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

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 NOx 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 NOx limits the use of the observed signal to a local area. Thus, a local background for NOx 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 NOx (∆NOx) 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 ∆NOx time series to construct continuous ∆ffCO2 estimates. The uncertainty of the ∆NOx-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. ∆NOx 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 ∆NOx to ∆ffCO2 ratios.