3.1.2. Kinetic Analysis at High NaNO<sup>2</sup> Concentrations

Time-dependent concentrations of 3MC in the presence of different initial NaNO<sup>2</sup> concentrations (i.e., from 0.5 to 5 mM) are shown in Figure 2a–d. The lines are the calculated concentration profiles according to the two proposed model functions (Equations (1a) and (4)). By using *k*app, the experimental data are represented well in all cases; thus, its values were further used to derive *k*2nd according to Equation (2b) for each experiment individually (see Table 2). Yet, a global fit was also performed to get a more reliable second-order rate constant, *k*2nd(global), by which the kinetics of 3MC decay at all treated conditions is still well described (black lines in Figure 2). *Atmosphere* **2020**, *11*, 131 7 of 17 according to the two proposed model functions (Equations (1) and (4)). By using *k*app, the experimental data are represented well in all cases; thus, its values were further used to derive *k*2nd according to Equation (2a) for each experiment individually (see Table 2). Yet, a global fit was also performed to get a more reliable second-order rate constant, *k*2nd(global), by which the kinetics of 3MC decay at all treated conditions is still well described (black lines in Figure 2).

**Figure 2.** 3-methylcatechol (3MC) degradation in aqueous solution at different initial concentrations of NaNO2 (pH = 4.5–5, T = 25 °C). The concentrations of NaNO2 were 0.5 (**a**), 1.0 (**b**), 2.0 (**c**), and 5.0 mM (**d**). Experimental data (symbols) and calculated time-dependent concentration profiles (lines) for the **Figure 2.** 3-methylcatechol (3MC) degradation in aqueous solution at different initial concentrations of NaNO<sup>2</sup> (pH = 4.5–5, T = 25 ◦C). The concentrations of NaNO<sup>2</sup> were 0.5 (**a**), 1.0 (**b**), 2.0 (**c**), and 5.0 mM (**d**). Experimental data (symbols) and calculated time-dependent concentration profiles (lines) for the pseudo-first order (*k*app) and global second order (*k*2nd (global)) fits.

The rate constant for 3MC decay in the simulated sunlight (*k*2nd(global) = 0.075 M−1·s−1) is twice as high as in the dark (*k*2nd = 0.032 M−1·s−1) under similar initial conditions (Table 2). As mentioned above, several different degradation mechanisms are possible in the sunlight that are prevented in the dark. The rate constant for 3MC decay in the simulated sunlight (*k*2nd(global) = 0.075 M−<sup>1</sup> ·s −1 ) is twice as high as in the dark (*k*2nd = 0.032 M−<sup>1</sup> ·s −1 ) under similar initial conditions (Table 2). As mentioned above, several different degradation mechanisms are possible in the sunlight that are prevented in

Similarly, under the sunlight, *k*2nd for the 3MC decay at the lowest NaNO2 concentration (0.1 mM) is almost twice as that at higher NaNO2 concentrations (0.5−5 mM). The reason might be in the

pseudo-first order (*kapp*) and global second order (*k*2nd (global)) fits.

the dark. Similarly, under the sunlight, *k*2nd for the 3MC decay at the lowest NaNO<sup>2</sup> concentration (0.1 mM) is almost twice as that at higher NaNO<sup>2</sup> concentrations (0.5–5 mM). The reason might be in the competition between NO<sup>2</sup> − and 3MC for OH radicals and other reactive species at these experimental conditions. When HNO2/NO<sup>2</sup> <sup>−</sup> is in excess, OH radicals preferentially react with NO<sup>2</sup> −, whereas only a minor fraction reacts with 3MC, which results in its slower degradation [37,39].

### *3.2. Absorption Spectra and BrC Formation*
