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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 17, issue 20 | Copyright
Atmos. Chem. Phys., 17, 12553-12571, 2017
https://doi.org/10.5194/acp-17-12553-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 24 Oct 2017

Research article | 24 Oct 2017

Contributions of foreign, domestic and natural emissions to US ozone estimated using the path-integral method in CAMx nested within GEOS-Chem

Alan M. Dunker1, Bonyoung Koo2, and Greg Yarwood2 Alan M. Dunker et al.
  • 1LLC, 727 Robinhood Circle, Bloomfield Hills, MI 48304, USA
  • 2Ramboll Environ, 773 San Marin Dr., Suite 2115, Novato, CA 94945, USA

Abstract. The Goddard Earth Observing System global chemical transport (GEOS-Chem) model was used at 2° × 2.5° resolution to simulate ozone formation for a base case representing year 2010 and a natural background case without worldwide anthropogenic emissions. These simulations provided boundary concentrations for base and natural background simulations with the Comprehensive Air Quality Model with Extensions (CAMx) on a North American domain (one-way nested) at 12km × 12km resolution over March–September 2010. The predicted maximum daily average 8h (MDA8) background ozone for the US is largest in the mountainous areas of Colorado, New Mexico, Arizona, and California. The background MDA8 ozone in some of these locations exceeds 60ppb, when averaged over the 10 days with the largest base-case ozone (T10base average). The background ozone generally becomes both a larger fraction of the base-case ozone in the western US and a smaller fraction in the eastern US when proceeding from spring to summer to the T10base average. The ozone difference between the base and background cases represents the increment to ozone from all anthropogenic sources. The path-integral method was applied to allocate this anthropogenic ozone increment to US anthropogenic emissions, Canadian/Mexican anthropogenic emissions, and the anthropogenic components of the lateral and top boundary concentrations (BCs). Using the T10base average MDA8 ozone, the relative importance of the sources is generally US emissions > anthropogenic lateral BCs > Canadian/Mexican emissions ≫ anthropogenic top BCs. Specifically, for 10 US urban areas, the source contributions were 12–53ppb for US emissions, 3–9ppb for lateral BCs, 0.2–3ppb for Canadian/Mexican emissions, and  ≤ 0.1ppb for top BCs. The contributions of the lateral BCs are largest for the higher-elevation US sites in the Intermountain West and along the western boundary of the domain. In Denver, for example, the lateral BCs contribute 44% to the T10base anthropogenic increment. Averaging over the 10 days in the background case with the largest MDA8 ozone (T10bkgd), the contribution from US emissions is reduced to 3–13ppb at the 10 urban sites, leading to a reduction in the anthropogenic ozone increment. The contribution of the Canadian/Mexican emissions remains about the same (0.3–2ppb), and the contribution from the lateral BCs increases (5–10ppb), especially at the lower-elevation urban sites. The net effect at the urban sites is that the relative importance of the anthropogenic lateral BCs is significantly greater for the T10bkgd average than the T10base average. This is also true for rural sites studied, where the anthropogenic lateral BC contribution becomes as large as 68% of the anthropogenic increment for the T10bkgd average. In addition to the source apportionment, we also used surface and ozonesonde measurements to evaluate GEOS-Chem and CAMx performance.

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Using global and regional models, we determined the anthropogenic increment to ozone (base-case simulation minus a background simulation with only natural emissions) for the US and allocated the increment to anthropogenic emissions inside and outside the US. For the larger ozone concentrations in the base case, the relative importance of the sources is generally US emissions > anthropogenic lateral boundary concentrations (BCs) > Canadian/Mexican emissions ≫ anthropogenic top BCs.
Using global and regional models, we determined the anthropogenic increment to ozone (base-case...
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