Potential interference bias in ozone standard compliance monitoring

摘要

The U.S. Environmental Protection Agency has established a federal reference method (FRM) for ozone (O-3) and allowed for designation of federal equivalent methods (FEMs). However, the ethylene-chemiluminescence FRM for O-3 has been replaced by the UV photometric FEM by most state and local monitoring agencies because of its relative ease of operation. Accumulating evidence indicates that the FEM is prone. to bias under the hot, humid, and stagnant conditions conducive to high O-3 formation. This bias may lead to overreporting hourly O-3 concentrations by as much as 20-40 ppb. Measurement bias is caused by contamination of the O-3 scrubber, a problem that is not detected by dry air calibration. An adequate wet test has not been codified, although a procedure has been proposed for agency consideration. This paper includes documentation of laboratory tests quantifying specific interferant responses, collocated ambient FRM/FEM monitoring results, and smog chamber comparisons of the FRM and FEMs with alternative scrubber designs. As the numbers of reports on monitor interferences have grown, interested parties have called for agency recognition and correction of these biases.