Marine sources of formaldehyde in the coastal atmosphere

Elevated concentrations of formaldehyde and other carbonyl compounds are frequently observed in the marine atmosphere but are often significantly underestimated by atmospheric models. To evaluate the potential impact of marine sources on atmospheric formaldehyde, high-resolution measurements were conducted at a coastal site (∼15 m from the sea) during the summer in Qingdao, China. Observed formaldehyde levels averaged 2.4 ± 0.9 ppbv (1 ppbv = 10^-9 L L^-1), with peaks reaching 6.8 ppbv. Backward trajectories indicate that formaldehyde concentrations remained high in marine air masses. Formaldehyde exhibited weak correlations with primary pollutants such as NO and CO but showed strong correlations with marine tracers, notably methyl ethyl ketone and 1-butene. Chamber experiments confirmed that the photodecomposition of Enteromorpha released large amounts of formaldehyde and marine tracer species. When normalized to acetylene, the levels of formaldehyde, 1-butene, and MEK increased by factors of 3.8, 8.1, and 3.5, respectively. Results from an observation-based chemical box model simulation, which utilizes the Master Chemical Mechanism (MCM), revealed that formaldehyde contributes 56% to the primary source of HO₂ radicals, while neglecting formaldehyde chemistry would lead to a 15% reduction in coastal ozone production rates. This study interlinks oceanic biology and atmospheric chemistry, advancing the understanding of the ocean's role as a significant source of organic compounds and its contribution to carbon cycling.