This paper develops the model and methodology to estimate the marginal external cost of urban road transport, which is necessary for analysing optimal urban transport prices. Four major marginal external costs analysed in this paper include the marginal external costs for congestion, air pollution, road accidents and noise.

Human activities have more than doubled the amount of nitrogen(N) circulating in the biosphere. One major pathway of thisanthropogenic N input into ecosystems has been increased regionaldeposition from the atmosphere.

In arid environments such as deserts, nitrogen is often themost limiting nutrient for biological activity. The majorityof the ecosystem nitrogen flux is typically thought to be drivenby production and loss of reactive nitrogen species by microorganismsin the soil.

Attempts to halt and reverse a decade of worsening air quality in London are being held up by political indecision. After steady rises in air quality up until the turn of the century, London should be well on the way to tackling one of the most serious public health concerns today.