(a) Scope and frequency. If you use an analyzer that measures only NO to determine NOX, you must use an NO2-to-NO converter upstream of the analyzer. Perform this verification after installing the converter, after major maintenance and within 35 days before an emission test. This verification must be repeated at this frequency to verify that the catalytic activity of the NO2-to-NO converter has not deteriorated.

(b) Measurement principles. An NO2-to-NO converter allows an analyzer that measures only NO to determine total NOX by converting the NO2 in exhaust to NO.

(c) System requirements. An NO2-to-NO converter must allow for measuring at least 95% of the total NO2 at the maximum expected concentration of NO2.

(d) Procedure. Use the following procedure to verify the performance of a NO2-to-NO converter:

(1) Instrument setup. Follow the analyzer and NO2-to-NO converter manufacturers' start-up and operating instructions. Adjust the analyzer and converter as needed to optimize performance.

(2) Equipment setup. Connect an ozonator's inlet to a zero-air or oxygen source and connect its outlet to one port of a three-way tee fitting. Connect an NO span gas to another port, and connect the NO2-to-NO converter inlet to the last port.

(3) Adjustments and data collection. Perform this check as follows:

(i) Set ozonator air off, turn ozonator power off, and set the analyzer to NO mode. Allow for stabilization, accounting only for transport delays and instrument response.

(ii) Use an NO concentration that is representative of the peak total NOX concentration expected during testing. The NO2 content of the gas mixture shall be less than 5% of the NO concentration. Record the concentration of NO by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as xNOref.

(iii) Turn on the ozonator O2 supply and adjust the O2 flow rate so the NO indicated by the analyzer is about 10 percent less than xNOref. Record the concentration of NO by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as xNO + O2mix.

(iv) Switch the ozonator on and adjust the ozone generation rate so the NO measured by the analyzer is 20 percent of xNOref or a value which would simulate the maximum concentration of NO2 expected during testing, while maintaining at least 10 percent unreacted NO. This ensures that the ozonator is generating NO2 at the maximum concentration expected during testing. Record the concentration of NO by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as xNOmeas.

(v) Switch the NOX analyzer to NOX mode and measure total NOX. Record the concentration of NOX by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as xNOxmeas.

(vi) Switch off the ozonator but maintain gas flow through the system. The NOX analyzer will indicate the NOX in the NO + O2 mixture. Record the concentration of NOX by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as xNOx + O2mix.

(vii) Turn off the ozonator O2 supply. The NOX analyzer will indicate the NOX in the original NO-in-N2 mixture. Record the concentration of NOX by calculating the mean of 30 seconds of sampled data from the analyzer and record this value as xNOxref. This value should be no more than 5 percent above the xNOref value.

(4) Performance evaluation. Calculate the efficiency of the NOX converter by substituting the concentrations obtained into the following equation:

(5) If the result is less than 95%, repair or replace the NO2-to-NO converter.

(e) Exceptions. The following exceptions apply:

(1) You may omit this verification if you can show by engineering analysis that for your NOX sampling system and your emission calculations procedures, the converter always affects your brake-specific NOX emission results by less than 0.5% of the applicable NOX standard.

[70 FR 40516, July 13, 2005, as amended at 73 FR 37313, June 30, 2008; 73 FR 59330, Oct. 8, 2008; 76 FR 57447, Sept. 15, 2011]