Micrometeorological Measurements of the Urban Heat Budget and CO2 Emissions on a City Scale

Direct measurements of urban CO2 emissions and heat fluxes are presented, made using the eddy covariance technique. The measurements were made from the top of a tower, approximately 65 m above the street level of Edinburgh, Scotland, and the fluxes are representative of footprint source areas of several square kilometers. The application of a stationarity test and spectral analysis techniques shows that, at this height, the stationarity criterion for eddy covariance is fulfilled for wind directions from the city center for 93% of the time, while for other wind directions this declines to 59%, demonstrating that pollutant fluxes from urban areas can be measured. The average CO2 emission from the city center was 26 μmol m-2 s-1 (10 kt of C km-2 yr-1), with typical daytime peaks of 50−75 and nighttime values of 10 μmol m-2 s-1. The correlation between CO2 emission and traffic flow is highly significant, while residential and institutional heating with natural gas are estimated to contribute about 39% to the emissions during the day and 64% at night. An analysis of the energy budget shows that, during the autumn, fossil fuel combustion within the city contributed one-third of the daily anthropogenic energy input of 3.8 MJ m-2 d-1, with the remainder coming from other energy sources, dominated by electricity. Conversely, the total energy input in late spring (May/June) was found to be approximately half this value.