Abstract
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.
Highlights
Background values forCO2 mixing ratios were assumed to be constant for the duration of the campaign and were taken to be the average of the daily minimum hourly values at a site on Palos Verdes Peninsula overlooking the Pacific Ocean (393.1 ppm; Fig. 1a)
There is a peak on 2–3 June with gradually decreasing mixing ratios over the eight days followed by an increase through the end of the campaign period, roughly inverse to the time series for planetary boundary layer height (PBLH) (Fig. 2b)
Surface CO2 concentrations increase at night and remain high until sunrise, probably due to respiration of the biosphere into the shallow nighttime stable layer, and quickly drop as the boundary layer grows after sunrise, entraining air with lower CO2
Summary
CO2 mixing ratios were assumed to be constant for the duration of the campaign and were taken to be the average of the daily minimum hourly values at a site on Palos Verdes Peninsula overlooking the Pacific Ocean (393.1 ppm; Fig. 1a). This site is on a steep hillside ∼ 1.3 km from the ocean and ∼ 0.33 km a.s.l., with prevailing wind from the WSW. For CO, we used a time-varying background derived using NOAA Earth System Research Laboratory (ESRL) data from Pacific marine boundary layer and Pacific aircraft sites, as described in Appendix B1
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