“…Brost [1988] proposed a method for evaluating numerically the sensitivity of model output variables to perturbations in the input variables. Derivation of source‐receptor relationships in the presence of non‐linear chemistry can be facilitated by using distinct tracers to simulate the same species emitted from different sources [Kleinman, 1987]; a more elaborate approach suggested by Hsu and Chang [1987] is to tag individual sources with a small oscillatory signal, which can be retrieved by Fourier analysis of concentrations at the site.…”