Patent Publication Number: US-2022234002-A1

Title: Air Purification Apparatuses, Systems, and Methods for Removing Particulates, Volatile Organic Compounds, and Nitrous Oxide-Containing Compounds

Description:
CROSS-REFERENCE 
     This application claims benefit of priority from U.S. Provisional Patent Application Ser. No. 63/140,684 filed Jan. 22, 2021 and incorporated by reference herein in its entirety as if made a part of the present specification. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to air filtration and purifying air. More particularly, the present disclosure relates to the sensing of air quality and the removal of particulates, Volatile Organic Compounds (VOC), and nitrous oxide-containing compounds from air, preferably from air in an enclosed environment, including vehicles, and enclosed terrestrial transportation environments. 
     BACKGROUND 
     Volatile organic compounds (VOC) are a class of organic chemicals characterized by a high vapor pressure at room temperature, typically resulting from a low boiling point. They include non-methane hydrocarbons (NMHC) and oxygenated NMHC (e.g., alcohols, aldehydes, and organic acids). VOCs emanate from off-gassing of foams, plastics, fabrics, and other manufactured products, particularly when they are new, and from the solvents in many cleaning products. VOCs are also produced as byproducts of human metabolic processes. Over 200 VOCs have been identified in human alveolar breath. In a closed environment full of people, such as a passenger aircraft, endogenously produced VOCs dominate. 
     VOCs cannot be removed by typical air filtration methods such as HEPA filtration. Existing systems to reduce VOC concentration in the cabin environment include activated carbon. 
     Further air contaminants can include various particulates, and can further include nitrous oxide-containing compounds that can contaminate an air supply, for example, as byproducts of production from reduction in nitrogen-containing compounds, particularly in enclosed environments, for example, due to emissions from cigarette and other tobacco-related products, emissions from e-cigarettes, the combustion of vehicle fuel, etc. that can migrate into, linger in, and otherwise accumulate in regions within and proximate to the enclosed and substantially enclosed environments, etc. Enclosed and substantially enclosed environments can include terrestrial transportation buildings, terrestrial and non-terrestrial rooms, and other substantially enclosed environments. 
     The Background and Technical Field sections of this document are provided to place aspects of the present disclosure in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted as prior art merely by its inclusion in the Background section. 
     SUMMARY 
     The following presents a simplified summary of the disclosure to provide a basic understanding to those of skill in the art. This summary is not an extensive overview of the disclosure and is not intended to identify key/critical elements of aspects of the disclosure or to delineate the scope of the disclosure. The sole purpose of this summary is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later. 
     According to present aspects, air filtration apparatuses, systems, and methods are disclosed that address previous problems relating to the need to remove different types of contaminants from air contained within an enclosed or substantially enclosed environment. According to present aspects, apparatuses include, in synergistic combination, filtering and contaminant eliminating capabilities to simultaneous purify ambient air in an enclosed or substantially enclosed environment. More particularly, the presently disclosed apparatuses, systems, and methods filter or otherwise remove, in combination, from ambient air in an enclosed environment non-volatile organic compound particulates, volatile organic compounds (VOCs), and nitrous oxide-containing compounds. 
     According to the present disclosure, a “substantially enclosed environment” can include rooms and buildings with doors that are periodically opened, underground garages and surface garages that can retain air impurities for periods of time although entrances and exits may remain open for extended periods of time, hallways, meeting areas, meeting rooms, conference halls, as well as space stations, etc. As used herein, the term “enclosed environment” includes a “substantially enclosed environment”. 
     According to one or more aspects of the present disclosure described and claimed herein, an air filtration unit enables the removal of particulates, including accumulated tars and other compounds resulting from, for example, smoked tobacco products, e-cigarettes, etc.; VOCs; and nitrous oxide-containing compounds from air in an enclosed environment, including where air can be directed through a duct. 
     According to present aspects, transverse to a longitudinal axis of an air ducts, an air filtration unit incorporating a HEPA filter, a VOC removal unit, and a nitrous oxide-containing compound removal unit is provided that can be positioned within or proximate to an air duct in order, for example, to protect the lifespan of the photocatalytic oxidation (PCO) component in the VOC removal unit. 
     A further aspect discloses an air filtration unit including an air duct, with the air duct including an air inlet at an air duct first end and an air outlet at an air duct second end, a high efficiency particulate air filter oriented proximate to or otherwise incorporated in the air inlet, and an airflow controller in communication with the air inlet. The air filtration unit further includes a carbon dioxide sensor in communication with the airflow controller, a pressure sensor in communication with the airflow controller. The air filtration unit includes an integrated ultraviolet light reactor located downstream from the high efficiency particulate air (HEPA) filter. The ultraviolet light reactor includes a plurality of baffles, with each baffle having a plurality of airflow spaces allowing airflow therethrough, with the baffles disposed at spaced locations within the duct between the air inlet and air outlet, and with the baffles being generally transverse to the longitudinal axis. The air filtration unit further includes a plurality of ultraviolet light emitting diodes mounted on each baffle, a porous and permeable photocatalytic oxidation filter module disposed between each pair of baffles, positioned generally transverse to the longitudinal axis, such that air flows through the photocatalytic oxidation filter module, and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), TiO 2  doped with carbon (C—TiO 2 ), or combinations thereof which, when illuminated by ultraviolet light, are operative to chemically reduce volatile organic compounds to non-volatile organic compounds. The air filtration unit further includes a porous and permeable nitrous oxide-adsorbing filter that can be, for example, a nitrous oxide-containing compound adsorbing chamber comprising a nitrous oxide-containing compound adsorbent, with the adsorbent in the adsorbent filter configured into an adsorbent bed, stack, etc. The nitrous oxide-adsorbing filter is disposed downstream of the UV light reactor. The terms “air duct” and “duct” are equivalent terms that have the same meaning herein and are interchangeable. 
     In another aspect, the air duct comprises a longitudinal axis. 
     In a further aspect, the carbon dioxide sensor is in communication with a controller. 
     In another aspect, the carbon dioxide sensor is in communication with the air inlet. 
     In another aspect, the carbon dioxide sensor is in communication with at least one of the air inlet and the air outlet. 
     In another aspect, the pressure sensor is in communication with the air inlet. 
     In a further aspect, the pressure sensor is in communication with at least one of the air inlet and the air outlet. 
     In another aspect, the pressure sensor is in communication with the controller. 
     In another aspect, the porous and permeable nitrous oxide-adsorbing filter comprises a solid amine-containing adsorbent. 
     In another aspect, the porous and permeable nitrous oxide-adsorbing filter comprises a packed solid adsorbent, with the packed solid adsorbent comprising at least one of: a cellular monolith arrangement and a granular media arrangement. 
     In another aspect, the porous and permeable nitrous oxide-adsorbing filter comprises a packed solid adsorbent, with the packed solid adsorbent comprising at least one of; an amine-containing compound, a metal-organic containing compound, and a zeolite. 
     In a further aspect, the porous and permeable nitrous oxide-adsorbing filter is oriented proximate to the air outlet. 
     In another aspect, the air filtration unit is configured to be removable from the air duct for replacement. 
     In another aspect, the air filtration unit is configured to be replaceable. 
     In a further aspect, one or more components of the air filtration unit are configured to be individually removable from the air duct. 
     In another aspect, one or more of the high efficiency particulate air filter, the ultraviolet light reactor, the photocatalytic oxidation filter module, and the nitrous oxide-adsorbing filter can be configured to be integrated into the air filtration unit as discrete components that are configured to be removable and replaceable. 
     In a further aspect, the air filtration unit further comprises one or more heat sink, wherein the one or more heat sink is configured to be disposed within the duct, and with the one or more heat sink further adapted to conduct heat away from the ultraviolet light emitting diodes. 
     In another aspect, the plurality of ultraviolet light emitting diodes are disposed both around a periphery of each baffle and between the airflow spaces, with the plurality of ultraviolet light emitting diodes configured to maximize ultraviolet illumination of an adjacent photocatalytic oxidation filter module. 
     In another aspect, the plurality of ultraviolet light emitting diodes are mounted on interior sides of baffles adjacent the air inlet and outlet, and the plurality of ultraviolet light emitting diodes are mounted on both sides of baffles, with the plurality of ultraviolet light emitting diodes configured to illuminate each photocatalytic oxidation filter module from both sides. 
     In another aspect, the photocatalytic oxidation filter module is spaced apart from the baffles such that both surfaces of each photocatalytic oxidation filter module are illuminated by ultraviolet light. 
     In a further aspect, the photocatalytic oxidation filter module comprises a plurality of filters, with each filter including one or more of a coarse foam, a fine foam, a fused quartz filament felt, or combination thereof, and wherein each filter is loaded with a catalyst including one or more of pure titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), TiO 2  doped with carbon (C—TiO 2 ), or combination thereof. 
     In another aspect, the photocatalytic oxidation filter module comprises a plurality of catalyst-loaded filters selected, arranged, and configured to maximize ultraviolet illumination of each filter and through a complete depth of each filter layer. 
     In another aspect, the ultraviolet light reactor comprises four baffles and three photocatalytic oxidation filter modules, and wherein the photocatalytic oxidation filter modules comprise, in order from air inlet to air outlet,
         1) R25-CTR-TA-R25;   2) CTR-TA-Q25-R25-R25; and   3) R25-CTR-TA-R25, wherein   R25 is a coarse foam loaded with pure TiO 2 ;   Q25 is a fused quartz filament felt loaded with pure TiO 2 ;   CTR is a coarse foam loaded with C—TiO 2  and   TA is a fine foam loaded with pure TiO 2 .       

     In another aspect, the ultraviolet light reactor comprises four baffles and three photocatalytic oxidation filter modules, and wherein the photocatalytic oxidation filter modules comprise, in order from air inlet to air outlet,
         1) R25-TA-FTR-CTR;   2) R25-CTA-FTR-Q25-R25; and   3) R25-TA-FTR-CTR; wherein   R25 is a coarse foam loaded with pure TiO 2 ;   Q25 is a fused quartz filament felt loaded with pure TiO 2 ;   CTR is a coarse foam loaded with C—TiO 2 ;   TA is a fine foam loaded with pure TiO 2 ; and   FTR is a coarse foam loaded with Fe—TiO 2 .       

     Aspects of the present disclosure further disclose a method for filtering air in an enclosed environment, with the method including monitoring carbon dioxide concentration in an enclosed environment, initiating an air purification cycle, directing an airflow to an air inlet of an air filtering unit, with the air filtration unit including an air duct having a longitudinal axis, with the air duct comprising an air inlet at an air duct first end and an air outlet at an air duct second end, a high efficiency particulate air (HEPA) filter unit oriented proximate to the air inlet, an airflow controller in communication with the air inlet, a carbon dioxide sensor in communication with the airflow controller, and a pressure sensor in communication with the airflow controller. The air filtration unit further includes an ultraviolet light reactor, with the ultraviolet light reactor further including a plurality of baffles, with each of the plurality of baffles having a plurality of airflow spaces allowing airflow therethrough the plurality of baffles, and with the plurality of baffles disposed at spaced locations within the air duct between the air inlet and air outlet, and with the baffles being generally transverse to the longitudinal axis. The air filtration unit and the ultraviolet light reactor further includes a plurality of ultraviolet light emitting diodes mounted on each baffle, a porous and permeable photocatalytic oxidation filter module disposed between each pair of baffles, generally transverse to the longitudinal axis, such that air flows through the photocatalytic oxidation filter module, and a porous and permeable nitrous oxide-adsorbing filter disposed downstream of the UV light reactor, and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising at least one of titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), TiO 2  doped with carbon (C—TiO 2 ), or combinations thereof which, when illuminated by ultraviolet light, are operative to chemically reduce volatile organic compounds in the airflow to non-volatile organic compounds. The method further includes removing an amount of particulate from the airflow upstream from the photocatalytic oxidation filter modules, illuminating the photocatalytic oxidation filter modules with ultraviolet light from ultraviolet light emitting diodes mounted on each baffle, chemically reducing volatile organic compounds in the airflow to non-volatile organic compounds, removing an amount of nitrous oxide-containing compounds from the airflow downstream from the photocatalytic oxidation filter modules where the nitrous oxide-containing compounds can be produced due to nitrogen compound reductions. 
     In another aspect, the carbon dioxide sensor is in communication with the air inlet. 
     In a further aspect, a method further comprises illuminating the photocatalytic oxidation filter modules with ultraviolet light comprises disposing the ultraviolet light emitting diodes both around a periphery of each baffle and between the airflow spaces, so as to maximize the ultraviolet illumination of adjacent photocatalytic oxidation filter modules. 
     In another aspect, a method further comprises disposing the ultraviolet light emitting diodes on the baffles comprises disposing the ultraviolet light emitting diodes on interior sides of baffles adjacent the air inlet and outlet, and on both sides of baffles, so as to illuminate each PCO filter module from both sides. 
     In another aspect, a method further comprises illuminating each photocatalytic oxidation filter module with ultraviolet light comprises spacing each photocatalytic oxidation filter module apart from the baffles such that an entirety of both surfaces of each photocatalytic oxidation filter module is illuminated by ultraviolet light. 
     In another aspect, the method further includes conducting heat from the ultraviolet light emitting diodes away from the ultraviolet light emitting diodes via one or more heats sinks, with the one or more heat sinks disposed within the air duct. 
     In a further aspect, the method further includes selecting and arranging a plurality of catalyst-loaded filters to form the photocatalytic oxidation filter module so as to maximize illumination of the plurality of catalyst-loaded filters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which aspects of the disclosure are shown. However, this disclosure should not be construed as limited to the aspects set forth herein. Rather, these aspects are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout. 
         FIG. 1  is a perspective view of an example aircraft air filtration and VOC removal unit. 
         FIG. 2  is a plan view of the example VOC removal unit, showing dimensions according to one aspect. 
         FIG. 3  is a cross-section view of the example VOC removal unit, showing dimensions of the inlet/outlet and duct according to one aspect. 
         FIG. 4  is a side view of an example baffle. 
         FIG. 5  is a section view of an exemplary VOC removal unit showing the spatial relationship between baffles, UV LEDs, and filter modules, according to present aspects. 
         FIG. 6  is a perspective view of an exemplary VOC and nitrous oxide-containing particle removal unit showing the spatial relationship between baffles, UV LEDs, and filter modules, according to present aspects. 
         FIG. 7  is an end view of an air inlet section of an exemplary air filtration unit. 
         FIG. 8  is an end view of an air outlet section of an exemplary air filtration unit. 
         FIG. 9  is a flow diagram of an exemplary method of air filtration employing present apparatuses and systems. 
         FIG. 10  is a flow diagram of an exemplary method of air filtration employing present apparatuses and systems. 
         FIG. 11  is a flow diagram of an exemplary method of air filtration employing present apparatuses and systems. 
         FIG. 12  is a flow diagram of an exemplary method of air filtration employing present apparatuses and systems. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the present disclosure is described by referring mainly to an exemplary aspect thereof. In the following description, numerous specific details are set forth to provide a thorough understanding of the present disclosure. However, it will be readily apparent to one of ordinary skill in the art that the aspects of the present disclosure can be practiced without limitation to these specific details. In this description, well known methods and structures have not been described in detail so as not to unnecessarily obscure the present disclosure. 
     Present aspects are directed to apparatuses, systems, and methods for VOC removal from air in enclosed environments that can considered to be large, enclosed environments including, for example, terrestrial environments such as, for example, rooms, transportation terminals, smoke rooms, hallways, meeting areas, meeting rooms, conference halls, as well as non-terrestrial environments including, for example, extra-terrestrial rooms, buildings, space stations, etc. In addition, further present aspects are directed to apparatuses, systems, and methods for protected VOC and nitrogen-containing compound removal from the enclosed environments disclosed herein. 
       FIG. 1  depicts an example of an air filtration and VOC removal unit  10  (hereinafter referred to as simply a “VOC removal unit”  10 ), according to one aspect of the present disclosure. The VOC removal unit  10  operates to remove VOCs from cabin air by photocatalytic oxidation (PCO), as will be explained in greater detail herein. The design maximizes airflow through the VOC removal unit  10  and minimizes heat generation and a thermal gradient across it, consistent with the maximum achievable UV illumination of PCO filters. As used herein, with respect to the present aspects, the terms “photocatalytic oxidation filter module” and “porous and permeable photocatalytic oxidation filter module” are equivalent terms and are used interchangeably. 
       FIG. 1  is a perspective view of an example VOC removal unit  10 , with one side removed to reveal internal components.  FIGS. 2 and 3  are sectional views of the VOC removal unit  10  according to one aspect, and  FIG. 5  is a section view showing the spacing and relationship of various components. 
     The VOC removal unit  10  comprises an air inlet  12 , a duct  14  having a longitudinal axis  15 , and an air outlet  16 . In the representative aspect of the VOC removal unit  10  depicted in the figures, the air inlet  12  and outlet  16  have a circular cross-sectional shape, and the duct  14  has a square cross-section. However, those of skill in the art will readily recognize that other shapes can be utilized, within the scope of the present disclosure. 
       FIG. 5  depicts the overall operative structure of the VOC removal unit  10  incorporated into the present air filtration unit. A plurality of baffles  18  is disposed at spaced locations within the duct  14 . Between each pair of baffles  18 , a PCO filter module  30  comprises a plurality of filters, each of which is loaded with a photoactive catalyst. The number and spacing (as explained further below) of baffles  18  and PCO filter modules  30  is representative. In other aspects, more or fewer of each may be provided. In general, the number of baffles  18  will always exceed the number of PCO filter modules  30  by one; with baffles  18  disposed at both ends and in between each pair of PCO filter modules  30 . Ultraviolet (UV) light emitting diodes (LED)  24  mounted on the baffles  18  illuminate both sides of the filter modules  30  with UV light to maximize illumination of photocatalytic coatings on material in the filter modules  30 . Since the photocatalytic coatings require UV light as a catalyst to convert VOCs to non-VOC molecules, maximizing the illumination of PCO modules  30  with UV light maximizes the effectiveness of the VOC removal unit  10 . Cabin air directed through the VOC removal unit  10  passes through the baffles and PCO filter modules  30 . As explained further herein, UV light illuminating the PCO filter modules  30  photoactivates catalysts loaded therein, initiating a chemical photocatalytic oxidation process that reduces VOCs in the air to non-VOCs molecules, such as carbon dioxide and water. 
     The baffles  18  are disposed at spaced locations within the duct  14 , between the air inlet  12  and air outlet  16 . The baffles  18  are disposed generally transverse to the longitudinal axis  15  of the duct  14 . As depicted in  FIG. 4 , each baffle  18  has a plurality of airflow spaces  22  formed in it, allowing airflow therethrough. A plurality of UV LEDs  24  is mounted on each baffle. The UV LEDs  24  are disposed both around the periphery of each baffle  18 , and between the airflow spaces  22 , so as to maximize the UV illumination of adjacent filter modules  30 . The UV LEDs  24  are mounted on the interior sides of baffles  18  adjacent the air inlet  12  and outlet  16  and are mounted on both sides of all other baffles  18 , so as to illuminate the filter modules  30  from both sides. Mounting UV LEDs  24  on all baffles  18  ensures maximum illumination of all filter modules  30 , for maximum photocatalytic effect. 
     The efficacy of the VOC removal unit  10  is greatest when the UV LEDs  24  are operated at high power (˜500 mA), thus generating a large luminous flux of UV light to activate the photoactive catalysts in the filter modules  30 . However, this can generate heat, which warms the air flowing through the duct  14 , increasing the load on aircraft air conditioning equipment. In one aspect a heat sink is connected to at least one, and preferably to each baffle  18  that includes LEDs  24 , using heat sink mounting holes  26 . This is done to maintain the life of the UV-LED lights by maintaining lower temperatures on their surface in low flow conditions, prolonging their life. In certain filter configurations (designed for those with higher flow rates) the heat sinks may not be necessary. 
     A porous and permeable PCO filter module  30 , comprising a plurality or “stack” of filters, is disposed between each baffle  18 . The PCO filter modules  30  are disposed generally transverse to the longitudinal axis  15  of the duct  14 , such that air flows through the PCO filter module  30 . Each PCO filter module  30  contains one or more catalysts which, when illuminated by UV light, are operative to chemically reduce VOCs to non-VOC molecules. Maximum UV illumination of all filters in each filter module  30  is thus desired, to maximize the efficacy of VOC removal. Accordingly, the PCO filter modules  30  are spaced apart from the baffles such that the entirety of both surfaces of each PCO filter module is illuminated by UV light. 
     If a PCO filter module  30  were directly adjacent a baffle  18 , only spots on the surface of the PCO filter module  30  that contact a UV LED  24  would be illuminated. As the spacing between the PCO filter module  30  and the baffle  18  increases, the illumination spot sizes increase, and the photonic efficiency decreases. The optimal spacing is that distance at which the illumination spots just overlap, fully illuminating the entire facing surface of the PCO filter module  30 , increasing the spacing beyond this distance reduces the luminous flux of UV light. In the aspect depicted in  FIG. 5 , the distance between each face of a filter module  30  and the facing baffle  18  is 20 mm (dimension “b”); at this distance, 500 mA of power applied to the UV LEDs  24  yields a light intensity of over 20 mW/cm 2 . The filter modules  30  in this aspect are 40 mm thick (dimension “c”). In the aspect depicted, the baffles  18 , measured to the outermost protruding UV LEDs  24 , are 15 mm thick for those adjacent to the air inlet  12  and outlet  16 , which have UV LEDs  24  mounted on only the interior-facing sides (dimension “a”), and 30 mm thick for interior baffles  18 , which have UV LEDs  24  mounted on both sides (dimension “a*”). These dimensions are exemplary. Present aspects contemplate alternate relative spacing of components for VOC removal units of different size or shape. 
     The filters in each PCO filter module  30  are loaded with some form of titanium dioxide (TiO 2 ). Photocatalytic oxidation occurs in the VOC removal unit  10  by illuminating the TiO 2  in the filters with UV light, generating hydroxyl radicals (OH.) by reaction with water molecules in the air. The free radicals, in turn, oxidize VOCs into non-VOC molecules—primarily carbon dioxide (CO 2 ) and water (H 2 O). These are returned to the airflow, avoiding the accumulation of contaminants. 
     Titanium dioxide is a light-sensitive semiconductor, which adsorbs electromagnetic radiation in the near UV region. The most common natural form of TiO 2  is the mineral rutile. Other forms of TiO 2  are anatase (also known as octahedrite) and brookite (an orthorhombic mineral). TiO 2 , when used as a photoactive catalyst, is primarily anatase, with a small amount of rutile. The anatase form of TiO 2  requires higher light energy than the rutile form and shows a stronger photoactivity. The energy difference between the valence and the conductivity bands of a TiO 2  molecule in the solid state is 3.05 eV for rutile and 3.29 eV for anatase, corresponding to a photonic absorption band at &lt;415 nm for rutile and &lt;385 nm for anatase. 
     Absorption of light energy causes an electron to be promoted from the valence band to the conduction band. This electron, and the simultaneously created positive “electron hole,” can move on the surface of the solid, where it can take part in redox reactions. In particular, water molecules in vapor state in the air are adsorbed onto the TiO 2  surface where they react with the free electron, generating hydroxyl radicals (OH.). These radicals are uncharged, short-lived, highly reactive forms of hydroxide ions (OH−), bearing considerable oxidizing power. The OH. radicals can cause complete oxidation of organic compounds to carbon dioxide and water. In some aspects, the OH. radicals reduce VOCs to the following end products: 
     organic molecules→CO 2 +H 2 O
 
organic N-compounds HNO 3 +CO 2 +H 2 O
 
organic S-compounds H 2 SO 4 +CO 2 +H 2 O
 
organic Cl-compounds HCl+CO 2 +H 2 O.
 
     Although the primary application of photocatalytic oxidation in the VOC removal unit  10  is to reduce VOCs into non-VOC molecules, the process also kills contaminants in bioaerosols, such as bacteria, molds, and fungus. In general, reduction of VOC levels in cabin air enhances comfort of passengers. 
     The photoactivity of TiO 2  is known and has commercial applications. AEROXIDE® P25 is a nanostructured, fine-particulate pure titanium dioxide with high specific surface area. The product, available from Evonik Industries of Parsippany, N.J. (AEROXIDE® P25), is a fine white powder with hydrophilic character caused by hydroxyl groups on the surface. It consists of aggregated primary particles. The aggregates are several hundred nm in size and the primary particles have a mean diameter of approximately 21 nm. The Brunner-Emmett-Teller (BET) theory can be used to measure the surface area of the solid or porous material selected, optionally in conjunction with transmission electron microscopy (TEM) imaging to confirm pore size. Further, pore size distribution can be evaluated by Barrett-Joyner-Halenda (BJH) interpretation. The weight ratio of anatase to rutile is approximately 80/20. AEROXIDE® P25 is sold commercially as a photoactive catalyst. With its high purity, high specific surface area, and combination of anatase and rutile crystal structure, AEROXIDE® P25 is widely used for catalytic and photocatalytic applications. Other forms of pure TiO 2  may also be used in PCO filter modules  30  in the VOC removal unit  10 . 
     Additionally, the inventors have found that doping TiO 2  with iron (Fe—TiO 2 ) and carbon (C—TiO 2 ) yield superior photocatalytic results. The UV-PCO reactor relies on adsorption of the organic compounds onto the surface of the catalyst to enable breakdown of the compounds. Doping the TiO 2  with carbon or iron increases the sorption capacity of the catalyst, which allows for greater removal of VOCs from the airstream. Through doping with metal and non-metal agents the band gap energy level of TiO 2  is lowered and electron-hole pair mechanism is kept constant with a longer duration for higher light absorption capability which results in better efficiency. 
     Table 1 below lists pre- and post-filtering concentrations of various representative VOCs (i.e., ethanol, or EtOH; acetone; and limonene) for pure TiO 2 , Fe—TiO 2 , and C—TiO 2 , when loaded onto various filter media types. It is clear from these data that Fe—TiO 2 , and C—TiO 2  provide superior VOC removal results, as compared to pure TiO 2 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Relative Performance of Photoactive Catalysts 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Initial VOC 
                 Final VOC 
                   
               
               
                   
                 VOC 
                 Concentration 
                 Concentration 
                   
               
               
                 Substrate 
                 Type 
                 (ppb) 
                 (ppb) 
                 Efficiency 
               
               
                   
               
            
           
           
               
            
               
                 pure TiO 2   
               
            
           
           
               
               
               
               
               
            
               
                 Coarse Foam 
                 EtOH 
                 423.40 
                 350.22 
                 17.2% 
               
               
                   
                 Acetone 
                 117.30 
                  78.80 
                 32.8% 
               
               
                 Fine Foam 
                 EtOH 
                 NA 
                 NA 
                 NA 
               
               
                   
                 Acetone 
                 NA 
                 NA 
                 NA 
               
               
                 Coarse/Fine 
                 EtOH 
                 449.00 
                 320.20 
                 28.6% 
               
               
                 Foam 
                 Acetone 
                 NA 
                 NA 
                 NA 
               
            
           
           
               
            
               
                 Fe-doped TiO 2   
               
            
           
           
               
               
               
               
               
            
               
                 Coarse Foam 
                 EtOH 
                 389.20 
                 198.70 
                 48.9% 
               
               
                   
                 Acetone 
                  74.50 
                  30.30 
                 59.3% 
               
               
                 Fine Foam 
                 EtOH 
                 NA 
                 NA 
                 NA 
               
               
                   
                 Acetone 
                 NA 
                 NA 
                 NA 
               
               
                 Coarse/Fine 
                 EtOH 
                 499.30 
                 196.70 
                 60.6% 
               
               
                 Foam 
                 Acetone 
                 NA 
                 NA 
                 NA 
               
               
                   
                 Limonene 
                  39.30 
                  9.30 
                 68.7% 
               
            
           
           
               
            
               
                 C-doped TiO 2   
               
            
           
           
               
               
               
               
               
            
               
                 Coarse Foam 
                 EtOH 
                 NA 
                 NA 
                 NA 
               
               
                   
                 Acetone 
                 NA 
                 NA 
                 NA 
               
               
                 Fine Foam 
                 EtOH 
                 NA 
                 NA 
                 NA 
               
               
                   
                 Acetone 
                 NA 
                 NA 
                 NA 
               
               
                 Coarse/Fine 
                 EtOH 
                 417.40 
                 203.45 
                 51.2% 
               
               
                 Foam 
                 Acetone 
                 152.30 
                  80.90 
                 46.8% 
               
               
                   
               
               
                 NA: Not Available 
               
            
           
         
       
     
     The photoactive catalyst—whether AEROXIDE® P25 (considered to be a “pure TiO 2 ” according to the present disclosure), other pure TiO 2 , Fe—TiO 2 , or C—TiO 2 —is loaded into a porous and air- and light-permeable filter. In one aspect, the photoactive catalyst is adhered to all surface area of the filter, including within pores and passages running throughout the volume of the filter medium. In at least one aspect, the catalyst is deposited by a dip coating method, followed by drying at 80-100° C. Other methods of catalyst deposition may be used. 
     The type of substrate and coating methods have important effects on coating stability, photocatalytic, and mechanical performance of the filters. Porous metal substrates offer better toughness, better malleability, and lower cost than ceramic substrates. However, using metal substrate generally results in peeling coatings with cracks. This occurs at heating stage and due to difference in thermal expansion coefficients between the TiO 2  and the substrate metal. 
     Porous TiO 2  filters are commonly employed to avoid this problem. Such filters are commonly prepared by coating a TiO 2  sol, slurry, or precursor liquid onto ceramic substrates, metal meshes or ceramic or metallic foam by different coating methods. After coating application, heat treatment necessary for photocatalysts activity around 500° C. is generally conducted. 
     In one aspect, three filter types are used in PCO filter modules  30 : coarse foam, fine foam, and fused quartz filament felt. Both the porosity (number and size of pores, or voids) and the permeability (ability of fluid to flow through, which is related to interconnectivity of the pores) of each type of filter type are selected based on a contemplated use relative to VOCs of interest. The Brunner-Emmett-Teller (BET) theory can be used to measure the surface area of the solid or porous material selected, optionally in conjunction with transmission electron microscopy (TEM) imaging to confirm pore size. Further, pore size distribution can be evaluated by Barrett-Joyner-Halenda (BJH) interpretation. Porosity contributes to the surface area for adhering more photocatalytic coatings. Permeability impacts the selected volume of air that can flow through the PCO filter modules  30  to remove VOCs from the cabin air of a large aircraft, for example. 
     The coarse foam is a relatively open foam, with average pores size of approximately 2540 um, and high permeability. The coarse foam filter is approximately 10 mm thick. A suitable coarse foam is available from Recemat BV of the Netherlands. This material can be uniformly coated with catalyst. The coarse foam filter allows much of the incident UV light to penetrate the foam, thus illuminating successive filters. For this reason, in some aspects, a coarse foam filter is at both exterior positions of a “stack” of filters forming a PCO filter module  30 . The position of coarse foam filter is also maintained on the outside of whole filter stack to start the VOC degradation in a lower specific surface area to very high specific surface area in fine foam filters. 
     The fine foam is a denser foam, with average pores size of approximately 800 um, and lower permeability as compared to coarse foam but with a higher surface area. The Brunner-Emmett-Teller (BET) theory can be used to measure the surface area of the solid or porous material selected, optionally in conjunction with transmission electron microscopy (TEM) imaging to confirm pore size. Further, pore size distribution can be evaluated by Barrett-Joyner-Halenda (BJH) interpretation. Accordingly, the fine foam filter is thinner than the coarse foam filter, at approximately 2-4 mm, to maintain robust airflow. This material is more difficult to coat uniformly with catalyst. A suitable fine foam is available from Alantum GMBH of Germany. 
     Another type of material is made from fused quartz filaments. A suitable felt of this type is QUARTZEL® felt, available from Magento of the USA. The QUARTZEL® felt is difficult to coat uniformly and presents a high resistance to air flow. Accordingly, it is used sparingly. In one aspect, only one of three PCO filter modules  30  in a VOC removal unit  10  include a fused quartz filament filter, and that module  30  includes only a single such filter. Each PCO filter module  30  comprises a plurality of catalyst-loaded filters selected and arranged to maximize UV illumination of all of the filters. That is, according to present aspects, in the present air filtration unit, the photocatalytic oxidation filter module comprises a plurality of catalyst-loaded filters selected and arranged to maximize ultraviolet illumination of each filter and through a complete depth of each filter layer. 
     With, in some aspects, three filter media and four types of photoactive catalysts, there are a dozen combinations of PCO filters from which to select. The number of permutations of which of these filters to stack into a filter module  30 , in which order, is very large. Furthermore, different PCO filter modules. That is, different selections and arrangements of photoactive-catalyst-loaded filters can be placed in different locations along the duct  14  of the VOC removal unit  10 . 
     In one aspect, as depicted in  FIG. 5 , an air filtration and volatile organic compound (VOC) removal unit  10  includes an air duct  14  having a longitudinal axis  15 , an air inlet  12  at one end, and air outlet  16  at the other end; a plurality of baffles  18  ( FIG. 4 ), each having a plurality of open spaces  22  allowing airflow therethrough, disposed at spaced locations within the duct  14  between the air inlet  12  and air outlet  16 , the baffles  18  being generally transverse to the longitudinal axis  15 ; a plurality of ultraviolet (UV) light emitting diodes (LED)  24  mounted on each baffle  18 ; and a porous and permeable photocatalytic oxidation (PCO) filter module  30  disposed between each pair of baffles  18 , generally transverse to the longitudinal axis  15 , such that air flows through the PCO filter module  30 . Each PCO filter module  30  contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), TiO 2  doped with carbon (C—TiO 2 ), or combinations thereof, which, when illuminated by UV light, are operative to chemically reduce VOCs to non-VOC molecules. This provides a compact, lightweight, low-power means for removing VOCs. 
     In one aspect, the VOC removal unit further includes one or more heat sinks disposed within the duct and adapted to conduct heat away from the UV LEDs away and maintain their lifespan. This prevents overheating of the LEDs to prolong their life in a low flow situation. 
     In one aspect, the UV LEDs are disposed both around a periphery of each baffle and between the airflow spaces so as to maximize the UV illumination of adjacent PCO filter modules. The UV LEDs are mounted on interior sides of baffles adjacent the air inlet and outlet and are mounted on both sides of all other baffles, so as to illuminate the PCO filter modules from both sides. The PCO filter modules are spaced apart from the baffles such that the entirety of both surfaces of each PCO filter module is illuminated by UV light. These features ensure maximum and even illumination of the PCO filter modules by UV light. 
     Each PCO filter module comprises a plurality of filters, each filter selected from the group consisting of a coarse foam, a fine foam, and a fused quartz filament felt, and each filter is loaded with a catalyst which may be one or more of pure titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), and TiO 2  doped with carbon (C—TiO 2 ), and combinations thereof. In one aspect, each PCO filter module comprises a plurality of catalyst-loaded filters selected and arranged to maximize UV illumination of all of the filters. These materials have high durability and the arrangements facilitate the removal of VOCs. 
     The VOC removal unit  10  comprises four baffles  18  and three PCO filter modules  30 . The PCO filter modules  30  comprise, in order from air inlet  12  to air outlet  16 , 
     1) R25-CTR-TA-R25; 
     2) CTR-TA-Q25-R25-R25; and 
     3) R25-CTR-TA-R25; wherein
 
R25 is a coarse foam loaded with pure TiO 2 ;
 
Q25 is a fused quartz filament felt loaded with pure TiO 2 ;
 
CTR is a coarse foam loaded with C—TiO 2  and
 
TA is a fine foam loaded with pure TiO 2 .
 
     In another aspect, with the same numbers of baffles  18  and PCO filter modules  30 , the PCO filter modules  30  comprise, in order from air inlet  12  to air outlet  16 , 
     1) R25-TA-FTR-CTR; 
     2) R25-CTA-FTR-Q25-R25; and 
     3) R25-TA-FTR-CTR; wherein
 
FTR is a coarse foam loaded with Fe—TiO 2 .
 
     Based on the information disclosed herein, those of skill in the art may devise numerous other selections and arrangements of both photoactive catalyst-loaded filters in each PCO filter module  30 , and the PCO filter modules  30  in the VOC removal unit  10 , within the scope of the present disclosure. 
     According to further aspects,  FIG. 7  shows an exemplary air purification system and apparatus that can implement the PCO filter modules of the types shown, for example, in  FIG. 5 , in combination with presently disclosed selected additional filter system components integrated into the system to further enhance particulate removal (including non-VOC particulate removal) upstream from the PCO filter modules, and further effect nitrous oxide-containing compound removal from an airflow downstream from the PCO filter modules. According to present aspects, placement of an HEPA filter upstream of the UV-PCO portion of the present air filtration unit removes tar/particles from the airflow directed into the present air filtration unit that may otherwise impact the surfaces of the PCO, and that could substantially reduce the efficiency of the ultraviolet light reactor(s). 
     According to present aspects, nitrous oxide (NO 2 , NOR, NO) adsorbent such as, for example, an amine adsorbent, is oriented in the nitrous oxide-adsorbing filter downstream of the ultraviolet light reactor(s), as nitrous oxides are a byproduct of reactions between ammonia (a product of, for example, smoking), and oxygen in the presence of ultraviolet light. Likewise, nitrous oxides are also highly present within indoor enclosed environments including, for example, an airport, a bus terminal, a railway terminal, a planetary habitat, and other terrestrial transportation environments, etc. To further purify ambient air in such environments, present aspects reduce the exposure to nitrous oxide-containing compounds as the contemplated nitrous oxide removal region, including a nitrous oxide-adsorbing filter, in the presently disclosed air purification unit significantly reduces the nitrous oxide concentrations in the ambient air present in, for example, an occupied area of a terrestrial transportation environment including, for example, an airport, a smoking room in an airport, etc. 
     As shown in  FIG. 6 , according to present aspects, an air filtration apparatus  200  incorporates a filter  204  that can be a high-efficiency particulate air HEPA filter located proximate to or otherwise incorporated into an air inlet  12  of a VOC removal unit  10   a  of the type shown in  FIG. 5  and described herein. According to present aspects, the HEPA filter is oriented upstream of the VOC removal unit  10   a . HEPA filters are also known as high-efficiency particulate adsorbing and high-efficiency particulate arresting filters representing an efficiency standard of air filter. Filters meeting the HEPA standard must satisfy certain levels of efficiency. Common standards require that a HEPA air filter must remove from the air passing therethrough of at least 99.95% (European Standard) or 99.97% (ASME, U.S. D.O.E.) of particles having a diameter equal to 0.3 μm or greater, with the filtration efficiency increasing for particle diameters both less than and greater than 0.3 μm. 
     According to present aspects, HEPA filters can comprise a mat of ordered or randomly arranged fibers. The fibers can include fiberglass having diameters between 0.5 and 2.0 μm. The air space between HEPA filter fibers is typically much greater than 0.3 μm. Unlike sieves or membrane filters, where particles smaller than openings or pores can pass through, HEPA filters are designed to target a range of particle sizes. These particles are trapped (they stick to a fiber) through a combination of the mechanisms including diffusion, interception, and impaction. As used in accordance with the present aspects, and as shown in  FIG. 6 , airflow introduced into the air filtration apparatuses and systems first encounters the HEPA filter that is located upstream from the VOC removal unit  10   a  for the purpose of removing particulates from the introduced airflow upstream of the VOC removal unit that incorporates the PCO filter modules (that, in turn, employ the L V photocatalysis) with the VOC removal unit being primarily responsible for the subsequent removal of VOCs from the airflow flowing through unit. 
     According to present aspects, the disclosed apparatuses, systems, and methods can implement a variable speed or diverted air system based upon the CO 2  of the occupied space in an enclosed environment to minimize exposure of the air purifier to contaminants; having the potential to increase the lifespan of the technology and minimize maintenance costs. When the NOx is primarily produced within the unit during operation, diverting an airflow can assist in preventing the exposure of the unit to a selected level of CO 2  that can be directed into the air filtration unit from the occupied space in, for example, an enclosed environment. 
       FIG. 6  shows an airflow controller  202  that is in communication with a first and second carbon dioxide (CO 2 ) sensors  206 ,  206   a , respectively, with the CO 2  sensors  206 ,  206   a  further comprising or otherwise in communication with first and second pressure sensors  208 ,  208   a  respectively incorporated into or proximate to the air inlet  12 . Pressure sensors  208 ,  208   a  are in communication with airflow controller  202  with the pressure sensors  208 ,  208   a  configured to monitor and determine changes in system pressures, with the pressure sensors  208 ,  208   a  further able to monitor the performance of the HEPA filter, for example, to output information regarding to potential clogging of, for example, a HEPA filter. 
     As shown in  FIG. 6 , the airflow controller can initiate, terminate, modify, and otherwise direct and control air in a surrounding environment to form an airflow into the air filtration apparatus  200 . Air in a surrounding environment can include air that is, for example, within an enclosed environment, and the enclosed environment can be, for example, a room in a building, a terminal, a warehouse etc., and that can be an enclosed environment in or proximate to a terrestrial transportation environment such as, for example, an airport terminal, a railway terminal, a bus terminal, a raceway, etc. 
     While the presently disclosed particulate and VOCs removed from ambient air in a particular surrounding can accomplish a certain desired level of purification of ambient air, as shown in  FIG. 6 , the air filtration unit  200  further comprises a nitrous oxide-adsorbing filter  210  that can be a porous and permeable nitrous oxide-adsorbing chamber further comprising, for example, a bed, stack etc. that is disposed downstream from the VOC removal unit  10   a  for the purpose of removing nitrous oxide-containing compounds from the air directed through the air filtration apparatus. The nitrous oxide-containing compounds that can be adsorbed by the nitrous oxide-adsorbing filter can include, for example, NO 2 , NO x , NO. The nitrous oxide-adsorbing filter can be oriented within the air filtration unit at a point downstream from the VOC removal unit, and can be located proximate to the air outlet, or otherwise incorporated into the air outlet. 
     The nitrous oxide-adsorbing filter can include a solid form amine-containing packed bed, a cellular monolith, a granular media set-up, etc., and the nitrous oxide-adsorbing filter can have a separate frame housing the nitrous oxide-adsorbing filter, or the frame can be built into or otherwise incorporated and formed as a part of (e.g., integral with) the air filtration unit air outlet. The amine will be immobilized into a solid form (e.g., a monolithic contractor), rather than utilizing a liquid membrane. While amines and amine-containing compounds are contemplated for use in the nitrous oxide-adsorbing filter, other potential nitrous oxide adsorbents can include metal organic compounds, zeolites, etc., alone or in combination. 
       FIG. 7  shows an end view of the air filtration unit  200  showing the air inlet  12  framed by duct  14 .  FIG. 8  shows an end view of the air outlet end of a presently disclosed air filtration unit  200 , with the air outlet  16  framed by the duct  14 . While the air intake is shown in a circular configuration and the duct perimeter and air outlet are shown as substantially square, it is understood that presently disclosed filtration units can be configured into any geometry as desired and according to the constraints presented by ducts in ductwork, including ducts that can be pre-existing ducts into which the presently disclosed air filtration units can be retrofitted, for example. 
       FIGS. 9, 10, 11, 12  are flowcharts showing exemplary methods implementing the apparatuses and systems shown at least in  FIGS. 5, 6, 7, and 8 . 
     As shown in  FIG. 9 , a presently disclosed method is disclosed for filtering air in an enclosed environment, with the method  300  including monitoring  302  carbon dioxide concentration in an enclosed environment, initiating  304  an air purification cycle, directing  306  an airflow to an air inlet of an air filtration unit, with the air filtration unit including an air duct having a longitudinal axis, said air duct comprising an air inlet at a first end and an air outlet at a second end, a high efficiency particulate air (HEPA) filter unit oriented proximate to the air inlet, an airflow controller in communication with the air inlet, a carbon dioxide sensor in communication with the airflow controller, and a pressure sensor in communication with the airflow controller. The air filtration unit further includes an ultraviolet light reactor that further includes a plurality of baffles, with each baffle having a plurality of airflow spaces allowing airflow therethrough, disposed at spaced locations within the duct between the air inlet and air outlet, and with the baffles being generally transverse to the longitudinal axis. The air filtration unit further includes a plurality of ultraviolet light emitting diodes mounted on each baffle, a porous and permeable photocatalytic oxidation filter module disposed between each pair of baffles, generally transverse to the longitudinal axis, such that air flows through the photocatalytic oxidation filter module, and a porous and permeable nitrous oxide-adsorbing filter disposed downstream of the UV light reactor, and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), TiO 2  doped with carbon (C—TiO 2 ), or combinations thereof which, when illuminated by ultraviolet light, are operative to chemically reduce volatile organic compounds to non-volatile organic compounds. The method further includes removing  308  an amount of particulate from the airflow upstream from the photocatalytic oxidation filter modules, illuminating  310  the photocatalytic oxidation filter modules with ultraviolet light from ultraviolet light emitting diodes mounted on each baffle, chemically reducing  312  volatile organic compounds in the airflow to non-volatile organic compounds (thus removing VOCs from the airflow), and removing  314  an amount of nitrous oxide-containing compounds from the airflow downstream from the photocatalytic oxidation filter modules and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), or TiO 2  doped with carbon (C—TiO 2 ) which, when illuminated by UV light, are operative to chemically reduce volatile organic compounds in the airflow to non-volatile organic compounds. 
       FIG. 10  is a flowchart outlining a presently disclosed method for filtering air in an enclosed environment, with the method  400  including monitoring  302  carbon dioxide concentration in an enclosed environment and monitoring  402  pressure of the system for the purpose of sensing a pressure drop indicating a need for adjusting a system airflow capacity, for example. The pressure can be monitored at an air intake or an air outlet, with the pressure monitored by a sensor that can send a signal to a controller that can adjust airflow capacity through the unit and system, etc. 
     Method  400  shown in  FIG. 10  further shows initiating  304  an air purification cycle, directing  306  an airflow to an air inlet of an air filtration unit, with the air filtration unit including an air duct having a longitudinal axis, said air duct comprising an air inlet at a first end and an air outlet at a second end, a high efficiency particulate air (HEPA) filter unit oriented proximate to the air inlet, an airflow controller in communication with the air inlet, a carbon dioxide sensor in communication with the airflow controller, and a pressure sensor in communication with the airflow controller. The air filtration unit further includes an ultraviolet light reactor that further includes a plurality of baffles, with each baffle having a plurality of airflow spaces allowing airflow therethrough, disposed at spaced locations within the duct between the air inlet and air outlet, and with the baffles being generally transverse to the longitudinal axis. The air filtration unit further includes a plurality of ultraviolet light emitting diodes mounted on each baffle, a porous and permeable photocatalytic oxidation filter module disposed between each pair of baffles, generally transverse to the longitudinal axis, such that air flows through the photocatalytic oxidation filter module, and a porous and permeable nitrous oxide-adsorbing filter disposed downstream of the UV light reactor, and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), TiO 2  doped with carbon (C—TiO 2 ), or combinations thereof which, when illuminated by ultraviolet light, are operative to chemically reduce volatile organic compounds to non-volatile organic compounds. The method further includes removing  308  an amount of particulate from the airflow upstream from the photocatalytic oxidation filter modules, illuminating  310  the photocatalytic oxidation filter modules with ultraviolet light from ultraviolet light emitting diodes mounted on each baffle, chemically reducing  312  volatile organic compounds in the airflow to non-volatile organic compounds (thus removing VOCs from the airflow), and removing  314  an amount of nitrous oxide-containing compounds from the airflow downstream from the photocatalytic oxidation filter modules and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), or TiO 2  doped with carbon (C—TiO 2 ) which, when illuminated by UV light, are operative to chemically reduce volatile organic compounds in the airflow to non-volatile organic compounds. 
       FIG. 11  is a flowchart outlining a presently disclosed method for filtering air in an enclosed environment, with the method  500  including monitoring  302  carbon dioxide concentration in an enclosed environment, monitoring  402  pressure of the system for the purpose of sensing a pressure drop indicating that the need for adjusting a system airflow capacity, for example. The pressure can be monitored at an air intake or an air outlet, with the pressure monitored by a sensor that can send a signal to a controller that can adjust airflow capacity through the unit and system, etc. 
     Method  500  shown in  FIG. 11  further shows initiating  304  an air purification cycle, directing  306  an airflow to an air inlet of an air filtration unit, with the air filtration unit including an air duct having a longitudinal axis, said air duct comprising an air inlet at a first end and an air outlet at a second end, a high efficiency particulate air (HEPA) filter unit oriented proximate to the air inlet, an airflow controller in communication with the air inlet, a carbon dioxide sensor in communication with the airflow controller, and a pressure sensor in communication with the airflow controller. The air filtration unit further includes an ultraviolet light reactor that further includes a plurality of baffles, with each baffle having a plurality of airflow spaces allowing airflow therethrough, disposed at spaced locations within the duct between the air inlet and air outlet, and with the baffles being generally transverse to the longitudinal axis. The air filtration unit further includes a plurality of ultraviolet light emitting diodes mounted on each baffle, a porous and permeable photocatalytic oxidation filter module disposed between each pair of baffles, generally transverse to the longitudinal axis, such that air flows through the photocatalytic oxidation filter module, and a porous and permeable nitrous oxide-adsorbing filter disposed downstream of the UV light reactor, and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), TiO 2  doped with carbon (C—TiO 2 ), or combinations thereof which, when illuminated by ultraviolet light, are operative to chemically reduce volatile organic compounds to non-volatile organic compounds. The method further includes removing  308  an amount of particulate from the airflow upstream from the photocatalytic oxidation filter modules, illuminating  310  the photocatalytic oxidation filter modules with ultraviolet light from ultraviolet light emitting diodes mounted on each baffle, chemically reducing  312  volatile organic compounds in the airflow to non-volatile organic compounds (thus removing VOCs from the airflow), and removing  314  an amount of nitrous oxide-containing compounds from the airflow downstream from the photocatalytic oxidation filter modules and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), or TiO 2  doped with carbon (C—TiO 2 ) which, when illuminated by UV light, are operative to chemically reduce volatile organic compounds in the airflow to non-volatile organic compounds. Method  500 , shown  FIG. 11  further includes conducting  404  heat away from LEDs via at least one heat sink, with the at least one heat sink disposed within the duct and adapted to conduct heat away from the UV LEDs away and maintain the lifespan of the UV LEDs to, for example, prevent overheating of the UV LEDs and prolong the life of the UV LEDs (e.g., in a low airflow situation). 
       FIG. 12  is a flowchart outlining a presently disclosed method for filtering air in an enclosed environment, with the method  600  including monitoring  302  carbon dioxide concentration in an enclosed environment, monitoring  402  pressure of the system for the purpose of sensing a pressure drop indicating that the need for adjusting a system airflow capacity, for example. The pressure can be monitored at an air intake or an air outlet, with the pressure monitored by a sensor that can send a signal to a controller that can adjust airflow capacity through the unit and system, etc. 
     Method  600  shown in  FIG. 12  further shows initiating  304  an air purification cycle, directing  306  an airflow to an air inlet of an air filtration unit, with the air filtration unit including an air duct having a longitudinal axis, said air duct comprising an air inlet at a first end and an air outlet at a second end, a high efficiency particulate air (HEPA) filter unit oriented proximate to the air inlet, an airflow controller in communication with the air inlet, a carbon dioxide sensor in communication with the airflow controller, and a pressure sensor in communication with the airflow controller. The air filtration unit further includes an ultraviolet light reactor that further includes a plurality of baffles, with each baffle having a plurality of airflow spaces allowing airflow therethrough, disposed at spaced locations within the duct between the air inlet and air outlet, and with the baffles being generally transverse to the longitudinal axis. The air filtration unit further includes a plurality of ultraviolet light emitting diodes mounted on each baffle, a porous and permeable photocatalytic oxidation filter module disposed between each pair of baffles, generally transverse to the longitudinal axis, such that air flows through the photocatalytic oxidation filter module, and a porous and permeable nitrous oxide-adsorbing filter disposed downstream of the UV light reactor, and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), TiO 2  doped with carbon (C—TiO 2 ), or combinations thereof which, when illuminated by ultraviolet light, are operative to chemically reduce volatile organic compounds to non-volatile organic compounds. The method further includes removing  308  an amount of particulate from the airflow upstream from the photocatalytic oxidation filter modules, illuminating  310  the photocatalytic oxidation filter modules with ultraviolet light from ultraviolet light emitting diodes mounted on each baffle, chemically reducing  312  volatile organic compounds in the airflow to non-volatile organic compounds (thus removing VOCs from the airflow), and removing  314  an amount of nitrous oxide-containing compounds from the airflow downstream from the photocatalytic oxidation filter modules and wherein each photocatalytic oxidation filter module contains one or more catalysts comprising titanium dioxide (TiO 2 ), TiO 2  doped with iron (Fe—TiO 2 ), or TiO 2  doped with carbon (C—TiO 2 ) which, when illuminated by UV light, are operative to chemically reduce volatile organic compounds in the airflow to non-volatile organic compounds. Method  600  further includes selecting and arranging  406  a plurality of catalyst-loaded filters in the photocatalytic oxidation (PCO) component in the VOC removal unit to maximize illumination of the catalyst-loaded filters. 
     The method  600 , though not shown in  FIG. 12 , can further include (as shown in method  500  shown in  FIG. 11 ) conducting  404  heat away from LEDs via at least one heat sink, with the at least one heat sink disposed within the duct and adapted to conduct heat away from the UV LEDs away and maintain the lifespan of the UV LEDs to, for example, prevent overheating of the UV LEDs and prolong the life of the UV LEDs (e.g., in a low airflow situation). 
     According to present aspects, the airflow rates of air delivered through the present air filtration units, and according to presently disclosed methods, can be, for example, from about 10 to about 15 ft 3 /minute (CFM) per occupant of purified airflow, with the understanding that the overall unit and system sizing and scale can be configured to accommodate and service enclosed environments (e.g., rooms, hallways, buildings, warehouses, garages, terrestrial transportation building environments, etc.). 
     Present apparatuses, systems, and methods are further understood to monitor, determine, and respond to CO 2  levels determined by the CO 2  sensors, and the system pressures observed, monitored, and detected by the pressure sensors. The combined factors of sensed CO 2  in an environment, and the changes in sensed CO 2  levels while the present systems are in operation, and further in view of sensed system pressures, can be relayed to one or more airflow controllers to adjust, in real time, airflow to be directed into the present systems. 
     The present disclosure can, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the disclosure. The present aspects are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.