The impacts of emissions, meteorology and climate change on pollution transport

Long-range transport (LRT) of pollutants is known to exacerbate local and regional air quality problems. Questions remain, however, as to the spatial and temporal variation of intercontinental influence and its importance relative to pollution produced at the local or regional scales at present and in the future. A global chemical transport model (MOZART), a general circulation model (AM3) as well as recent field observations are applied to investigate the impacts of pollutant emissions, synoptic meteorology and climate change on the magnitude and variability of LRT.;Surface O3 is a key air pollutant and it is mainly controlled by the supply of NOx on the global or regional scale. U.S. anthropogenic NOx emissions decreased notably since the 1990s due to emission regulations to improve the O3 air quality. Model sensitivity studies show a less-than-linear response of NOy (NOx and its oxidation products) export to NOx emission controls during summer time and suggest a need to control a larger percentage of emissions for a desired change in O3 precursor export. Recent observations suggest that northern mid-latitudes lightning NOx source may be underestimated in the current generation of models. If these models are used to estimate the contribution of O3 LRT, they will likely overestimate the fractional contribution of PAN to total NOy export and the northern mid-latitudinal regional mean O3 production efficiency per unit NOx. The resulting O3 response to foreign anthropogenic emissions will be overestimated. Future observational studies of lightning NOx are needed to constrain model estimates of the O3 response to anthropogenic NOx changes.;Field campaigns over the past decades have identified cyclone passages and associated warm conveyor belts as key mechanisms for pollutant export from the United States. Using MOZART simulated CO as a tracer of pollution, a quantitative relationship between strong daily export events and migratory mid-latitude cyclones is established and these strong export events are estimated to contribute 25% to total pollutant export. Although cyclone passages are the primary driver for strong daily export events, export during days without cyclone passages also makes a considerable contribution (>30%) to the total export and thereby to the global pollutant budget. The meteorological conditions leading to export on these days should be studied further in future field campaigns.;Pollutant transport is closely related to synoptic meteorology and therefore is sensitive to climate change. In a warmer future climate, reduced boundary layer ventilation and wet deposition contribute to a degradation of surface air pollution. Tighter emission regulations are thus required to reach a desired target for air quality as climate warms. Changes in the global total, large-scale or convective precipitation cannot serve as a predictive index for changes in soluble pollutants. Rather, the regional and seasonal changes in large-scale (as opposed to convective) precipitation control the changes in soluble pollutant distribution, at least in our model. Different parameterizations of wet deposition may cause the relative importance of large-scale versus convective wet deposition to vary across models. To obtain a robust prediction of future changes in soluble pollutant distribution and transport, a better process-level understanding of wet deposition on different scales is highly recommended.