The ARM method was originally developed by Chu and Meyer
(1991). They analyzed historical ambient NO and NO2 monitoring
data to establish a conservative conversion factor for estimating the
portion of the model predicted total NOX concentration that is in
the form of NO2 for annual averaging intervals They reviewed
ambient monitoring data from a large number of sites throughout
the US for the period 1987e1989, and for each site calculated the
annual average ambient NO2 and NOX concentrations and the
ambient NO2/NOX ratio (i.e., the observed conversion factor). The
90th percentile of the observed conversion factorswas selected as a
conservative, upper bound estimate that established the current
EPA recommended Tier 2 conversion factor of 0.8 for annual
average NO2 AERMOD modeling.