The cycle of atmospheric nitrous oxide

Abstract

An attempt has been made to construct an N 2 O balance of the troposphere in order to demonstrate the state of knowledge of the cycle of atmospheric N 2 O. This balance has been constructed considering the parameters relevant for atmospheric N 2 O as far as known such as the horizontal and vertical distribution in the atmosphere and in the oceans, the variation of the atmospheric mixing ratio with time, the fluxes in the air-sea and in the air-soil systems, and man-made fluxes. The total atmospheric burden of N 2 O appears to be in the range 1600-2100 Mt, the tropospheric turn-over time between 4 and 12 years. Most of the atmospheric N 2 O seems to originate from microbial activities at the Earth’s surface with the oceans apparently being the major source with about 35 % of the total source as obtained from the tropospheric N 2 O mass and turn-over time (range of uncertainty 5-100 %) followed by soil and fresh water with about 25 % of the total source (range of uncertainty 4-100 %) including the contribution of industrial nitrogen fertilizers with about 10 % of the total source (range of uncertainty 2-24 %). Direct man-made N 2 O sources seem to be relatively small with about 2 % of the total source (range of uncertainty 0.5-5 %), but may grow considerably in the future. The N 2 O production by chemical reactions in the troposphere itself (e.g. electrical discharges) is not known; a figure of between 3 and 70 % of the total source has been inferred from laboratory experiments. The sinks of atmospheric N 2 O are poorly known. The only known major N 2 O destruction appears to occur in the stratosphere. The rate of transport into the stratosphere, however, limits this sink to about 10 % of the total amount of N 2 O to be destroyed under steady state conditions (range of uncertainty 3-22 %). The major problem of the cycle of atmospheric N 2 O is the unknown main sink. Since the stratosphere does not seem to come into question, the missing sink should be found somewhere in the troposphere, pedosphere, and/or hydrosphere.