This study investigates how lakes affect diurnal PM_2.5 variation in nearby urban areas. Using advanced computer simulations comparing scenarios with and without lakes, we discovered special pollution patterns. During daytime, lakes accumulated higher PM_2.5 concentrations than expected, which then spread to surrounding cities. At night, urban areas near lakes had lower pollution levels compared to simulations without lakes. This pattern contradicts traditional understanding of pollution distribution. The research reveals that lakes create stable air conditions during the day that trap pollutants over water surfaces, while at night they enhance urban pollutants mixing that reduces ground-level pollution in nearby cities. Temperature and humidity differences between lake and land surfaces significantly affect how secondary pollutants form and disperse. These findings provide important insights for understanding and managing air quality in regions with lakes and could help improve pollution control strategies in complex urban-lake environments.