Impacts of transported background pollutants on summertime Western US air quality: model evaluation, sensitivity analysis and data assimilation
1Center for Global and Regional Environmental Research, University of Iowa, Iowa City, IA 52242, USA
2NOAA/OAR/ARL, Silver Spring, MD 20910, USA
3NOAA/NESDIS, Madison, WI 53706, USA
4NOAA/ESRL, Boulder, CO 80305, USA
5University of Washington, Bothell, WA 98011, USA
6Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
7California Air Resource Board, Sacramento, CA 95812, USA
8National Center for Atmospheric Research, Boulder, CO 80305, USA
9School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 30332, Atlanta, GA, USA
10NASA Langley Research Center, Hampton, VA 23681, USA
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 extensively affect Western US surface O3, and can contribute to >50% of the total O3, varying among different geographical regions and land types. The stratospheric O3 impacts are weak. Ozone is the major contributor to surfaceO3 among the TBG pollutants, and TBG peroxyacetyl nitrate is the most important O3 precursor species. Compared to monthly mean daily maximum 8-h average O3, the secondary standard metric "W126 monthly index" shows larger responses to TBG perturbations and stronger non-linearity to the size of perturbations. Overall the model-estimated TBG impacts negatively correlate to the vertical resolution and positively correlate to the horizontal resolution. The estimated TBG impacts weakly depend on the uncertainties in US anthropogenic emissions.
Ozone sources differ at three sites spanning ~10° in latitude. Mt. Bachelor (MBO) and Trinidad Head (THD) O3 are strongly affected by TBG, and occasionally by US emissions, while South Coast (SC) O3 is strongly affected by local emissions. The probabilities of airmasses originating from MBO (2.7 km) and THD (2.5 km) entraining into the boundary layer reach daily maxima of 66% and 34% at ~3:00 p.m. PDT, respectively, and stay above 50% during 9:00 a.m.–4:00 p.m. for those originating from SC (1.5 km). Receptor-based adjoint sensitivity analysis demonstrates the connection between the surface O3 and O3 aloft (at ~1–4 km) at these sites 1–2 days earlier.
Assimilation of the surface in-situ measurements significantly reduced (~5 ppb in average, up to ~17 ppb) the modeled surface O3 errors during a long-range transport episode, and is useful for estimating the upper-limits of uncertainties in satellite retrievals (in this case 5–20% and 20–30% for Tropospheric Emission Spectrometer (TES) and Ozone Monitoring Instrument (OMI) O3 profiles, respectively). Satellite observations identified this transport event, but assimilation of the existing O3 vertical profiles from TES, OMI and THD sonde in this case did not efficiently improve the O3 distributions except near the sampling locations, due to their limited spatiotemporal resolution and possible uncertainties.