This research article analyzes trends in PM2.5 concentrations in five major Chinese cities between 1999-2008. The study finds that PM2.5 levels decreased overall during this period, especially in warmer seasons, but remained highest during evening hours and winter months. Additionally, PM2.5 concentrations were still above Chinese national air quality standards, particularly in northern regions during winter. Stronger government measures are needed to reduce emissions, especially in northern China during colder periods.