, converting organic substances into water and carbon dioxide. This is a non-toxic and odorless decomposition process. During a chemical reaction, many substances change their form and generate energy. However, photocatalysts continuously generate energy without consuming materials. This means that it can be used permanently, making photocatalysts a sustainable material suitable for air purification.

The photocatalytic process requires ultraviolet (UV) light activation. In the context of the Gyeongbu Expressway, leveraging the ample sunlight, especially during equinoxes and solstices, is essential for maximizing the efficiency of this process. In particular, at the midpoint between Seocho IC and Banpo IC, more than 10 hours of sunlight is secured on average during the spring equinox and summer solstice. High levels of sunlight are maintained even during rush hours when vehicle traffic is highest. Natural light, that is, ultraviolet rays, is essential to activate Tio2. Therefore, this area has the most suitable conditions for ventilation purification shafts.

Multiscale Environmental Purification Strategy

Urban Scale: During peak hours, the Gyeongbu Expressway experiences traffic of approximately 10,400 vehicles. This traffic contributes to substantial air pollution, emitting around 13,000 ppm of Volatile Organic Compounds (VOCs) and 17,680 ppm of Particulate Matter (PM) per minute. To address this, a sustainable ventilation purification system spanning 6.8 km of the tunnel has been proposed. This system not only purifies the air but also reduces the cost of mechanical ventilation by 20-35%. Furthermore, it transforms the space into a cultural and environmental haven for Seoul’s citizens, providing cleaner air and a rejuvenated urban environment.

Building Scale: The proposed system utilizes Heliostat Pipes for natural light transportation, where concentrated sunlight increases the temperature within the tunnel pipes. This creates a pressure differential between the inside and outside of the tunnel, facilitating air circulation. Additionally, these pipes introduce natural light into otherwise dark tunnels, reducing the risk of accidents at tunnel entrances and providing an artistic illumination.

System Scale 1: A module that tracks and captures sunlight directs vast amounts of solar energy into the system, activating the photocatalytic purification process.


System Scale 2: The photocatalytic coating surface area is maximized through a corrugated design, allowing for a greater volume of pollutants to be processed per unit area.
Grain Scale: A mesh surface ensures that all particulate matter (PM10/2.5) and VOCs necessarily pass through the coated surface for treatment.

Molecule Scale: Fine filters are employed to trap particulate matter, filtering out 11,592,520.68 ppm/min of contaminants.
Electron Scale: Hydroxyl radicals (OH Radical), known for their strong oxidative properties, break down harmful organic compounds into non-toxic, odorless substances such as carbon dioxide and water, processing 7,479,045.6 ppm/min of pollutants.

Electron Scale: Hydroxyl radicals (OH Radical), known for their strong oxidative properties, break down harmful organic compounds into non-toxic, odorless substances such as carbon dioxide and water, processing 7,479,045.6 ppm/min of pollutants.

Principle of Air Purification & Light
A. Pollutants & Light Circulation: Atmospheric pollutants emitted from vehicles are transferred through intake ports into the ventilation system. For photocatalysis, it’s crucial to maintain the efficiency of organic material degradation. Thus, the intake section is equipped with a device that removes inorganic substances and dust, preventing the build-up on the TiO2 surface, which can decrease efficiency. The Heliostat Pipes (natural light transport pipes) gather and direct natural light to illuminate the tunnel, particularly brightening the entrance to prevent accidents.
B. Re-direct Sunlight: A module that tracks sunlight reflects UV and infrared light into the Heliostat Pipes. This system creates areas beneath the structure reminiscent of tree shade, while the Heliostat Pipes secure a total of 208m2 of sunlight exposure. The intensity of light within these pipes is significantly amplified, maximizing the efficiency of the photocatalytic reaction.
C. Air Purification Process: The Heliostat Pipes, equipped with photocatalytic and fine particulate matter filters, have gaps allowing UV light to reach the TiO2 coated surfaces, facilitating photocatalytic reactions. The pollutants (VOCs, PM2.5, and PM10) drawn into the tunnel undergo multiple filtration stages, leading to their decomposition. This process breaks down 7,479,045.6 ppm of VOCs per minute and filters 11,592,520.68 ppm of PM2.5/10, effectively offsetting the pollutant emission rate of vehicles on the Gyeongbu Expressway and resulting in a positive environmental impact. The remaining light, after the photocatalytic reaction, is emitted outside through structural louvers, providing a calming, lantern-like ambiance under the trees for users. The intensity of light decreases progressively towards the bottom of the shaft due to the gaps. Therefore, the four main Heliostat Pipes surrounding the primary ducts provide additional light energy, securing a total sunlight exposure of 1,040m2.

Sun Duration
This study investigates the duration of sunlight exposure and its implications on the design and functionality of photocatalytic systems in the expressway’s tunnel. A comprehensive sun duration study was conducted focusing on the equinoxes and solstices to determine the optimal periods of UV light availability. The study involved detailed observations and recordings of sunlight patterns along the Gyeongbu Expressway. Additionally, a case study examining the influence of structural shapes on sunlight capture and distribution was carried out. This investigation aimed to understand how different designs can enhance or impede the effectiveness of sunlight utilization for photocatalytic processes.