Sectoral attribution of greenhouse gas and pollutant emissions using multi-species eddy covariance on a tall tower in Zurich, Switzerland

Abstract. Eddy covariance measurement of species that are co-emitted with carbon dioxide (CO2), such as carbon monoxide (CO) and nitrogen oxides NO and NO2 (NOx), provides an opportunity to attribute a net flux to individual source or sink categories. We present eight months of continuous simultaneous measurements of fluxes (F) of CO2, CO, NOx, methane (CH4), and nitrous oxide (N2O) from a tall tower (112 m a.g.l.) in Zurich, Switzerland. Median daily fluxes of FCO2 were 1.47 times larger in the winter (November–March) as opposed to summer (August–October) months (10.9 vs. 7.4 µmol m−2 s−1); 1.08 times greater for FCO (30 vs. 28 nmol m−2 s−1); 1.08 times greater for FNOx (14 vs. 13 nmol m−2 s−1); 1.01 times greater for FCH4 (13.5 vs. 13.3 nmol m−2 s−1); and not statistically significantly different for FN2O. Flux ratios of FCO/FCO2 and FNOx/FCO2 are well characterised by inventory emission ratios of stationary combustion and road transport in cold months. In warm months both FCO/FCO2 and FNOx/FCO2 systematically exceed expected inventory ratios during the day, while no statistically significant seasonal difference is observed in FNOx/FCO, indicating biospheric photosynthetic activity. A linear mixing model is proposed and applied to attribute half-hourly FCO2, FCO, and FNOx to stationary combustion and road transport emission categories as well as determine the biospheric FCO2. Flux attribution is reasonable at certain times and from certain wind directions, but over-attributes CO and NOx fluxes to road traffic and CO2 fluxes to stationary combustion, and overestimates photosynthetic CO2 uptake.