Estimating vehicle carbon dioxide emissions from Boulder, Colorado, using horizontal path-integrated column measurements.

We performed 7.5 weeks of path-integrated concentration measurements of CO, CH, HO, and HDO over the city of Boulder, Colorado. An open-path dual-comb spectrometer simultaneously measured time-resolved data across a reference path, located near the mountains to the west of the city, and across an over-city path that intersected two-thirds of the city, including two major commuter arteries. By comparing the measured concentrations over the two paths when the wind is primarily out of the west, we observe daytime CO enhancements over the city. Given the warm weather and the measurement footprint, the dominant contribution to the CO enhancement is from city vehicle traffic. We use a Gaussian plume model combined with reported city traffic patterns to estimate city emissions of on-road CO as (6.2 ± 2.2) × 10 metric tons (t) CO yr after correcting for non-traffic sources. Within the uncertainty, this value agrees with the city's bottom-up greenhouse gas inventory for the on-road vehicle sector of 4.5 × 10 t CO yr. Finally, we discuss experimental modifications that could lead to improved estimates from our path-integrated measurements.