Coupling aerosol-cloud-radiative processes in the WRF-Chem model: Investigating the radiative impact of elevated point sources

摘要

The local and regional influence of elevated point sources on summertimeaerosol forcing and cloud-aerosol interactions in northeastern North Americawas investigated using the WRF-Chem community model. The direct effects ofaerosols on incoming solar radiation were simulated using existing modulesto relate aerosol sizes and chemical composition to aerosol opticalproperties. Indirect effects were simulated by adding a prognostic treatmentof cloud droplet number and adding modules that activate aerosol particlesto form cloud droplets, simulate aqueous-phase chemistry, and tie atwo-moment treatment of cloud water (cloud water mass and cloud dropletnumber) to precipitation and an existing radiation scheme. Fully interactivefeedbacks thus were created within the modified model, with aerosolsaffecting cloud droplet number and cloud radiative properties, and cloudsaltering aerosol size and composition via aqueous processes, wet scavenging,and gas-phase-related photolytic processes. Comparisons of a baselinesimulation with observations show that the model captured the generaltemporal cycle of aerosol optical depths (AODs) and produced clouds ofcomparable thickness to observations at approximately the proper times andplaces. The model overpredicted SO mixing ratios and PM mass, butreproduced the range of observed SO to sulfate aerosol ratios,suggesting that atmospheric oxidation processes leading to aerosol sulfateformation are captured in the model. The baseline simulation was compared toa sensitivity simulation in which all emissions at model levels above thesurface layer were set to zero, thus removing stack emissions.Instantaneous, site-specific differences for aerosol and cloud relatedproperties between the two simulations could be quite large, as removingabove-surface emission sources influenced when and where clouds formedwithin the modeling domain. When summed spatially over the finest resolutionmodel domain (the extent of which corresponds to the typical size of asingle global climate model grid cell) and temporally over a three dayanalysis period, total rainfall in the sensitivity simulation increased by31% over that in the baseline simulation. Fewer optically thin clouds,arbitrarily defined as a cloud exhibiting an optical depth less than 1,formed in the sensitivity simulation. Domain-averaged AODs dropped from 0.46in the baseline simulation to 0.38 in the sensitivity simulation. Theoverall net effect of additional aerosols attributable to primaryparticulates and aerosol precursors from point source emissions above thesurface was a domain-averaged reduction of 5 W m in mean daytimedownwelling shortwave radiation.