## 4 Discussion
### Summary of the UV-A Photocatalysis under the Livestock Environment

Table 15: Summary of the percent reduction of target gases investigated in the previous study with UV-A photocatalysis. Bold signifies statistical significance. The UV-A photocatalysis mitigates odor, VOCs, NH\({}_{3}\), H\({}_{2}\)S, N\({}_{2}\)O and O\({}_{3}\). Targeted gases were significantly reduced depending on the UV dose (the light intensity and the treatment time) and the catalyst coating thickness. The mitigation of NH\({}_{3}\) ranged from 6%-35% (UV-A dose: 5.8-970 mJ\(\cdot\)cm\({}^{-2}\)). While the results were statistically significant, the mitigation effect was relatively low. The range of UV dose required to reduce NH\({}_{3}\) varied, likely due to the different approaches used to estimate the UV light intensity (I) in each study. To be specific, the light intensity (I) in the previous papers was either (1) measured on the surface irradiated directly by the UV lamp (one-dimension aspect) or (2) averaged over all three-dimensions by the UV lamp. The 2 nd approach was used to measure the light intensity in this study (as described in greater detail in [18]), but if the UV dose was calculated using the 1 st method, it would increase to \(\sim\)0.12 J\(\cdot\)cm\({}^{-2}\). Therefore, it is difficult to compare the UV doses measured in previous studies directly, but it is considered that a minimum of 5.8 mJ\(\cdot\)cm\({}^{-2}\) is required to reduce NH\({}_{3}\). Furthermore, higher UV-A doses will likely be necessary to mitigate NH\({}_{3}\) on the farm-scale (inside the barn). The mitigation of H\({}_{2}\)S ranged from 4%-40% (UV-A dose: 0.6-5.3 mJ\(\cdot\)cm\({}^{-2}\)). It is challenging to make an accurate comparison within previous research due to the differences in the TiO\({}_{2}\) coating thicknesses and I (averaging over one vs. three dimensions). Several papers proved the mechanism of H\({}_{2}\)S oxidation with UV-A photocatalysis based on TiO\({}_{2}\) coating [9; 10; 11]. However, it is not easy to propose an accurate UV dose to mitigate H\({}_{2}\)S based on the currently collected data. This is because the H\({}_{2}\)S concentration did not show significant mitigation with a higher UV dose on the lab-scale and a lower UV dose on the farm-scale, respectively. Mitigating H\({}_{2}\)S on the farm-scale could be confounded by interactions with other compounds or factors and should be investigated further. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Ref. **} & \multicolumn{1}{c}{**Experimental Conditions**} & \multicolumn{1}{c}{**Catalyst (Dose)**} & \multicolumn{1}{c}{**UV Wavelength**} & \multicolumn{1}{c}{**UV Dose**} & \multicolumn{1}{c}{**Target Gas (Percent**} \\  & & & & **(intensity)** & **Reduction)** \\ \hline \multirow{10}{*}{[2]} & \multirow{6}{*}{Pilot-scale (poultry farm): T: 28 \(\pm\) 3 \({}^{\circ}\)C; RH: 56\%} & \multirow{6}{*}{TiO\({}_{2}\) (10 \(\mu\)g\(\cdot\)cm\({}^{-2}\))} & \multirow{6}{*}{365 nm} & NH\({}_{3}\) (**8.7**) & \multirow{6}{*}{**N\({}_{2}\)O  
N\({}_{2}\)O was consistently mitigated by 9%-14% under UV-A photocatalysis with 1.9 mJ\(\cdot\)cm\({}^{-2}\) or higher doses. There was no statistically significant change in CH\({}_{4}\). A few papers reported significant decreases and increases in CH\({}_{4}\) concentration, but most report random generation and mitigation regardless of the UV-A dose. Mitigation of CO\({}_{2}\) was reported in fewer papers, while most report CO\({}_{2}\) generation. In general, CO\({}_{2}\) is the oxidative endpoint for photocatalytic decomposition of virtually all C-containing compounds under conditions like those used here, and thus UV-A is not expected to mitigate it. The O\({}_{3}\) showed more reduction at the farm and pilot scale (100%) than at the laboratory scale (48%). It is hypothesized that the O\({}_{3}\) (if present) would be rapidly utilized by reactions with odorous VOCs present in actual farm conditions. The mitigation of odor ranged from 32%-63% (UV-A dose: 2.5-5.8 mJ\(\cdot\)cm\({}^{-2}\)).