ARM Version 2 (ARM2) has been developed using 1-h ambient
monitoring data to address this variation in the conversion factor as
a function of total NOX concentration. ARM2 is based on observed
hourly NO2/NOX concentration ratios from a large data set with
diverse source-monitor distances, atmospheric ozone concentrations,
and atmospheric dispersion conditions. This large measured
data set was used to develop an empirical equation that estimates a
conservative upper bound of the conversion factor as a function of
total NOX concentration. The performance of the ARM2 method has
been evaluated using other, independent monitored data sets as
well as comparisons to the EPA Tier 3 conversion methods.