Photochemical Production and Biological Consumption of CO in the SML of Temperate Coastal Waters and Their Implications for Air‐Sea CO Exchange

Abstract

AbstractThe sea surface microlayer (SML), less than 1,000‐μm uppermost layer of the ocean water column, is located at the air‐sea interface and plays critical roles in global biogeochemical cycles and climate change through air‐sea gas exchange. To clarify the significance of the dynamics of carbon monoxide (CO) in the SML, where active photochemical and biological processes are expected, in air‐sea CO exchange, the production and consumption of CO in the SML and its sea‐air emission were investigated in temperate coastal waters. In the SML, the light‐normalized photochemical CO production rate was relatively high from spring to autumn (median: 2.57 nM [kWh m−2]−1) when relatively high absorbance of chromophoric dissolved organic matter (0.55 m−1) was observed. Biological CO consumption rate constant in the SML showed relatively high values from spring to autumn (mean ± standard deviation: 0.060 ± 0.010 h−1) during the period of relatively high water temperature (22.3 ± 2.7 °C). The calculated sea‐air CO flux (F) varied similarly to CO concentration in the subsurface water. Comparison among the production, consumption, and sea‐air emission of CO in the SML suggests that biological consumption in the SML can be ignored in air‐sea CO exchange throughout the year whereas photochemical production in the SML enhances F during summer under intense light, active biological production, and weak wind conditions. Further, seawater warming experiments found the tendency of the stimulation of biological CO consumption by water temperature increase from spring to autumn, which suggests negative but insignificant feedback on global warming.