Carbon exchanges in cities and woodlands of southern England

By Sue Grimmond

Cities are major contributors to recent carbon dioxide (CO₂) rises in the atmosphere.  Emissions come from multiple sources, including vehicles, domestic heating/cooling, industrial activities and directly from people. While a large number of studies have focused on carbon exchanges from vegetated ecosystems, direct measurements from urban areas remain relatively limited. To address this we have undertaken long term measurements at two contrasting sites in southern England – in London and Swindon.  This is part of a broader programme of work on urban climates, with a particular focus on London.

The measurements described here were undertaken at three sites 70-100 km apart (Figure 1): central London (U); a residential suburban site in Swindon (S); and a deciduous oak woodland at the Alice Holt Research site (W).

Figure 1. Photographs and aerial images (2009, ©GeoPerspectives) of the (a-b) London, (c-d) Swindon and (e-f) Alice Holt sites with the positions of measurement towers shown with crosses.

The data were collected using state-of-the-art equipment mounted on tall towers. Here we present data from January 2011 to April 2013. The site in London remains operational and daily values can be seen here.

The results show striking contrasts in both the magnitude and seasonal and daily variability of the observed carbon fluxes between the three sites, as expected (Figure 2). At the forested site, the annual cycle is dominated by the balance of photosynthesis (which takes up CO₂) and respiration (which releases it). Over a year net uptake by the understorey is evident early in the year, followed by a sudden increase in net uptake which coincides with leaf-out of the oak canopy. Overall the site is a net sink (i.e. it takes up carbon).

In contrast, central London is a major source of CO₂ all year round. Larger emissions occur during the day than the night; the highest emissions occur during winter afternoons; the lowest during the early morning hours in summer.

Similarly, Swindon’s emissions are largest in winter. At this site the effects of rush hours (peaks in the morning and afternoon) are clear, reflecting the importance of traffic emissions. During the summer, however, the pattern of daytime CO₂ fluxes is more similar to the woodland, reflecting the significant amount of vegetation at the site.

These data provide clear evidence on the impacts of increasing urban density and changing land use on CO₂ exchanges and providing insights to reduce or mitigate emissions.

Figure 2. Monthly median diurnal cycles and interquartile ranges (shaded) of 30 minute CO2 fluxes for the three sites: urban, suburban and woodland.

For further information on this study and all those who support it, please see:

Ward, H. C., Kotthaus, S., Grimmond, C. S. B., Bjorkegren, A., Wilkinson, M., Morrison, W. T. J.,Evans, J. G., Morison, J. I. L. and Iamarino, M., 2014. Effects of urban density on carbon dioxide exchanges: observations of dense urban, suburban and woodland areas of southern England. Environmental Pollution, 198. pp. 186-200. ISSN 0269-7491 doi:10.1016/j.envpol.2014.12.031