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. Air quality on our planet has been changing in particular since the industrial revolution (1750s) because of anthropogenic emissions.It is becoming increasingly important to realize air cleanliness, since clean air is as valuable a resource as clean water.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).The CII is a simple index defined by the normalization of the amount of a set of individual air pollutants.A CII value of 1 indicates completely clean air (no air pollutants), and 0 indicates the presence of air pollutants that meet the numerical environmental criteria for the normalization.In this time, the air pollutants used in the CII were taken from the Air Quality Guidelines (AQG) set by the World Health Organization (WHO), namely O3 , particulate matters, NO2 , and SO2 .We chose Japan as a study area to evaluate CII because of the following reasons: (i) accurate validation data, as the in situ observation sites of the Atmospheric Environmental Regional Observation System (AEROS) provide highly accurate values of air pollutant amounts, and (ii) fixed numerical criteria from the Japanese Environmental Quality Standards (JEQS) as directed by the Ministry of the Environment (MOE) of Japan.We quantified air cleanliness in terms of the CII for the all 1896 municipalities in Japan and used data from Seoul and Beijing to evaluate Japanese air cleanliness.The amount of each air pollutant was calculated using a model that combined the Weather Research and Forecasting (WRF) and Community Multiscale Air Quality (CMAQ) models for 1 April 2014 to 31 March 2017.The CII values calculated by the WRF–CMAQ model and the AEROS measurements showed good agreement.The mean of the correlation coefficient for the CII values of 498 municipalities where the AEROS measurements operated was 0.66±0.05 , which was higher than that of the Air Quality Index (AQI) of 0.57±0.06 .The CII values averaged for the study period were 0.67, 0.52, and 0.24 in Tokyo, Seoul, and Beijing, respectively; thus, the air in Tokyo was 1.5 and 2.3 times cleaner, i.e., lower amounts of air pollutants, than the air in Seoul and Beijing, respectively.The average CII value for the all Japanese municipalities was 0.72 over the study period.The extremely clean air, CII ≈0.90 , occurred in the southern remote islands of Tokyo and to the west of the Pacific coast, i.e., Kochi, Mie, and Wakayama prefectures during summer, with the transport of clean air from the ocean.We presented the top 100 clean air cities in Japan as one example of an application of CII in society.We confirmed that the CII enabled the quantitative evaluation of air cleanliness.The CII can be useful and valuable in various scenarios such as encouraging sightseeing and migration, investment and insurance business, and city planning.The CII is a simple and fair index that can be applied to all nations.

Novel index to comprehensively evaluate air cleanliness: the Clean aIr Index (CII)