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.
The Earth system has undergone extensive change during the last century, with important implications for human health, resource management, and the environment. The ability to predict these changes and their impacts on time scales of months to a decade is becoming increasingly important. Key to improving the predictability of Earth system over these time scales is an improved understanding of the coupling between the water cycles and those involving the biology, geology, and chemistry of the earth system, the latter of which is often referred to as the "biogeochemical cycle." This coupling is the main focus of our NSF Emerging Topics in Biogeochemical Cycles project, entitled "Exploring forest ecosystem response to water availability and the impact on biogeochemical and water cycles." The project consisted of coordinated laboratory and field studies that explored the roles that carbon-containing gases and particles emitted from ecosystems play in the processes that couple vegetation physiology and climate. The proposed coupling between these processes is shown in Figure 1. The diagram shows that biogenic volatile organic compounds (BVOCs), which are carbon-containing chemicals emitted from plants and soils, contribute to the growth of secondary organic aerosol (SOA), which are then capable of functioning as the nuclei upon which liquid cloud droplets can form. In additional, primary biological aerosol particles (PBAP) emitted from plants and soils, such as spores and bacteria, are capable of functioning as nuclei upon which ice cloud droplets form. Since these processes modify liquid and ice cloud properties, precipitation may likewise be affected, which in turn is capable of affecting emissions from ecosystems and altering aerosol number concentrations. This forms a feedback loop. With the linkages shown in Figure 1 in mind, we set out to understand these processes in a water-limited ecosystem in the Colorado Rocky Mountains, with the ultimate goal of providing the tools necessary for simulating these processes in computer models. Two major community field measurement campaigns were co-organized by the project team at our Manitou Experimental Forest Observatory site, located near Woodland Park, CO. The Rocky Mountain Organic Carbon Study (ROCS) in 2011 focused on the biosphere-atmosphere exchange of reactive organic gases that serve as aerosol precursors and control many chemical processes in the atmosphere. The Rocky Mountain Biogenic Aerosol Study (RoMBAS) shared the same research goals as this project, that is, understanding interactions between plant emissions, aerosols, and climate. ROCS and RoMBAS each featured the participation of about 10-15 US universities, 4 US research labs, and 4–6 international groups, and the combined scientific outcome from these projects will be unprecedented in the field of forest-atmosphere interactions. We also performed long-term measurements of meteorology, trace gases, and aerosols at the Manitou site, which showed how season and meteorology impact the relative amounts of different carbon-containing gas and aerosol sources. We found that not all emitted BVOCs are equally effective in contributing to cloud formation. Since different BVOCs ultimately lead to products with unique chemical properties and varying degrees of volatility, they will exhibit different capacities to form aerosol particles of specific size and affinity to water. These different tendencies for specific BVOCs to impact climate through aerosol processes translates into a varying capacity of landscapes and ecosystems in the Western U.S. to influence regional precipitation, and different roles of these ecosystems in the feedbacks induced by plant emissions and clouds depicted in Figure 1. Other significant achievements of this project include progress in understanding processes involving the BVOCs themselves. When BVOCs are emitted from plants the first thing that usually occurs in the atmosphere is reaction with an oxidant, and one of the most important atmospheric oxidants is the hydroxyl radical (OH). We determined that there is a significant missing reactant of OH in the Manitou Forest ecosystem; nevertheless, the OH budget could be reasonably well simulated when key components were constrained by our observations. Thus, we conclude that the missing OH reactant is likely the products of BVOC atmospheric oxidation rather than missing BVOCs. We also observed surprisingly large formaldehyde emissions above this forest ecosystem. It is not a primary emission but represents within-canopy processing of BVOCs. Finally, we studied the impacts of bark beetle infestation on BVOC emissions, and found that bark beetles can stimulate BVOC emissions from this forest ecosystem. Both the magnitude and the composition of the BVOCs change as a result of bark beetle infestation, and these likely have an impact on aerosol production.
