CALPOST, the post-processing model of the CALPUFF modeling system, has long had the option to estimate concentrations of nitrogen dioxide (NO2) from model-calculated concentrations of total nitrogen oxides (NOX). The user provided a single ratio which was applied to all NOX concentrations. The latest update to the CALPUFF modeling system, Version 7 (released in June 2015) contained an update in CALPOST which allows users to specify ratios which vary with NOX concentration.
In Lakes Environmental’s CALPUFF View, all CALPOST model options are handled via the CALPOST Wizard window. The last tab of this window, NO2/NOx, contains the available ratio options. When the Version 7 modeling system is selected, a new option appears in the list: Specify Short and Long Term NO2/NOx ratios that vary with NOx concentration in PPB.
Choosing this option will enable two separate tables where users can supply varying ratios.
The user must first define the minimum number of hours for using Long Term Ratios (default value of 8000 hours). The Short Term Ratios will then apply to all averaging periods of a shorter term.
NOx Concentrations are defined by increasing magnitude, and the ratio used for a particular NOx concentration is interpolated from the values provided in the table. The ratio for the smallest tabulated NOx concentration (the first) is used for all NOx concentrations less than the smallest user-defined value. In the example above, all NOx concentrations less than 100 PPB (53.2 µg/m3 based on the default conversion factor of 1.88) would have a NO2/NOx ratio of 0.90.