The goal of this study is to develop a gradient-independent method for modeling surface gas flux using surface gas concentration data. The proposed method is built on the relationship between gas flux and the time history of surface gas concentration, known as half-order derivative (HOD), when the transport of gas in the boundary layer is described by a diffusion equation. The new model is tested using in-situ data of CO2 concentration time series at half hour (or hour) intervals from Ameriflux Network at several locations in US, Canada, Mexico and Brazil to estimate CO2 fluxes. In this research the estimated CO2 flux using HOD method are compared with observed CO2 fluxes from four study sites out of 160+ sites with diverse vegetation cover, geographic and climatic conditions to test the generality of model within reasonable endeavor. The sites are Cedar Bridge National Forest, New Jersey, Delta Junction 1920 Control, Alaska, Lucky Hills Shrub land, Arizona, and LBA Tapajos Mature Forest, Brazil. The modeled CO2 flux demonstrates close agreement with field observations confirming the usefulness and potential of HOD model for estimating CO2 gas fluxes.

A series of detailed laboratory investigations were conducted to examine low solubility gas transfer across wind-forced wavy air-water interfaces. The study focuses on the increase in gas flux associated with the microphysical interfacial wind momentum exchange and the complex wave coupled hydrodynamics. Key elements of the laboratory investigations included the measurement of hydrodynamic behaviour within the aqueous viscous sub-layer using a particle image velocimetry (PIV) system and the development of a Laser Induced Fluorescent (LIF) system capable of measuring reliable dissolved oxygen concentration profiles to within 28??m of the air-water interface. Major achievements and findings included: 1. The first phase resolved gas flux measurements along wind forced microscale waves, indicating the highest mean gas fluxes are located in the wave troughs. This finding demonstrated the relative importance of wave orbital straining in gas flux enhancement; a wave coupled hydrodynamic process whose significance has previously been neglected. 2. The relative contributions to gas flux from wind shear, wave orbital straining, increased surface area of the waves, parasitic capillary ripples and microscale breaking are quantified with respect to friction velocity, wave steepness and an efficiency of microscale wave breaking. The parasitic capillary ripples are shown to have a negligible role in gas enhancement. A hybrid model is developed to estimate the gas flux based on both wind and wave characteristics. 3. Gas enhancement due to microscale wave breaking and the significance of the highly localised subduction at the toe of the spilling region on the leeward face of the wave crests was investigated using data from the LIF experiments. The highly localised subduction was shown to substantially reduce the thickness of the diffusion sub-layer, resulting in an increase in gas flux when waves transitioned from the incipient breaking to the microscale breaking wave form. 4. Consideration of previously unidentified optical distortions in LIF imagery due to non-linear effects is presented that is critical for robust LIF data processing and experimental design. A formal mathematical description of optical distortions has been developed and presented.