Ventilation in a Residential Building Brings Outdoor NO_x Indoors with Limited Implications for VOC Oxidation from NO₃ Radicals

Energy-efficient residential building standards require the use of mechanical ventilation systems that replace indoor air with outdoor air. Transient outdoor pollution events can be transported indoors via the mechanical ventilation system and other outdoor air entry pathways and impact indoor air chemistry. In the spring of 2022, we observed elevated levels of NO_x (NO + NO₂) that originated outdoors, entering the National Institute of Standards and Technology (NIST) Net-Zero Energy Residential Test Facility through the mechanical ventilation system. Using measurements of NO_x, ozone (O₃), and volatile organic compounds (VOCs), we modeled the effect of the outdoor-to-indoor ventilation of NO_x pollution on the production of nitrate radical (NO₃), a potentially important indoor oxidant. We evaluated how VOC oxidation chemistry was affected by NO₃ during NO_x pollution events compared to background conditions. We found that nitric oxide (NO) pollution introduced indoors titrated O₃ and inhibited the modeled production of NO₃. NO ventilated indoors also likely ceased most gas-phase VOC oxidation chemistry during plume events. Only through the artificial introduction of O₃ to the ventilation duct during a NO_x pollution event (i.e., when O₃ and NO₂ concentrations were high relative to typical conditions) were we able to measure NO₃-initiated VOC oxidation products, indicating that NO₃ was impacting VOC oxidation chemistry.