Towards a spatial CO 2 budget of a metropolitan region based on textural image classification and flux measurements

Abstract

A project on monitoring urban CO 2 budgets has been conducted since the year 2000 focusing on the Metropolitan area of Copenhagen, Denmark. Methodologically, the project combines remote sensing with CO 2 fluxes measured by eddy covariance technique from the top of a 40-m mast located in the center of Copenhagen. These data are supplemented by flux measurements from a mobile system on a 10-m mast which is moved between different urban types, including a major entrance road, and residential and industrial areas. By comparing the time series of vertical CO 2 exchange and the number of cars on the major entrance roads, it is demonstrated that the traffic intensity has a major impact on the urban carbon budget. The spatial distribution of the CO 2 emission rates is examined through texture-based classification of Landsat-TM satellite images. Local traffic intensity and local heating is seen as a function of specific local urban land use and activities, and the corresponding satellite image texture is used as a proxy for the CO 2 emission from these components. The urban scene is divided into urban land use classes that constitute homogenous areas in terms of main types of activity and these are linked to specific levels of CO 2 emission. For this purpose, a multi-scale approach based on co-occurrence matrices has been developed and applied. The paper outlines how the CO 2 exchange from the urban sources and sinks can be estimated from continuous flux measurements in central Copenhagen. It is shown that traffic is the largest single CO 2 source in the city. The mobile measurements demonstrate that the emission rates ranges from less than 0.8 g CO 2 m −2 h −1 in the residential areas up to a maximum of 16 g CO 2 m −2 h −1 along the major entrance roads in the city center. An average annual CO 2 exchange rate of 35 g CO 2 m −2 day −1 is calculated by assigning fluxes to each land use type and excluding the effect of remote sources (power plants, air and sea traffic). This value can be compared to a carbon budget recently calculated from national statistics showing that the local urban sources (road traffic, industry, service and household) have a comparable net emission rate of 38 g CO 2 m −2 day −1. The perspective of having more precise knowledge of the distribution of sources and sinks is finally discussed in relation to changing land use patterns.