Reply

Abstract

[1] We thank Stockwell and Goliff [2002] for their comments on, and interest in, the paper by Srivastava et al. [2001]. Seldom do atmospheric chemists give so much interest to a paper dealing with a new transport algorithm. Their comments about the chemical mechanism being simplified are quite correct, and we caution the readers directly in the above paper that the mechanism is not considered realistic. That caveat alone should keep anyone from considering the mechanism for use in an engineering model. The use of a simplified mechanism is to identify problems with a numerical transport algorithm that may result from nonlinear chemical interactions between species. For that purpose, the mechanism provided works fine, and even more simplified versions have been used successfully for this purpose in the past [e.g., Hov et al., 1989; Odman and Russell, 1991; Chock and Winkler, 1994]. Srivastava et al. [2001] added an additional reaction to provide greater realism than past efforts. Specifically, the mechanism was augmented with a HO2 + HO2 → H2O2 + O2 radical termination reaction to ensure that concentrations of NO during simulations do not drop to very low and unrealistic values. The reason one uses simplified mechanisms, not increasingly complex ones, is that the solution of the chemical dynamics starts taking a bulk of the computational effort and hinders the advancement and testing of the horizontal transport solver, which is the focus of Srivastava et al. [2001]. Also, the greater the complexity of the chemical mechanism, the more difficult it is to track down which aspects of the transport algorithm may be causing problems. After one has fully tested the transport algorithm and identified/fixed any problems, then the transport algorithm is used in an actual application, where it is linked with a comprehensive chemical mechanism. Likewise, when atmospheric chemists test their mechanisms, they do not do so by embedding their mechanism in a full three-dimensional model. They do so in zero dimensions. If they tested their mechanism in a full three-dimensional model at first, it would take much longer and also obscure problems. [2] Indeed, the mechanism proposed by Stockwell and Goliff [2002] probably is chemically more comprehensive and more closely depicts the actual chemistry in the atmosphere. It does this at the expense of increasing by about 50% the number of reactions; the rate constants become more complex, etc. The need to consider the pressure dependence when testing a horizontal transport algorithm is misplaced: This is not a model application. There is nothing in the comment that suggests that the simplified mechanism used by Srivastava et al. [2001] is not suitable for testing horizontal transport algorithms and that it does not capture the nonlinear interactions between species. Indeed, prior tests show that it does. It is not meant to be, and again, the reader is directly warned against, using the mechanism for atmospheric simulations. [3] More comprehensive mechanisms can be employed as one is willing to devote more time to solving the chemistry and less to actually testing the horizontal advection algorithm, which obviously is not the point of the work discussed here. Indeed, the mechanism proposed, since it is still relatively simple, may be useful for future testing of advection routines. The concern of Stockwell and Goliff [2002] that our simplified chemical schemes, which are useful for testing model transport, may ultimately end up in an air quality model could also be directed at their mechanism. Consequently, we should be striving for even greater chemical fidelity in air quality models.