Feasibility of determining surface emissions of trace gases using an inverse method in a three‐dimensional chemical transport model

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Abstract
We investigate the feasibility of using an inverse method based on a linear Kalman filter to determine regional surface fluxes through comparisons between observations and predictions in a three‐dimensional atmospheric transport model. The ability of the present ALE/GAGE observation sites to quantify the regional fluxes of anthropogenic trace gases is studied also. These investigations are done in the low‐resolution spectral model of Golombek and Prinn (1986) using CFCl3 as the test tracer since its sources are relatively well known. The first of these investigations is done with the transport model being perfect in the sense that the “observations” were produced by running the model with the CFCl3 emissions derived from industry data. The inverse method used is capable in this case of accurately determining regional surface fluxes using the present ALE/GAGE sites and to converge to the correct solution within a year or two even using initial conditions very different from the final solution. We also investigate how well the Kalman filter approach works with a less than perfect chemistry circulation model by using the ALE/GAGE observations Of CFCl3 for the inversion. The success of this inversion depends largely on the ability of the model circulation to predict observed concentrations of CFCl3 since its chemistry is reasonably well understood. The larger the difference between the model and the observed values using the real (industry) emissions then the larger the bias will be in the estimated emissions. Such studies can help to understand the inherent biases in the model when used in an inverse scheme before trying to use the model to estimate unknown surface fluxes such as those for methane, nitrous oxide, and carbon dioxide. We also investigate where additional observational stations could be placed to enhance the capability of the present ALE/GAGE network for determining regional net fluxes. If we are prepared to accept the circulation in the spectral low‐resolution model as sufficiently realistic, it appears that Hateruma (24N, 123E) and to a lesser extent Kamchatka (51N, 156E) are very promising locations for new stations and that Hateruma is superior to the ALE/GAGE Oregon station in providing information about Asian sources. This type of analysis done with a realistic circulation model can aid the process of choosing observation sites by addressing how well each site contributes to the different goals for use of the data.