This study investigates the impact of the National Ambient Air Quality Monitoring Network (NAAQMN) on corporate greenwashing behaviour, focusing on Chinese A-share companies from 2009 to 2020. Utilizing a staggered Difference-in-Differences model, we find that the NAAQMN significantly increases greenwashing among firms, as the monitoring function and enforcement were limited. Notably, from a geographic distance perspective, firms located farther from monitoring stations are more likely to engage in greenwashing due to reduced deterrence. We find that firms facing high financial constraints, operating in highly competitive markets, and managed by younger executives are particularly prone to greenwashing. The evidence also reveals that high public environmental concern and corporate digital transformation can mitigate firms' greenwashing by enhancing scrutiny and information transparency. This research underscores the need for improved monitoring technologies and public awareness to promote genuine sustainability efforts. These findings contribute to an understanding of the complex interplay among environmental policy, monitoring functions, public attentions and business strategic responses.