## 5 Conclusions  
We investigated UV-A photocatalysis treatment to mitigate gaseous emissions at the farm-scale. Specifically, we tested the UV-treatment at a mobile laboratory-scale capable of treating \(\sim\)0.2-0.8 m\({}^{3}\cdot\)s\({}^{-1}\) of barn exhaust air. The targeted gaseous emissions were significantly (\(p<\) 0.05) mitigated up to:  
* 40% reduction of odor
* 32%-66% reduction of key compounds responsible for downwind odor, i.e., dimethyl disulfide, isobutyric acid, butanoic acid, p-cresol, indole and skatole
* 40% reduction of hydrogen sulfide (H\({}_{2}\)S)
* 100% reduction of ozone (O\({}_{3}\))
* 13% reduction of nitrous oxide (N\({}_{2}\)O)
* The PM mitigation effect was not significant. No formation of formaldehyde was detected in these experiments. However, as expected under oxidizing conditions, additional CO\({}_{2}\) was observed (up to 21%, \(p<\) 0.05). The percent reduction of odorous targeted gases depended on the UV dose, UV wavelength and PM concentration in the air. Especially, the percent reduction of targeted gases decreased as the airborne particulate matter increased. The simultaneous chemical and sensory analysis confirmed that UV-A treatment changed the overall nuisance odor character of swine barn emissions into 'toothpaste' and'mint'. The smell of benzoic acid generated in UV-A treatment was likely one of the compounds responsible for the less-offensive overall odor character of the UV-treated emissions. Results are needed to inform the design of future real farm work, where the interior barn walls will be covered with the photocatalyst, and foul air will be passively treated as it moves through the barn. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multirow{2}{*}{**Targeted Gas**} & **UV Dose** & **Targeted Gas Emission (\(E_{r}\) mg\(\cdot\)min\({}^{-1}\),** & **Cost (USD\(\cdot\)kg\({}^{-1}\) for NH\({}_{3}\) and Odor = OU\({}_{E}\)-min\({}^{-1}\))** & **H\({}_{2}\)S; USD\(\cdot\)g\({}^{-1}\) for Butan-1-ol,** \\  & & **Control** & **UV Treatment** & **N\({}_{2}\)O and O\({}_{3}\))** \\ \hline \multirow{4}{*}{[18]} & NH\({}_{3}\) & 3.9 & 746 & 676 & 53.4 \\  & NH\({}_{3}\) & 5.8 & 763 & 676 & 62.5 \\  & & 2.5 & 31.5 & 25.3 & 442 \\  & Butan-1-ol & 3.9 & 30.9 & 20.3 & 352 \\  & & 5.8 & 32.9 & 19.4 & 403 \\  & NH\({}_{3}\) & 5.8 & 64.2 & 60.2 & 1300 \\  & N\({}_{2}\)O & 3.9 & 8.14 & 7.79 & 10.6 \\  & N\({}_{2}\)O & 5.8 & 8.06 & 6.92 & 4.72 \\
[19] & O\({}_{3}\) & 1.3 & 0.01 & 0.00 & 18.9 \\  & O\({}_{3}\) & 5.8 & 0.01 & 0.00 & 60.0 \\  & & 3.9 & 9210 & 3910 & - \\  & Odor & 5.8 & 9200 & 3430 & - \\  & H\({}_{2}\)S & 5.3 & 8.55 & 3.58 & 1090 \\  & N\({}_{2}\)O & 3.9 & 9.30 & 8.43 & 3.43 \\  & N\({}_{2}\)O & 5.3 & 9.31 & 8.21 & 4.93 \\  & O\({}_{3}\) & 5.3 & 0.16 & 0.00 & 33.7 \\  & Odor & 4.0 & 5480 & 3320 & - \\  & & 5.3 & 6290 & 3830 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 16: Summary of the estimated cost of electric energy needed to mitigate targeted gases with UV-A photocatalysis using the mobile lab. **Author Contributions:** Conceptualization, J.A.K. and W.S.J. ; methodology, J.A.K. ; validation, M.L., B.C. and J.A.K. ; formal analysis, M.L., W.M. and B.C. ; investigation, M.L. and W.M., B.C., P.L. and C.B. ; resources, M.L., W.M., BC, P.L., C.B. and J.A.K. ; data curation, M.L. and J.A.K. ; writing--original draft preparation, M.L. ; writing--review and editing, M.L., C.B., J.A.K. and W.S.J. ; visualization, M.L. ; supervision, J.A.K. ; project administration, J.A.K. and W.S.J. ; funding acquisition, J.A.K. and W.S.J. All authors have read and agreed to the published version of the manuscript. This research was supported by Iowa Pork Producers Association Project #18-089 \"Employing environmental mitigation technology and/or practices: Treating swine odor and improving air quality with black light.\" In addition, this research was partially supported by the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project no.