The sources and sinks of isocyanic acid (HNCO), a toxic gas, are largely uncharacterized in indoor environments. In particular, cigarette smoke has been identified to be a significant source of HNCO and possible precursor compounds. Identification of indoor sources and conditions that lead to elevated HNCO are necessary to understand the health risk for occupants. In this study, controlled smoking of tobacco cigarettes was investigated both in a residence in Toronto, Canada and in an environmental chamber using online chemical ionization mass spectrometry. Fast measurements, inside and outside of the Toronto residence, allowed for the determination of the emission rates of HNCO from cigarette smoke, the background conditions in the house, and comparison to outdoor concentrations. Chamber measurements explored possible indoor-relevant reactions - oxidation by ozone, hydroxyl radical (OH), and photo-reaction under indoor fluorescent light - that could lead to secondary production of HNCO from cigarette smoke precursors. The HNCO emission ratio from side-stream cigarette smoke was determined, from both the residence and chamber studies, to be in a similar range to emission ratios from previously reported biomass burning smoke sources. Side-stream smoke from a single cigarette introduced a large pulse of HNCO to the indoor environment, increasing the HNCO mixing ratio by up to a factor of ten above background conditions of 0.15 ppb. Even in the absence of smoking, the indoor HNCO mixing ratio in the Toronto residence was elevated compared to concurrent outdoor measurements by approximately a factor of two. Oxidation of the cigarette smoke by OH was the only investigated process to produce secondary HNCO. After 30 minutes of oxidation by OH (1.1 x 10<sup>7</sup> molecules per cm<sup>3</sup>), the HNCO mixing ratio was observed to double and continued to increase with further oxidation. Loss of HNCO by partitioning or reaction at indoor surfaces was identified to be significant, in addition to mixing within the house and ventilation to outdoors.

Sources of isocyanic acid (HNCO) Indoors: A focus on cigarette smoke

ACS Fall 2020 Virtual Meeting

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