CII: Clean aIr IndexCII: Clean aIr Index

Clean air is as valuable a resource as clean water. Air pollution, due to increase of PM2.5, is one of the most serious environmental issues in the world. A global standard for quantifying the level of air cleanliness is swiftly required, and we defined a novel concept, namely the Clean aIr Index (CII). We hope that the CII index will be commonplace and widely used such as for tourism and city planning all over the world.

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What is CII

CII is an index that quantify the air cleanliness, and is represented by a number of 1 or less. A higher CII value indicates cleaner air, with a maximum of 1 indicating the absence of air pollutants. The CII value decreases as the amount of air pollutants increases, with a value of 0 indicating that the amount of air pollutants is equal to the numerical criteria and a negative value indicating that the amount of air pollutants is larger than the numerical criteria.


Where x[i] is the amount of ith air pollutant, s[i] is the numerical criteria for the normalization of x[i], and N is the number of air pollutants considered in the CII. The numerical criteria vary, depending on the type of air pollutants. Normalization is a process that unifies the amount of air pollutants, not the amount of air pollutants themselves, but how much or less they are compared to the standard. We considered ozone (O3), suspended particulate matter (SPM), nitrogen dioxide (NO2), and sulfur dioxide (SO2), which are mentioned by the Air Quality guidelines (AQG) set by the World Health Organization (WHO). The air pollutant amounts are calculated by combining models called WRF (Weather Research and Forecasting) and CMAQ (Community Multiscale Air Quality).

When is the season of clean air?
Where is the city of clean air?

Figure 1 shows the daily mean CII values from April 2014 to March 2017. The horizontal axis is time (day), and the vertical axis shows the national code of each municipality. The lower the number, the more north the municipality. We can clearly see that the air cleanliness in Japan changes with the seasons. In spring, the CII value is relatively low because air pollutants come to Japan on the wind blowing from the Eurasian continent. In summer, the CII value is high because clean air comes to Japan from the Pacific Ocean.

Figure 1: Seasonal changes in daily mean CII in each region
Figure 1: Seasonal changes in daily mean CII in each region

Table 1 compares the CII values for Tokyo, Seoul, and Beijing. The air in Tokyo is relatively cleaner among the large cities in East Asia.

Table 1: Comparison of average CII values for 3 cities
(23 wards)
Soul Beijing
0.67 0.52 0.24

Figure 2 is a map showing the daily mean CII value for the three years. You can see that the places with particularly clean air (light blue) in Japan are concentrated in the eastern part of Hokkaido.

Figure 2: Map of average CII values over 3 years Figure 2: Map of average CII values over 3 years
Figure 2: Map of average CII values over 3 years

Furthermore, we took the daily average for 30 days with the highest CII value, and selected the "100 Clean Air Cities" (Table 2).

Table 2: 100 clean air cities
Prefectures Municipality name
Hokkaido Nemuro City, Kushiro Town, Akkeshi Town, Hamanaka Town
Tokyo Niijima Village, Kozushima Village, Miyake Village, Mikurajima Village, Hachijo Town, Aogashima Village, Ogasawara Village
Wakayama Tanabe City, Minabe Town, Shirahama Town, Kamitonda Town, Nachikatsuura Town, Kozagawa Town, Kushimoto Town
Tokushima Kaiyo Town
Ehime Uwajima City, Seiyo City, Matsuno Town, Kihoku Town, Ainan Town
Kochi Kochi City, Aki City, Nankoku City, Tosa City, Susaki City, Yadoge City, Tosashimizu City, Shimanto City, Kami City, Kami Town, Toyo Town, Nahanri Town, Tano Town, Yasuda Town, Kitagawa Village, Umaji Village, Geishi Village, Inomachi, Niyodogawa Village, Nakatosa Town, Takamura Tsuno Town, Shimanto Town, Ochi Town, Mihara Village, Kuroshio Town
Kumamoto Taragi Town, Yunomae Town, Mizukami Village, Asagiri Town
Oita Saiki City
Miyazaki Miyazaki City, Tsuno City, Nobuoka City, Hinokage City, Kobayashi City, Hinokage City, Kushima City, West City, Ebino City, Mitsumata Town, Takaharu Town, Shintomi Town, Nishimera Village, Kijo Town, Kawaminami Town, Tsuno Town, Kadogawa Town, Morozuka Village, Shiiba Village, Misato Town, Takachiho Town, Hino Kage Town, Gokase Town
Kagoshima Kanoya City, Pillowzaki City, Shinan City, Nishino Omote City, So City, Kirishima City, Fushi City, Amami City, Minamitane City, Osaki Town, Yusui Town, Osaki Town, Higashikushira Town, Kinko Town, Minamitane Town, Kimotsuki Town, Nakase Town, Minamitane Town, Yakushima Town, Yamato Village, Kikai Town
Okinawa Minamidaito Village, Kitadaito Village, Miyakojima City, Kunigami City, Higashi Village, Tarama Village, Yonaguni Town

CII has already begun to permeate society. For example, Weathernews Inc. has distributed an article about the cleanliness of Japanese air using CII.

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