Gaseous emissions from livestock and poultry barns include components known collectively as greenhouse gases (GHGs, e.g., CO₂), volatile organic compounds (VOC, e.g., formaldehyde), along with others not typically included in those groupings, such as hydrogen sulfide (H₂S) and ammonia (NH₃). Other noxious components of the exhausts include the particulate matter (PM) of various sizes and airborne microbes. Cumulatively, these have a detrimental effect on human health, environment, climate and quality of life in rural communities ^{[1][2][3][4][5]}[2,3,4,5,6].

UV treatment technology was tested in the early 2000s because of its easy application and expected mitigation effects in barns. The UV spectrum is divided into four ranges because the energy of each photon is inversely related to its wavelength; different materials absorb and react in each range. Traditionally, the ranges are known as “A” (315–400 nm), “B” (280–315 nm), “C” (200–280 nm) and “vacuum” (100–200 nm) ^[6][10]. The term “vacuum” arises because those wavelengths are absorbed by the components of ordinary air, and thus, the transmission of the light requires a vacuum. Although each of these causes photodamage to humans and animals on exposure, in broad terms, ultraviolet light-A (UV-A) is the least toxic, and vacuum ultraviolet (V-UV) is the most toxic. For example, UV-A is used in the commercial tanning industry, while standard germicidal bulbs are in the ultraviolet light-C (UV-C) range. In a variety of related pollutant mitigation applications, the most typical combination is the use of UV-A with a semiconductor photocatalyst that absorbs the light, causing the formation of reactive intermediates that in turn degrade the unwanted pollutants.

The photocatalysis reaction is initiated when photons of sufficient energy (more than the bandgap) are absorbed by the photocatalyst, resulting in electron (e⁻)/hole (h⁺) pair generation ^{[7][8][9][10]}[11,12,13,14]. Nanosized titanium dioxide (TiO₂) is commonly applied to surfaces as a catalyst material, and it is effective with all UV with wavelengths below 400 nm ^{[7][8][11][12][13][14][15][16]}[11,12,15,16,17,18,19,20]. Although the detailed mechanism of photocatalysis varies with different target pollutants, it is commonly agreed that the primary reactions responsible are interfacial redox reactions of electrons and holes with adsorbed pollutants or mediators, such as water ^[8][17][18][12,21,22].

The applicability of UV-A photocatalytic technology to the farm has been evaluated for mitigating odorous gases and fine particulate matter concentrations, as well as for increasing the feed conversion rates ^{[19][20][21][22][23][24][25][26][27][28][29][30]}[1,8,23,24,25,26,27,28,29,30,31,32]. 

2. Mechanism of UV-A Photocatalysis

Nanosized titanium dioxide (TiO₂) is commonly used as a photocatalyst material due to its relatively efficient photoactivity, high stability and lowest cost in the industry ^{[7][8][11][13]}[11,12,15,17]. There are TiO₂ crystalline forms of anatase, rutile and brookite, but anatase and rutile of TiO₂ are widely used commercially due to high photocatalytic activity ^[8][12][13][12,16,17]. Activation of TiO₂ is initiated at wavelengths <400 nm ^[14][15][16][18,19,20]. Photocatalysis is, by definition, the acceleration of a photochemical reaction in the presence of a catalyst, typically, but not exclusively, with UV-A light. The primary photochemical event caused by the absorption of a photon (Figure 1) is the promotion of an electron from the valence band to the conduction band of the catalyst, causing charge separation, also frequently referred to as producing a valence band “hole” (h⁺) and conduction band electron (e⁻) ^{[8][9][10][12][13][17][18][31]}[12,13,14,16,17,21,22,33]. At its most basic level of description, holes (h⁺) react with water molecules to generate hydroxyl radicals (·OH) or oxidize organic materials through direct reaction ^[9][13]. The electrons generated in the conduction band react with molecular oxygen (O₂) to form superoxide ions (O₂^•−) ^[9][10][13][13,14,17]. Although a complete description of the cascade of reactions is complex, the photon’s energy is used to convert an oxidatively neutral surface to one with strong oxidizing power and modest reducing power. In the absence of water, O₂ and other substrates, the conduction band electron will collapse back to the valence band, “wasting” the absorbed energy in the form of heat release. These processes are qualitatively illustrated in Figure 1.

The reported mitigations of NH₃ ranged from 5 to 35% (UV-A dose: 3.9–970 mJ·cm⁻², Table 13). Statistically significant reductions were observed from lab-scale to farm-scale studies. However, it is difficult to quantitatively evaluate the minimum UV dose and TiO₂ coating thickness required to mitigate NH₃ in real farm conditions. This is because the information on the irradiance (light intensity) and UV dose was omitted in some works ^[22][28][24,30] that report a relatively high mitigation rate, and the methods used for measuring light intensity were different in other reports. (As noted previously, issues of actual surface geometry could also come into play.) As implied in the Discussion, this does mean that existing data are not of sufficient quality to be able to put a universally meaningful dollar-per-gram price on pollutant removal. In addition, some inconsistencies suggest that not all variables were accounted for. For example, although significant NH₃ mitigation was reported on the pilot scale with 5.8 mJ·cm⁻² of UV dose ^[24][26][26,28], there was no significant mitigation at the farm scale with a similar 5.3 mJ·cm⁻² of UV dose ^[25][27]. It is expected that a higher UV dose is needed on the actual farm due to the harsh environmental conditions (high airborne dust and high relative humidity). Furthermore, higher UV-A doses and thicker TiO₂ coating will likely be necessary to mitigate NH₃ efficiently on the farm scale (inside the barn).

[27] observed a significant change in the perceived overall odor “character” for swine barn emissions after UV-A photocatalysis. The research team described the smell of UV-A photocatalysis treated air as a mix of less-offensive “disinfectant”, “minty” or “swimming pool” scents with a weaker smell of swine manure in the background. In addition, the research team described the compound that is believed to have changed the characteristic smell as benzoic acid (or 1-octanol) based on simultaneous chemical and sensory analysis of the GS-MS olfactory test. Although further research on odor character change is still needed, it is interesting that it is the first study to track odor character change after UV-A photocatalysis.

It is important to underline the generation of some targeted compounds for all UV doses in the previous studies ^[25][27]. The generated compounds (several in the VFAs group, DMDS and phenol) are odorants that are considered slightly less impactful than p-cresol, skatole and indole (representative phenolic compounds), and the generated compounds appear to be partial degradation products from compounds known to be in the original mixtures. Therefore, it is feasible to hypothesize that the generated compounds offset the overall odor mitigation.

For the GHGs (Table 35), the previous lab- and pilot-scale study did not find significant mitigation in CH₄ under UV-A photocatalysis; however, moderate mitigations (15–27%) were observed with the farm scale and high TiO₂ coating thickness ^[21][22][23,24]. It is widely understood from closely related research in photocatalysis in both air and water that hydrocarbons are indeed oxidized (ultimately to CO₂) by TiO₂ ^[35][36][45,54]; the lack of mitigation in the lab- and pilot-scale reports may reflect insufficient mass transport or other competitive reactions that stop the expected reduction in methane levels. There is little doubt that, in principle, methane should be oxidized.