Attenuated sensitivity of ozone to precursors in Beijing–Tianjin–Hebei region with the continuous NOx reduction within 2014–2018

Facing the elevated surface summer O3 over North China in recent years with the continuous NOx reduction, we conducted the ozone-precursor sensitivity study in summer (July) in Beijing-Tianjin-Hebei region (BTH) under the 2018 and 2014 emissions, based on WRF-Chem model. On 2018 emission condition, 30% precursor reduction simulations presented the positive contribution of VOCs and the negative contribution of NOx to daytime O3. The occurrence probabilities of VOCs-sensitive, NOx-titration, mixed sensitive, NOx-sensitive, and non-sensitive regimes respectively reached 3-49%, 2-82%, 0-7%, 0-6% and 14-82% in the urban grids, and 2-32%,1-19%, 1-6%, 0-5% and 54-86% in the rural grids. For several widely used photochemical indicators, their values in VOCs-sensitive regime were well separated from those in NOx-sensitive regime, but the NOx-sensitive values were very similar to the non-sensitive values, which implied the inefficiency of these indicators in indicating NOx-sensitive regime. Finally, VOCs-sensitive regime was discerned based on the indicator HCHO/NO2, occupying about a third of areas in morning and dusk but shrinking to about a tenth of areas in 11:00-16:00 in BTH. And these areas apparently decreased from 2014 emission to 2018 emission. However, the rest areas of this region were under non-sensitive regime but not NOx-sensitive regime, for the noticeable O3 drop never happened in NOx reduction scenario. Meanwhile, the modeled O3/PAN in the areas under non-sensitive regime exceeded 60, which also implied the minor impact of local photochemistry on O3 there. Additionally, the responses of daytime O3 to precursor emissions in the urban grids were calculated, declining by 20.8% for NOx and 6.9% for VOCs from 2014 to 2018. Therefore, to solve the ozone pollution problem of BTH, the cross-region strategy coupled with the VOCs and NOx co-control will be essential.