Biosphere-atmosphere exchange of carbon dioxide (CO2) has been on the scientific agenda for several decades and new technology now also allows for high-precision, continuous monitoring of fluxes of other trace gases such as methane (CH4), carbon monoxide (CO) and nitrous oxide (N2O).
Compared to the natural environment, flux measurements in the urban environment, which is home to over 50% of the population globally, are still rare despite high densities of anthropogenic sources of pollutants.
Direct measurements can be used to study the temporal dynamics and spatial distributions of pollutant sources and can also provide an independent validation of emission budgets estimated by atmospheric emission inventories. Atmospheric emissions inventories are used to construct budgets based on assumed spatial distributions and source strengths of a range of pollutants. Inventorying is currently the only accepted method for constructing the annual emission budgets the COP21 signatory countries report to the United Nations Framework Convention on Climate Change (UNFCCC).
Despite the widespread use of this approach, there is a need for independent validation of the estimated emissions budgets, which can be done by a "top down" approach such as the one used at the BT tower observatory.