HONO Emission from Marine Algae

Nitrous acid (HONO) is a key precursor to hydroxyl radicals (OH) and a reservoir of reactive nitrogen, yet the processes sustaining elevated daytime HONO in marine and coastal environments remain poorly understood. We combine coastal field observations with irradiation chamber experiments and atmospheric modeling to identify abiotic photodecomposition of marine algae as a previously unrecognized HONO source. During Ulva prolifera green tides, daytime HONO levels in the coastal atmosphere closely followed tidal cycles and peaked at low tide, in contrast to typical inland nocturnal peaks. Chamber experiments confirm that common algae (Ulva prolifera, Bryopsis plumosa, Chaetomorpha spiralis Okam., Sargassum, and Silvetia siliquosa) directly emit HONO under irradiation, with fluxes increasing with light intensity and algal surface area. Measured HONO fluxes of 1.08 × 10^–7 to 2.31 × 10^–6 mol m^–2 h^–1are comparable to reported soil HONO emissions and exceed marine NO fluxes by 2 to 3 orders of magnitude. Incorporating this source into an atmospheric model increases HONO concentrations, enhancing OH and ozone production and accelerating the oxidative loss of dimethyl sulfide and methane. As eutrophication and warming intensify algal blooms worldwide, algal photodecomposition is likely to become an increasingly important driver of coastal reactive nitrogen emissions and oxidation capacity.