In many climate and air quality models, the sulfate phase is not modeled because of the difficulties in tracking the particle RH trajectory. In this project, using GEOS-Chem and the CRH data measured in the lab, we developed methods of explicitly modeling the aerosol phase transition, and quantitatively investigate its implications to aerosol forcing, visibility, cirrus cloud formation, and heterogeneous chemistry.

2. Result
We have so far completed the modeling of sulfate phase transition in sulfate-water system. Please see details in our manuscript. We are now looking into the sulfate phase impact on cirrus formation.
The following figure shows the seasonal and geographic distributions of tropospheric sulfate burdens in aqueous and solid phases, and corresponding aerosol optical thicknesses τaq and τsd. The numbers located to the upper right of each panel show globally averaged quantities.

We select the Central America as our study region. During every Spring season, large amount of smoke from biomass-burning fires in this region is transported to the southeastern United States, degrading the air quality, visibility, and affecting the regional environment and climate. The smoke events in 1998 is the largest one in last decade.  As reported by CNN on May 13, 1998, "AUSTIN, Texas (AP) -- Smoke and air pollution from fires in Mexico, Honduras and Guatemala is drifting across much of the U.S. Southeast, prompting Texas officials to issue a health warning to residents throughout much of south Texas. The Texas warning was the first related to fire pollutants in the state in nearly 30 years. …". In addition air quality effect, these smoke aerosols also affect the surface and air temperature through their scattering and absorbing of sunlight. Recently, it was also found that these smoke aerosols alter the cloud properties and might intrigue the lighting over the Texas area [Lyons et al., 1998, Science; Murray et al., 2000, GRL]. The relatively high density of observation sites through the IMPROVE, EPA AIRS, and DOE ARM networks in the Southeastern U.S. provides us a golden opportunity to test the utility of satellite-based aerosol products for the modeling of smoke transport and smoke radiative effects on climate. This is the exact goal of Central American Biomass Burning - Smoke Transport And Radiation Study (CABB- STARTS) project that was originally started as part of my Ph.D. work funded by the NASA Earth System Science Graduate Fellowship Program, and now is supported by the NOAA Global Change and Climate Postdoctoral Fellowship Program under the administration of UCAR visiting scientist program.

2. Introduction to RAMS-AROMA
RAMS-AROMA is built upon the CSU RAMS model, but has newly developed capabilities of Assimilation and Radiation Online Modeling of Aerosols. The AROMA module is developed by Wang et al [2004], and Wang et al [2005]. In addition to all capabilities that RAMS has, RAMS-AROMA has several new features. In RAMS-AROMA, we have replaced the original RTM in RAMS with a δ-four stream (δ-4S) RTM in which aerosol radiative effects can be explicitly considered. RAMS-AROMA also includes an aerosol transport model in which aerosol advection (using the tracer advection module in RAMS), dry and wet deposition are all considered. Aerosol source functions in RAMS-AROMA are specified by assimilating the satellite-based AOT and emissions. An aerosol optical model (including optical properties of dust and smoke) and cloud optical properties are tabulated in the RAMS-AROMA as well, so that aerosol mass is converted into the AOT, which in turn is used by δ-4S RTM to include the aerosol radiative impact during the radiative transfer calculation. Because all the new features and newly developed models are built upon the RAMS, the coupling among the aerosol transport, aerosol radiation, and meteorology are achieved simultaneously at each time step and each model grid. These unique feathers allow us to simulate the aerosol distribution with high spatiotemporal resolution, and examine the aerosol radiative feedbacks on the surface energy budget, temperature profile, as well as other boundary layer processes such as evolution of boundary layer height.

Although RAMS-AROMA has been applied successfully in several case studies [Wang et al., 2004, 2005], it, just like other models, has various limitations. The further development is ongoing, including nesting this regional model with the Harvard's GEOS-CHEM model to improve aerosol chemistry component in RAMS-AROMA.

3. Results Highlights

see details in Wang, et al., J. Geophys. Res., 2006. (pdf file)