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

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 at 12 km × 12 km resolution over March–September 2010. The predicted maximum daily average 8-hour (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 60 ppb, when averaged over the 10 days with the largest base-case ozone. The ozone difference between the base and background cases represents the increment to ozone from all anthropogenic sources. Using the Path-Integral Method, the anthropogenic ozone increment was allocated to US anthropogenic emissions, Canadian/Mexican anthropogenic emissions, and the anthropogenic components of the lateral and top boundary concentrations (BCs). For the larger MDA8 ozone concentrations in the base case, the relative importance of the sources is generally US emissions > anthropogenic lateral BCs > Canadian/Mexican emissions ≫ anthropogenic top BCs. The contributions of the lateral BCs are largest for the higher elevation US sites in the Intermountain West and sites closest to the boundaries. If the focus instead is on the larger ozone concentrations in the background case, the contribution from US emissions is reduced leading to a reduction in the anthropogenic ozone increment. The contribution of the Canadian/Mexican emissions remains about the same, and the contribution from the lateral BCs increases at lower elevation urban sites. The net effect is that the relative importance of the anthropogenic lateral BCs is significantly increased for the days with the largest background concentrations. In addition to the source apportionment, we also used surface and ozonesonde measurements to evaluate GEOS-Chem and CAMx performance.