A reexamination of the impact of anthropogenically fixed nitrogen on atmospheric N₂O and the stratospheric O₃ layer

The impact of anthropogenic nitrogen fixation on atmospheric N 2 O is estimated using the approach of the 1970s, which assumed that some fraction β of anthropogenically fixed nitrogen is rapidly denitrified back to the atmosphere, with a significant fraction α of the end product as N 2 O. Appropriate values for β and α are discussed and applied to current anthropogenic nitrogen fixation rates, which are dominated by synthetic fertilizer and crop production. These calculations yield an N 2 O source of about 3.5 Tg N/yr associated with anthropogenic nitrogen fixation, which accounts for most of the observed atmospheric N 2 O increase of 3–5 Tg N/yr. This simple nitrogen cycle‐based approach toward estimating anthropogenic N 2 O sources provides a useful check on the more complex approaches employed today, in which emissions from a large number of small, independent sources are estimated by extrapolating measured emissions coefficients. Such approaches can be inconsistent with considerations of the global nitrogen cycle and likely have underestimated the fertilizer N 2 O source and double counted other sources. A box model of atmospheric N 2 O which assumes an anthropogenic N 2 O source proportional to past and projected future rates of anthropogenic nitrogen fixation can reproduce much of the historic growth in N 2 O. Continued growth in the rate of anthropogenic nitrogen fixation could increase atmospheric N 2 O to 400–500 ppbv by the year 2100. Two‐dimensional model calculations suggest that the corresponding increase in stratospheric NO x would cause a small loss of O 3 , which would be superimposed upon a larger recovery due to the phaseout of anthropogenic halocarbons. An increase in N 2 O could put more NO x into the middle and upper stratosphere than supersonic aircraft, although the relevant time scale is considerably longer. To better understand the impact of anthropogenic nitrogen on atmospheric N 2 O and the stratospheric O 3 layer, more information is needed about future anthropogenic nitrogen fixation rates, the N 2 O yields of denitrification and nitrification, net storage/loss of naturally and anthropogenically fixed nitrogen, and NO x chemistry in the stratosphere.