The Role of Intercontinental Transport in a Changing Climate

Intercontinental transport of pollution is of major concern as it can offset the impact of local emission control efforts in certain regions. This study aims to determine what role the intercontinental transport of pollution from East Asia will play in global air quality and climate under current and future emissions and climate scenarios. The model simulations are performed using the Global-through-Urban Weather Research and Forecasting Model with Chemistry for the spring months (March, April, and May) of the years 2001 and 2050, during which both the enhanced vertical and horizontal motion make the impact of intercontinental transport more pronounced. To access the impact of the pollution from East Asia on the globe we conduct six simulations, including three baseline simulations of 2001, 2050 with projected climate change and emissions, and 2050 with climate change only (2050_CCO) and three sensitivity simulations in which East Asian anthropogenic emissions (EAAEs) are removed. The results show that East Asian pollution export follows two primary pathways: a northward track into the Arctic region and a westerly track into the North Pacific and western North America. Under the current scenario, ozone (O₃) generated by EAAEs contributes to 1.2 ppb of the surface maximum 8-hr average O₃ mixing ratio on a global average (3-33.5 ppb in East Asia, 1-2 ppb in western North America, and 1-3 ppb in the Arctic). Under the future scenario, EAAEs produce ~2 ppb of O₃ on a global average (4-44.6 ppb in East Asia and 2-5 ppb in western North America and the Artic). This is a result of increased East Asian emissions of O₃ precursors in the future, as 2050_CCO shows a contribution of 1.2 ppb on global average (3-37.7 ppb in East Asia, 0-3 ppb in western North America, and 1-3 ppb in the Arctic). The differences in O₃ between the 2001 and 2050_CCO simulations are the result of the 1-3 ⁰C increase in temperature over most of East Asia (except south east China with cooling by ~1 ⁰C) and changes in the atmospheric flow pattern. The increased temperature results in increased O₃ production thus making the contribution in East Asia slightly larger in 2050_CCO than 2001, additionally the 2050_CCO flow pattern diverts EAAEs O₃ from the western U.S. (reducing the contribution of EAAEs to O₃ mixing ratios by 1 ppb in that region) into two separate tracks. The northerly track leads to an increased O₃ contribution of ~1 ppb in portions of Alaska and western Canada. The southerly track flows into western Mexico increasing the role of EAAEs generated O₃ by ~1 ppb in some areas. The impact on PM_2.5 especially in the planetary boundary layer (PBL) is not homogenous as the removal of EAAEs alters the wind speed and PBL height through cloud and radiative feedbacks. The wind speed alterations over the oceans lead to dipole increases and decreases in natural sea salt emissions. Additionally, EAAEs contribute to an increase in PBLH through atmospheric feedback mechanisms over Scandinavia where atmospheric conditions are stagnant in 2001, resulting in a decrease of 0.5-3 µg m^-3 in the local PM_2.5 level. However, EAAEs contribute 0.3-0.4 µg m^-3 to the global average PM_2.5 concentration. The removal of EAAEs reduces the biogenic secondary organic aerosol (BSOA) in East Asia by 0.1-1.2 µg m^-3 (10-81%) with large areas experiencing a decrease of 0.5-1.2 µg m^-3 (50-81%) under both current and future scenarios as a result of a reduction in the atmosphere's oxidation capacity. In addition, EAAEs of aerosol and greenhouse gases such as CH₄ and N₂O affect climate, particularly over East Asia and North Pacific. The EAAEs-induced aerosols contribute 0.5 x 10⁸ - 1.2 x 10⁹ cm^-2 to the column cloud condensation nuclei level at a supersaturation of 0.5% in these regions, which translate into a 1 x 10⁵ – 2.7 x 10⁶ cm^-2 contribution to the cloud droplet number concentration. Such enhancements account for a 0.5-4.0 larger cloud optical thickness which in turn leads to incoming solar radiation that is lower by 8-21% in the base cases than that in the simulation without EAAEs. These results show that EAAEs have a large impact on global air quality and climate, especially on downwind regions, demonstrating a need to synergize global emissions control efforts.