Carbon monoxide emission from a Mauritanian upwelling filament

During a Lagrangian study in the Mauritanian upwelling (April 15 to May 27 2009), we investigated the biogeochemical cycling of carbon monoxide (CO) through a series of experiments, field observations and a simple 1-D model. We carried out ten photochemical experiments in a solar simulator and a further dark experiment in order to determine the magnitude of the photochemical source and microbial sink for CO. Parallel irradiation experiments using a long-pass filter (390 nm cutoff) showed that CO photoproduction was dominated by UV-light. Assuming that the apparent quantum yield (AQY) of CO photoproduction followed an exponential decrease with increasing wavelength, we applied a non-linear fit to derive the AQY for each of 10 experiments. The average AQY at 350 nm from our irradiation experiments was 1.4 × 10− 5 mol CO produced per mol photons absorbed. One of our photochemical experiments showed a distinct dissolved organic matter absorption ‘shoulder’ in the UV-B, consistent with the presence of mycosporine-like amino acids. Though this feature was rapidly photobleached during irradiation, CO AQY did not differ from other experiments. During the microbial CO oxidation experiment we found first-order loss of CO with a rate constant of 0.17/h. We further used our experimental data to parameterise a 1-D steady state model which included photochemical and dark production of CO, microbial oxidation, mixing and sea to air loss. Our sea–air flux estimates were in the range of 4.6–9.6 μmol CO per square metre per day. The estimated annual flux of CO to the atmosphere from the Mauritanian upwelling was 0.04 Tg CO yr− 1.