Heterogeneous Reactions of Nitrogen Dioxide and Guaiacol on Indoor Surface Materials: Surface Adsorbed Nitro-phenolic Compound Formation and Gas-Phase HONO Yields

Abstract

Indoor air quality and indoor environments have become important topics in relation to human health. The role of heterogeneous surface reactions is recognized to play an important role in indoor air chemistry. HONO is considered a household pollutant due to its nearly 5-fold increased concentration indoors compared to outdoors. It is well recognized that the heterogeneous hydrolysis of nitrogen dioxide (NO2) on indoor surfaces results in the production of HONO in the gas phase. Despite this knowledge, there are several factors that need to be further understood, including the chemical and physical characteristics of building material surfaces and their coatings on HONO formation. Wildfires near urban locations are occurring with increasing frequency and intensity and chemicals present in wildfire smoke can adsorb and react on indoor surfaces from smoke infiltration into indoor environments. This current study aims to investigate the effects of guaiacol, an organic compound found in wildfire smoke, on gas-phase HONO concentrations in indoor environments and to better understand surface reactions of guaiacol and NO2. Surfaces of different building materials were exposed to guaiacol, including kaolinite, titanium dioxide, and gypsum. The impact of guaiacol on HONO formation was monitored via cavity enhanced absorption spectroscopy. Our results show that the effect of guaiacol on gas-phase HONO yields varies depending on the specific surface material. Guaiacol-exposed kaolinite surface films show more than 3 times increase in HONO concentrations compared to kaolinite alone, whereas guaiacol-exposed titanium dioxide had 1.7 times increase in HONO. Guaiacol-exposed gypsum surface films showed no effect on HONO concentrations relative to bare gypsum surface films. We also characterized new surface products that show that NO2 can react with adsorbed guaiacol to produce nitro-phenolic compounds, which are known to be toxic. Overall, we have identified some considerations of the role of specific surface materials in indoor chemistry relevant to HONO and how this research may be used in the construction and design of indoor environments to improve indoor air quality.