Continuous Measurements of Greenhouse Gases and Atmospheric Oxygen in the Namib Desert

A new, near-coastal background site was established for observations of greenhouse gases (GHGs) and atmospheric oxygen in the central Namib Desert near Gobabeb, Namibia. The location of the site was chosen to provide observations in a data-poor region in the global sampling network for GHGs. Semi-automated, continuous measurements of carbon dioxide, methane, nitrous oxide, carbon monoxide, atmospheric oxygen, and basic meteorology are made at a height of 21 m a.g.l., 50 km from the coast at the northern border of the Namib Sand Sea. Atmospheric oxygen is measured with a differential fuel cell analyzer. Carbon dioxide and methane are measured with an early-model cavity ring-down spectrometer; nitrous oxide and carbon monoxide are measured with an off-axis integrated cavity output spectrometer. Instrument-specific water corrections are employed for both instruments in lieu of drying. The representativity of the site was assessed within the context of atmospheric transport. During austral summer, strong equatorward winds are present as a result of the Hadley circulation. This brings marine boundary layer air inland to Gobabeb. In austral winter, the descending branch of the southern Hadley cell is at the same latitude as NDAO, which encourages the establishment of anticyclonic conditions over southern Africa. The variability of air mass history during this time of year is quite high, alternating between marine and terrestrial air masses, as well as air that was recently in contact with the surface and air that had descended from heights greater than 2 km. NOAA ask samples taken at Gobabeb from 1996 to the present appeared to respond to these seasonal patterns in atmospheric dynamics, when compared to other marine background sites at the same latitude as NDAO. Two years of data are presented from the observatory. Diurnal variability was noted at times for all species, particularly for atmospheric oxygen. Through stoichiometry and phasing, this was attributed primarily to the local wind system, which features a prominent sea breeze, and daily boundary layer oscillations. Large anomalies in carbon monoxide and methane were observed in the time series on a synoptic time scale, during the ascending portion of the seasonal cycle. These were attributed to an alternation between polluted air masses from the continental interior and marine boundary layer air. The continental air masses were progressively in uenced by biomass burning as the fire season developed. The concentration of re activity close to the station increased throughout the year, peaking in September, a fact re ected in the enhancement ratio of CH4 to CO. During such synoptic events the molar exchange ratio of O2 to CO2 also supported this interpretation. Finally, the NDAO time series was used to make top-down estimates of air-sea fl uxes of the main measurands from the Lüderitz and Walvis Bay upwelling cells in the Benguela Current region, during upwelling events. Flux densities were evaluated using shipboard measurements within the study area, showing good agreement with the top-down estimates. Average flux densities for CO2 were 0.450.4 µmol m-2 sec-1, -3.92.6 µmol m-2 sec-1 for O2, 6.05.0 nmol m-2 sec-1 for CH4, 0.50.4 nmol m-2 sec-1 for N2O, and 2.71.7 nmol m-2 sec-1 for CO. N2O uxes were fairly low, in accord with previous work, suggesting that the evasion of this gas from the Benguela is smaller than in other upwelling systems. Conversely, methane release was very high for the marine environment, which adds to mounting evidence of a large sedimentary source of methane in the Walvis Bay area. Carbon dioxide and oxygen uxes were substantial and probably not accounted for in current budgets.


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