Formaldehyde in remote marine air and rain: Flux measurements and estimates

The tropospheric trace constituent formaldehyde, HCHO, was measured in rain and in the gas phase during the wet season at Enewetak Atoll, a remote marine site in the central equatorial Pacific. Rainwater averaged 8 ± 2 µg/kg; the gas phase averaged 0.4 ± 0.2 ppbv (0.5 µg/m³). These values, especially the rain, are among the lowest reported to date. The formaldehyde flux to the sea by rainout and washout extrapolates to 0.010 g m−2 y−1. The gaseous flux into the sea surface is estimated to be 0.05 g m−2 y−1 by an air‐sea exchange calculation that takes into account enhanced uptake by hydroxide‐catalyzed formaldehyde hydration. The measured mixing ratio is close to the 0.18 ppbv prediction of a tropospheric chemistry model calculation. The methane oxidation chain probably is the sole formaldehyde source in the Enewetak area. The total formaldehyde flux as carbon into the ocean is ∼ 2% of the estimated total organic carbon from rainout and washout. About 2‐4% of the calculated column formaldehyde production is removed from the atmosphere by these processes.