Terry Keating

A multi‐model study of the hemispheric transport and deposition of oxidised nitrogen

Fifteen chemistry-transport models are used to quantify, for the first time, the export of oxidised nitrogen (NOy) to and from four regions (Europe, North America, South Asia, and East Asia), and to estimate the uncertainty in the results. Between 12 and 24% of the NOx emitted is exported from each region annually. The strongest impact of each source region on a foreign region is: Europe on East Asia, North America on Europe, South Asia on East Asia, and East Asia on North America. Europe exports the most NOy, and East Asia the least. East Asia receives the most NOy from the other regions. Between 8 and 15% of NOx emitted in each region is transported over distances larger than 1000 km, with 3–10% ultimately deposited over the foreign regions.

Technical note: Coordination and harmonization of the multi-scale, multi-model activities HTAP2, AQMEII3, and MICS-Asia3: simulations, emission inventories, boundary conditions, and model output formats

We present an overview of the coordinated global numerical modelling experiments performed during 2012–2016 by the Task Force on Hemispheric Transport of Air Pollution (TF HTAP), the regional experiments by the Air Quality Model Evaluation International Initiative (AQMEII) over Europe and North America, and the Model Intercomparison Study for Asia (MICS-Asia). To improve model estimates of the impacts of intercontinental transport of air pollution on climate, ecosystems, and human health and to answer a set of policy-relevant questions, these three initiatives performed emission perturbation modelling experiments consistent across the global, hemispheric, and continental/regional scales. In all three initiatives, model results are extensively compared against monitoring data for a range of variables (meteorological, trace gas concentrations, and aerosol mass and composition) from different measurement platforms (ground measurements, vertical profiles, airborne measurements) collected from a number of sources. Approximately 10 to 25 modelling groups have contributed to each initiative, and model results have been managed centrally through three data hubs maintained by each initiative. Given the organizational complexity of bringing together these three initiatives to address a common set of policy-relevant questions, this publication provides the motivation for the modelling activity, the rationale for specific choices made in the model experiments, and an overview of the organizational structures for both the modelling and the measurements used and analysed in a number of modelling studies in this special issue.

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 (NAAQS) 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 (SR) relationships among three continents in the Northern 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. STEM’s top and lateral chemical boundary conditions were downscaled from three global chemical transport models’ (i.e., GEOS-Chem, RAQMS, and ECMWF C-IFS) base and sensitivity simulations in which the East Asian (EAS) anthropogenic emissions were reduced by 20 %. The mean differences between STEM surface O3 sensitivities to the emission changes and its corresponding boundary condition model’s are smaller than those among its boundary condition models, in terms of the regional/period-mean (<10 %) and the spatial distributions. An additional STEM simulation was performed in which the boundary conditions were downscaled from a RAQMS (Realtime Air Quality Modeling System) simulation without EAS anthropogenic emissions. The scalability of O3 sensitivities to the size of the emission perturbation is spatially varying, and the full (i.e., based on a 100% emission reduction) source contribution obtained from linearly scaling the North American mean O3 sensitivities to a 20% reduction in the EAS anthropogenic emissions may be underestimated by at least 10 %. The three boundary condition models’ mean O3 sensitivities to the 20% EAS emission perturbations are ~8% (May–June 2010)/~11% (2010 annual) lower than those estimated by eight global models, and the multi-model ensemble estimates are higher than the HTAP1 reported 2001 conditions. GEOS-Chem sensitivities indicate that the EAS anthropogenic NOx emissions matter more than the other EAS O3 precursors to the North American O3, qualitatively consistent with previous adjoint sensitivity calculations. In addition to the analyses on large spatial–temporal scales relative to the HTAP1, we also show results on subcontinental and event scales that are more relevant to the US air quality management. The EAS pollution impacts are weaker during observed O3 exceedances than on all days in most US regions except over some high-terrain western US rural/remote areas. Satellite O3 (TES, JPL–IASI, and AIRS) and carbon monoxide (TES and AIRS) products, along with surface measurements and model calculations, show that during certain episodes stratospheric O3 intrusions and the transported EAS pollution influenced O3 in the western and the eastern US differently. Free-running (i.e., without chemical data assimilation) global models underpredicted the transported background O3 during these episodes, posing difficulties for STEM to accurately simulate the surface O3 and its source contribution. Although we effectively improved the modeled O3 by incorporating satellite O3 (OMI and MLS) and evaluated the quality of the HTAP2 emission inventory with the Royal Netherlands Meteorological Institute–Ozone Monitoring Instrument (KNMI–OMI) nitrogen dioxide, using observations to evaluate and improve O3 source attribution still remains to be further explored.

The Incorporation of the US National Emission Inventory into Version 2 of the Hemispheric Transport of Air Pollutants Inventory

EPA’s 2008 National Emission Inventory has been incorporated into version 2 of the Hemispheric Transport of Air Pollutants Inventory. This work involves the creation of a detailed mapping of EPA Source Classification Codes (SCC) to the International Nomenclature for Reporting System (NFR). The mapping of SCC codes to the NFR system allows for comparison of USA emission inventories with other national inventories on a consistent basis. We summarize the emission estimates for 2008 and 2010 and provide a useful reference for linking USA inventories to Global inventories for use in regional and global chemical transport models.

Source contributions of sulfur and nitrogen deposition – an HTAP II multimodel study on hemispheric transport

Abstract. With rising emissions by human activities, enhanced concentrations of air pollutants have been detected in hemispheric air flows in recent years, aggravating the regional air pollution and deposition burden. However, contributions of hemispheric air pollution to deposition at global scale have been given little attention in the literature. In this light, we assess the impact of hemispheric transport on sulfur (S) and nitrogen (N) deposition for 6 world regions: North America, Europe, South Asia, East Asia, Middle East and Russia in 2010, by using the multi-model ensemble results from the 2 nd phase of Task Force Hemispheric Transport of Air Pollution (HTAP II) with and without 20 % emission perturbation experiments. About 27–58 %, 26–46 % and 12–23 % of local S, NO x and NH 3 emissions are transported and removed by deposition outside of the source regions annually, with 5 % higher fraction of export in winter and 5 % lower in summer. For receptor regions, 20 % emission reduction in source regions affects the deposition in receptor regions by 1–10 % for continental non-coastal regions and 1–15 % for coastal regions and open oceans. Significant influences are found from North America to the North Atlantic Ocean (5–15 %), from South Asia to western East Asia (2–10 %) and from East Asia to the North Pacific Ocean (5–10 %) and western North America (5–8 %). The impact on deposition caused by transport between neighbouring regions (i.e. Europe and Russia) occurs throughout the whole year (slightly stronger in winter), while that by transport over long distances (i.e. from East Asia to North America) mainly takes place in spring and fall, which is consistent with the seasonality found for hemispheric transport of air pollutants. Deposition in emission intense regions such as East Asia is dominated (~ 80 %) by own region emission, while deposition in low emission regions such as Russia is almost equally affected by own region emission (~ 40 %) and foreign impact (~ 23–45 %). We also find that deposition on the coastal regions or near coastal open ocean is twice more sensitive to hemispheric transport than non-coastal continental regions, especially for regions (i.e. west coast of North America) in the downwind location of major emission source regions. This study highlights the significant impact of hemispheric transport on deposition in coastal regions, open ocean and low emission regions. Further research is proposed for improving ecosystem and human health in these regions, with regards to the enhanced hemispheric transport.