I. Introduction
Precisely forecasting the value of air quality has a great significance for social decision making. The governments can set the environment policies about pollutant emission referring to the future values of air quality, and local residents might make their out-going plans according to accurate and trustworthy air quality predictions. Many cities have built air quality monitoring stations to inform citizens the real-time (hourly) air quality index (AQI), and provide hourly or daily forecasting services. A long-term prediction is required to support superior planning and decision making.