2011年末，一場持續數天的霧霾天氣席捲北京，環保部門卻並未提供相關污染物PM2.5的監測數據。相關法律法規的落後，以及環保部門的不作為，激起民眾的不滿。隨著美國駐華大使館、環保NGO、微博名人和廣大民眾的持續參與與推動，政府相關資訊政策也逐漸變化，最終將PM2.5納入空氣質量標準并公佈監測數據。本文以此次霧霾污染事件作為「焦點事件」切入點，首先藉由事件回顧討論“誰來監測”的問題，探討了環境信息公開的理論與實踐；然後分析政策變化的過程，歸納「政策變化模型」，討論政策變化形成的原因，分析各行為者在政策變化中的角色和作用；最後，由此事件得出這一政策轉變的影響以及啟示，歸納目前環境資訊公開領域制度層面所存在的缺失，并就此事件的經驗對於政府和環境NGO給出建議。 / In the end of 2011, the city of Beijing was hit by a long-lasting haze. However, the city’s Environmental Protection Bureau was unable to provide the public with the data of a critical pollutant – PM2.5. The political inaction of the government has caused great discontent among the public. With the continuous efforts from the US Embassy in China, environmental NGOs, Sina Weibo celebrity users and the public, the official information disclosure policy has seen a big change – the government has adopted a higher standard and taken steps to publish the data of PM2.5. This article takes the haze incident as a “focusing event”. It firstly reviews the case and discusses the theories and practices of the environmental information disclosure. It then analyzes the process of this policy change and forms a policy change model to determine the contributing factors and the roles of different actors. The concluding part summarizes the omission of the current information disclosure system, and makes recommendations to the government and the NGOs.