## 2 Materials and Methods
### Material
#### 2.1.2 Data Sampling  
Odors in pig houses are caused by incomplete digestion of nutrients in feed and anaerobic fermentation from long-term storage of livestock manure [17; 18]. Therefore, evaluation of odor using a single substance is not appropriate, as the odors in the pig houses are generated by a mixed gas in which multiple substances exist in combination [19]. Therefore, in this study, complex odors and 15 species of odor. The measured odor compounds were Ammonia (NH\({}_{3}\)), Hydrogen Sulfide (H\({}_{2}\)S), methyl mercaptan (MM), Dimethyl sulfide (DMS), Dimethyl disulfide (DMDS), Acetic acid (ACA), Propionic acid (PPA), Butyric acid (BTA), Iso-Butyric acid (IBA), Valeric acid (VLA), Iso-Valeric acic (IVA), Phenol (Ph), para-Cresol (p-C), Indole (ID), and Skatole (SK). When dealing with high concentrations of ammonia, attention should be paid to the sampling rate of ammonia. For ammonia sampling, we used two series-connected impingers. Although Korea's standard methods for odor examination require sampling at the flow rate of the pump at 10 L/min and a sampling time of 5 min, there is a risk of breakthrough. Therefore, the ammonia sample was collected by passing the sample through the absorbent solution (5 g of boric acid, 50 mL of boric acid solution prepared with 1 L of distilled water) using two series-connected impingers at a rate of 5 L/min for 10 min. Complex odor and sulfur compounds were sampled using the aluminum Tedlar bag. The complex odor was calculated according to the air dilution olfactory method of Korea's standard methods for the examination of odor samples and was analyzed within 48 h, five panelists participated and the ammonia was measured using an ultraviolet-visible light spectrophotometer (UV-2700, Shimadzu, Kyoto, Japan) at a wavelength of 640 nm. Meanwhile, sulfur compounds were analyzed using a GC with a pulsed-flame photometric detector (PFPD) (456-GC, Scion instruments, Goes, The Netherlands) equipped with a CP-Sil 5 CB column and thermal desorption (TD, Unity + air server, Markes, Bridgend, UK) system with a cold trap. The oven temperature was maintained at 60 \({}^{\circ}\)C for 3 min, increased to 160 \({}^{\circ}\)C at 8 \({}^{\circ}\)C/min, and then maintained at 160 \({}^{\circ}\)C for 9 min. All samples were taken at least in triplicate, and every experiment was repeated three times at each condition [20]. The volatile organic compounds (VOCs), VFAs, phenols, and indoles were sampled at 0.1 L/min for 5 min using the 3-bed tube (Carbopack C:Carbopack B:Carbopack X, 1:1:1). The analysis system utilized a system in which thermal desorption (TD) and GC/MSD are connection systems. The TD (unity + air server, Markes, Bridgend, UK) cold trap condition increases a low temp of 5 \({}^{\circ}\)C to a high temp of 300 \({}^{\circ}\)C, the split flow was 10:1, and the flow path temperature was maintained at 150 \({}^{\circ}\)C. VOCs, VFAs, phenols, and indoles were analyzed using a GC (6890N/5973N, Agilent, Santa Clara, CA, USA) equipped with a CP-Wax52 CB column (60 m \(\times\) 0.25 mm \(\times\) 0.25 mm) and an MSD. The oven temperature was initially 45 \({}^{\circ}\)C for 5 min, increasing to 250 \({}^{\circ}\)C at 5 \({}^{\circ}\)C/min, which was then held at 250 \({}^{\circ}\)C for 4 min. The ion source temperature was maintained at 230 \({}^{\circ}\)C [18]. Table 1 summarizes the conditions of the odor substance analysis device. Table 2 presents a summary of the collected complex odors and odor substances. The types of all variables are float, and 57 were collected for each variable. In addition, missing  
\begin{table}
\begin{tabular}{c c c c c} \hline  
data occurred in 11 variables out of 15 explanatory variables. \"Dimethyl disulfide\" (91.2%), \"Dimethyl sulfide\" (63.2%), \"Methyl mercaptan\" (54.4%), \"Indole\" (36.8%), \"Skatole\" (14%), \"Phenol\" (3.5%), \"Pro-panoic acid\" (1.8%), \"Isobutyric acid\" (1.8%), \"Normality butyric acid\" (1.8%), \"Normality valeric acid\" (1.8%), and \"P-Cresol\" (1.8%).