An evaluation of oceanic emissions of CH4 and N2O

Abstract

Methane (CH4) and nitrous oxide (N2O) are important greenhouse gases in the troposphere. Ocean environments appear to be an important natural source of atmospheric N2O and a minor source of CH4. However, their emission rates of the ocean show large range due likely to heterogeneous production rates in different water column. A series of IPCC reports describe oceanic emission rate of CH4 as 5 &#8211

25 Tg (CH4) yr-1. However, basin-wide measurements provides utmost ~3 Tg (CH4) yr-1. Regarding N2O, ocean emits at 1 &#8211

7 TgN yr-1, while much higher source strength of 7 &#8211

11 TgN yr-1 has been reported. To provide more concrete source strength of the ocean for these two gases, we steamed the Atlantic in 1998 and measured their concentrations in the surface mixed layer and the overlying air from 50°N to 50 °S. The cruise covers the open ocean, coastal regions, the west Africa upwelling region, the equatorial upwelling zone, and the Rio de la Plata estuarine region. Throughout the cruise track, CH4 and N2O were saturated or supersaturated in the surface waters. Dissolved CH4 was highly supersaturated in the Rio de la Plata estuarine region. The coastal regions are significant CH4 source, but no increase of CH4 concentration was observed in the equatorial upwelling zone. Dissolved N2O concentrations were highly supersaturated in the west Africa upwelling region and the equatorial upwelling zone, which indicates that the upwelled water delivered a large amount of N2O from below the surface mixed layer. Based on the measurements of CH4 and N2O in the surface water and the overlying air, I estimated emission rate from the ocean as 0.3 &#8211

1.9 Tg(CH4) yr-1 for CH4 and 0.5 &#8211

2.6 TgN yr-1 for N2O. The emission rate of CH4 is far lower than the values in IPCC reports, but is comparable to the value estimated by other basin-wide measurements. Our estimate of N2O emission is at the lower bound of the range in IPCC report.