## 2 Experimental Section
### Experimental Methods  
All experiments were performed at atmospheric pressure and the temperature of 325 \(\pm\) 5 K. The simulated flue gas, which was supposed to be outlet flue gas after NO oxidation, was simulated by mixing proper amount of NO\({}_{2}\), O\({}_{2}\), SO\({}_{2}\), and Ar from compressed gas cylinders. The basic simulated  
Figure 1: Schematic diagram of the experimental setup. MFC: Mass flow controller; IC: Ion chromatography; FTIR: Fourier transform infrared spectrometry; GC: Gas chromatography.  
flue gas compositions were 260 ppm NO\({}_{2}\), 300 ppm SO\({}_{2}\), 6% O\({}_{2}\) and Ar. The gas flow rate was adjusted by mass flow controller (MFC), and the four gas streams with a total flow rate of 4 L min\({}^{-1}\) were completely mixed in a mixing chamber before being fed into the absorption reactor. The volume of Na\({}_{2}\)SO\({}_{3}\) solution (0.02 M) stored in the stirred bubbling device was 2 L. pH of absorption solution was maintained at 6-7 by adding 1 M HCl or 1 M NaOH solution. The liquid samples at different times for IC analysis were attained from the sampling port. Qualitative analysis of liquid phase (NH\({}_{4}\)\({}^{+}\), NO\({}_{3}\)\({}^{-}\), NO\({}_{2}\)\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\)) was carried out by ion chromatography (IC, Metrohm 792, Herisau, Switzerland). The concentrations of NO\({}_{2}\), NO, SO\({}_{2}\), and O\({}_{2}\) in the tail gas were monitored by a flue gas analyzer (Testo350-pro, Schwarzwald, Germany). Sulfite ions (SO\({}_{3}\)\({}^{2-}\)) in the liquid phase were measured by an iodometric titration method. N\({}_{2}\) formed in the absorption reaction was monitored using gas chromatography (GC, Agilent 7890, Santa Clara, CA, USA) with a Thermal conductivity detector (TCD, Agilent, Santa Clara, CA, USA), and 5 A molecular sieve chromatographic column (Agilent, Santa Clara, CA, USA), and the temperature of column oven and detector were 40 and 180 \({}^{\circ}\)C, respectively. Other N compositions of outlet gas were analyzed by Fourier transform infrared spectrometry (FTIR, Nicolet 6700, Waltham, MA, USA) equipped with a high-sensitivity mercury cadmium telluride detectors (MCT, Nicolet, Waltham, MA, USA) cooled by liquid nitrogen. The outlet gas stream passed through a 2.5 L gas cell, which had optical path length of 10 m, accordingly averaged spectra after 32 scans at 4 cm\({}^{-1}\) resolution were collected in the 3000-650 cm\({}^{-1}\) ranges.  
In the current system, inlet NO\({}_{2}\), SO\({}_{2}\) concentrations, and initial sulfite concentration were previously set, without other sources of nitrogen and sulfur compositions. The molar quantity of N and S compositions in a closed loop system from absorption reaction could be calculated by graphical integration during a fixed period, and the theoretical values in this loop-locked system were expressed as follows:  
\[TN_{t}=\int_{0}^{t}\frac{Q(C_{0}-C_{t})\times 10^{-6}}{V_{m}}dt \tag{1}\]  
\[TS_{t}=\int_{0}^{t}\frac{Q(C_{0}-C_{t})\times 10^{-6}}{V_{m}}dt+0.04 \tag{2}\]  
where \(TN_{t}\) and \(TS_{t}\) are the total molar quantity of N and S compositions absorbed into reaction system during \(t\) absorption time (mol). \(C_{0}\) and \(C_{t}\) are inlet and outlet concentrations of NO\({}_{2}\) or SO\({}_{2}\) (ppm), respectively. \(Q\) is the total flow rate of the flue gas (L min\({}^{-1}\)). \(V_{m}\) is the molar volume of NO\({}_{2}\) or SO\({}_{2}\) in 325 K (L mol\({}^{-1}\)), and \(t\) is the absorption time (min). Meanwhile, 0.04 in Equation (2) is the original molar quantity of SO\({}_{3}\)\({}^{2-}\) in stirred bubbling device (mol).