The effect of portable air purifiers and cabin air filters on particulate matter and nitrogen oxides concentrations in vehicle cabins

In this study, portable air filters ranging in size from small desktop units with ∼80 lpm (∼3 CFM) flow rates to large plug-in filters with ∼8500 lpm (∼300 CFM) flow rates were tested to evaluate their performance in reducing particulate matter (PM) and nitrogen oxide (NO_X) concentrations (as a sum of NO and NO₂, or individually) inside vehicle cabins. Aftermarket cabin air filters with various types of carbon trap features were tested to evaluate their performance in reducing NO_X concentrations inside vehicle cabins. The first goal was to determine the minimum size (in terms of flow rates) for a supplemental portable air purifier to aid the existing cabin air filter such that it enhances air quality without excessive energy consumption or space requirements. Tests were conducted with each portable air purifier in a vehicle cabin in different HVAC settings, all using the fresh air mode. The second goal was to determine the effectiveness of the carbon trap elements inside the portable air purifiers as well as after-market cabin air filters that have built-in carbon trap layers. The Cabin Air Quality Index for particle number (CAQI_PN) was determined as a function of vehicle HVAC fan speeds. Baseline results without a portable air purifier showed that the built-in cabin filter efficiency for particles drops significantly as the vehicle HVAC fan speed increases. The CAQI values for NO, NO₂, and NO_X were determined with the lowest HVAC fan speed for commercially available OEM and aftermarket adsorption filters. The portable air purifier with ∼270 lpm (∼10 CFM) showed no reduction of CAQI_PN compared to the stock cabin filter alone. The plug-in type air purifier with ∼1400 lpm (∼60 CFM) showed 37% and 51% reduction in CAQI_PN at low and medium fan speed settings, respectively. The minimum effective flow rate for an additional air purifier in a vehicle was seen to be between 280 lpm (10 CFM) and 1400 lpm (60 CFM). Two OEM (Original Equipment Manufacturer) cabin filters with the activated carbon layer showed 64–67% reduction in NO₂. The inexpensive aftermarket cabin air filters showed no reduction in NO₂ concentrations. To the best of the authors' knowledge, this is the first study to: (1) demonstrate that built-in cabin filters can be highly inefficient when the vehicle's HVAC fan speed is set to high, (2) evaluate the effectiveness of portable air purifiers within vehicle cabins, and (3) assess and compare commercially available adsorption-type filters for vehicle cabin air quality concerning the cabin air quality index.