The University of Denver used its remote sensor for motor vehicle carbon monoxide emissions to measure carbon monoxide (CO) emissions in the Lynwood area of the South Coast Air Basin during eleven days in December 1989. The work was performed in support of a larger study to investigate the reasons for high CO concentrations near Lynwood on winter nights. Vehicles were measured in a mix of driving conditions ranging from deceleration preceding a traffic light through idling in heavy congestion to acceleration and cruise on a freeway on-ramp. The ARB validated the performance of the instrument in a doublet blind test using an instrumented vehicle provided by General Motors Research Laboratories. Over 27,000 valid CO measurements were collected during the study - 16,000 of these were matched to license plate information through the California Department of Motor Vehicles to obtain make and model year information. The data collected indicate that, for the driving modes encountered, more than half of the CO was emitted by only 11% of the vehicles. These vehicles emitted more than 5% CO in their exhaust, and are referred to as 'gross polluters.' A model of vehicle emissions based on vehicle age suggests that emissions increase linearlymore » with average age of the fleet, and that the linear increase is dominated by the steady increase in the fraction of gross polluters with age. The model helps explain the higher concentration of CO near Lynwood, as the vehicle fleet in the Lynwood area is older by two to three years than the fleets in other areas measured.« less