Quantification of the enhanced effectiveness of NO<sub><i>x</i></sub> control from simultaneous reductions of VOC and NH<sub>3</sub> for reducing air pollution in Beijing-Tianjin-Hebei region, China

Abstract

Abstract. As one common precursor for both PM2.5 and O3 pollution, NOx gains great attention because its controls can be 15 beneficial for reducing both PM2.5 and O3. However, the effectiveness of NOx controls for reducing PM2.5 and O3 are largely influenced by the ambient levels of NH3 and VOC, exhibiting strong nonlinearities characterized as NH3-limited/-poor and NOx-/VOC-limited conditions, respectively. Quantification of such nonlinearities is prerequisite to making suitable policy decisions but limitations of existing methods were recognized. In this study, a new method was developed by fitting multiple simulations of a chemical transport model (i.e., Community Multi-scale Air Quality Modeling System (CMAQ)) with a set of polynomial functions 20(denoted as "pf-RSM") to quantify responses of ambient PM2.5 and O3 concentrations to changes in precursor emissions. The accuracy of the pf-RSM is carefully examined to meet the criteria of a mean normalized error within 2% and a maximal normalized error within 10% by using forty training samples with marginal processing. An advantage of the pf-RSM method is that the nonlinearity in PM2.5 and O3 responses to precursor emission changes can be characterized by quantitative indicators, including (1) peak ratio (denoted as PR) representing VOC-limited or NOx-limited condition, (2) suggested reduction ratio of VOC to NOx 25(denoted as VNr) to avoid increasing O3 under VOC-limited condition, (3) flex ratio (denoted as FR) representing NH3-poor or NH3-rich condition, and (4) enhanced benefits in PM2.5 reductions from simultaneous reduction of NH3 with the same reduction rate of NOx. A case study in Beijing-Tianjin-Hebei region suggested that most urban areas present strong VOC-limited condition with PR from 0.4 to 0.8 in July, implying that the NOx emission reduction rate need be greater than 20%-60% to pass the transition from VOC-limited to NOx-limited. A simultaneous VOC control (VNr is about 0.5-1.2) can avoid increasing O3 during the 30 transition. For PM2.5, most urban areas present strong NH3-rich condition with PR from 0.75-0.95, implying the NH3 is sufficiently abundant to neutralize extra nitric acid produced by an additional 5%-35% of NOx emissions. Enhanced benefits in PM2.5 reductions from simultaneous reduction of NH3 were estimated to be 0.04-0.15 µg m -3 PM2.5 per 1% reduction of NH3 along with NOx, with greater benefits in July when the NH3-rich condition is not as strong as in January. Thus, simultaneously reducing NH3 and VOC emission along with NOx reduction is recommended to assure the control effectiveness of PM2.5 and O3. 35Atmos. Chem. Phys. Discuss., https://doi