Anthropogenic greenhouse gas (GHG) emissions enhance the atmospheric greenhouse effect, tend to increase the average global temperature, and contribute to global climate change. Those consequences motivate the establishment of regulatory frameworks to control and reduce GHG emissions. The credibility of emissions regulations depends on reliable, independent methods for long-term monitoring, verification and accounting of the actual emissions of market participants. Therefore the objectives of the present study are: (1) to evaluate the performance of state of the art trace gas dispersion models for the atmospheric boundary layer; (2) to develop novel measurement and modeling techniques for quantifying GHG emissions from spatially distributed sources using a top-down approach. Top-down methods combine atmospheric measurements of GHG concentration with meteorological data, and inverse transport models to quantify emissions sources. The ability of Weather Research and Forecasting, large-eddy simulation (WRF-LES) to model passive scalar dispersion from continuous sources in the atmospheric boundary layer was investigated. WRF-LES profiles of mean and fluctuating concentration in the daytime convective boundary layer were similar to data from laboratory experiments and other LES models. Poor turbulence resolution near the surface in neutral boundary layer simulations caused under prediction of mean dispersion in the crosswind direction, and over prediction of concentration variance in the surface layer. WRF-LES simulations also showed that the concentration intermittency factor is a promising metric for detecting source location using atmospheric measurements. A source determination model was developed to predict the location and strength of continuous, surface level, trace gas sources using concentration and turbulence measurements at two locations. The need for measurements at only two locations is advantageous for GHG monitoring applications where large sensor arrays are unfeasible due to high equipment costs and practical constraints on sensor placement. Atmospheric measurements of turbulence and methane concentration made during an outdoor, controlled release experiment were used to demonstrate the feasibility of the source determination model. The model predicted trace gas flux with less than 50% uncertainty, and provided an upper bound for fluxes from localized sources. The model can be used for detection and continuous, long-term monitoring of fugitive GHG emissions

The eddy covariance technique (EC) is used at hundreds of field sites worldwide to measure trace gas exchange between the surface and the atmosphere. Data quality and correction methods for EC have been studied empirically and theoretically for many years. The recent development of new gas analyzers has led to an increase in technological options for users. Open-path (no inlet tube) and closed-path (long inlet tube) sensors have been used for a long time, whereas enclosed-path (short inlet tube) sensors are relatively new. We tested the comparability of fluxes calculated from five different gas analyzers including two open-path (LI-7500 A from LI-COR, IRGASON from Campbell), two enclosed-path (CPEC200 from Campbell, LI-7200 from LI-COR), and one closed-path (2311-f from Picarro) analyzers, which were all located on a single tower at an irrigated alfalfa field in Davis, CA. To effectively compare sensors with different tube characteristics we used three different spectral correction methods. We found that all sensors, regardless of type, can be used to measure fluxes if appropriate corrections are applied and quality control measures are taken. However, the comparability strongly depended on the gas (CO2 or H2O) and the correction method. Average differences were below 4% for CO2 fluxes using any spectral correction method, but for H2O average differences were between 4% and 13% for the different methods. The magnitude of corrections also varied strongly, especially for water vapor fluxes. This study does not identify the best sensor, but rather weighs the benefits and difficulties of each sensor and sensor type. Our findings show that enclosed and closed-path gas analyzers that measure water vapor with inlet tubes experience large high frequency attenuation and should be corrected with empirical correction methods. This information presented here about different the diverse sensors be considered by investigators when choosing a sensor for a site or when analyzing EC measurements from multiple sites.