Attribution of Tropospheric Ozone to NO_<i>x</i> and VOC Emissions: Considering Ozone Formation in the Transition Regime

Abstract

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 ( R_ts) and end ( R_te) 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. R_ts and R_te 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 = ( P_H2O2 + P_ROOH)/ P_HNO3, which is based on the production rates of H₂O₂, HNO₃ and organic hydroperoxides (ROOH), the recommended R_ts and R_te 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.