DISCUSSION OF A LAGRANGIAN TRAJECTORY MODEL DESCRIBING LONG-RANGE TRANSPORT OF OXIDES OF NITROGEN, THE INCORPORATION OF PAN IN THE CHEMICAL MECHANISM, AND SUPPORTING MEASUREMENTS OF PAN AND NITRATE SPECIES AT RURAL SITES IN ONTARIO, CANADA

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Abstract

The derivation of a highly simplified parameterization of NOx chemistry for use with the AES Lagrangian trajectory model is described. Two versions, both employing essentially independent first-order kinetics are discussed: (1) a one step mechanism: NO2 → inorganic nitrate and (2) a somewhat more involved reaction scheme including PAN chemistry. Evaluation of model performance is attempted with data from the Canadian Air and Precipitation Monitoring Network (APN). It is shown that both model versions are able to reproduce trends of annual and monthly average atmospheric nitrate concentrations, although the models tend to suggest transport over larger distances than is apparent from the field data. The inclusion of PAN chemistry appears to improve the model performance. However, the model predicts a winter maximum and a summer minimum for the annual profile of PAN, and this is in direct contradiction with field measurements of PAN elsewhere.

Initial results from a PAN measurement program conducted at three sites in Ontario are also reported and compared with the second model version. Observations for summer 1982 are predicted quite well, but data collected during March 1983 are substantially overpredicted. It is indicated that during periods of rain, PAN constitutes the major oxidized NOx component. The suggestion is made that this observation is due to a much more efficient precipitation scavenging of nitrate species than of PAN.

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