Diurnal tracking of anthropogenic CO₂ emissions in the Los Angeles basin megacity during spring 2010

Abstract. Attributing observed CO₂ variations to human or natural cause is critical to deducing and tracking emissions from observations. We have used in situ CO₂, CO, and planetary boundary layer height (PBLH) measurements recorded during the CalNex-LA (CARB et al., 2008) ground campaign of 15 May–15 June 2010, in Pasadena, CA, to deduce the diurnally varying anthropogenic component of observed CO₂ in the megacity of Los Angeles (LA). This affordable and simple technique, validated by carbon isotope observations and WRF-STILT (Weather Research and Forecasting model – Stochastic Time-Inverted Lagrangian Transport model) predictions, is shown to robustly attribute observed CO₂ variation to anthropogenic or biogenic origin over the entire diurnal cycle. During CalNex-LA, local fossil fuel combustion contributed up to ~50% of the observed CO₂ enhancement overnight, and ~100% of the enhancement near midday. This suggests that sufficiently accurate total column CO₂ observations recorded near midday, such as those from the GOSAT or OCO-2 satellites, can potentially be used to track anthropogenic emissions from the LA megacity.