A reexamination of the impact of anthropogenically fixed nitrogen on atmospheric N2O and the stratospheric O-3 layer

The impact of anthropogenic nitrogen fixation on atmospheric N2O is estimated using the approach of the 1970s, which assumed that some fraction beta of anthropogenically fixed nitrogen is rapidly denitrified back to the atmosphere, with a significant fraction a of the end product as N2O. Appropriate values for beta and alpha are discussed and applied to current anthropogenic nitrogen fixation rates, which are dominated by synthetic fertilizer and crop production. These calculations yield an N2O source of about 3.5 Tg N/yr associated with anthropogenic nitrogen fixation, which accounts for most of the observed atmospheric N2O increase of 3-5 Tg N/yr. This simple nitrogen cycle-based approach toward estimating anthropogenic N2O 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 N2O source and double counted other sources. A box model of atmospheric N2O which assumes an anthropogenic N2O source proportional to past and projected future rates of anthropogenic nitrogen fixation can reproduce much of the historic growth in N2O. Continued growth in the rate of anthropogenic nitrogen fixation could increase atmospheric N2O to 400-500 ppbv by the year 2100. Two-dimensional model calculations suggest that the corresponding increase in stratospheric NOx 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 N2O could put more NOx 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 N2O and the stratospheric O-3 layer, more information is needed about future anthropogenic nitrogen fixation rates, the N2O yields of denitrification and nitrification, net storage/loss of naturally and anthropogenically fixed nitrogen, and NOx chemistry in the stratosphere.