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 es timated using the approach of the 1970s, which assumed that some fract ion beta of anthropogenically fixed nitrogen is rapidly denitrified ba ck to the atmosphere, with a significant fraction a of the end product as N2O. Appropriate values for beta and alpha are discussed and appli ed to current anthropogenic nitrogen fixation rates, which are dominat ed by synthetic fertilizer and crop production. These calculations yie ld an N2O source of about 3.5 Tg N/yr associated with anthropogenic ni trogen fixation, which accounts for most of the observed atmospheric N 2O 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 extrapola ting measured emissions coefficients. Such approaches can be inconsist ent with considerations of the global nitrogen cycle and likely have u nderestimated the fertilizer N2O source and double counted other sourc es. A box model of atmospheric N2O which assumes an anthropogenic N2O source proportional to past and projected future rates of anthropogeni c 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-dimensi onal model calculations suggest that the corresponding increase in str atospheric NOx would cause a small loss of O-3, which would be superim posed upon a larger recovery due to the phaseout of anthropogenic halo carbons. An increase in N2O could put more NOx into the middle and upp er stratosphere than supersonic aircraft, although the relevant time s cale is considerably longer. To better understand the impact of anthro pogenic nitrogen on atmospheric N2O and the stratospheric O-3 layer, m ore information is needed about future anthropogenic nitrogen fixation rates, the N2O yields of denitrification and nitrification, net stora ge/loss of naturally and anthropogenically fixed nitrogen, and NOx che mistry in the stratosphere.