Urea SCR system is one of the most remarkable solution for air pollution caused by cars in Japan. Like in many other countries in Asia, air particulate matters such as PM2.5 are concerned seriously in Japan through many years. According to the ministry of the environment in Japan, the amount of SPM and PM2.5 are reduced by Japanese government's regulations of gas emissions in the last decade.

However, it still can affect our health and environment. Since main cause of PM2.5 is gas emission by diesel cars, Japanese car companies are making effort to reduce air pollution. I'll explain the mechanism of this system simply. As you know, PM2.5 is caused by air pollutants such as SOx, NOx, and VOC. So, this system use urea to clean up these air pollutants by a chemical reaction. To know more about this system, please read this report written by researchers in Mazda, Japanese famous car company.

http://www.mazda.com/contentassets/87fc47b4d41f4dcf9f09ac85d7a69c2e/files/2010_no010.pdf

reference: Japanese ministry of the environment official website (http://www.env.go.jp/en/index.html),