## 3 Methods
### VOCs  
The air samples were collected in 1 L gas sampling glass bulbs (Supelco) using portable sampling pumps. Each gas sampling glass bulb was flushed with DI water and baked in the oven overnight before the gas sampling day. Immediately after each sample collection, 1 \(\upmu\)L of internal standard (100 ppm Hexane) was injected into each gas sampling glass bulb onsite to check for leakage after being analyzed by the GC-MS, as shown in Figure 15. The gas bulbs were stored in a cooler, brought to the laboratory, and were analyzed using a multidimensional GC-MS within 24 h of collection.  
The gas samples were analyzed using the Agilent 6890 (Agilent Technologies, Santa Clara, CA, USA) customized multidimensional gas chromatograph (Microanalytics, a part of volatile analysis Corporation, Round Rock, TX, USA). On the chromatography part, there were two capillary columns. A 5% phenyl polysilphenylene-siloxane (30 m \(\times\) 0.53 mm inner diameter \(\times\) 0.5 \(\upmu\)m thickness, Trajan Scientific, Austin, TX, USA) column, followed by a polar column bonded polyethylene glycol in a Sol-Gel matrix (30 m \(\times\) 0.53 mm inner diameter \(\times\) 0.5 \(\upmu\)m thickness, Trajan Scientific, Austin, TX, USA). The first column was fixed with a restrictor precolumn (connected to an injector), and the midpoint of the two columns was maintained at a constant pressure of 0.39 atm by a pneumatic switch. For this research, the multidimensional capability of the GC was not needed for the separations of target VOCs. Yet, the unusual coupling of columns was still in place out of convenience, and all effluent from the nonpolar column was directed to the polar column. Ultra-high pure He (99.999%, Airgas, Des Moines, IA, USA) was used as the carrier gas. The discharge from the second column was simultaneously directed to a single quadrupole Agilent mass spectrometer (MS) (Model 5973N, Santa Clara, CA, USA).  
The GC oven temperature was programmed at an initial 40 \({}^{\circ}\)C for 3 min, followed by an increase to 240 \({}^{\circ}\)C at a rate of 7 \({}^{\circ}\)C/min, where it was maintained for 8.43 min. The quadrupole MS was set to an electron ionization mode, with ionization energy of 70 eV during operation and a full scan range of 34 to 350 m/z. The system automation software was Multitrax v. 6.0.1 (Microanalytics, Round Rock, TX, USA), and the data acquisition software was ChemStation E.01.01.335 (Agilent Technologies, Santa Clara, CA, USA). The multidimensional GC-MS schematic and a comprehensive description were published elsewhere [10].  
Gas samples from the gas bulbs were extracted using a 2 cm 50/30 \(\upmu\)m DVB/PDMS/Carboxen fused silica SPME fiber (57248-U, Supelco, Bellefonte, PA, USA) at lab temperature (23-24 \({}^{\circ}\)C) for 50 min. The SPME fibers which were loaded with the VOCs from the samples were injected to the GC injector and heated at 260 \({}^{\circ}\)C to thermally desorb the VOCs to the GC columns. Then, the fibers were separated by the GC column and analyzed by the mass spectrometer to measure the relative abundance of each targeted VOC, which followed the similar protocols from previous studies [11; 12]. The samples were analyzed in the SIM mode because of its higher sensitivity and lower detection limit as opposed to the total scan mode. To identify the compounds, the NIST mass spectral library was (with at least 80% spectral match) used in this study. Further, to verify the retention time of the VOCs studied, pure standards of all 15 VOCs were analyzed and calibrated [13; 14]. The VOC concentrations were not quantified. A surrogate metric of VOC abundance (measured with peak area counts) was used to assess the performance of manure additives to mitigate emissions from swine manure. The dataset could be used to estimate the mitigation effect by comparing the VOC abundance in the treatment and control.