| Abstract: |
Aerosol particles are well known to cause the largest uncertainty in predicting climate impacts of energy-related species. In particular, their indirect effects on cloud properties are especially uncertain. The organic component of particles is a major determinant of the water uptake and growth of particles, and hence their ability to act as cloud condensation nuclei (CCN). For example, surfactants on the surface of aerosol particles can inhibit water uptake, while highly oxidized organics have been thought to enhance water uptake and particle growth. However, it has become clear during recent years that this simplified view is often not correct; there are increasing examples of highly oxidized organic films or particles not becoming hydrophilic as originally expected. As a result, it is critical to understand the nature of organic particles and coatings at the molecular level, including their formation, fates and in some cases, portioning between the gas and particle phases.
While there is some understanding of the chemistry that leads to organic aerosols generated by gas-to-particle conversion either in the absence of existing particles or in the presence of pre-existing sulfate particles, little is known about the potential for organic aerosol formation associated with nitrate particles. This is a critical gap because (1) nitrate is a major component of tropospheric aerosols, often exceeding sulfate by a significant amount; (2) nitrate in aqueous solutions is known to reside largely at the air-liquid interface, and photolyzes to generate OH radicals at the interface.
Laboratory studies using a unique flow tube designed for this research are being carried out to measure the change in particle properties and composition upon exposure of nitrate particles to volatile organic compounds (VOC) in the presence of light. These studies mimic the changes that occur as an air mass moves downwind from an urban core. Use of selected single VOC and mixtures of VOCs capture synergisms in the chemistry that lead to different products and combinations of products in the particles. To aid in interpretation of the results, studies of reactions with O3 and mixtures of O3 and NO2 (which generate NO3) in the dark are also carried out. The VOCs are representative of biogenic compounds as well as anthropogenic species with carefully selected structures that will provide insight into the relationship between the precursor organics and changes in the aerosol properties. In addition, unique aerosol "marker compounds" are probed that can be targets of field measurement programs and used to estimate the contribution of biogenic versus anthropogenic sources to the organic component of particles.
The understanding gained from these studies are very useful in guiding field studies of aerosol transformations as the air mass moves downwind from source regions and is distributed regionally and globally. By judicious choice of the VOCs in terms of their structure and reactivity, relationships between the structure of the gas phase parent molecule and the properties of the aerosol are elucidated and parameterized. This parameterization can then be used as input to climate models designed to track the impacts of large aerosol source regions on regional and global climate. |
