NOx emission reduction and its effects on ozone during the 2008 Olympic Games.

We applied a daily assimilated inversion method to estimate NO(x) (NO + NO(2)) emissions for June-September 2007 and 2008 on the basis of the Aura Ozone Monitoring Instrument (OMI) observations of nitrogen dioxide (NO(2)) and model simulations using the Regional chEmistry and trAnsport Model (REAM). This method allows for estimating emission changes with a finer temporal resolution than previous studies and shows that the progression of the emission reduction corresponds roughly to the scheduled implementation of emission controls over Beijing. OMI column NO(2) reductions are approximately 45%, 33%, and 14% over urban Beijing, rural Beijing, and the Huabei Plain, respectively, while the corresponding anthropogenic NO(x) emission reductions are only 28%, 24%, and 6%, during the full emission control period (July 20-Sep 20, 2008). Meteorological changes from summer 2007 to 2008 are the main factor contributing to the column NO(2) decreases not accounted for by the emission reduction. The surface ozone changes due to NO(x) emission reduction are negligible using a standard VOC emission inventory. When using enhanced VOC (particularly aromatics) emissions derived from in situ observations, urban Beijing shifted O(3) production from the VOC-limited regime toward the NO(x)-limited regime resulting in a more substantial ozone decrease (up to 10 ppbv).