Diurnal and Seasonal Attribution of Anthropogenic CO2 Emissions over Two Years in the Los Angeles Megacity

Abstract:

Human activities in urban regions are the largest contributors to anthropogenic greenhouse gas emissions, and CO₂ is the dominant species emitted, creating large variations that are easily observed. As governments continue to sign treaties limiting emissions in order to curb global warming, they must understand the temporal distribution of the signals. Bottom-up inventories account for emissions at all times of day from various sectors but are based only on economic data, with no observational confirmation. Top-down observations from satellites generally record variations at one time of day only a few times per month for any given location. Therefore we must use long-term in situ measurements to develop higher temporal resolution data sets. Attributing the observed emissions to different sources requires modeling using bottom-up emission data products and/or isotopic and tracer/tracer ratio measurements. Here we report results of two years (June 2013 – June 2015) of hourly average measurements of CO, CO₂, and δ¹³C from Pasadena, CA, to examine diurnal and seasonal variations in emissions from the biosphere and from natural gas and petroleum combustion. COxs/CO₂xs (excess over background) values can be used to determine the amount of fossil fuel-derived CO₂ relative to that from the biosphere. We calibrate this with mid-afternoon data using radiocarbon. We then use continuous measurements of ð¹³C to differentiate between petroleum and natural gas combustion, which are characterized by distinct isotopic compositions. The results of this study show that the relative amounts of emissions of the three sources are different at different times of day as well as different times of year.