## 4 Discussion  
### Summary of the UV-A Photocatalysis under the Livestock Environment  
Table 15 summarizes previous research on the mitigation of selected gases via photocatalysis with UV-A in livestock-relevant context and conditions. The summary shows that UV-A photocatalysis yields significant reductions of targeted gases, but the magnitude varies for specific compounds. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Ref.** & **Experimental Conditions** & **Catalyst (Dose)** & **UV Wavelength** & **UV Dose** & **Target Gas (Percent** \\  & & & & **(Intensity)** & **Reduction)** \\ \hline
[20] & Swine farm (farrowing rooms) T: 24 \({}^{\circ}\)C (18.9–27.3): RH: 54\% & TiO\({}_{2}\) (7 mg\(\cdot\)cm\({}^{-2}\)) & 315–400 nm & Not reported &
\begin{tabular}{c} **NH\({}_{3}\) (30.5)** \\ **CH\({}_{4}\) (10.8)** \\ **CO\({}_{2}\) (15.3)** \\ \end{tabular} \\ \hline
[21] & Swine farm (weaning units) T: 26 \({}^{\circ}\)C (24.2–29.9); RH: 56\% (52–90\%) & TiO\({}_{2}\) (7 mg\(\cdot\)cm\({}^{-2}\)) & 315–400 nm & Not reported &
\begin{tabular}{c} **CH\({}_{4}\) (27.4)** \\ **PM 10 (17.0)** \\ \end{tabular} \\ \hline
[7] & Lab-scale (simulated livestock & & & & (0.46 mW\(\cdot\)cm\({}^{-2}\)) \\  & farm); T: 24 \({}^{\circ}\)C; RH: 50\% & TiO\({}_{2}\) (0.7 \(\mu\)g\(\cdot\)cm\({}^{-2}\)) & 365 nm & \
\begin{tabular}{c} Not reported \\ (0.46 mW\(\cdot\)cm\({}^{-2}\)) \\ \end{tabular} &
\begin{tabular}{c} **NH\({}_{3}\) (35)** \\ **MT (80–87)** \\ **DMS (92–95)** \\ **DMS (83–91)** \\ **Butan-1-01 (93-95)** \\ **AA (81–89)** \\ **PA (97–98)** \\ **BA (98–99)** \\ **VA (98–99)** \\ **DMDS (40)** \\ **DEDS (81)** \\ **DMTS (76)** \\ **BA (87)** \\ **BA (87)** \\ **PA (87)** \\ **BA (88–99)** \\ **VA (98–99)** \\ **DMDS (40)** \\ **DEDS (81)** \\ **DMTS (76)** \\ **BA (87)** \\ **BA (87)** \\ **BA (87)** \\ **BA (87)** \\ **BA (87)** \\ **BA (87)** \\ **BA (87)** \\ **BA (87)** \\ **CA (100)** \\ **PA (16)** \\ **_p-Cresol (22)_ \\ \end{tabular} \\ \hline
[1] & Lab-scale (simulated poultry farm); T: 25 \(\pm\) 3 \({}^{\circ}\)C; RH: 12\% & TiO\({}_{2}\) (10 \(\mu\)g\(\cdot\)cm\({}^{-2}\)) & 365 nm &
\begin{tabular}{c} **NH\({}_{3}\) (19)** \\ **N\({}_{2}\) (10)** \\ **CO\({}_{2}\) (3.8)** \\ **O\({}_{3}\) (48)** \\ **N\({}_{2}\) (0.87)** \\ **CO\({}_{2}\) (3.1)** \\ **Odor (16)** \\ **_p-Cresol (22)_ \\ \end{tabular} \\ \hline
[16] & Pilot-scale (swine finishing rooms); T: 22–26 \({}^{\circ}\)C; RH: 36–80\% & TiO\({}_{2}\) (10 \(\mu\)g\(\cdot\)cm\({}^{-2}\)) & 365 nm &
\begin{tabular}{c} **NH\({}_{3}\) (30.5)** \\ **CH\({}_{4}\) (10.8)** \\ **CO\({}_{2}\) (15.3)** \\ **Odor (16)** \\ **_p-Cresol (22)_ \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 14: Results of simultaneous chemical and smell sensory characterization of selected VOCs emitted from swine manure and treated with UV-A. Panelist responses are the ’odor character (i.e., ’what it smells like’) and the (hedonic tone) (on an ‘unpleasant-neutral-pleasant’ scale). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**RT** & **Compound** & **Abundance** & **Control** & & & & **UV-A Treatment** & & & \\  & & **Panelist 1** & **Panelist 2** & **Panelist 3** & **Abundance** & **Panelist 1** & **Panelist 2** & **Panelist 3** & **\% R** \\ \hline
15.6 & \begin{tabular}{c} Benzoic \\ acid \\ (1-Octanol) \\ \end{tabular} & \begin{tabular}{c} Alcoholic \\ (1120 \(\times\) 10\({}^{3}\) \\ \end{tabular} & \begin{tabular}{c} Alcoholic \\ (Unpleasant) \\ \end{tabular} & \begin{tabular}{c} Acid (Unpleasant) \\ \end{tabular} & \begin{tabular}{c} Acid (Unpleasant) \\ \end{tabular} & \begin{tabular}{c} 1750 \(\times\) 10\({}^{3}\) \\ \end{tabular} & \
\begin{tabular}{c} Toothpaste \\ (Pleasant) \\ \end{tabular} &
\begin{tabular}{c} Mint \\ (Neutral) \\ \end{tabular} & \(-\)55.6 \\ \hline \hline \end{tabular}  
Note: RT = GC column retention time, min; Abundance = peak area counts; arbitrary units; % R = percent reduction (negative value signifies compound generation by the UV-A treatment).  
\end{table}