Scaling of infrared remote sensor measurements for motor vehicle emission inventory calculations

Infrared (IR) remote sensors calibrated with propane understate volatile organic compound (VOC) concentrations in vehicle exhaust by 30−70% when compared to flame ionization detectors (FID). The difference depends on VOC composition and arises because many organic compounds in vehicle exhaust absorb less IR radiation than propane on a per-carbon basis. This study demonstrates an approach for scaling infrared measurements to reflect more accurately total exhaust VOC emissions from on-road motor vehicle fleets. Infrared versus flame ionization detector response to individual VOC was measured in the laboratory for methyl tert-butyl ether and a range of alkanes, alkenes, and aromatics that are prominent in vehicle exhaust. Overall IR/FID response to real exhaust mixtures was calculated by summing the response contributions of all individual VOC constituents. Average IR/FID response factors were calculated for typical on-road vehicle fleets based on VOC speciation profiles measured in several U.S. roadway tunnels. Results indicate that hydrocarbon concentrations measured by remote sensors with 3.4 μm filters should be multiplied by a factor of 2.0 ± 0.1 for light-duty vehicles using either California or federal reformulated gasoline blends and by 2.2 ± 0.1 when conventional gasoline is used.