Evaluating the impact of emissions regulations on the emissions reduction during the 2015 China Victory Day Parade with an ensemble square root filter

In order to guarantee good air quality for the 15th International Association of Athletics Federations World Championships (22-30 August) and the China Victory Day parade (3 September) in Beijing, a series of comprehensive emissions regulations were implemented in Beijing and the surrounding provinces from 20 August to 3 September 2015. During this period, the intensity of the emissions regulation was enhanced step by step in the run-up to the event. This period therefore allowed us to quantitatively estimate the reduction in emissions as a result of the emissions regulations under different intensity and strategy scenarios. The ensemble square root filter was employed to optimize the initial conditions and the emissions simultaneously. Numerical results showed that the variations of the anthropogenic emissions before, during, and after the emissions regulations were reasonably simulated. The updated emissions were about 80% and 50% of the prior anthropogenic emissions during the regular- and stringent-regulations periods, respectively. As a result, the PM2.5 concentrations consistently reduced during the regulations period, and the largest reduction occurred under the stringent-regulations period. Sensitivity simulations forced by the averaged emissions before the regulations and during the regular- and stringent-regulation periods further highlighted the valuable impacts of emissions regulations, under different strategy and intensity scenarios, on the improvement of air quality. More intense emissions regulation will lead to a greater reduction in PM2.5 concentrations.