| Abstract: |
The effect of atmospheric aerosols on climate may be as large as that of greenhouse gases but much more uncertain. The high uncertainties are due to the currently inadequate representation of the complex aerosol interactions with climate, and the intrinsically complex aerosol composition. Aerosols affect climate through direct and indirect interactions. Through the indirect effect, hydrophilic aerosols may serve as cloud condensation nuclei (CCN) affecting cloud cover and hence the radiation balance. Through direct effects clear air aerosols containing black carbon (BC) such as soot aerosols, absorb incoming light heating the atmosphere, while most other aerosols scatter light and produce cooling. Even though BC represents only 1-2% of the total annual emissions of particulate mass to the atmosphere, it has been estimated that the direct radiative effect of BC is the second-most important contributor to global warming after absorption by CO2. The estimates however are highly uncertain.
During our current DOE funding period, the Boston College/Aerodyne Research group has been performing a range of studies aimed at understanding the role of atmospheric aerosol particles in climate change. The goal of our program is to reduce uncertainties in aerosol radiative forcing in the two areas identified by DOE ASP; namely (1) the indirect effects of aerosols on clouds and (2) the effect on climate of black carbon and organic aerosols. (BC is contained mainly in soot particles).
In this connection we note that soot particles in the atmosphere become coated, often rapidly, with organic and/or inorganic compounds that affect particle morphology, CCN activity, and optical properties. Freshly generated soot particles do not exhibit CCN activity. A hydrophylic coating may convert soot particles into effective cloud condensation nuclei. Coatings may likewise cause dramatic changes in the light scattering and absorption properties of the particles. Therefore, to reliably assess the effect of aerosols on climate, it is particularly important to understand the effect of such atmospheric coatings on particle morphology, CCN activity, and optical properties.
A series of systematic experiments, extending is in progress to study the optical and CCN effects of coatings composed of relevant organic, inorganic and mixed compounds. The results of these studies will be useful in interpreting field experiments, and also in the development of a quantitative model predicting the effect of coatings on the optical and CCN properties of particles.
Another aspect of the proposed studies is instrument development. Available instruments designed to measure parameters relevant to aerosol-climate interactions need to be reliably calibrated and new instruments need to be developed. Our research group has continued and expanded our recent successful instrument development and instrument inter-comparison activities.
Key components of our laboratory apparatus are a well-characterized source of soot aerosol particles developed at Boston College, and the Aerodyne Aerosol Mass Spectrometer (AMS) developed at Aerodyne Research Inc., with the participation of Boston College researchers. The Aerodyne AMS measures, in real time, the non-refractory organic and inorganic mass composition and vacuum aerodynamic diameter of submicron aerosol particles. The AMS is now widely used in field studies with ~ 80 such instruments in service performing field studies in various parts of the world. The AMS coupled to a broad range of other instruments in our laboratory can accurately characterize the aerosol particle size, morphology and composition. We can experimentally simulate realistic atmospheric processes that transform soot aerosols, altering their optical properties and converting initially hydrophobic particles to cloud condensation nuclei (CCN). The effect of highly oxidized organics is being studied in collaboration with Prof. Brune (Penn State) using the PAM technique developed by his group. Our suite of laboratory instruments can quantitatively and systematically characterize the transformation in the CCN and optical (absorption and scattering) properties of the aerosols. Because the laboratory experiments and the field studies are performed with the same AMS instrument, the laboratory data are directly useful in interpreting results of field studies as has been demonstrated. |
