A new inverse method for trace gas flux estimation: 2. Application to tropospheric CFCl3 fluxes

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Abstract
An initial application of a new inverse method for the estimation of flux strengths of long‐lived atmospheric trace gases is presented. CFCl3 is studied using the Australian National University's Chemical Transport Model. Unit‐pulse responses are derived from the model, and used to identify a time‐varying state‐space model of tropospheric CFCl3. This in turn is used in a Kaiman filter to perform two input‐estimation studies. The first uses model generated measurements to estimate known flux strengths. This demonstrates the robustness of the method, although it is found that instantaneous stratospheric loss rates are not well‐estimated using only surface concentration measurements. Emissions however, are robustly estimated. A time‐aver aged tropospheric lifetime can be estimated however, with an accuracy of ±5 years. Thirteen years of Atmospheric Lifetime Experiment Global Atmospheric Gases Experiment CFCl3 measurements are used in the second flux estimation experiment. The estimated fluxes though are outside known physical limits for CFCl3, and it is concluded that either a more accurate/appropriate transport model, or more measurement locations, are needed to obtain useful information regarding regional tropospheric CFCl3 fluxes.