Does Regional Aerosol Loading Affect Terrestrial Carbon Exchange?

In this study we test a proposal that the aerosol induced radiative feedback could be an important modulator of terrestrial carbon exchange. It is hypohtesized that, aerosols can influence radiation and radiative processes that affect vegetation - atmosphere interactions. Indeed, one of the important feedback pathways in the carbon cycle is the vegetation - atmosphere interactions as part of the terrestrial ecosystem. Hence, understanding this interaction is important not only because of its magnitude but also because of the uncertainty and the variability associated with the carbon source / sink values. In this study, we hypothesize that the potential of the vegetated land surface to be a source or sink for carbon will depend on the basal vegetation characteristics (such as C3 or C4 photosynthesis pathways), and the environmental feedbacks from hydrological and radiative effects. To test this hypothesis, we present the first results based on CO2 observations taken from an AmeriFlux site in Oak Ridge, TN under pre-selected hydrological and radiative conditions. Observations of CO2 flux from the Walker Branch AmeriFlux site in Oak Ridge, TN were clustered according to dominant aerosol loading influencing the study site. The dominant aerosol loading is based on air mass classifications that uses air particle trajectories. The air masses are classified as highly polluted (HP), continental (C), and marine (M), based on SO2 and NOx emission inventory. The HP, C, and M regimes correspond to high, medium, and low aerosol loading values. With higher aerosol loading the Diffuse-to-Direct Solar Irradiance Ratio (DDR) increases. The CO2 flux measurements were then further grouped for each of the three regimes, descriptive statistics, and significance tests between each of the three air mass classifications were considered. Based on the initial analysis, the different aerosol loading corresponding to HP, M, and C air mass classifications respectively, appear to significantly (at 95% or higher level of confidence) affect the surface CO2 flux. The initial findings of this study suggest aerosol loading does appear to significantly affect observed field scale CO2 fluxes. More studies are needed on different locations, landscapes, and hydrological conditions to understand the feedback between the environmental processes and the terrestrial carbon exchange.