Recirculation Mode Research Articles

Effects of Tunnel and Its Ventilation Modes on the Aerodynamic Drag of a Subway Train

This paper reports an in situ measurement on the effects of a tunnel and its ventilation modes on the aerodynamic drag of a subway train with eight carriages during its routine operation. The train speed (V) varied continually from 0 to 22 m/s. Two modes of tunnel ventilation were examined, i.e., recirculation and free-cooling modes. The former mode is associated with pumping cooled air into the tunnel to provide extra cooling, while the latter is not. The friction coefficient Cf of the train surface was estimated using two hotwire probes mounted on the roof of the first and last carriages, respectively. The front- and rear-stagnation pressures (Pf and Pl) were measured using two pressure taps located at the center of the forward surface of the first carriage and the backward surface of the last carriage, respectively. It has been found that the presence of a tunnel significantly increases both Cf and Pf. For example, at V = 20.5 m/s, Cf and Pf were 30.2% and 24.5% higher, respectively, in the tunnel than their counterparts in open air. The tunnel ventilation mode also has remarkable effects on Cf. The recirculation mode resulted in 23.5% higher Cf than the free-cooling mode. On the other hand, the tunnel ventilation mode does not seem to have an appreciable effect on Pf. The physics behind these observations is also discussed.

CFD simulations of effects of recirculation mode and fresh air mode on vehicle cabin indoor air quality

Drivers in hot and humid countries tend to operate the vehicle air conditioner system in the recirculation mode when driving. However, this causes the concentration of carbon dioxide (CO2) in the cabin to rise sharply, potentially resulting in dizziness, drowsiness and other traffic safety hazards. In the fresh air mode, suspended particles and contaminants are introduced into the cabin along with the outside air, posing further health risks to the occupants. Thus, it is important to evaluate the optimal ventilation strategy for vehicle cabins. Accordingly, the present study performs computational fluid dynamics (CFD) simulations to investigate the indoor air quality (IAQ) in the cabin of a compact passenger vehicle under various ventilation/circulation modes. The validity of the CFD model is confirmed by comparing the simulation results for the IAQ in the cabin with the experimental data reported in the literature. An air-conditioning ventilation strategy is proposed for maintaining the CO2 concentration in the cabin between 1000 ppm and 2000 ppm. The feasibility of the proposed strategy is investigated for various numbers of occupants in the cabin (1, 2, and 5 people) and air filters with different filtration efficiencies of 20%, 40%, and 80%, respectively. The results show that as the number of people in the cabin increases, the frequency at which switching between the two ventilation modes is required increases. Moreover, the relative proportion of time for which the fresh air mode is needed also increases. The concentration of suspended particles in the vehicle cabin increases as the duration of the fresh air mode increases. However, the particle concentration reduces significantly as the filtration efficiency of the air filter is increased.

Development of a new rotating photocatalytic reactor for the degradation of hazardous pollutants

Abstract The aim of this study was to develop a new rotating photocatalytic reactor operating in recirculation mode with light sources placed outside the photoreactor vessel. The photoreactor with cylindrical geometry was equipped with four artificial lamps used to simulate solar irradiation (2.4% UVB and 12% UVA; 300–700 nm). The photocatalyst was immobilized on abrasive material used as a support and placed on the central (inner) photoreactor tube, which was connected to a power-driven shaft that allowed rotation at a desired speed. A suitable modification of the commercial TiO2 P25 photocatalyst was carried out to reduce its band gap energy and electron-hole recombination and to extend the visible light response range of TiO2. The main task of this research was to apply the basic principles of process intensification methodology, i.e. to explore the influence of rotational hydrodynamics, which allows good access of reactants to the photocatalyst surface, good irradiation of the photocatalytic surface and reduction of mass transfer resistance, leading to increased process efficiency. The homemade photoreactor was used for the photocatalytic degradation of one of the major types of neonicotinoid insecticides, acetamiprid. The influence of various working conditions, such as initial solution pH, rotation speed, recirculation flow rate and initial concentration of acetamiprid on the photocatalytic degradation process was investigated. The optimum degradation conditions were found at a recirculation flow rate of 200 cm3 min−1 and a rotation speed of 200 rpm, indicating that the mass transfer process strongly contributes to the photocatalytic degradation rate at the conditions used in this study. The results obtained during the photocatalytic degradation of acetamiprid in a rotating photoreactor were compared with those obtained under similar operating conditions in a flat-plate photoreactor, and the corresponding conclusions were drawn based on the performed kinetic analysis.

Optimization-based technoeconomic comparison of multi-stage membrane distillation configurations for hypersaline produced water desalination

Unconventional oil and gas production raises concerns regarding sustainable management of high salinity wastewaters generated in this process. Membrane distillation (MD) is a thermal desalination process capable of treating hypersaline brines such as produced water. The low single pass recovery in MD systems operated in a single stage requires a large recycle stream to achieve the desired recovery, resulting in high energy consumption and operating cost. Multistage configurations in continuous recirculation operation mode offer the potential to reduce the energy intensity of MD systems. However, rigorous analysis is needed to assess the performance of MD configurations when operating in multi-stage mode. We present an optimization-based comparison of economic and energetic performance for five configurations of MD (including DCMD, AGMD, PGMD, CGMD, and VMD) operating in multi-stage continuous recirculation mode. Our findings demonstrate that multistage operation reduces the treatment cost and energy intensity of all MD configurations compared to single stage MD, with the greatest benefit for VMD and PGMD. AGMD with five stages outperforms other configurations with 3.58 $/m3feed treatment cost, followed by VMD, CGMD, DCMD, and PGMD with 3.8, 4.2, 5.4, and 9.06 $/m3feed treatment costs corresponding to twelve, eight, eight, and sixteen stages, respectively.

Anaerobic Digestion of Pig-Manure Solids at Low Temperatures: Start-Up Strategies and Effects of Mode of Operation, Adapted Inoculum, and Bedding Material.

The objective of this study was to obtain start-up strategies for the operation of a dry anaerobic digestion (DAD) system treating pig-manure (PM) solids at low-temperatures, and evaluate the effects of operation mode, adapted inoculum, and bedding material on the performance. A DAD system coupled with an inoculum system (two-stage DAD) was operated at 20 ± 1 °C to digest PM solids (Total Solids, TS: 27%) with wheat straw or woodchips as bedding materials (TS substrate-mixture: 45%) using a liquid inoculum. Static DAD was also operated in parallel for comparison purposes. Overall, the percolation–recirculation mode of operation was superior to the static mode; the former had more than a 3-fold increase in specific methane yield in cycle 3. Using the adapted inoculum in cycle-2 improved methane yield by 7% and 26% for cycles 1 and 3, respectively, under the percolation–recirculation mode of operation. In addition, the digestate resulting from the digestion of woodchips + PM solids had better physical characteristics than wheat straw + PM solids. Thus, anaerobic digestion of pig-manure solids at low-temperatures with appropriate start-up strategies, inoculum, and bedding material is a promising technology for transforming PM solids into biogas and using its digestate as biofertilizer.

Treatment of petroleum refinery wastewater by adsorption using activated carbon fixed bed column with batch recirculation mode

Water pollution and the lack of access to clean water are general global problems that result from the expansion of industrial and agricultural activities. Petroleum refinery wastewaters consider as a major challenge to the environment and their treatment is mandatory. The present work concerns with the removal of chemical oxygen demand (COD) from petroleum refinery wastewater taken from Iraq's Al-Diwaniyah petroleum refinery plant by using an activated carb fixed-bed column operated at a batch recirculation mode. The fixed bed column used in this work was composed of three sections: upper, central, and bottom compartments. The bottom compartment serves as a feeding chamber to the central adsorption chamber while the upper compartment serves as a collecting effluent chamber. By adopting response surface methodology (RSM), in the pacts of various operational parameters such as packing level, pH, and time on the COD removal efficiency were investigated. The optimal conditions were an activated carbon packing level of 80%, pH of 5.7, and adsorption time of 73 min approximately, which resulted in a COD removal efficiency of 96.70%. The results indicated that the packing level of activated carbon had a major effect on COD elimination followed by pH, while time had a minor effect. The model equation's adequacy was demonstrated by its strong R2 value (0.975). The present study demonstrates that the adsorption system by activated carbon is an effective method for removingcondomODom Al-Diwaniyah petroleum refinery wastewaters.

INTENSIFICATION OF THE ABSORPTION OF OXYGEN BY WATER USING A ROTOR-PULSATING APPARATUS

One of the important components of the quality of food products is the quality of water used. A common way to remove unwanted impurities from water is aeration, i.e. the oxidation of chemical compounds in water by oxygen. Aeration devices with mechanical energy input in comparison with other groups of devices, namely with energy input with gas phase and with liquid phase, have low power consumption and additional mixing of the processed medium. The work presents a study of oxygen absorption in water in an experimental setup with rotor-pulsating apparatus for water treatment in beverage production technology. In this paper, the energy and technological parameters of aeration of a model aqueous solution of a certain concentration of sodium sulfide in an experimental setup with a rotor-pulsating apparatus as an aerator are determined. The experimental aeration setup allows conducting research in several modes and consists in particular of a vessel, a rotor-pulsating apparatus, two ejectors - one at the entrance to the rotor-pulsating, the other at the outlet, the recirculation pipeline. Air from the atmosphere enters each of the ejectors through a separate air duct. The aeration of the studied water took place in the recirculation mode for 20 minutes. Determination of the oxygen mass transfer rate is determined by the iodometric titration method on the rate of oxidation of sodium sulfite. Experiments were conducted without using a catalyst. It is determined that when placing the ejector unit at the rotor-pulsating apparatus inlet at the angular rotor unit velocity of 240.02; 270.18, and 300.02 s-1, the oxygen mass transfer rate is 1.39; 1.49 and 1.73 kg m3/h. At the location of the ejector unit at the outlet of the rotor-pulsating apparatus, the velocity of the oxygen mass transfer under the same conditions is 1.17; 1.36 and 1.63 kg m3/h respectively. However, the power consumption of the second scheme exceeds the power consumption by the first scheme by 50%.

Downstream recovery of Li and value-added metals (Ni, Co, and Mn) from leach liquor of spent lithium-ion batteries using a membrane-integrated hybrid system

The end-of-life management of a large number of discarded lithium-ion batteries (LiBs) has become a global issue because of the steady increase in their usage every year. In this study, a novel membrane-integrated hybrid system was developed for recycling commercially valuable metals while treating acidic leach liquor obtained from spent LiBs (mostly LiFePO4 type). Alkaline (NaOH and NH4OH)-pretreated leach liquor (pH adjusted to 5.53) was ultrafiltered to ensure the effective removal of Fe and Al and reduce the turbidity (∼1.6 NTU) prior to processing in a nanofiltration system. The nanofiltration membrane (VNF2) was applied successfully to obtain the rejection values of 92.5% (Ni2+), 94.6% (Co2+), and 95.8% (Mn2+) while permeating > 89.6% of Li+ with 7.5 L/m2·h of flux under optimized conditions of transmembrane pressure (10 bar) and crossflow rate (2.25 m3/h) in the recirculation mode. The fractionation of monovalent ions from bivalent ions was performed in a concentrated mode to enrich the bivalent metal ions, Ni2+, Co2+, and Mn2+ from 0.74, 0.52, and 0.63 g/L to 6.14, 4.59, 5.62 g/L, respectively, at 90% recovery of the feed solution. The Li+ (21.1 g/L) that was contained in the nanofiltrate permeate stream was crystallized into Li2CO3; a purity of 99.5 wt% was obtained at 88.2% recovery using 4 M K2CO3 at an operating temperature of 70 °C. Hence, the proposed novel system can lead to the development of a clean and sustainable process for the recycling of precious metal ions from end-of-life LiBs for reuse on a commercial scale.

Treatment of petroleum refinery wastewater by adsorption using activated carbon fixed bed column with batch recirculation mode

Treatment of petroleum refinery wastewater by adsorption using activated carbon fixed bed column with batch recirculation mode

Intensification of the Process of Drying Fruits and Vegetables in a Recirculating Solar Dryer

To intensify the drying process and refine the modes of drying fruits and vegetables, the authors have developed a recirculating solar dryer with a recuperative heat exchanger for drying fruit and vegetable products with a loading capacity of 30 kg for fresh fruit. The solar drying plant contains a drying chamber equipped with a ventilation system, an autonomous power supply unit (solar panel), a control panel and automatic control systems. The recirculating solar dryer works only with solar energy. In this solar dryer, the kinetics of fruit drying was studied in the drying agent recirculation mode with and without a heat exchanger. It has been established that in the recirculation mode, the drying time of fruits is reduced by 1.5-2 times compared to that in a conventional solar dryer.

Multiple processes modeling and identification for a cleaner supercritical power plant via Grey Wolf Optimizer

Validated mathematical modeling of clean coal power generation technologies is an essential subject of research that has a realized importance in the scientific communities working on the energy production and environment protection. This study presents a complete multiple processes modeling and simulation of a practical cleaner coal supercritical power plant (SCPP). The model covers a wider range of operation than ever published models before, which focus either on startup or on once-through operation. The model in this paper rather embeds the whole journey from recirculation mode during startup process up to the maximum produced power, then to emergency shut-down process in the same model. It has been found that slight adaptations in the once-through model parameters are sufficient to switch from one mode of process to another, which is useful in retaining simplified structure of the model. The fixed parameters of the model have been optimized by modern meta-heuristic technique of Grey Wolf Optimizer (GWO) and compared with Genetic Algorithms (GA). It has been proved that GWO has a sort of superiority over the GA for parameter identification of the SCPP model.

Reverse osmosis and nanofiltration opportunities to concentrate multicomponent mixtures of volatile fatty acids

The application of reverse osmosis (RO) and nanofiltration (NF) to concentrate and/or fractionate volatile fatty acids (VFAs, occurring in acidogenic effluents) was systematically assessed for the first time using three commercial spiral wound modules. A bench scale plant was used to test the polyamide membrane cartridges, namely: AG1812-34D, DK1812-34D and NFS-3B-1812F. The effects of main operational parameters (pH, temperature, applied pressure) were firstly studied in total recirculation mode, using different total and relative VFA compositions. Thereafter, batch concentration tests were carried out (at constant applied pressure, ca. 30 bar) to fully characterize the RO and NF modules. AG1812-34D performed as an RO module and it allowed concentrating target solutions almost 3 times without relevant VFA losses (<4%) in the permeate side. All tested parameters were observed to affect concentration performances. However, typical pH levels occurring in acidogenic effluents allowed to achieve fully satisfactory VFA rejections. DK1812-34D and NFS-3B-1812F performed as NF modules and could represent suitable solutions for VFA fractionation. Separation factors significantly increased by pH rise; in particular, high acetic and propionic acid separation factors over butyric acid were achieved at pH 9, with DK1812-34D module.

Mathematical modelling of active chlorine based UV/Electro-Fenton-like process: Elucidation of reactor engineering implications to guide scaling-up

Electrochemically-driven processes are emerging as competitive solution for environmental remediation. When considering scaling of processes accurate engineering modeling can aid on process engineering design for real application. This work defines step stones on electrochemical reactor engineering understanding and sets guidelines for UV/Electro-Fenton-like process modelling to ease scale-up to pilot plant. The UV/Electro-Fenton-like process considers the use of a flow-by cell with parallel plate electrodes to electrogenerated in situ active chlorine species. The turbulent convection-diffusion equation was solved for mass transport simulations considering for the first time a simplified kinetic model in recirculation mode of operation to remove a model contaminant. Tertiary current distribution was solved to simulate the electrochemical production of HClO. The theoretical analysis evidences a quasi-uniform current distribution in the working electrode, a continuous consumption of chloride, and sustained production of HClO for effective treatment at larger scale system level. Comparisons between numerical and experimental results evidence high predictability of the designed model that can be effectively translated for other electrochemically-driven processes.

A numerical evaluation of a novel recovery fresh air heat pump concept for a generic electric bus.

Since the outbreak of the worldwide COVID-19 pandemic, public transportation networks have faced unprecedented challenges and have looked for practical solutions to address the rising safety concerns. It is deemed that in confined spaces, operating heating units (and cooling) in non-re-circulation mode (i. e., all-fresh air mode) could reduce the airborne transmission of this infectious disease, by reducing the density of the pathogen and exposure time. However, this will expectedly increase the energy demand and reduce the driving range of electric buses. To tackle both the airborne transmission and energy efficiency issues, in this paper a novel recovery heat pump concept, operating in all-fresh air mode, was proposed. The novelty of this concept lies in its potential to be applied to already manufactured/in-service heat pump units as it does not require any additional components or need for redesigning the heating systems. In this concept, the cabin exhaust air is directed to pass through the evaporator of the heat pump system to recover part of the waste heat from the cabin and to improve the efficiency of the system. In this paper, a 0D/1D coupled model of a generic single-deck cabin and a heat pump system was developed in the Simulink environment of MATLAB (R2020b) software. The model was run in two different modes, namely the all-fresh air (as a baseline and a recovery heat pump concepts), and the air re-circulation mode (as a conventional heat pump concept with a 50% re-circulation ratio). The performance of these concepts was investigated to evaluate how an all-fresh air policy could affect the performance of the system, as well as the energy-saving potential of the proposed recovery concept. The performance of the system was studied under different ambient temperatures of −5 °C, 0 °C, and 5 °C, and for low and moderate occupancy levels. Results show that implementing the all-fresh air policy in the recovery and baseline concepts significantly improved the ventilation rate per person by at least 102% and at most 125%, compared to the air-re-circulating heat pump. Moreover, adopting the recovery concept reduced the power demand by at least 8% and at most 11%, compared to the baseline all-fresh air heat pump, for the selected fan and blower flow rates. The presented results in this paper along with the applicability of this concept to in-service mobile heat pumps could make it a feasible, practical, and quick trade-off solution to help the bus operators to protect people and improve the energy efficiency of their service.

SARS in Cars: Carbon Dioxide Levels Provide a Simple Means to Assess Ventilation in Motor Vehicles.

Background:Poorly ventilated enclosed spaces pose a risk for airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other respiratory viruses. Limited information is available on ventilation in motor vehicles under differing driving conditions.Methods:We conducted carbon dioxide measurements to assess ventilation in motor vehicles under varying driving conditions with 2 to 3 vehicle occupants. During routine driving, carbon dioxide produced by the breathing of vehicle occupants was measured inside 5 cars and a van under a variety of driving conditions with or without the ventilation fan on and with windows open or closed. Carbon dioxide readings above 800 parts per million (ppm) were considered an indicator of suboptimal ventilation.Results:Carbon dioxide levels remained below 800 ppm in all vehicles if the ventilation fan was on and/or the windows were open while parked or during city or highway driving. With the ventilation system set on non-recirculation mode, carbon dioxide levels rose above 800 ppm in all vehicles when the fan was off and the windows were closed while parked and during city driving, and in 2 of the 6 vehicles during highway driving. With the ventilation system set on recirculation mode, carbon dioxide rose above 800 ppm within 10 minutes in all vehicles tested.Conclusion:Carbon dioxide measurements could provide a practical and rapid method to assess ventilation in motor vehicles. Simple measures such as opening windows, turning on the fan, and avoiding the recirculation mode greatly improve ventilation.

Improvements in azithromycin recrystallization using ultrasound for size reduction

The primary motive of the current work is to achieve smaller mean particle size with narrow size distribution that can enhance the bioavailability of azithromycin (ARZ), an essential requirement due to its poor water solubility. Recrystallization of ARZ was evaluated using cooling as well as antisolvent crystallization approaches in the presence of ultrasonic irradiation with detailed study into effect of different parameters such as ultrasonic power, time and temperature. Ultrasound assisted antisolvent crystallization at low temperatures (<10℃) yielded best size reduction up to 80% with narrower distribution and also gave better yield of the product, that too within 5 min of sonication. With scale up considerations, recirculation mode of operation was also evaluated which offered promising results for the size reduction. Images captured using optical microscope and SEM revealed a nearly uniform rod/plate-shaped geometry. Increase in amorphous nature of ARZ was confirmed based on XRD analysis. FTIR analysis showed no significant changes in the functional groups when compared to the original sample. Overall, the work demonstrated an improved reprocessing approach based on the use of ultrasound with insights into effect of operating parameters and effect of ultrasound on various characteristics.

Batch sorption and fixed-bed elution for Pd recovery using stable amine-functionalized melamine sponge

Sorption-based techniques have been widely studied for metal recovery. Conventionally, sorption and desorption steps are operated in fixed-bed column system under one-pass mode (FO); however, such sorption requires a large volume of solution passing the bed when the feed concentration is low. In this study, highly stable amine-coated alginate melamine (ML/APG) sponge was prepared and applied in a combined process involving sorption under fixed-bed recirculation mode (FR, simulating batch system) and desorption under FO for Pd(II) concentration. The efficiency was evaluated in terms of both recovery of Pd(II) and utilization of the sponge. Results show that ML/APG sponge is stable in 3 M of nitric acid, sodium hydroxide and sodium chloride after shaking at 300 rpm for 72 h with mass loss less than 0.9%. The maximum sorption capacity obtained by Langmuir model for Pd(II) increases from 9.7 mg g−1 to 134.3 mg g−1 after amine functionalization. The sorption equilibrium could be achieved by a less amount of solution and less time when using FR compared to FO. For example, it only needs 0.7 L of Pd(II) solution with a metal concentration of 20 mg L−1 to achieve the maximum sorption capacity under FR, while the required volume is 1.8 L at flow rate of 1 mL min−1 and more than 5 L at 10 mL min−1 under FO. This means that FR improved the rational use of the sorbent and thus increased sorption efficiency. For elution step, FO only required 40 mL to completely desorb Pd(II) from exhausted sponge, while that needed more than 200 mL under FR. The combined process allows both effective sorption and elution. The reusability of ML/APG sponge and Pd(II) recovery from low-concentration solution were studied by combining FR for sorption step and FO for elution step. Results show that the sorption efficiency maintains more than 96% after 100 min of reaction during 10 cycles. The elution performance is stable with a 100% desorption efficiency by passing 50 mL of acidic thiourea solution. The concentration factor (CF, ratio of metal concentration in the eluate over its concentration in the sorption used for sorption), is above or close to 70 for the first 10 mL-sample and much more than 40 for the second 10 mL-sample.

Air Quality and Comfort Characterisation within an Electric Vehicle Cabin in Heating and Cooling Operations.

This work is aimed at the experimental characterisation of air quality and thermal profile within an electric vehicle cabin, measuring at the same time the HVAC system energy consumption. Pollutant concentrations in the vehicle cabin are measured by means of a low-cost system of sensors. The effects of the HVAC system configuration, such as fresh-air and recirculation mode, on cabin air quality, are discussed. It is shown that the PM concentrations observed in recirculation mode are lower than those in fresh-air mode, while VOC concentrations are generally higher in recirculation than in fresh-air mode. The energy consumption is compared in different configurations of the HVAC system. The novelty of this work is the combined measurement of important comfort parameters such as air temperature distribution and air quality within the vehicle, together with the real time energy consumption of the HVAC system. A wider concept of comfort is enabled, based on the use of low-cost sensors in the automotive field.

Optimization-based modeling and economic comparison of membrane distillation configurations for application in shale gas produced water treatment

Membrane distillation (MD) is an emerging membrane technology with great potential for treatment of hypersaline wastewater generated by unconventional (shale) oil and gas reservoirs. However, the low energy efficiency of this technology makes the operating cost of MD systems relatively high, especially in the absence of waste heat. There are several MD configurations with inherent advantages and disadvantages and varying performance. As such, there is a need for thermo-economic optimization of MD systems in a systematic manner to assess their economic performance. We present an optimization framework to model and compare the performance of six MD configurations (DCMD, AGMD, PGMD, CGMD, SGMD, and VMD) in continuous recirculation mode for treatment of hypersaline wastewater. The optimization results show that AGMD with small gap size operated at low stream Reynolds number outperforms all other configurations with treatment cost of 4.57 US $/m3 of feed. However, restricting the system design to more practically relevant operating conditions, such as higher Reynolds number and larger gap size, diminishes the cost superiority of AGMD over other configurations. We also observed that treatment cost using PGMD configuration approaches those of CGMD and DCMD, particularly when modules with small gaps are used.

The effect of built-in and portable ionizers on in-cabin ozone concentrations in light-duty vehicles.

This paper investigates the effects of ionizers on the ozone concentration within vehicle cabins by using a series of measurements combined with a kinetic box model. Testing consisted of measuring ozone concentration during static tests where the ventilation of the test vehicle was turned on and off depending on the test. This testing was repeated for three different portable ionizers and two vehicles with built-in ionizers. Ionizer A produced ozone at a rate of ∼0.04 ppb s-1 (∼0.68 mg h-1), which increased the in-cabin O3 concentrations of a Mitsubishi Mirage to ∼10 ppb with the fan off and ∼6 ppb in the recirculation mode. In the fresh air mode, in-cabin O3 concentrations were dominated by outdoor-to-indoor transport. Ionizer B and C produced O3 at a rate of less than 0.008 ppb s-1 (<0.14 mg h-1); however, during retesting, ionizer C was shown to emit large amounts of ozone for short amounts of time while being tested up close. The same testing was completed on vehicles with built-in ionizers; these produced <0.01 ppb s-1 (<0.32 mg h-1 in the Buick Enclave and <0.25 mg h-1 in the Hyundai Genesis), and in-cabin O3 concentrations were again dominated by outdoor-to-indoor transport with fresh air ventilation. While ionizers are currently regulated, the negative impact they have on in cabin air quality is important to continue monitoring.