Experimental and kinetic model evaluation of HONO production from surface nitrate photolysis

Solar irradiation plays a key role in promoting nitrous acid (HONO) production from surface enhanced photochemical reactions such as photolysis of nitrate. Nitrate anions are ubiquitous in ambient particles and on various outdoor and indoor surfaces, with reported surface density from 10−7 to 10−3 mol m−2. In this work, we measured HONO and NO2 production from surface nitrate photolysis under eight narrow bandwidth light sources with peak emission wavelengths ranging from 300 to 605 nm. We defined the probability of HONO and NO2 release per surface nitrate absorbing unit photon density of particular wavelength as “reactive cross section” of surface nitrate. Relative to the absorption cross section of gaseous HNO3, the reactive cross section of surface nitrate was found enhanced by up to 970-fold. The reactive cross section determined in our laboratory decreased from (6.3 ± 0.5) × 10−20 cm2 molecule−1 at 300 nm to (1.0 ± 0.3) × 10−24 cm2 molecule−1 at 605 nm, and showed a nonlinear relationship with the surface density of nitrate. Outdoor HONO production was mostly attributed to solar UV irradiation as expected, while underappreciated indoor HONO accumulation was also feasible considering HONO production induced by visible light and large specific surface area indoors. We then constructed a dynamic model to further evaluate the indoor HONO source in typical indoor environments. Photochemical properties of this HONO source lead to diurnal profiles in indoor HONO accumulation, with daily average HONO accumulation ranging from 1.1 to 17.8 ppb under various conditions. The model calculation confirmed that visible light substantially promotes surface nitrate photolysis and indoor HONO accumulation, with an average daily strength that is 0.28–0.34 mg h−1 at surface nitrate of 10−4 mol m−2 under simulated sunlight illumination. Our results call for direct measurements of indoor HONO in a variety of real indoor environments.