In recent years, Beijing has experienced severe air pollution which has caused widespread public concern. Compared to the same period in 2014, the first three quarters of 2015 exhibited significantly improved air quality. However, the air quality sharply declined in the fourth quarter of 2015, especially in November and December. During that time, Beijing issued the first red alert for severe air pollution in history. In total, 2 red alerts, 3 orange alerts, 3 yellow alerts, and 3 blue alerts were issued based on the adoption of relatively temporary emergency control measures to mitigate air pollution. This study explored the reasons for these variations in air quality and assessed the effectiveness of emergency alerts in addressing severe air pollution. A synthetic analysis of emission variations and meteorological conditions was performed to better understand these extreme air pollution episodes in the fourth quarter of 2015. The results showed that compared to those in the same period in 2014, the daily average emissions of air pollutants decreased in the fourth quarter of 2015. However, the emission levels of primary pollutants were still relatively high, which was the main intrinsic cause of haze episodes, and unfavorable meteorological conditions represented important external factors. Emergency control measures for heavy air pollution were implemented during this red alert period, decreasing the emissions of primary air pollutants by approximately 36% and the PM_2.5 concentration by 11%?21%.