Elevated Outdoor O3 Can Sustain New Particle Formation and Growth Indoors

Ultrafine particles and submicron aerosol mass negatively impact human health; while indoor primary sources are known, secondary aerosol formation from oxidation of volatile organic compounds (VOCs) is an important, but less understood, exposure source. Previous studies have suggested that indoor new particle formation (NPF) and growth are ozone-limited. However, we find that in an authentic aged unoccupied office in a region with high outdoor O3 concentrations, sufficient O3 is transported indoors that NPF and growth are instead limited by the availability of VOCs. Mechanical ventilation continuously supplies O3 in the summertime and sustains the subsequent growth so that particle growth rates upon the addition of limonene range from 20 to 100 nm h-1─rates 1-2 orders of magnitude greater than those observed outdoors. Additionally, we observe meaningful mass contributions from the growth of newly formed particles. Household use of limonene-containing products in rooms with elevated ozone─either from ozone-producing remediation and germicidal UV technologies, or during outdoor ozone exceedance events─will produce ultrafine particles and increase particle mass. The effect of ventilated O3 from outdoors on particle formation and growth highlights the value of continued efforts to reduce outdoor O3 pollution for moderating both indoor and outdoor sources of secondary pollutants.