Abstract
Abstract. The local and regional influence of elevated point sources on summertime aerosol forcing and cloud-aerosol interactions in northeastern North America was investigated using the WRF-Chem community model. The direct effects of aerosols on incoming solar radiation were simulated using existing modules to relate aerosol sizes and chemical composition to aerosol optical properties. Indirect effects were simulated by adding a prognostic treatment of cloud droplet number and adding modules that activate aerosol particles to form cloud droplets, simulate aqueous-phase chemistry, and tie a two-moment treatment of cloud water (cloud water mass and cloud droplet number) to precipitation and an existing radiation scheme. Fully interactive feedbacks thus were created within the modified model, with aerosols affecting cloud droplet number and cloud radiative properties, and clouds altering aerosol size and composition via aqueous processes, wet scavenging, and gas-phase-related photolytic processes. Comparisons of a baseline simulation with observations show that the model captured the general temporal cycle of aerosol optical depths (AODs) and produced clouds of comparable thickness to observations at approximately the proper times and places. The model overpredicted SO2 mixing ratios and PM2.5 mass, but reproduced the range of observed SO2 to sulfate aerosol ratios, suggesting that atmospheric oxidation processes leading to aerosol sulfate formation are captured in the model. The baseline simulation was compared to a sensitivity simulation in which all emissions at model levels above the surface layer were set to zero, thus removing stack emissions. Instantaneous, site-specific differences for aerosol and cloud related properties between the two simulations could be quite large, as removing above-surface emission sources influenced when and where clouds formed within the modeling domain. When summed spatially over the finest resolution model domain (the extent of which corresponds to the typical size of a single global climate model grid cell) and temporally over a three day analysis period, total rainfall in the sensitivity simulation increased by 31% 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 0.46 in the baseline simulation to 0.38 in the sensitivity simulation. The overall net effect of additional aerosols attributable to primary particulates and aerosol precursors from point source emissions above the surface was a domain-averaged reduction of 5 W m−2 in mean daytime downwelling shortwave radiation.
Highlights
Current understanding of how aerosols affect weather and climate contains large uncertainties that must be reduced in order to better estimate the impact of anthropogenic emissions on the atmosphere
This coupling allows for fully interactive feedbacks; do aerosols affect cloud droplet number and cloud radiative properties, but clouds alter aerosol size and composition via aqueous processes, wet scavenging, and gas-phase photolytic processes
During the summer of 2004 several independent atmospheric field measurement programs were conducted in North America, each focusing on separate aspects of climate change and air quality issues
Summary
Current understanding of how aerosols affect weather and climate contains large uncertainties that must be reduced in order to better estimate the impact of anthropogenic emissions on the atmosphere. Emissions from industrial and powerplant stacks likely have a similar effect, but their impact is more difficult to quantify since such point sources are often located in the vicinity of other large anthropogenic sources that contribute to background pollution and CCN levels These sources influence downwind aerosol radiative forcing, and climate. We use WRF-Chem to investigate the shortterm impact of elevated anthropogenic point sources on the net radiative forcing (direct, indirect, and semi-direct) over the northeastern US during a summer period This region was selected because elevated industrial and power-plant stack emissions (Frost et al, 2006) contribute a large fraction of the overall precursors of particulates. We discuss the implications of point source emissions on cloud-aerosol interactions in the model simulations
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