Global impact of fossil fuel combustion on atmospheric NOx

Fossil fuel combustion is the largest global source of NOx to the troposphe re. This source is concentrated in polluted continental boundary layers, an d the extent to which it impacts tropospheric chemistry on a global scale i s uncertain. We use a global three-dimensional model of tropospheric chemis try and transport to study the impact of fossil fuel combustion on the glob al distribution of NOx during nothern hemisphere summer. In the model, we t ag fossil fuel NOx and its reservoir NOy species in order to determine the relative contribution of fossil fuel combustion to NOx concentrations in di fferent regions of the world. Our model includes a detailed representation of NOx-O-3-nonmethane hydrocarbon (NMHC) chemistry, which is necessary to p roperly simulate the export of reactive nitrogen, including organic nitrate s such as peroxyacyl nitrates (PANs), from the continental boundary layer. We find that fossil fuel combustion accounts for over 40% of NOx concentrat ions in the lower and middle troposphere throughout the extratropical north ern hemisphere. PANs are shown to provide an important mechanism for transp orting NOx from source regions to the remote troposphere, accounting for ov er 80% of the fossil fuel NOx in the lower troposphere over most of the oce an. 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 f or 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 atmosphe re.