KIEAE Journal

The function and structure (layout and shape) of a city is one of the factors that greatly influence PM concentration. Areas adjacent industrial facilities are at higher risk of exposure to PM. Commercial districts and residential complexes with high population densities tend to have high concentrations of NO_x (which cause secondary PM) due to the relatively high traffic volume. The layout and shape of a building can also affect the flow of air, with the inclusion of particulate matter. In high-density areas where skyscrapers are located, PM is likely to accumulate or stagnate because the wind flow is not constant. On the other hand, fresh air can be introduced into the city in the region where wind paths are established through spatial planning.

Spatial elements that make up a city (e.g., vegetation, tunnels, intersections and road pavement) could directly and indirectly affect urban particulate matter. Green-blue infrastructures, such as neighborhood parks, buffered green spaces, roadside trees and water spaces, can play a role in suppressing and reducing particulate matter activities. This could also introduce air flow by evaporative action creating a difference in temperature in urban spaces. On the other hand, impermeable pavement or tunnels cause urban heat island phenomenon and prevent fresh air from entering the city. This accumulates particulate matter in a specific space that increases the exposure risk.

Urban temperatures, humidity, wind speed and direction have a direct impact on particulate matter activity. The reversal of temperature accompanied by urbanization causes air flow to stall and expose residents to high levels of particulate matter. Moreover, in areas with high humidity, water vapor particles are combined with atmospheric pollutants, such as ozone and ammonia, causing secondary PM generation in a city. The effects of natural and climatic conditions make it difficult to predict particulate matter concentrations as well as increase damage.

Standards for action need to be established or strengthened to curb PM emissions. In 1990, Korea began to manage air pollutants separately for air quality management. PM is registered and managed as an environmental reference substance under the 「Air Conservation Act」. In addition, the 「Special Act on the Improvement of the Metropolitan Air Environment」, which is the ground law for the management of particulate matter, was established in 2003[11].

In Korea, the environmental standard for PM10 (10㎍/m³ or less) was established in 1993. The PM2.5 criteria was introduced in 2015, much later than in the US (2006), EU (2010), Japan (2009) and China (2012). Moreover, the daily and annual averages for the PM2.5 concentrations were below those of the major countries as well as the WHO recommendations at the time. Since March 2018, the daily and annual average standards have been strengthened to 35㎍/m³ and 15㎍/m³, respectively. Since particulate matter has a great impact on human health, it is necessary to continue to strengthen domestic standards in consideration of international recommendations.

Spatial unit management is a way to manage the spaces where urban residents work around the roads, under the premise that urban PM emitted or generated by road movement pollutants is relatively high. There are ways to designate a zone that restricts entry of vehicles at certain times or parking.

A representative example is the German environmental zone (Umweltzonen). It was implemented since 2005 in accordance with the German Federal Environmental Pollution Act (BImSchG). As of April 2017, only vehicles with the green sticker (Euro 4) are allowed to access 55 areas in Germany[12]. This has resulted in a reduction in the share of traffic from PM10 sources in urban areas from 23% (1998) to 15% (2013)[13]. Low Emissions Zone (LEZ) is not only operating in Germany but also in other European countries, such as Italy, UK, Netherlands and Sweden. In the UK, the LEZ has been implemented since 2008. Recently, the LEZ has further been reinforced, and Ultra LEZ (ULEZ) will be introduced in 2020 in central London, which permits only the Euro 6 level. This aims to reduce PM and NOx by about 45% in road transport emissions by 2020[14].

Accurate measurement and information are very important ways to cope with emitted PM. Microclimatic conditions of the space in which people live and breathe in a city varies depending on the location and shape of buildings, and PM may be congested in a specific space. In addition, the particulate matter measurement at the top may be less accurate. This is because the PM density tends to be lower at the higher levels from the ground due to the spreading by wind. Therefore, PM concentrations in cities need to be measured and managed within the sources of emissions, in particular around roads.

Moreover, it is essential to provide and manage information on primary and secondary PM generation as well as those on the number of times exceeding environmental standards. The information should be provided in real-time at places that are easily accessible to urban residents. Moreover, it should be provided with accurate numerical values rather than ambiguous phrases, such as ‘good’, ‘bad’, ‘very bad’, etc.

Ecological solutions (i.e., greening in the city) are the most sustainable methods to reduce PM. Plants are basically capable of absorbing and adsorbing PM. Since the ability to absorb and adsorb PM differs depending on the plant species, it is necessary to select plants that are effective in reducing particulate matter and to make a list for them. In addition, the location, shape and structure of planting should consider the characteristics of space.

In general, the more the planting area is secured, the better it is to reduce PM. Therefore, the existing greening should be maintained and preserved. Moreover, the space for additional greening should be considered as much as possible. For example, building facades, roofs and fences can be used. Modularization and weight lightening are required for high variability and applicability of additional greening with easy and simple installation.

Urban greening needs to consider new plans and designs, or the application of facilities or equipment. However, it may be a good idea to utilize the existing infrastructure because areas that are already densely populated with elements of the city are not enough to secure new facilities or space. Green infrastructure needs to be actively utilized because its basic function contribute to the improvement of the quality of life as well as reducing urban PM.

Physical solutions to reduce particulate matter include filter and electrical precipitators. The filter refers to a filter medium for removing small solids in the gas or liquid phases. Fiber filters mainly used to control the PM are very effective for separating particulate contaminants from gases[15]. The principle of the filter is a combination mechanism of inertia, blocking, diffusion and gravity. It is possible to collect various types of particulate matter by manipulating the filter structure and handling capacities according to the design scale[16]. Furthermore, it can be operated under various resistance conditions and has the advantage of being cheaper.

An electrical precipitator is a device that collects PM by moving them to the surface of the collecting electrode after being charged by a corona discharge[17]. Since PM is collected by an electrostatic force, it is effective to remove particles having small diameters and has the advantage of being able to operate for a long time[18]. The principle of electrical precipitator is a combination mechanism of electric and inertial forces, and gravity. In the electric force, the most important action consists of charging of particles, movement of charged particles and collection.

Chemical solutions to reduce PM include the use of photo-catalysts and CMA (calcium magnesium acetate). Photocatalyst refers to a substance that decomposes various bacteria and contaminants by promoting the catalytic reaction (oxidation-reduction) with light as an energy source[19]. In particular, TiO₂ can be used as the most common photocatalyst because it can be used semi-permanently, and it can oxidize all organic materials and decompose them into CO₂ and water. The TiO₂ may be coated on the road or surface of buildings and infrastructures to remove PM precursors, such as NOx. Recently, photocatalysts have been introduced to reduce PM in crosswalks, pedestrian sidewalk blocks, subway entrances, apartment walls and tunnels, etc.[20].

CMA is widely used as an environmentally friendly antacid to prevent freezing of road surfaces in winter[21]. CMA was recently highlighted as an economical chemical binder that has an excellent effect of adsorbing PM[22]. Moreover, it is biodegradable, non-corrosive and harmless, so it does not affect the ecology and human health. Various projects are under way to reduce PM in cities in EU countries, such as Austria, Italy and Germany. In the near future, it is expected that CMA could be applied to diverse urban spaces to effectively reduce particulate matter and prevent road freezing.

The ecological solutions selected in this study aimed to reduce particulate matter by using plants with various techniques. All three cases are applicable to roads, which are ‘hot spots’ of PM. City tree has the advantages of mobile systems. Therefore, it can also be applied to open spaces, such as parks and plazas. Enka®-Moss is known as a pilot project based on the proven results of moss absorption and adsorption effects. The case of ‘Via Verde’, designed to utilize reclaimed rainwater, is an additional idea that increases the sustainability of ecological measures.

It was confirmed that ivy or moss was selected for all the cases. This is because they are classified as plants having excellent ability to adsorb and adsorb particulate matter in Europe. In addition, in two cases except Enka®-Moss, plant pockets were developed or pots were used to make room for plants to absorb their basic nutrients and root. The system was automated, monitored and remotely controlled.

City tree is meaningful in that it combines the functions of street furniture to secure a new greening area. The other two cases are significant in that they have obtained additional greening area from existing infrastructure.

A representative case of the physical solution is the ‘Filter Cubes Ⅲ’ pilot project. This is a fixed standalone installation unlike the movable ‘City tree’. It is planned to be applied adjacent to roads where NOx or particulate matters are highly generated.

‘Filter Cubes Ⅲ’ is 3.6m high and consists of three cubes. It is equipped with a PM collecting filter and energy efficient fans. In addition, the control unit can adjust the operation of the filter according to the real-time air quality. Integrated sensors can record atmospheric and weather data, and upload it to cloud services for analysis[28].

A representative case of the chemical solution is the ‘Torre de Especialidades’ hospital using TiO₂ on the facade of a building that forms a vertical plane in an urban space.

The tile on the building’s surface is coated with TiO₂, which uses ultraviolet light to break down pollutants into non-toxic chemicals (e.g., CO₂ and water)[29][30]. The facade of the building is designed in a honeycomb geometric tile network structure and applied in the form of a double skin to make a higher surface area (2,500㎡)[31]. This structure doubles the surface area compared to the planar form[31].