HONO emission from marine algae

Nitrous acid (HONO) is a key precursor to hydroxyl radicals (OH) and a reservoir of reactive nitrogen. Here, we identify abiotic photodecomposition of marine algae as a previously unrecognized HONO source. During Ulva prolifera green tides, daytime HONO levels closely followed tidal cycles, peaking at low tide, contrasting with typical inland nocturnal peaks. Chamber experiments confirm that common algae (e.g., U. prolifera and Sargassum ) emit HONO under irradiation, with fluxes increasing with light intensity and algal surface area. This light-driven, abiotic process is mechanistically distinct from microbial soil HONO production. Measured fluxes (1.08 × 10 −7 to 2.31 × 10 −6 mole per square meter per hour) are comparable to soil HONO emissions and exceed marine NO fluxes by two to three orders of magnitude. Incorporating this source into atmospheric models substantially elevated HONO concentrations, enhancing OH and ozone production and oxidation of climate-relevant gases. With intensifying global algal blooms driven by eutrophication and climate warming, this process is expected to become increasingly important.