Impacts of transported background pollutants on summertime Western US air quality: model evaluation, sensitivity analysis and data assimilation
The impacts of transported background (TBG) pollutants on western US ozone (O-3) distributions in summer 2008 are studied using the multi-scale Sulfur Transport and dEposition Modeling system. Forward sensitivity simulations show that TBG contributes similar to 30-35 ppb to the surface Monthly mean Daily maximum 8-h Average O-3 (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 similar to 10-17 ppm-h to the secondary standard metric "W126 monthly index" over EPA Region 9 and similar to 3-4 ppm-h over Region 10. The strongest TBG impacts on W126 occur over the grass/shrub-covered regions. Among TBG pollutants, O-3 is the major contributor to surface O-3, while peroxyacetyl nitrate is the most important O-3 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 O-3 and O-3 aloft (at similar to 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 similar to 03:00 p. m. Pacific Daylight Time (PDT), respectively, and stay above 50% during 09:00 a.m.-04:00p.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 O-3 during a long-range transport episode by similar to 5 ppb on average (up to similar to 17 ppb) and increased the estimated TBG contributions by similar to 3 ppb. Available O-3 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 O-3 distributions except near the sampling locations, due to their limited spatiotemporal resolution and/or possible uncertainties.