Effect of seasonal variability and land use on particle number and CO 2 exchange in Helsinki, Finland

•Joint aerosol particle and CO2 fluxes above urban and suburban areas were analysed.•The response of the two seemingly similar scalars to environmental factors was different.•CO2 flux was more sensible to changes in land use than particle flux.•Particle emissions were more sensitive to changes in air temperature than CO2.•In city centre, enhanced turbulent transfer of CO2 was seen.