City-Scale Traffic Emissions from Long-Path Measurements

Accurate, long-term measurement of greenhouse gas emissions at the city scale is becoming increasingly important as an ever-growing fraction of the world’s population moves to urban areas. Here we present the quantification of vehicle emissions from a medium-sized city using a novel instrument, an open-path dual frequency comb spectrometer. We measure simultaneously over two km-scale open-air paths for two months: a reference path that measures clean air from the mountains, and an over-city path that crosses the city of Boulder, CO and primarily measures on-road emissions. We observe enhancements of CO₂ in the city measurements relative to the reference measurements and use a 0-D box model to quantify the city traffic emissions. We extrapolate these to a yearly total to compare with the city of Boulder traffic emissions inventory value and find good agreement to within about a factor of two. This technique shows potential for determining emissions at the facility to medium-city scales and for observing temporal variability in the emissions.