Richard C. Easter

A physically based estimate of radiative forcing by anthropogenic sulfate aerosol

Estimates of direct and indirect radiative forcing by anthropogenic sulfate aerosols from an integrated global aerosol and climate modeling system are presented. A detailed global tropospheric chemistry and aerosol model that predicts concentrations of oxidants as well as aerosols and aerosol precursors, is coupled to a general circulation model that predicts both cloud water mass and cloud droplet number. Both number and mass of several externally-mixed aerosol size modes are predicted, with internal mixing assumed for the different aerosol components within each mode. Predicted aerosol species include sulfate, organic and black carbon, soil dust, and sea salt. The models use physically-based treatments of aerosol radiative properties (including dependence on relative humidity) and aerosol activation as cloud condensation nuclei. Parallel simulations with and without anthropogenic sulfate aerosol are performed for a global domain. The global and annual mean direct and indirect radiative forcing due to anthropogenic sulfate are estimated to be -0.3 to -0.5 and -1.5 to -3.0 W m-2, respectively. The radiative forcing is sensitive to the models horizontal resolution, the use of predicted vs. analyzed relative humidity, the prediction vs. diagnosis of aerosol number and droplet number, and the parameterization of droplet collision/coalescence. About half of the indirect radiativemorexa0» forcing is due to changes in droplet radius and half to increased cloud liquid water.«xa0less

Analysis of Mid-Tropospheric Carbon Monoxide Data Using a Three-Dimensional Global Atmospheric Chemistry Numerical Model

Carbon monoxide is an important atmospheric trace species. It has long been recognized as a major contributor to urban air quality and in high concentrations is known to adversely affect health (Seinfeld, 1986). CO is the third most abundant carbon-containing species in the atmosphere and its reaction with hydroxyl radical (OH) represents a 2000–3000 Tg/yr (1 Tg = 1012 g) source of carbon dioxide. On a global basis, through its reaction with OH, CO plays a significant role in the troposphere’s overall oxidative capacity (Crutzen and Zimmerman, 1991). Furthermore, depending on the local abundance of nitrogen oxides, CO can participate in reactions that either increase or decrease the formation of tropospheric ozone (Logan et al., 1981).

Simulation of Climate Forcing by Aerosols

The largest source of uncertainty in estimates of the radiative forcing governing climate change is in the radiative forcing due to anthropogenic aerosols. Current estimates of the global mean of the aerosol radiative forcing range from –0.3 to –3.0 watts per square meter (Wm-2 ) which is opposite in sign and possibly comparable in magnitude to the +2 Wm-2 forcing due to increasing greenhouse gases. We have developed a global aerosol and climate modeling system that provides arguably the most detailed treatment of aerosols and their impact on the planetary radiation balance of any model, but our estimates of radiative forcing have been hindered by our lack of access to high performance computing resources. We propose to use the MSCF to conduct a series of simulations with and without emissions of a variety of aerosol particles and aerosol precursors. These extensive simulations will enable us to produce much more refined estimates of the impact of anthropogenic emissions on radiative forcing of climate change. To take full advantage of the parallelism available on the MSCF MPP1, we will apply the Global Array Toolkit to dynamically load balance the reactive chemistry component of our model. We will adapt our modifications of the morexa0» serial NCAR Community Climate Model CCM2 to the parallel NCAR CCM3.10. «xa0less