As part of a major field study to define the conditions that cause frequent exceedance of the California and federal carbon monoxide (CO) standards at certain monitoring locations in the of Los Angeles basin, monitoring was performed during the winter of 1989-1990. Intensive CO monitoring was conducted using integrated bag samplers at 36 locations. Sixteen integrating CO samplers were placed within 3 kilometers of the Lynwood AQMD station, and the other twenty samplers were placed within a 30-kilometer grid centered on the same site including measurements of CO concentration at 15, 30 and 45 meters were made in Lynwood, and tethersonde soundings for wind speed, wind direction and temperature at a height of 100 meters. This study also used tracer and traffic measurements to determine the regional air and traffic flows during the sampling period. Perfluorocarbon tracer gases were released at the start of each intensive at four locations within 5 kilometers around Lynwood.Traffic volumes, coupled with complex meteorology and, possibly, topographical features appear be the major factors influencing the high CO concentrations. This observation and the apparent motion of the CO cloud during episodes suggest that transported CO may also play a role in the episode buildups.