Numerical Study on the Dispersion of Urban Emissions and Downwind Measurements

This numerical study models the dispersion of urban carbon-dioxide (CO2) plumes, and assesses the feasibility of emission quantification through downwind surface measurements. As a first step, a large-eddy simulation of a convective boundary layer (CBL) is performed under clear and shallow cumulus-topped conditions. A passive tracer is released both from surface and elevated point sources to model the dispersion of CO2. Tracer fluxes are tracked at various downwind locations to estimate emission rates. The uncertainties of surface measurements are quantified in terms of boundary layer mixing, cloud venting, and both spatial and temporal unsteadiness related to plume meandering and diffusion. Some guidelines on the optimal downwind distance and crosswind spread of surface stations, as well as averaging times are recommended based on CBL length and time scales. Putting the lessons learned from the idealized study to practice, we perform nested-simulations to model CO2 dispersion from the city of Indianapolis. The simulations obtain realistic initial and boundary conditions through grid nesting from the North American Mesoscale-Reanalysis. The spatially and temporally varying CO2 emissions are set by the Vulcan emission inventory. Two cases study with clear and shallow-cumulus-topped conditions are performed at high resolution, with 200 m horizontal grid spacing. In addition to confirming the results from the idealized study, this real-world simulation allows for quantification of additional uncertainties from the unsteadiness of the winds and the diurnal cycle.