The impact of marine organic emissions on global climate and coastal air qiuality

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Description

The emissions of primary organic matter (POM) of marine biogenic origin and secondary organic aerosol (SOA) from the oxidation of phytoplankton-produced volatile organic compounds (VOC) can lead to changes in chemical composition and size distribution of marine aerosol, cloud droplet number concentration (CDNC), coastal air quality, and climate. Recent studies suggest that due to poor characterization of marine aerosols over remote regions, global climate models with prognostic treatment of CDNC could have up to 80% uncertainty in simulated aerosol indirect effect. Here the global and regional effects of marine biogenic trace gases and organic carbon (OC) aerosol emissions are explored using the NCAR Community Atmosphere Model, coupled with the PNNL Modal Aerosol Model (CAM-MAM) and the Community Multiscale Air Quality (CMAQ) modeling system Version 4.7. 10-year CAM-MAM model simulations are conducted at a grid resolution of 1.9°×2.5° with 26 vertical layers. The CMAQ simulations are performed for the months of June¬–August, 2005 over the western U.S. at a horizontal resolution of 12×12 km2. Remotely sensed chlorophyll-a concentration, laboratory measurements, and model meteorology are used to calculate marine emissions of isoprene and monoterpenes. Marine POM emissions in sub- and super-micron modes are calculated by connecting organic mass fraction of sea spray with remotely sensed wind speed and the sea surface concentration of dissolved organic carbon (DOC). Both sub- and super-micron marine POM are assumed to be mostly water-insoluble, while marine SOA is assumed to be 50% water-soluble. Preliminary results show that different marine aerosol emissions and cloud droplet activation mechanisms yield 10%–20% increase in CDNC of global maritime shallow clouds. Changes associated with cloud properties raise shortwave forcing by -0.3Wm-2 to -0.7W m-2. Simulations suggest small effect of marine organic emissions on a regional scale with 0.1–0.2% increase in ozone and up to 3% increase in PM2.5 concentrations. Nevertheless, marine sources are shown to comprise 30% and 50% of the simulated average surface OC aerosol over the coastal areas and open ocean, respectively. By using different emission scenarios, SOA formation mechanisms, and droplet activation parameterizations, this study suggests that addition of marine primary aerosols and biologically generated reactive gases could reduce uncertainty in future global climate simulations and air quality studies.