Principles, applications and limitations of eddy covariance flux measurements in cities – IG3IS Urban Good Practice Guidelines

The eddy-covariance (EC) method is unique in terms of urban greenhouse gas (GHG) emission monitoring as it provides direct net vertical fluxes from an area upwind of the measurement - termed the flux footprint. Generally, EC observations in cities can be broadly classified into three spatial scales with increasing measurement height and hence increasing size of the spatial extent of the footprint: (i) Plot-scale EC measurements to quantify emissions and uptake from a specific individual urban land cover patch (e.g. lawn, green roof or landfill), (ii) local-scale EC measurements to measure net fluxes from an archetypical urban districts (e.g. residential or commercial area, park), and (iii) tall-tower EC systems integrating over large parts of cities and e.g. enabling the derivation of typical urban emissions. In all three cases, fluxes measured by EC are quasi-continuous and quasi-instantaneous at sub-hourly resolution, hence time-profiles of emissions, emission ratios, and emission factors can be obtained under different conditions (e.g. weather, day of the week, seasons). Drawing from examples across the world, we will discuss how EC measurements of GHGs and co-emitted species can contribute to integrated emissions monitoring in cities: We will show how tower-based local-scale EC have been used to assess and monitor long-term and short-term emission trends from urban areas over decades. We will further show how EC measurements in combination with flux footprint models, allow for the evaluation of gridded emission inventories, emission factors and identification of unaccounted sources and sinks, e.g. carbon dioxide uptake by vegetation or methane emissions. We also show how flux observations of GHGs and their isotopologues allow an understanding of sectoral attribution and processes controlling emissions and uptake dynamics. The contribution will summarize limitations and challenges of EC flux observations in cities as discussed in the latest Practice Guidelines.