One of the biggest changes to the AERMOD modeling system in the 18081 model update was the removal of the original Ambient Ratio Method (ARM) for analyzing conversion of NOx to NO2. This option, based on Chu & Meyer’s 1991 work presented at that year’s Air & Waste Management Association’s annual conference, used a single ratio to convert all NOx concentrations to NO2. The national default value was 75% for annual averages, and subsequent guidance recommended a value of 80% for the hourly standard.
The removal of ARM leaves the Ambient Ratio Method 2 (ARM2) as the only regulatory Tier 2 modeling demonstration. Let’s take a closer look at this model option.
The Ambient Ratio Method 2 was added as a BETA option to AERMOD as part of the 13350 model update. It was based on work done by Podrez (Atmospheric Environment, 2015) in which a 10-year record of ambient data from the Air Quality System was analyzed to characterize typical NO2/NOx ambient ratios. When analyzing the 98th percentile values for the entire AQS data set, the resulting plot demonstrated a polynomial fit upon which the ARM2 conversion factor equation was developed.
98th Percentile NO2/NOx Ambient Ratios as Calculated by ARM2
These calculated ratios were constrained to a maximum of 0.90 (based on the current EPA recommendation for maximum equilibrium ratio) and a minimum value of 0.20 (based on data analysis and consideration of typical in-stack ratios).
Current guidance from the U.S. EPA recommends a minimum ratio of 0.50 to maintain an appropriate level of conservatism in the results.
To enable ARM2 in AERMOD View, proper option selection must be made on the Control Pathway.
1. On the Pollutant/Averaging options, select NO2 from the Type drop-down list.
2. On the NOx to NO2 Options, select the ARM2 option from the Tier 2 group. Options are available to modify the Minimum and Maximum NO2/NOx Ratios, but these values are constrained to an absolute minimum of 0.50.