Attribution of Tropospheric Ozone to NOx and VOC Emissions: Considering Ozone Formation in the Transition Regime

An improved three-regime (3R) O-3 attribution technique for O-3 source apportionment in regional chemical transport models is developed to divide the entire range of VOC-NOx-O-3 formation sensitivity to VOC-limited, transition, and NOx-limited regimes based on the value of a regime indicator R. The threshold R values to mark the start(R-ts) and end (R-te) of the transition regime are defined at the point where O-3-NOx sensitivity turns from negative to positive and where O-3-NOx sensitivity is ten times higher than O-3-VOC sensitivity, respectively. R-ts and R-te are determined using NO and VOC sensitivity simulations in a box model with a modified SAPRC-11 mechanism. For the widely used indicator ration R = (P-H2O2 +P-ROOH)/PHNO3, which is based on the production rates of H2O2, HNO3 and organic hydroperoxides (ROOH), the recommended Rts and Rte values are 0.047 and 5.142, respectively. Parameterized attribution functions, depending only on the values of R, are developed to apportion modeled in situ O-3 formation in the transition regime to NO and VOCs. The new 3R and the traditional two-regime (2R) schemes are incorporated into the Community Multiscale Air Quality (CMAQ) model to quantify NOx and VOC contributions to regional O-3 concentrations in China in August 2013. The 3R approach predicts approximately 5-10 ppb and up to 15 ppb higher NOx contributions to 8 h O-3 in in the North China Plain, the Yangtze River Delta and the Pearl River Delta than the 2R approach. The big differences in O-3 attribution between 2R and 3R can have significant policy implications for air pollution emission controls.