HONO Production from Gypsum Surfaces Following Exposure to NO₂ and HNO₃: Roles of Relative Humidity and Light Source

Abstract

Nitrous acid (HONO) is a toxic household pollutant and a major source of indoor OH radicals. The high surface-to-volume ratio and diverse lighting conditions make the indoor photochemistry of HONO complex. This study demonstrates surface uptake of NO2 and gaseous HNO3 followed by gas-phase HONO generation on gypsum surfaces, model system for drywall, under reaction conditions appropriate for an indoor air environment. Tens of parts per billion of steady-state HONO are detected under these experimental conditions. Mechanistic insight into this heterogeneous photochemistry is obtained by exploring the roles of material compositions, relative humidities, and light sources. NO2 and HNO3 are adsorbed onto drywall surfaces, which can generate HONO under illumination and under dark conditions. Photoenhanced HONO generation is observed for illumination with a solar simulator as well as with the common indoor light sources such as compact fluorescence light and incandescent light bulbs. Incandescent light sources release more HONO and NO2 near the light source compared to the solar radiation. Overall, HONO production on the gypsum surface increases with the increase of RH up to 70% relative humidity; above that, the gaseous HONO level decreases due to surface loss. Heterogeneous hydrolysis of NO2 is predicted to be the dominant HONO generation channel, where NO2 is produced through the photolysis of surface-adsorbed nitrates. This hydrolysis reaction predominantly occurs in the first layer of surface-adsorbed water.

Nitrous Acid (HONO) Formation from the Irradiation of Aqueous Nitrate Solutions in the Presence of Marine Chromophoric Dissolved Organic Matter: Comparison to Other Organic Photosensitizers

“…The simultaneous and quantitative measurements of gaseous HONO and NO 2 that formed in these studies were accomplished using an in-house designed and constructed incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS). , A simple schematic of the instrument is shown in Figure . The spectrometer consists of a 75 cm long cavity made of polytetrafluoroethylene (PTFE) with an inner diameter of 2.5 cm.…”

“…The reflectivity and effective path length are determined by comparing the loss of light due to Rayleigh scattering by He and N 2 ; this calculation can be found in the Supporting Information. To validate the instrument, the r 2 value of the actual NO 2 concentration versus instrument derived NO 2 concentration in the range from 50 to 125 ppb was determined to be 0.96 …”

“…The spectra were averaged 10 times and were integrated for 20 s, giving information every 200 s. The data were deconvoluted and analyzed using the Differential Optical Absorption Spectroscopy Intelligent System (DOASIS) software . The software fits the experimental data and separates the contribution from different components of the gas mixtures from the transmittance into the absorption due to HONO and NO 2 by comparing them to their respective cross section reference spectra, and the rest is fit to a polynomial. , More information on this can be found in a previously published paper …”

“…Trace chemical oxidizers like nitrous acid (HONO) are highly reactive and, as such, play a large role in the chemistry of the atmosphere. Reactions that lead to nitrous acid formation have been studied extensively for over the past 30 years due to the rapid photolysis of HONO into OH and NO at wavelengths between 300 and 400 nm, which falls within the solar spectrum as well as some indoor light sources, relevant to photochemistry in indoor environments. − It has been reported that, in some conditions, HONO photolysis can be the major source of OH formed outdoors . Understanding sources of OH is important as it is the most significant oxidant in the atmosphere and contributes to the oxidation of volatile organic carbons (VOCs) to form secondary organic aerosols. , …”

The Positive Effect of Formaldehyde on the Photocatalytic Renoxification of Nitrate on TiO<sub>2</sub> Particles

“…Under humid conditions, generated NO 2 (ads) continued to react with H 2 O adsorbed on the surface to form HONO(ads). HONO was desorbed from the surface and released into the gas phase (Zhou et al, 2003;Bao et al, 2018;Pandit et al, 2021), providing gaseous HONO to the reaction system. Because the NO x concentration remained high, the effect of HONO on NO x analyzer results was negligible (Shi et al, 2021a).…”