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City clusters play an important role in air pollutant and greenhouse gas (GHG) emissions reduction in China, primarily due to their high fossil energy consumption levels. The “2 + 26” Cities, i.e., Beijing, Tianjin and 26 other perfectures in northern China, has experienced serious air pollution in recent years. We employ the Greenhouse Gas and Air Pollution Interactions and Synergies model adapted to the “2 + 26” Cities (GAINS-JJJ) to evaluate the impacts of structural adjustments in four major sectors, industry, energy, transport and land use, under the Three-Year Action Plan for Blue Skies (Three-Year Action Plan) on the emissions of both the major air pollutants and CO2 in the “2 + 26” Cities. The results indicate that the Three-Year Action Plan applied in the “2 + 26” Cities reduces the total emissions of primary fine particulate matter with an aerodynamic diameter of ≤ 2.5 μm (PM2.5), SO2, NOx, NH3 and CO2 by 17%, 25%, 21%, 3% and 1%, respectively, from 2017 to 2020. The emission reduction potentials vary widely across the 28 prefectures, which may be attributed to the differences in energy structure, industrial composition, and policy enforcement rate. Among the four sectors, adjustment of industrial structure attains the highest co-benefits of CO2 reduction and air pollution control due to its high CO2 reduction potential, while structural adjustments in energy and transport attain much lower co-benefits, despite their relatively high air pollutant emissions reductions, primarily resulting from an increase in the coal-electric load and associated carbon emissions caused by electric reform policies.

A coaxial nonthermal plasma (NTP) reactor was used for the removal of nitric oxide in mist. The effects of discharge gap, thorn number, tooth slice number as well as discharge polarity on NO oxidation and NOx removal were investigated. Decreasing the discharge gap is favorable for the NO oxidation and NOx removal. The mist is almost captured completely even under a relative low input energy. Increasing the thorn number of discharge slice or tooth slice number facilitates the energy input in the plasma reactor and thus enhances the NO oxidation and NOx removal under a fixed applied voltage. The energy efficiency for the positive DC (4.5 g NO/kWh) is three times as much as that for the negative DC (1.5 g NO/kWh), corresponding to NO oxidation efficiency of around 80%.

A coaxial nonthermal plasma (NTP) reactor was used for oxidizing and removing NO in mist. The effects of gas/mist components on O3 formation, NO oxidation, and NOx removal by NTP in simulated flue gas after wet desulfurization were investigated. The presence of mist results in the significant decrease of O3 concentration. Formation of active species, such as OH and HO2, would promote the reactions between NO and OH/HO2. Thus, the NO oxidation, NOx removal, and energy efficiency are significantly improved in the presence of mist. The promotion effects of mist component on the NO oxidation and NOx removal follow the order: Na2SO3 mist > FeSO4 mist > Na2SO4 mist. Increase of the mist pH can enhance the absorption of nitrogen oxides with the mist. Adding SO2 to the gas mixtures or raising the NO2/NOx ratio of simulated gas increases the NOx removal efficiency. This process demonstrates that NO oxidation by NTP and subsequently absorption by mist are feasible and have great value to practical application.

NH3 synthesis from H2O + N2, H2 + N2 or H2O + H2 + N2 by dielectric barrier discharge combined with an Ru/Al2O3 catalyst is studied at atmospheric pressure and room temperature. Additionally, the effects of reaction gas composition, energy density, and discharge frequency on NH3 yield and by-product formation are investigated. The results show that NH3 can be formed from reaction gases consisting of H2O and N2. The NH3 yield of the reaction gas containing H2 is much higher than that of the reaction gas consisting of H2O and N2. The presence of H2O has a promotion effect on NH3 synthesis from H2 and N2, especially for reaction gases with an H2 content less than 10%. The NH3 yield first increases and then decreases with an increase in the energy density. The maximum yield of 680 mg kW−1 h−1 occurs at the energy density of 1400 J L−1 and in the reaction gas of 0.14% H2O, 40% H2 and N2. The discharge frequency has a great effect on the NH3 yield, and maximum NH3 yield is obtained at the frequency of 13 kHz. Besides NH3, some by-products, such as N2O and NO2, are also formed with reaction gases containing H2O. However, their formation can be suppressed in the presence of H2, which is attributed to the reduction effect of H2 on the by-products.

The simultaneous removal of nitrogen oxides (NOx) and sulfur dioxide (SO2) by absorption is considered to be one of the most promising technologies for flue gas treatment, and sulfite is the main component of the absorption solution. To understand the chemical behaviors of the NO2 absorption in sulfite solution, the absorption time dependences of concentrations of nitrogen and sulfur compositions in both gas phase and liquid phases were investigated by flue gas analyzer, Ion chromatography (IC), Gas chromatography (GC), and Fourier transform infrared spectrometry (FTIR) methods using a bubbling reactor. The mass equilibrium of the N and S compositions were also studied. The results indicate that sulfite concentration plays a vital role in NO2 absorption. The main absorption products are NO3− and NO2−, and NO2− can be converted into NO3− in the presence of oxygen. Besides, about 4% to 9% by-products of S compositions are formed, and 4% to 11% by-products of N compositions such as NO, N2, N2O5, N2O, and HNO3 in the gas phase were detected in the emissions from the bubbling reactor. On the basis of N and S compositions, a possible pathway of NO2 absorption in sulfite solution was proposed.