Global Model for Depth‐Dependent Carbonyl Photochemical Production Rates in Seawater

Abstract

A photochemical model was used to quantify the global contribution of carbonyl photoproduction in the photodegradation of marine dissolved organic carbon (DOC). As model input, wavelength-and temperature-dependent apparent quantum yields (AQYs) for the photochemical production of carbonyl compounds were determined in seawater collected from the Northwest Atlantic Ocean. These AQY data and published AQY data from the North Pacific were used with remotely sensed seawater optical properties and solar irradiance data in a global model to calculate depth-resolved, mixed-layer photochemical fluxes of acetaldehyde and glyoxal in seawater. Based on this model, the annual global surface mixed-layer photochemical production is 89.7 ± 36 Tg year −1 for acetaldehyde and 20.0 ± 8.0 Tg year −1 for glyoxal. This work significantly improves our understanding of the impact of photochemistry on the cycling of DOC in the surface oceans. Low-molecular-weight carbonyl compounds represent the second largest carbon flux among all known carbon products that are produced during the photolysis of DOC. The annual photoproduction of carbonyl-compound carbon is~110 ± 23 Tg C year −1 , comprising approximately 9.6% of the total carbon and 22% of the biologically labile carbon that are produced globally from the photolysis of marine DOC.Plain Language Summary Ultraviolet and visible solar irradiation absorbed by dissolved organic matter in seawater causes a myriad of chemical transformations including the photochemical production of atmospherically important and biologically labile organic compounds. In the current study, we determined the photochemical production of acetaldehyde, glyoxal, and methylglyoxal, in the surface oceans. A global-scale seasonal model of surface and depth-dependent photochemical production rates was developed for acetaldehyde and glyoxal using wavelength-and temperature-dependent photoproduction data determined in open ocean, oligotrophic seawater from the North Pacific and Northwest Atlantic Oceans, along with modeled solar irradiance and satellite-retrieved optical properties of seawater. Methylglyoxal was not modeled because its wavelength-dependent photochemical production could not be determined in open ocean waters. Results of this study were used to show that the photochemical production of carbonyl compounds represents an important fraction of marine organic matter that is photochemically degraded on an annual basis. Key Points:• Wavelength and temperature dependence for carbonyl photoproduction was determined in seawater from four stations in the NW Atlantic Ocean • Depth-integrated, surface mixed-layer rates of acetaldehyde and glyoxal photochemical production were calculated globally • Carbonyl compounds represent 9.6% and 22% of the total and biologically labile carbon produced from the photolysis of marine organic carbon Supporting Information:• Supporting Information S1

Photochemical Production and Photolysis of Acrylate in Seawater

“…The photon exposure in our sunlight-exposure experiments ranged between 161−581 μmol quanta cm −2 , with all except one sample exposed to less than ∼400 μmol quanta cm −2 (Table S1). We have previously shown for carbonyl compounds 46 that photobleaching was only an issue for extended photon exposures when the quantum yields for carbonyl photoproduction decreased (i.e., reciprocity was no longer obeyed). The main uncertainties in our results, and indeed that for anyone who studies photoprocesses in natural waters, are the unknown photobleaching history of samples used in the photochemical experiments and whether CDOM absorption can be used as a proxy to follow/predict the rate of a photochemical reaction in seawater.…”

New Perspectives on the Marine Carbon Cycle–The Marine Dissolved Organic Matter Reactivity Continuum

“…Using carbon monoxide as an example, they estimated that surface ocean marine carbon monoxide fluxes amounted to 41 Tg C yr −1 , in line with previous estimates of 30 to 84 Tg C yr −1 . 57,58 More recently, this approach was used with temperature-dependent AQY spectra for acetaldehyde and glyoxal to estimate their annual surface ocean production, 59 and it was found that photoproduction of carbonyl carbon (i.e., glyoxal, methylglyoxal, formaldehyde, acetaldehyde) amounts to 110 Tg C yr −1 . Since these LMW compounds (and CO) are known biological substrates 60 or lost to the atmosphere, their production should also be considered a sink for marine DOM.…”

Airborne glyoxal measurements in the marine and continental atmosphere: Comparison with TROPOMI observations and EMAC simulations

“…However, their contribution to the atmospheric glyoxal budget is poorly constrained. For instance, acetaldehyde is largely produced in seawater (Zhu and Kieber, 2020) and a net flux to the atmosphere is expected (Zhu and Kieber, 2019). Estimates of the global oceanic source of acetaldehyde range from 34 to 57 Tg yr −1 (Millet et al, 2010;Wang et al, 2019).…”