The study aims to identify and quantify linkages between the biogenesis of aerosols that can function as cloud nuclei and the feedback through altered precipitation patterns on vegetation processes that generate more aerosols. The approach combines experiments, field measurements and laboratory investigations. Each represents a key component of the overall cycle: 1) surveys of important sources of biogenic volatile organic compounds along a broad transect of sites in the Southwestern U.S.; 2) atmospheric surveys of biological aerosol particles, which will take place at one of the sites and include the interesting idea of DNA sequencing of particles to identify their sources; 3) two field studies to manipulate water and investigate impacts on vegetation, one involving imposition of drought in individual ponderosa pine trees and an investigation of their VOC emissions and the other on a separate (but nearby) set of plots to impose different moisture regimes to quantify the impact of moisture on soil/litter VOC emissions; 4) studies of biogenic ice nuclei and cloud condensation nuclei. These four investigations will be linked together in a fifth activity involving a model: Model of Emissions of Gases and Aerosols from Nature (MEGAN).
One of the principal broader impacts of this work will occur through the improvement of a coupled surface-atmosphere model that is available to the broader scientific community from the National Center for Atmospheric Research (NCAR) web site, and is capable of supporting future studies involving plant-cloud interactions. The project will also support diversity in science initiatives at NCAR and the University of Colorado by involving at least six underrepresented-group undergraduate students in the research project. To extend the impact of project into the public sector and the K-12 community, web-based educational materials will be developed that will explain the impact of climate change on the forest ecosystem, and will include real-time data from the Manitou Forest Observatory. Ultimately, the results will lead to new strategies for assessing the impact of regional climate change on ecosystems in the Western U.S. and managing those impacts through recognition of the role ecosystems play in amplifying or dampening further climate change.
This project was an interdisciplinary study of the connections between the carbon and water cycles in a semi-arid region of the Western U.S. The study included a variety of activities, all of which were aimed at the goal of using observations to assess the influence of ecosystem processes, and their responses to climate, on the number and composition of climatically-relevant biogenic aerosols and their potential to function as cloud condensation nuclei (CCN) and ice nuclei (IN), which in turn can influence regional clouds and precipitation. This work was done within the context of the National Center for Atmospheric Research (NCAR) Bio-hydro-atmosphere-interactions of Energy, Aerosols, Carbon, H2O, Organics, and Nitrogen (BEACHON) project. CCN are particles on which water condenses to form clouds droplets. The ability of a particle to serve as an effective CCN at typical humidities in clouds depends both on its size and hygroscopicity. IN are those particles that catalyze ice nucleation in the atmosphere, a critical step in initiating precipitation in mixed phase clouds. Varying levels of CCN and IN can impact cloud density, cloud lifetime, cloud moisture levels, and precipitation. The role of our work in this interdisciplinary project was to quantify CCN and IN number concentrations and compositions and to relate these measurements to biogenic emissions. All work was carried out at Manitou Experimental Forest Observatory (MEFO) in Colorado, a mountainous semi-arid ponderosa pine-dominated forest. We collected a full annual cycle of size-resolved CCN data at MEFO, which to our knowledge is the first dataset of its kind. Data were analyzed to determine both number concentrations and hygroscopicity of the ambient particles. Along with particle size distributions, these data provide a simple description of aerosol-water interactions which can be incorporated into models for investigating aerosol effects on warm cloud formation. We found that there was generally little change in hygroscopicity for the smallest particles at the site, and that these particles were dominated by organics of biogenic origin. At larger sizes, the aerosol had a stronger influence from anthrogenic sources, but was still dominated by organics derived from biogenic emissions. We also observed new particle formation events throughout the year, the growth of which appeared to be driven by biogenic organics, and these events had a clear impact on CCN number concentration and hygroscopicity. We were able to validate our inferred aerosol composition with measurements from two mass spectrometers during a one month study (BEACHON RoMBAS, Rocky Mountain Biogenic Aerosol Study), when measurements overlapped. Some seasonal variability was observed, with lesser contributions from biogenic emissions in the fall, winter and spring. Thus, even in this semi-arid, high altitude location, biologically driven aerosol production and growth is an important mechanism linking the biosphere, hydrosphere and atmosphere. In addition to CCN, the number concentrations of IN were determined as a function of temperature during BEACHON RoMBAS. IN measurements were made nearly every day of the study, covering a range of ambient conditions and processing temperatures. We also collected residual IN using the CFDC and analyzed the elemental composition and biological content of the particles. These measurements indicated that IN number concentrations at temperatures colder than -20°C were comparable to, but somewhat higher than, mean IN number concentrations often used in modeling studies, as determined from more than a decade of IN measurements. These number concentrations tended to decrease nearly exponentially with increasing temperature, as has been observed previously. Both mineral dust and carbonaceous particles contributed to the IN population at the site, with a major contribution coming from primary biological aerosol particles. DNA sequencing results suggest that a diverse population of micro-organisms contributed to the IN population, with DNA from 11 bacteria and 11 fungi identified. A highlight result from this study was the observation that ground level IN concentrations were enhanced during rain events, by up to a factor of forty, and these particles were primarily biological (bacteria and fungal spores). If these surface IN are transported to higher levels in the atmosphere, they may influence subsequent precipitation events. Such a feedback between biogenic cloud active particles and precipitation suggests a strong link between the biosphere, hydrosphere, and atmosphere. Finally, all of the IN data collected from BEACHON-RoMBAS were used to further develop a mathematical description of atmospheric IN, which can be used in atmospheric models to better describe how aerosol impacts cloud formation in the atmosphere.
