Revisiting HONO Formation Mechanism by NO₂ Conversion on Mineral Dust Surface

Mineral dust particles are omnipresent in the atmosphere all over the globe. Nitrogen dioxide (NO2) can be adsorbed on the dust surface and converted to nitrous acid (HONO), which in turn represents one of the most important sources of hydroxyl radicals (OH) driving the oxidation capacity of the atmosphere. Here, we evaluate the conversion of NO2 to HONO on mineral dust samples from different regions of the world. We reveal that the synergistic effects of relative humidity (RH), UV-light, titanium dioxide (TiO2), and microbes present on the mineral dust surface are responsible for the observed high HONO yields. The light-induced uptake coefficients of NO2 on mineral dust surface are 1 order of magnitude higher than the uptakes measured in the dark. Intriguingly, the uptakes of NO2 are higher in the absence of water vapor; however, the HONO yields increase with the increase of RH (0–90%), the NO2 concentration (10–50 ppb), and the light intensity (19–50.4 W m–2). Our findings demonstrate that mineral dust contributes to atmospheric HONO through light- and RH-dependent processes with high HONO yields (up to 80.3%) under realistic conditions. Global models must account for both uptake coefficients and HONO yields to accurately quantify this source, particularly in dust-prone regions.