Impact of intercontinental pollution transport on North American ozone air pollution: an HTAP phase 2 multi-model study

The recent update on the US National Ambient Air Quality Standards of the ground-level ozone (O3) can benefit from a better understanding of its source contributions in different US regions during recent years. In the Hemispheric Transport of Air Pollution experiment Phase 1 (HTAP1), various global models were used to determine the O3 source-receptor relationships among three continents in the North Hemisphere in 2001. In support of the HTAP Phase 2 (HTAP2) experiment that studies more recent years and involves higher-resolution global models and regional models’ participation, we conduct a number of regional scale Sulfur Transport and dEposition Model (STEM) air quality base and sensitivity simulations over North America during May-June 2010. The STEM top and lateral chemical boundary conditions were downscaled from three global chemical transport models’ (i.e., GEOS-Chem, RAQMS, and ECMWF C-IFS) base 39 and sensitivity simulations (in which the East Asian anthropogenic emissions were reduced by 40 20%). We perform analyses not only on large spatial/temporal scales relative to the HTAP1, but also on subcontinental- and event-scale that are more relevant to the US air quality management. The differences between STEM surface O3 sensitivities (including the 24h mean and the US policy-relevant maximum daily 8h average (MDA8) metric averaged throughout the study period and during a selected pollution transport event) and its corresponding boundary condition model’s are often smaller than those among its boundary condition models. The STEM sensitivities are also compared with the mean sensitivities estimated by multi- global models, which are higher han the HTAP1 reported 2001 conditions, as well as the 2001-2005 conditions studied using the tagged tracer approach. This indicates the increasing impacts of the East Asian anthropogenic pollution on North America during 2001-2010. The GEOS-Chem sensitivities indicate that the East Asian anthropogenic NOx emissions matter more than the other East Asian O3 precursors to the North American O3, qualitatively consistent with previous adjoint sensitivity calculations. An additional STEM simulation was performed in which the boundary conditions were downscaled from a global RAQMS simulation without East Asian anthropogenic emissions, to assess the scalability of O3 sensitivities to the size of the emission perturbation. The scalability is spatially varying, and the full source contribution obtained by linearly scaling the North American regional mean O3 sensitivity to the 20% reduction in the East Asian emissions may be underestimated. Satellite NO2 (KNMI OMI) and O3 (TES, JPL-IASI, OMI, MLS, and AIRS) products help detect pollution episodes, quantify or/and reduce the uncertainties in the bottom-up NOx emissions and the model transported background O3. Based on model calculations and satellite/surface observations during a selected event, we show the different influences from stratospheric O3 intrusions along with the transported East Asian pollution on O3 in the western and the eastern US. Future directions of using satellite data in air quality research are also suggested.JRC.D.5-Food Securit