Development of atmospheric N2O isotopomers model based on a chemistry-coupled atmospheric general circulation model

It is well known that isotopic information is useful to qualitatively understand cycles and constrain sources of some atmospheric species, but so far there has been no study to model N2O isotopomers throughout the atmosphere from the troposphere to the stratosphere, including realistic surface N2O isotopomers emissions. We have started to develop a model to simulate spatiotemporal variations of the atmospheric N2O isotopomers in both the troposphere and the stratosphere, based on a chemistry-coupled atmospheric general circulation model, in order to obtain more accurate quantitative understanding of the global N2O cycle. For surface emissions of the isotopomers, combination of EDGAR-based anthropogenic and soil fluxes and monthly varying GEIA oceanic fluxes are factored, using isotopic values of global total sources estimated from firn-air analyses based long-term trend of the atmospheric N2O isotopomers. Isotopic fractionations in chemical reactions are considered for photolysis and photo-oxidation of N2O in the stratosphere. The isotopic fractionation coefficients have been employed from studies based on laboratory experiments, but we also will test the coefficients determined by theoretical calculations. In terms of the global N2O isotopomer budgets, precise quantification of the sources is quite challenging, because even the spatiotemporal variabilities of N2O sources have never been adequately estimated. Therefore, we have firstly started validation of simulated isotopomer results in the stratosphere, by using the isotopomer profiles obtained by balloon observations. N2O concentration profiles are mostly well reproduced, partly because of realistic reproduction of dynamical processes by nudging with reanalysis meteorological data. However, the concentration in the polar vortex tends to be overestimated, probably due to relatively coarse wave-length resolution in photolysis calculation. Such model features also appear in the isotopomers results, which are almost underestimated, relative to the balloon observations, although the concentration is well simulated. The tendency has been somewhat improved by incorporating another photolysis scheme with slightly higher wave-length resolution into the model. From another point of view, these facts indicate that N2O isotopomers can be used for validation of the stratospheric photochemical calculations in model, because of very high sensitivity of the isotopomer ratio values to some settings such as the wave-length resolution in the photochemical scheme.Therefore, N2O isotopomers modeling seems to be not only useful for validation of the fractionation coefficients and of isotopic characterization of sources, but also have the possibility to be an index especially for precision in the stratospheric photolysis in model.