Global impact of fossil fuel combustion on atmospheric NOx

Abstract

Fossil fuel combustion is the largest global source of NOx to the troposphere. This source is concentrated in polluted continental boundary layers, and the extent to which it impacts tropospheric chemistry on a global scale is uncertain. We use a global three-dimensional model of tropospheric chemistry and transport to study the impact of fossil fuel combustion on the global distribution of NOx during nothern hemisphere summer. In the model, we tag fossil fuel NOx and its reservoir NOy species in order to determine the relative contribution of fossil fuel combustion to NOx concentrations in different regions of the world. Our model includes a detailed representation of NOx-O3-nonmethane hydrocarbon (NMHC) chemistry, which is necessary to properly simulate the export of reactive nitrogen, including organic nitrates such as peroxyacyl nitrates (PANs), from the continental boundary layer. We find that fossil fuel combustion accounts for over 40% of NOx. concentrations in the lower and middle troposphere throughout the extratropical northern hemisphere. PANs are shown to provide an important mechanism for transporting NOx from source regions to the remote troposphere, accounting for over 80% of the fossil fuel NOx in the lower troposphere over most of the ocean. Sources in the United States are found to contribute about half of the fossil fuel NOx over the North Atlantic Ocean. Emissions from China, which are expected to increase rapidly in the coming decades, currently account for about half of the fossil fuel NOx over the western North Pacific Ocean; the influence of these emissions extends into the tropics. Because of this tropical influence, emissions from China have more potential than emissions in the United States to perturb the global oxidizing power of the atmosphere.