Modeling intercontinental transport of ozone in North America with CAMx for the Air Quality Model Evaluation International Initiative (AQMEII) Phase 3

Abstract

Abstract. Intercontinental ozone (O 3 ) transport extends the geographic range of O 3 air pollution impacts and makes local air pollution management more difficult. Phase 3 of the Air Quality Modeling Evaluation International Initiative (AQMEII-3) is examining the contribution of intercontinental transport to regional air quality by applying regionalscale atmospheric models jointly with global models. We investigate methods for tracing O 3 from global models within regional models. The CAMx photochemical grid model was used to track contributions from boundary condition (BC) O 3 over a North American modeling domain for calendar year 2010 using a built-in tracer module called RTCMC. RTCMC can track BC contributions using chemically reactive tracers and also using inert tracers in which deposition is the only sink for O 3 . Lack of O 3 destruction chemistry in the inert tracer approach leads to overestimation biases that can exceed 10 ppb. The flexibility of RTCMC also allows tracking O 3 contributions made by groups of vertical BC layers. The largest BC contributions to seasonal average daily maximum 8 h averages (MDA8) of O 3 over the US are found to be from the mid-troposphere (over 40 ppb) with small contributions (a few ppb) from the upper troposphere-lower stratosphere. Contributions from the lower troposphere are shown to not penetrate very far inland. Higher contributions in the western than the eastern US, reaching an average of 57 ppb in Denver for the 30 days with highest MDA8 O 3 in 2010, present a significant challenge to air quality management approaches based solely on local or US-wide emission reductions. The substantial BC contribution to MDA8 O 3 in the Intermountain West means regional models are particularly sensitive to any biases and errors in the BCs. A sensitivity simulation with reduced BC O 3 in response to 20 % lower emissions in Asia found a near-linear relationship between the BC O 3 changes and surface O 3 changes in the western US in all seasons and across the US in fall and winter. However, the surface O 3 decreases are small: below 1 ppb in spring and below 0.5 ppb in other seasons.

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

“…Given these limitations, our performance is similar to what some others have obtained (Zhang et al, 2008;Nopmongcol, 2016). However, Zhang et al (2014), who reduced lightning NO x emissions, Travis et al (2016), who reduced lightning and anthropogenic NO x emissions, and Yan et al (2016), who used three two-way nested models within GEOS-Chem, obtained better performance than we did.…”

“…Figure 3 shows the comparisons for Trinidad Head and Hilo in April and At Trinidad Head, there is relatively good agreement between the G-base O 3 and the ozonesonde measurements at altitudes up to the mid-troposphere (∼ 7 km) that are most influential to ground-level O 3 in the Intermountain West and eastern US (Nopmongcol et al, 2017). However, the G-base O 3 is greater than the measurements in the marine boundary layer (< 1 km) in August and less than the measurements at upper altitudes in April.…”

“…McDonald-Buller et al (2011) review background and baseline O 3 and how the latter can be used to test model results. There are three common definitions of background O 3 : US background, North American background, and natural background, which correspond to elimination of US, North American, or worldwide anthropogenic emissions, respectively Fiore et al, 2014;Dolwick et al, 2015;Nopmongcol et al, 2016). The anthropogenic increment of O 3 is the difference between a base-case simulation with all emissions present and the chosen type of background simulation.…”

“…A limitation is that the sum of all the anthropogenic source contributions generally does not equal the anthropogenic increment due to the nonlinear chemistry. Another approach is to add reactive tracers (tagged species) to the base case for the emissions from anthropogenic sources and/or the secondary pollutants formed from the emissions and then use the tracers to estimate the contribution of the sources to the total O 3 concentration (Zhang et al, 2008;Lefohn et al, 2014;Baker et al, 2015;Dolwick et al, 2015;Nopmongcol et al, 2017). Chemistry causes interactions between the sources through direct and indirect effects.…”

“…Indirect effects are the photolysis of the NO 2 to recreate O 3 and the reaction of the OH with anthropogenic VOC emissions to form O 3 . Reactive tracers can follow some indirect effects Nopmongcol et al, 2017) but ultimately it is difficult to follow all the nonlinear chemical interactions among emissions from different sources. In addition, there is no requirement that the sum of tracer contributions ascribed to the anthropogenic sources equals the anthropogenic increment (as defined above).…”