Tracking the pulse of a North American megacity through city-scale top-down mapping of atmospheric carbon monoxide

Atmospheric carbon monoxide (CO) in cities is primarily produced by incomplete combustion of fossil fuels. Due to its moderately long lifetime and low background concentration, CO is an important tracer for atmospheric transport. In the Los Angeles (LA) basin, on-road mobile sources account for more than 70% of the total CO emissions. Therefore, the spatial and temporal patterns of CO can be an effective indicator of the pulse of human activity in the LA megacity.

CO mixing ratio (XCO) in the LA basin has been continuously measured by the California Laboratory for Atmospheric Remote Sensing-Fourier Transform Spectrometer (CLARS-FTS), located on top of Mt Wilson (1.67 km a.s.l.) and overlooking the basin. In SVO mode, CLARS-FTS measures solar radiation that has traversed the free troposphere. In LABS mode, the FTS points down at 33 surface target sites across the LA basin and measures reflected sunlight that has gone through a long path within the planetary boundary layer (PBL). XCO excess (XCOxs), the difference between LABS (PBL concentration) and SVO (background concentration) measurements, is calculated to infer the CO emissions associated with human activities within the PBL. We generated maps of XCOxs by interpolation between the surface targets. The results show that XCOxs in the LA basin shows strong diurnal and weekly cycles and seasonal variations driven by the variabilities of on-road mobile emissions and atmospheric transport. On a seasonal scale, XCOxs in summer is higher than that in winter, probably due to the relatively stagnant air in summer. On an interannual scale, the XCOxs has been decreasing over time because of the effectiveness of emission control policies. The results from these diurnal, weekly, monthly, and interannual XCOxs variabilities show that (1) the spatial and temporal XCOxs patterns are closely related to on-road mobile emissions associated with human activities, and (2) the spatial patterns are also significantly affected by atmospheric dynamics associated with sea-breeze circulation and by topographic effects in the LA basin.

These results provide insight into monitoring urban anthropogenic emissions with high spatial resolution on an urban scale and form a basis for verifying data products from future geostationary missions such as TEMPO and GeoCARB.