Natascha Kljun

Eddy covariance measurement of species that are co-emitted with carbon dioxide (CO₂), such as carbon monoxide (CO) and nitrogen oxides NO and NO₂ (NO_x), 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, NO_x, methane (CH₄), and nitrous oxide (N₂O) from a tall tower (112 m a.g.l.) in Zurich, Switzerland. Median daily fluxes of F_CO2 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 F_CO (30 vs. 28 nmol m⁻² s⁻¹); 1.08 times greater for F_NOx (14 vs. 13 nmol m⁻² s⁻¹); 1.01 times greater for F_CH4 (13.5 vs. 13.3 nmol m⁻² s⁻¹); and not statistically significantly different for F_N2O. Flux ratios of F_CO /F_CO2 and F_NOx /F_CO2 are well characterised by inventory emission ratios of stationary combustion and road transport in cold months. In warm months both F_CO /F_CO2 and F_NOx /F_CO2 systematically exceed expected inventory ratios during the day, while no statistically significant seasonal difference is observed in F_NOx /F_CO, indicating biospheric photosynthetic activity. A linear mixing model is proposed and applied to attribute half-hourly F_CO2, F_CO, and F_NOx to stationary combustion and road transport emission categories as well as determine the biospheric F_CO2. Flux attribution is reasonable at certain times and from certain wind directions, but over-attributes CO and NO_x fluxes to road traffic and CO₂ fluxes to stationary combustion, and overestimates photosynthetic CO₂ uptake.