We use a global 3-D coupled oxidant-aerosol model (GEOS-CHEM) to quantify natural and transboundary pollution influences on visibility degradation in the United States, and to assess the implications for the EPA Regional Haze Rule (RHR). The model is evaluated extensively with frequency distributions of aerosol concentrations and visibility degradation measured at the IMPROVE network of sites. We examine in particular the ability of the model to simulate the spread in the frequency distributions, which is a critical metric for application of the RHR. Model results show that transboundary transport of pollution, including transpacific transport of sulfate from Asia, makes the RHR aspiration of natural visibility in federal class I areas unachievable through domestic controls only. Transboundary pollution also affects significantly the schedule of emission reductions under Phase I implementation of the RHR (2004-2018). Satellite (MODIS) and ground-based (AERONET) measurements of aerosol optical depth confirm the model simulation of transpacific transport of sulfate pollution. In particular, we show that Asian dust events over the western U.S. are accompanied with a substantial Asian pollution enhancement. Sulfur is exported from Asia in the model mostly as SO2, which escapes precipitation scavenging. The resulting sulfate is acidic and highly hygroscopic, magnifying its contribution to visibility degradation.

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