Global oceanic emissions of nitrous oxide

Abstract

The global N2O flux from the ocean to the atmosphere is calculated based on more than 60,000 expedition measurements of the N2O anomaly in surface water. The expedition data are extrapolated globally and coupled to daily air‐sea gas transfer coefficients modeled at 2.8°×2.8° resolution to estimate a global ocean source of about 4 (1.2–6.8) Tg N yr−1. The wide range of uncertainty in the source estimate arises mainly from uncertainties in the air‐sea gas transfer coefficients and in the global extrapolation of the summertime‐biased surface N2O data set. The strongest source is predicted from the 40–60°S latitude band. Strong emissions also are predicted from the northern Pacific Ocean, the equatorial upwelling zone, and coastal upwelling zones occurring predominantly in the tropical northern hemisphere. High apparent oxygen utilization (AOU) at 100 m below the mixed layer is found to be correlated positively both to N2O production at depth and to the surface N2O anomaly. On the basis of these correlations, the expedition data are partitioned into two subsets associated with high and low AOU at depth. The zonally averaged monthly means in each subset are extrapolated to produce two latitude‐by‐month matrices in which monthly surface N2O is expressed as the deviation from the annual mean. Both matrices contain large uncertainties. The low‐AOU matrix, which mainly includes surface N2O data from the North Atlantic and the subtropical gyres, suggests many regions with positive summer deviations and negative winter deviations, consistent with a seasonal cycle predominantly driven by seasonal heating and cooling of the surface ocean. The high‐AOU subset, which includes the regions most important to the global N2O ocean source, suggests some regions with positive winter deviations and negative summer deviations, consistent with a seasonal cycle predominantly driven by wintertime mixing of surface water with N2O‐rich deep water. Coupled seasonal changes in gas transfer coefficients and surface N2O in these important source regions could strongly influence the global ocean source.

A global database of sea surface dimethylsulfide (DMS) measurements and a procedure to predict sea surface DMS as a function of latitude, longitude, and month

“…Thus, at least for the North Sea, bacteria and zooplankton seem to play a subordinate role in governing the DMS concentration in the water column. Given the complex situation described in the previous paragraphs, the task of making maps of DMS concentration seems difficult, but there is a precedent for mapping other biogeochemically relevant species in the ocean [Conkright et al, 1994;Nevison et al, 1995]. To make any map based on geophysical data, one needs point measurements and a scheme to extrapolate the measurements to a gridded field, in this case, the globe.…”

The role of the oceans in climate

“…The oceans are also a significant source of other important long-lived greenhouse gases, particularly CH 4 and N 2 O (Nevison et al, 1995;Bates et al, 1996;Lelieveld et al, 1998;Seitzinger and Kroeze, 1998;UpstillGoddard et al, 2000). Both gases are produced biologically within the oceans.…”

The FuGas 2.3 Framework for Atmosphere–Ocean Coupling: Comparing Algorithms for the Estimation of Solubilities and Gas Fluxes

“…Higher divergences reached +4.5% for CO 2 solubility in cooler marine waters, +5.65% for CH 4 solubility in warmer saltier waters, and −2.1% for N 2 O solubility in brackish waters. Hence, the algorithms diverge by as much as 0.045 mol·mol −1 of CO 2 , 0.0015 mol·mol −1 of CH 4 and 0.012 mol·mol −1 of N 2 O (i.e., mol of gas in the ocean surface per mol of gas in the atmosphere) in some of the most sensitive situations for Earth System Modelling and satellite data processing, namely: (i) the cooler marine sub-polar waters where the solubility pump traps greenhouse gases and carries them to the deep ocean [2], and (ii) the warmer waters at the coastal ocean and seas, which have regularly been observed having greenhouse gases and DMS dissolved in concentrations highly unbalanced with those of the atmosphere [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”

“…Nevertheless, differences from 10% to −5% for CO 2 , 11% to 5% for CH 4 , and 7% to −5% for N 2 O were observed in freshwaters. Determining which formulation provides more accurate estimates of greenhouse gas (GHG) solubilities in freshwater is fundamental for Earth System Modelling, because rivers and freshwater reservoirs are important sources of GHG to the atmosphere, even if only seasonably [7][8][9][10][11][12][13][14]16]. In marine waters, our simulations found differences of up to 12% for CO 2 , 9% for CH 4 and 7% for N 2 O, suggesting that for marine waters is also important to determine which formulation provides more accurate estimates of GHG solubilities.…”

“…On the other hand, at the surface of coastal waters the balances and fluxes of CO 2 , CH 4 , N 2 O and DMS are very heterogenic, mostly due to factors like upwelling, plankton productivity, and land-originated loads of both natural and anthropogenic cause. Consequently, besides DMS [3,4] the coastal ocean can be, at least seasonally, also a source of CO 2 [1,2,[5][6][7][8][9][10][11], CH 4 [2,10,12], and N 2 O [2,10,[13][14][15] to the atmosphere. Thus, the uncertainty on the atmosphere-coastal ocean exchange of CO 2 begs the question about the overall budget of the global ocean [1,3,6,16].…”