Modeling the effect of chlorine emissions on atmospheric ozone and secondary organic aerosol concentrations across the United States

This paper presents the modeled effects of natural and anthropogenic chlorine emissions on the atmospheric concentrations of ozone and secondary organic aerosol across the United States. The model calculations include anthropogenic molecular chlorine emissions, anthropogenic hypochlorous acid emissions from cooling towers and swimming pools, and natural molecular chlorine released from sea-salt aerosol. The anthropogenic molecular chlorine emissions are obtained from the 1999 National Emissions Inventory (version 3). Studies have suggested that cooling towers and swimming pools can emit substantial quantities of chlorine emissions into the atmosphere; these anthropogenic emissions are estimated using methods available in the literature and modeled in the form of hypochlorous acid. Recent studies suggest that heterogeneous chemical reactions between aerosol-phase chloride and certain gas-phase species may release substantial quantities of molecular chlorine to the marine boundary layer. We recently modeled the effect of chlorine emissions on atmospheric ozone across the United States using the Community Multiscale Air Quality (CMAQ) model. However, the CMAQ model used in that study did not include any sea-salt emissions; therefore heterogeneous reactions with aerosol-phase chloride could not be explicitly treated. Instead, the release of molecular chlorine from sea-salt aerosol was modeled using a constant emission rate over all ocean surfaces. In this study, a recently developed sea-salt emissions and chemistry module is linked with the gas-phase chemistry module of the CMAQ model using heterogeneous reactions that release molecular chlorine to the marine boundary layer. Sea-salt emissions fluxes from the open ocean and coastal surf zone are calculated using the best available parameterizations in the literature. The size distribution of sea-salt particles is adjusted instantaneously to the ambient relative humidity at the point of emission. Also, a set of chlorine reactions is combined with the Carbon Bond IV gas-phase chemical mechanism and incorporated into the CMAQ model. Air quality model simulations are performed for a selected summer period in 2001 and the results obtained with and without chlorine emissions are analyzed. The net effect of chlorine emissions on atmospheric ozone and secondary organic aerosol concentrations is presented in the accompanying article.

DISCLAIMER The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.