Abstract
Abstract. The impacts of transported background (TBG) pollutants on western US ozone (O3) distributions in summer 2008 are studied using the multi-scale Sulfur Transport and dEposition Modeling system. Forward sensitivity simulations show that TBG contributes ~30–35 ppb to the surface Monthly mean Daily maximum 8-h Average O3 (MDA8) over Pacific Southwest (US Environmental Protection Agency (EPA) Region 9, including California, Nevada and Arizona) and Pacific Northwest (EPA Region 10, including Washington, Oregon and Idaho), and ~10–17 ppm-h to the secondary standard metric "W126 monthly index" over EPA Region 9 and ~3–4 ppm-h over Region 10. The strongest TBG impacts on W126 occur over the grass/shrub-covered regions. Among TBG pollutants, O3 is the major contributor to surface O3, while peroxyacetyl nitrate is the most important O3 precursor species. W126 shows larger responses than MDA8 to perturbations in TBG and stronger non-linearity to the magnitude of perturbations. The TBG impacts on both metrics overall negatively correlate to model vertical resolution and positively correlate to the horizontal resolution. The mechanisms that determine TBG contributions and their variation are analyzed using trajectories and the receptor-based adjoint sensitivity analysis, which demonstrate the connection between the surface O3 and O3 aloft (at ~1–4 km) 1–2 days earlier. The probabilities of airmasses originating from Mt. Bachelor (2.7 km) and 2.5 km above Trinidad Head (THD) entraining into the boundary layer reach daily maxima of 66% and 34% at ~03:00 p.m. Pacific Daylight Time (PDT), respectively, and stay above 50% during 09:00 a.m.–04:00 p.m. PDT for those originating 1.5 km above California's South Coast. Assimilation of the surface in-situ measurements significantly reduced the errors in the modeled surface O3 during a long-range transport episode by ~5 ppb on average (up to ~17 ppb) and increased the estimated TBG contributions by ~3 ppb. Available O3 vertical profiles from Tropospheric Emission Spectrometer (TES), Ozone Monitoring Instrument (OMI) and THD sonde identified this transport event, but assimilation of these observations in this case did not efficiently improve the O3 distributions except near the sampling locations, due to their limited spatiotemporal resolution and/or possible uncertainties.
Highlights
Transported background (TBG) ozone (O3) and its precursors from the eastern Pacific and the lower stratosphere, together with the locally-formed O3 from anthropogenic and natural emissions, affect the O3 variability over the western United States (US)
Since time-varying boundary conditions (BCs) downscaled from results in a coarser grid may significantly affect the regional model results (Tang et al, 2007; Huang et al, 2010a; Pfister et al, 2011a), we evaluated the BCs used in this study by comparing Real-time Air Quality Modeling System (RAQMS) and 60 km Sulfur Transport and dEposition Model (STEM) results with: (1) O3, carbon monoxide (CO), Peroxyacetyl nitrate (PAN) and NOy sampled by the 22 June DC-8 flight over the eastern Pacific, and (2) Tropospheric Emission Spectrometer (TES) nadir O3 vertical profiles for the days that “step and stare” observations were available over the eastern Pacific (150–120◦ W, 30–60◦ N) (Fig. 1)
The predictions show higher positive biases along the coast, larger in the 60 km grid, which may be caused by uncertainties in emissions and the BCs, as well as the inaccuracies in predicted meteorology associated with complicated land-sea breezes and topography
Summary
Transported background (TBG) ozone (O3) and its precursors from the eastern Pacific and the lower stratosphere, together with the locally-formed O3 from anthropogenic and natural (e.g., biogenic/geogenic, lightning and biomass burning) emissions, affect the O3 variability over the western United States (US). The magnitude of TBG is expected to increase as the international emission sources grow (Task Force on Hemispheric Transport of Air Pollution (HTAP), 2010; National Research Council (NRC), 2009; Cooper et al, 2012). This trend is especially important in the context of US air quality standards, which tend to be tightened over time to further protect human health and ecosystems. The proposal was withdrawn in 2011 and the revision is expected to occur in 2013 based on the most recent scientific findings (The White House Office of the Press Secretary, 2011)
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