In 2013, the Chinese central government launched a war on air pollution. As a new and major source of information, the Internet plays an important role in diffusing environmental news emotion and shaping people's perceptions and emotions regarding the pollution. How could the government make use of the environmental news emotion as an informal regulation of pollution? The paper investigates the causal relationship between web news emotion (defined by the emotional tone of web news) and air pollution (SO2, NO2, PM2.5 and PM10) by exploiting the central government's war on air pollution. We combine daily monitoring data of air pollution at different levels (cities and counties, respectively the second and third administrative levels in China) with the GDELT database that allows us to have information on Chinese web news media (e.g. emotional tone of web news on air pollution). We find that a decrease of the emotional tone in web news (i.e. more negative emotions in the articles) can help to reduce air pollution at both city and county level. We attribute this effect to the context of China’s war on air pollution in which the government makes use of the environmental news emotion as an informal regulation of pollution.