Initial Results from the Los Angeles Megacity Carbon Project: Exploring Spatial and Temporal Variability of the In Situ CO2 Observations

The Megacities Carbon Project (megacities.jpl.nasa.gov) is a multi-national, multi-institution project aimed at quantifying multi-year carbon emissions trends from large urban areas. The Los Angeles Megacity greenhouse gas observation network includes fifteen surface in situ sites and was designed to capture spatial and temporal variability in carbon emissions across the South Coast Air Basin. The surface sites include a combination of towers and rooftops equipped with laser absorption spectrometers measuring carbon dioxide (CO2), with methane and carbon monoxide measurement capability at a subset of the sites. In this study, we report intial results from January-April 2015 collected at four tower sites: two "urban" sites, which predominantly exhibit urban CO2 enhancements, and two "background" sites, which are primarily influenced by unpolluted maritime or continental air masses and reflect local background CO2 levels. During January-April 2015, hourly-averaged in situ measurements ranged from roughly 395-650 ppm CO2, with the urban sites reflecting systematically higher CO2 levels compared to the background sites. The average CO2 enhancement observed at midday (12-4pm PST) ranged from 15-23 ppm CO2 excess at the urban sites, Compton and Granada Hills. The tower measurements also show diurnal, day of week, and monthly variability. Overall, the results from the in situ network are consistent with prior studies that utilized ground-based and space-based remote sensing data to demonstrate that the "urban CO2 dome" hypothesis applies to Los Angeles. The continuous measurement capability provided by the in situ network also highlights the fact that atmospheric CO2 enhancements vary in both space and time and are not uniform throughout the South Coast Air Basin. Future work will involve sectoral attribution of the emissions sources contributing to the urban CO2 enhancements using the Hestia high-resolution fossil fuel carbon emissions inventory. A forward model simulation using WRF-GHG is also planned for the January-April 2015 period.