“…A growing number of applications employ the 3D-Var technique [3,[65][66][67][68][69][70][71][72]. The 4D-Var approach has been used to adjust gas phase chemical tracer initial conditions [10,11,61,69,[73][74][75][76][77], to improve estimates of pollutant emissions, i.e., emission inversion, [15,78,79], and to improve aerosol fields [80][81][82]. Suboptimal Kalman filters have been employed successfully in chemical data assimilation for over a decade [63,[83][84][85][86][87][88][89].…”