Attribution of Tropospheric Ozone to NO x and VOC Emissions: Considering Ozone Formation in the Transition Regime.

An improved three-regime (3R) O3 attribution technique for O3 source apportionment in regional chemical transport models is developed to divide the entire range of VOC-NO x-O3 formation sensitivity to VOC-limited, transition, and NO x-limited regimes based on the value of a regime indicator R. The threshold R values to mark the start ( Rts) and end ( Rte) of the transition regime are defined at the point where O3-NO x sensitivity turns from negative to positive and where O3-NO x sensitivity is ten times higher than O3-VOC sensitivity, respectively. Rts and Rte are determined using NO x and VOC sensitivity simulations in a box model with a modified SAPRC-11 mechanism. For the widely used indicator ration R = ( PH2O2 + PROOH)/ 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 O3 formation in the transition regime to NO x and VOCs. The new 3R and the traditional two-regime (2R) schemes are incorporated into the Community Multiscale Air Quality (CMAQ) model to quantify NO x and VOC contributions to regional O3 concentrations in China in August 2013. The 3R approach predicts approximately 5-10 ppb and up to 15 ppb higher NO x contributions to 8 h O3 in in the North China Plain, the Yangtze River Delta and the Pearl River Delta than the 2R approach. The big differences in O3 attribution between 2R and 3R can have significant policy implications for air pollution emission controls.