Marine Algae Are an Overlooked Coastal Source of Nitrous Acid

Nitrous acid (HONO) is a key precursor of hydroxyl radicals (OH), the primary atmospheric oxidant, yet its sources in marine and coastal environments have long remained enigmatic. Writing in Science Advances, Shen et al. identify abiotic photodecomposition of marine algae as a previously unrecognised HONO source. During Ulva prolifera green tides in coastal Qingdao, North China, daytime HONO concentrations closely tracked tidal cycles, peaking at low tide when algae were exposed to sunlight. Controlled chamber experiments confirmed that five representative algal species emit HONO under irradiation, with fluxes comparable to soil emissions and exceeding marine nitric oxide (NO) fluxes by 2–3 orders of magnitude. Incorporating this source into atmospheric models substantially enhanced OH and ozone production and accelerated the oxidation of climate-relevant gases such as methane (CH4) and dimethyl sulfide (DMS). With intensifying global algal blooms driven by eutrophication and climate warming, this ocean–atmosphere nitrogen transfer pathway is expected to become increasingly important.

Reference: Shen, H., Zhong, X., Zhu, Y., et al. (2026). HONO emission from marine algae. Sci. Adv., 12, eaeb8486. https://doi.org/10.1126/sciadv.aeb8486