Satellite Based Investigations of Day-of-Week Variation in NOx Emissions

In the state of California, mobile sources account for more than 50 % of NOx emissions. Regulation focusing on passenger vehicles has led to dramatic reductions in total NOx emissions. Heavy-duty diesel vehicle emissions have, in contrast, not decreased and therefore represent a growing fraction of total NOx emissions. A pronounced weekend effect, marked by a substantial decrease in measured NO2 on weekends when compared with measurements on weekdays has been observed in some urban areas of the state due to variation in the activity patterns of these two types of vehicles. We examine this weekend effect using observations from the Ozone Monitoring Instrument (OMI). We developed a high resolution (5 X 5 km2) average of the OMI observations that permits a detailed view of spatial variations in emissions with day of week. Comparison of the observations to emission inventories suggests opportunities for improvements and for assessing differences between San Joaquin Valley and South Coast Air Basins.