Evaluation of air cleanliness using a novel index, "Clean aIr Index, CII"

Abstract:

Air quality on our planet has been changing because of anthropogenic emissions. The COVID-19 pandemic has drastically changed our lifestyle around the world, and unprecedented NO₂ column decrease was observed over widespread area in Jan. – Apr. 2020 using the satellite remote sensing [Bauwens et al., GRL, 2020]. Global standard to quantify the level of air cleanliness is swiftly required, and we defined a novel concept, namely "Clean aIr Index, CII" [Sato et al., GC, 2020]. The CII is a simple index defined by the normalization of the amount of individual air pollutants. A CII value of 1 indicates completely clean air (no air pollutants), and 0 indicates the presence of air pollutants up to numerical criteria for the normalization. The air pollutants used in the CII were taken from the WHO Air Quality Guidelines, namely O₃, particulate matters, NO₂ and SO₂. In this study, we chose Japan as a study area to evaluate CII because of the following reasons: i) accurate validation data provided by the Atmospheric Environmental Regional Observation System (AEROS), ii) obvious numerical criteria given by the Ministry of the Environment. The amount of each air pollutant was calculated using 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 showed good agreement with those by the AEROS measurements with a correlation coefficient of 0.66. Using AQI, the correlation coefficient between WRF-CMAQ and AEROS was 0.57, smaller than that of CII. The CII averages air pollutant amounts normalized by the numerical criteria, while the AQI employs only the highest effective air pollutant and the others are ignored. The WRF-CMAQ model showed that the extremely clean air, CII ~ 0.90, occurred in southern remote islands and around west of the Pacific coast during summer with transport of clean air from the ocean. The CII values averaged for the study period was 0.67, 0.52 and 0.24 in Tokyo, Seoul and Beijing, respectively, thus, the amounts of air pollutants in Tokyo was 1.5 and 2.3 times less than those in Seoul and Beijing, respectively. In this presentation, we also evaluate the air cleanliness around the world using CII based on the ground-based measurements. The CII is a simple and fair index that can be applied to all nations.