The influence of megacities on global atmospheric chemistry: a modelling study

We present the first study of the effects of megacities on global atmospheric chemistry using a global three-dimensional chemical transport model. The effects on air quality, radiative forcing and atmospheric oxidation capacity are disproportionately smaller than the proportion of anthropogenic emissions due to megacities. Disproportionately large effects of megacities are modelled for reactive nitrogen compounds, in particular PAN (peroxy acetyl nitrate), which has increased in abundance globally by 9% due to megacities under year 2000 conditions, with 23% of the Earth experiencing an increase of 10% or more. These influences decrease under two very different future emission scenarios. Under a low-emission future scenario, the influence of megacities is generally reduced, and under a high-emission future scenario, although the local influence of megacities is increased, the geographical extent of the influence becomes smaller. In our model, the individual grid cells that contain megacities respond to the megacity emissions differently depending on their latitude. Tropical megacity grid cells generally show increased ozone year-round, while northern extratropical megacities generally show reduced ozone year-round. Better parameterisation of the sub-grid effects of megacities is an important issue for future work.