## 1 Introduction  
As major poisonous air pollutants, nitrogen oxides (NO\({}_{x}\)) and sulfur dioxide (SO\({}_{2}\)) from the fossil fuel combustion are key contributors to acid deposition, photochemical smog and hazy weather, and bring out a great harm to the environment and human health [1]. NO\({}_{x}\) and SO\({}_{2}\) removal technologies, therefore, have been the research focus of air pollution control in recent decades. It is well known that the commonly practiced technologies of removing NO\({}_{x}\), such as low NO\({}_{x}\) burners, selective non-catalytic reduction (SNCR), and selective catalytic reduction (SCR), have been widely developed and used. SO\({}_{2}\) emissions have also been effectively controlled by wet flue gas desulfurization (WFGD), circulated fluid bed flue gas desulfurization (CFB-FGD), and so on. Although the combination of the commonly practiced de-NO\({}_{x}\) and de-SO\({}_{2}\) processes can meet the limitations of SO\({}_{2}\) and NO\({}_{x}\) emissions stipulated by environmental legislation, traditional NO\({}_{x}\) and SO\({}_{2}\) removal technologies in a stepwise manner have some drawbacks, such as the requirement for large construction site, high capital investment, and high operation costs. Thus, the emerging technologies for the simultaneous removal of NO\({}_{x}\) and SO\({}_{2}\) are to be extensively developed [2; 3].  
The integrated oxidation-absorption process using chemical reagents for simultaneous removal of NO\({}_{x}\) and SO\({}_{2}\) is considered as a cost-effective and environmental friendly technology for flue gas treatment and has gained increasing interest [4]. However, nitric oxide (NO), accounting for more than 95% of NO\({}_{x}\) in typical flue gas, is almost insoluble and low active. Thus, NO has to be oxidized into soluble nitrogen dioxide (NO\({}_{2}\)) at the first step. Recently, a few studies related to the NO oxidation have been reported [5; 6; 7; 8; 9; 10; 11; 12], including gas phase catalytic oxidation, ozone oxidation, corona discharge oxidation, and liquid oxidation. After oxidation, the soluble NO\({}_{2}\) can be further removed with SO\({}_{2}\) in the subsequent absorption process [13]. Till now, the NO\({}_{2}\) absorption process using chemical reagents have widely been studied to develop the industrial scale process. Many absorbents such as Ca(OH)\({}_{2}\), CaCO\({}_{3}\), Na\({}_{2}\)SO\({}_{3}\), (NH\({}_{4}\))\({}_{2}\)SO\({}_{3}\), and CaSO\({}_{3}\) have been tested and the reaction mechanism have been probed [1; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Among them, sulfite solutions have been considered as a cost-effective absorbent because of their high removal efficiency, their desulphurization byproduct characteristics, and their capacity of simultaneous removal of mercury and chlorine [23; 24; 25]. However, NO\({}_{2}\) and SO\({}_{2}\) absorption into sulfite absorbents involves complex chemical behaviors. Consequently, and the chemical behavior of absorption has not been widely generalized up to now. Furthermore, there are few reports on the analysis of the gas products from the NO\({}_{2}\) and SO\({}_{2}\) absorption in absorbents. Thus it is absolutely necessary to further understand the chemical behaviors of the absorption process to promote NO\({}_{2}\) and SO\({}_{2}\) removal by adsorption.  
In this study, Na\({}_{2}\)SO\({}_{3}\) solution was selected as sulfite absorbent. The time-dependences of concentrations of main nitrogen and sulfur (N and S) compositions in both gas phase and liquid phase were investigated by flue gas analyzer, Ion chromatography (IC), Gas chromatography (GC) and Fourier transform infrared spectrometry (FTIR). The mass equilibrium of the N and S compositions of NO\({}_{2}\) absorption into absorbent through mass balance calculation were also carried out. Moreover, a possible reaction pathway of NO\({}_{2}\) removal was proposed.