## 3 Results and Discussion
### Qualitative Analysis of N Compositions  
Besides NO\({}_{2}\), NO, NO\({}_{2}\)\({}^{-}\), and NO\({}_{3}\)\({}^{-}\), the nitrous products such as N\({}_{2}\)O, N\({}_{2}\)O\({}_{3}\), N\({}_{2}\)O\({}_{5}\), and N\({}_{2}\), can form during the NO\({}_{2}\) absorption into sulfite solution [10; 30]. IC, GC, and FTIR were used to detect these products in this study. Figures 6 and 7 present the GC and FTIR spectra of the gaseous products, respectively. It can be seen from Figure 6 that the gaseous N\({}_{2}\) is formed mainly in the initial absorption period, declines as the absorption proceeds, and finally disappears after 230 min. N\({}_{2}\) formation can be explained by the following reactions [31],  
\[2\text{NO}+2\text{SO}_{3}{}^{2-} \to 2\text{SO}_{4}{}^{2-}+\text{N}_{2}\] (R4) \[\text{N}_{2}\text{O}+\text{SO}_{3}{}^{2-} \to \text{SO}_{4}{}^{2-}+\text{N}_{2}\] (R5)  
The sulfite plays an important role in N\({}_{2}\) formation according to Reactions (4) and (5). Thus N\({}_{2}\) concentration decrease is due to the fact that sulfite concentration in reaction system gradually declines and finally disappears. Although N\({}_{2}\) concentration cannot contribute to the equilibrium analysis of nitrogen, the observed declining tendency of N\({}_{2}\) peaks with the absorption time indicates that N\({}_{2}\) is one of other nitrogen compositions. Figure 5: The equilibrium analysis of S compositions for SO\({}_{2}\) absorption in sulfite solution. Figure 6: GC spectra of gaseous N-products during NO\({}_{2}\) absorption. Based on the FTIR analysis, the effect of absorption time on the gaseous N-product distribution can be seen, as shown in Figure 7. Six distinct absorbance peaks are observed at 2930 cm\({}^{-1}\), 2287 cm\({}^{-1}\), 1720 cm\({}^{-1}\), 1600 cm\({}^{-1}\), 1326 cm\({}^{-1}\), and 887 cm\({}^{-1}\), and the bands are assigned to NO\({}_{2}\), N\({}_{2}\)O, N\({}_{2}\)O\({}_{5}\), NO\({}_{2}\), HNO\({}_{3}\), and HNO\({}_{3}\), respectively [34]. In the initial absorption period, the intensities of bands at 2287 cm\({}^{-1}\), 1720 cm\({}^{-1}\), 1326 cm\({}^{-1}\), and 887 cm\({}^{-1}\) almost keep unchanged, which shows that N\({}_{2}\)O, N\({}_{2}\)O\({}_{5}\), and HNO\({}_{3}\) as gaseous products are continuously generated from the absorption reactions. Then the amounts of N\({}_{2}\)O, N\({}_{2}\)O\({}_{5}\), and HNO\({}_{3}\) rapidly decrease after the absorption time of about 210 min and finally disappear after about 230 min, which indicates that the absorption equilibrium is established, and no sufficient sulfite is used to react with NO\({}_{2}\) to produce gaseous N\({}_{2}\)O, N\({}_{2}\)O\({}_{5}\), and HNO\({}_{3}\). These results are consistent with the equilibrium analysis of sulfur compositions (see Figure 5). In fact, not all sulfite would be converted to sulfate ions, nitrogen-sulfur compounds, and sulfite radical may exist in reaction system. In addition, the occurrence of N\({}_{2}\)O\({}_{5}\) is due to the reaction between NO\({}_{2}\) and O\({}_{2}\) in gas phase, or the decomposition of nitrogen-sulfur compounds. Furthermore, the gaseous HNO\({}_{3}\) is not only attributed to the fact that HNO\({}_{3}\) in absorption liquid enters gas phase, but also that gas phase reaction happens, which can be described by the following Reaction (6):  
\[3\text{NO}_{2}(\text{g})+\text{H}_{2}\text{O}(\text{g})\rightarrow\text{NO}( \text{g})+2\text{HNO}_{3}(\text{g}) \tag{10}\]  
Figure 7 also shows that the peak at 1600 cm\({}^{-1}\), assigned to NO\({}_{2}\), gradually increases and the peak at 1900 cm\({}^{-1}\), assigned to NO, gradually decreases after the absorption time of about 210 min. The results are also in accordance with those of gas phase composition analysis by blue gas analyzer (Testo350-pro, Germany), as shown in Figure 2.  
Figure 8 shows the distribution of nitrogen products for NO\({}_{2}\) absorption into sulfite solution, based on mass balance for a typical inlet gas with 260 ppm NO\({}_{2}\), as the sole N source.