The Design of an Urban Roadside Automatic Sprinkling System: Mitigation of PM_2.5–10 in Ambient Air in Megacities

Table 1

Comparison of RASS with existing systems for tackling negative externalities of urban system.

Strategies and systems	∗Primary control option efficiency for removal PM2.5–10	Lifecycle cost of deployment and operation	Advantages	Possible issues
RASS	Sprinkling system is used to purify air, washing away dust content and preventing generating and resuspending particulate matters caused by running vehicles, efficiency (85%–100%); function in removing snow and ice on the road, efficiency (95–100%)	Initial cost will be high to moderately high depending on the scale of installation. Operation cost and maintenance are small to moderate dependent on frequency of event, traffic volume, and situation of inclement weather.	(i) Can be operated at any frequency if needed (24/7); (ii) Labor cost is minimal; (iii) Has multiple functions; (iv) Does not affect traffic flow when being operated; (v) Ex-ante intervention; (vi) Cover as many areas as possible; (vii) Can be operated synchronously and asynchronously; (viii) Good controllability; (ix) Preventive action; (x) Without causing ice in winter; (xi) High robustness	(i) Water dependent; (ii) Need to be integrated into current urban engineering system (road infrastructure, underground utility system); (iii) Some blind areas may not be covered
∗∗Vacuum sweeping; water flushing and sweeping	Man-driving truck sweeper and sprinkling pressure water in all traffic lanes and sidewalks; vacuum sweeping (0–50%), waster flushing and sweeping efficiency (0–96%) [26–29]	Initial cost will be high depending on the number of trucks and drivers needed; operation and maintenance cost will be moderate to high depending on the size of the city	(i) No need to change current urban engineering system; (ii) Saving water; (iii) Mobility of sweeper truck enables it to work at locations where RASS cannot be installed	(i) Ex-post intervention; (ii) Labor intensive; (iii) Affected by traffic situation and influence traffic flow as well; (iv) Water dependent; (v) Cover very limited areas; (vi) Causing resuspension of fine and coarse particulate matters; (vii) Cannot be operated synchronously
Artificial precipitation	Using cloud seeding method to increase precipitation (rain or snow), raining or snowing, efficiency (42.6–100%) [30, 31]	Operation cost will be very high	(i) No blind pots within the whole area; (ii) Relief pollution, hot weather, and drought as well	(i) Bad controllability; (ii) Dependent on the availability of clouds; (iii) Possible land soil pollution from chemical materials such as silver iodide [30]
Electrostatic air cleaner	Using the mechanism of electrostatic precipitator,; efficiency (76.2%–99.7%) [32, 33]	Building cost is high and; operation cost is low to moderate	(i) Low operation cost; (ii) Can be operated synchronously and asynchronously; (iii) Good controllability; (iv) Does not affect ground traffic flow when being operated; (v) Can be operated at any frequency if needed (24/7); (vi) Labor cost is minimal	(i) Cannot prevent the fine and coarse particulate matters from resuspending; (ii) Effect of electrostatic discharge on telecommunication, air transportation [34–36]; (iii) Safety issues: gas station, and other possible dust explosion, electronic and electric equipment [37–40]; (iv) Adverse health impact on human being [41]
Other preventive measures, equipment, and systems	Other measures not called system such as covering trucks, improving fuel quality, and reducing emission from waste incineration	Costs ranging from very low to very high	(i) Handling the pollution from the origin of emission; (ii) Preventive action	(i) Too much variability; (ii) Hard to operate synchronously and synergistically
∗∗∗Desnowing and deicing system with similar mechanism; (referring to Figure 6)	Ground-installed warm-water sprinkler (Figure 6) [42, 43],; efficiency (90–95%)	Cost is high	Without causing land and soil pollution	(i) System must work continuously; otherwise water may become ice; (ii) Depending on the availability of warm water such as hot geyser in Japan where this system has been deployed (or water boiler)

∗Since there are very limited data available, efficiency and cost are based on current literature and estimation of authors.
∗∗Tucker reviewed most of the current knowledge on the origin and control strategies of fine particles in both urban and rural areas. Since this research is focusing on the removal and mitigation of PM_2.5–10 in urban areas, the citywide PM removal systems are compared with our proposed system in terms of the control efficiency, lifecycle cost of deployment and operation, advantages, and possible side effects.
∗∗∗Having the similar system structure as our proposed RASS but serving different function, artificial snow-melting systems have been employed in countries like Japan to desnow or deice urban area with the capability of increasing urban resilience of megacities when affected by snowfall, sleet, and icy weather.