Linking local air pollution to global chemistry and climate

We have incorporated a reduced-form urban air chemistry model in the Massachusetts Institute of Technology's two-dimensional land and ocean resolving coupled chemistry-climate model. The computationally efficient reduced-form urban model was derived from the California Institute of Technology-Carnegie Institute of Technology (at Carnegie Mellon University) Urban Airshed Model by employing the probabilistic collocation method. To study the impact of urban air pollution on global chemistry and climate, we carried out three simulations, each including or excluding the reduced-form urban model for the time period from 1977 to 2100. All three runs use identical total emissions; however, in the two runs involving the reduced-form urban model, the emissions assigned to urban areas are allocated in different ways, depending on the scenario assumed for the future development of polluted urban areas. These two simulations are compared to the reference, which does not utilize the reduced-form urban model. We find that the incorporation of urban air chemistry processes leads to lower global tropospheric NO_x, ozone, and OH concentrations, but a higher methane mole fraction, than in the reference. The tropospheric mole fraction of CO is altered either up or down depending on the projections of urban emissions. The global mean surface temperature is affected very little by implementation of the reduced-form urban model because predicted increases in CH₄ are offset in part by decreases in O₃, leading to only small changes in overall radiative forcing.