Analysis of sources and sinks of atmospheric nitrous oxide (N2O)

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Abstract
There are major unanswered questions about the sources of atmospheric nitrous oxide. Recent assessments of N2O sources are diverging rather than converging. Here I use a simple model to place constraints on the relative sizes of N2O sources in the northern and southern hemispheres (NH and SH). Using measurements of the rate of temporal increase of atmospheric nitrous oxide (N2O) concentrations and observations of their cross‐equator differences, I calculate the total sources of N2O for the (NH and SH). The NH source ΦN must exceed the SH source ΦS; their ratio R (= ΦN/ΦS) is between 1.7 and 2.4 for the case where the NH concentration is 1 ppb higher than that in the SH. When the NH concentration is 0.75 ppb higher, R is between 1.5 and 1.9. These values of R increase by about 50% when more rapid loss to the NH stratosphere is introduced. Slightly wider ranges of R are calculated for certain choices of parameters. For these calculations I have used a two‐box model which permits an array of assumptions to be tested; a range of values is employed for interhemispheric exchange times and for removal times due to stratospheric destruction of N2O. In addition to the common case of no uptake of atmospheric N2O by soils, I have tested the model sensitivity to two hypothesized nonzero soil sinks. Relatively small soil sinks would decrease the atmospheric residence time of N2O to values below those that are calculated from stratospheric removal alone.