Using NO_x as quantitative fossil fuel CO₂ proxy in urban areas: challenges and benefits

Abstract. Continuously monitoring local excess fossil fuel CO₂ concentrations remains challenging due to the absence of accurate, continuous ¹⁴CO₂ measurements. Continuous estimates of fossil fuel CO₂ (ffCO₂) are made by observing continuously measurable proxies that are co-emitted during fossil fuel combustion. This paper investigates the potential and challenges of using in situ NO_x observations in urban areas to quantitatively estimate hourly ffCO₂ concentration enhancements, using observations at the ICOS pilot station in Heidelberg, Germany. The short atmospheric lifetime of NO_x limits the use of the observed signal to a local area. Thus, a local background for NO_x and ffCO₂ was approximated using the Stochastic Time-Inverted Lagrangian Transport (STILT) model and bottom-up emission estimates from the Netherlands Organisation for Applied Scientific Research (TNO). Using ¹⁴CO₂ data from 185 hourly integrated flask samples between 2020 and 2021, mean ratios of local excess NO_x (∆NO_x) to local excess ffCO₂ (∆ffCO₂) concentrations of 1.40 ppb ppm⁻¹ for winter and 2.12 ppb ppm⁻¹ for summer were calculated. These ratios were applied to the ∆NO_x time series to construct continuous ∆ffCO₂ estimates. The uncertainty of the ∆NO_x-based ∆ffCO₂ record was estimated at 3.94 ppm. Comparisons with ¹⁴CO₂-based and ∆CO-based ∆ffCO₂ estimates showed good agreement, while still demonstrating distinct behaviour for individual events. ∆NO_x shows considerable potential as proxy for ffCO₂ and as useful addition to ∆CO-based estimates, as both proxies have different footprints due to their lifetimes. A key challenge remains in reliably determining the seasonal and diurnal cycle of average ∆NO_x to ∆ffCO₂ ratios.