Ventilation in a Residential Building Brings Outdoor NO x Indoors with Limited Implications for VOC Oxidation from NO 3 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 (NO + NO) 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, ozone (O), and volatile organic compounds (VOCs), we modeled the effect of the outdoor-to-indoor ventilation of NO pollution on the production of nitrate radical (NO), a potentially important indoor oxidant. We evaluated how VOC oxidation chemistry was affected by NO during NO pollution events compared to background conditions. We found that nitric oxide (NO) pollution introduced indoors titrated O and inhibited the modeled production of NO. NO ventilated indoors also likely ceased most gas-phase VOC oxidation chemistry during plume events. Only through the artificial introduction of O to the ventilation duct during a NO pollution event (i.e., when O and NO concentrations were high relative to typical conditions) were we able to measure NO-initiated VOC oxidation products, indicating that NO was impacting VOC oxidation chemistry.