Plume to global-scale atmospheric impacts of aviation emissions

Abstract

Commercial aircraft combustion emissions impact the atmospheric composition, alter the Earth’s climate by accounting for ~4% of anthropogenic radiative forcing [72, 49] and affect surface air quality, causing an estimated ~16,000 premature mortalities per year [7, 28]. These environmental impacts are driven by chemical, microphysical and transport processes that span different magnitudes of temporal and spatial scales, from near-field inplume chemistry that evolve over minutes and distances of ~100 m to global-scale phenomena taking place at the continental scale. To evaluate aviation’s environmental impacts, all temporal and spatial scales need to be captured. In this thesis I develop and evaluate numerical models that span all modeling scales. First, I quantify the role of plume-scale processes in the atmospheric impact of aviation emissions. Previous literature has indicated that current global-scale modeling of aircraft emissions overestimates aviation-attributable ozone by instantly diluting emissions at a coarse resolution [86]. To estimate the magnitude of the ozone discrepancy, I use a recently-developed aircraft plume model to calculate the nonlinear chemical conversions that occur in aircraft plumes. I then propagate the plume-scale results to the global atmospheric impact through the chemistry transport model (CTM) GEOS-Chem by embedding a plume-scale parameterization. After accounting for plumescale processes, I find a ~5% downward correction in the simulated aviation-attributable ozone response.

High-altitude emissions from current subsonic aviation or from potentially future supersonic aircraft modify the total column ozone, thus leading to either increases in tropospheric ozone or a decrease in stratospheric ozone, with the latter causing larger UV flux at the ground. Both changes affect human health and, in this thesis, I identify a column ozone-neutral altitude for subsonic and supersonic aviation. Adjoint models of CTMs have been developed to quantify receptor-oriented sensitivities of environmental metrics (e.g. population-weighted ozone exposure) to emissions. Adjoint modeling overcomes the numerical cost of source-oriented sensitivity analysis, as performed by forward models. However, adjoint models of atmospheric chemistry have historically been limited to the troposphere. In this thesis, I build upon previous work and extend the GEOS-Chem Adjoint to further include stratospheric processes, and then validate the sensitivities with multi-year scenarios. I then present adjoint-derived sensitivities to identify column ozone-neutral altitudes for subsonic and supersonic aviation, based on their respective emission characteristics. I find that the 12 - 15 km altitude band is approximately column ozone-neutral for aviation emissions. Neglecting the effects of plume-scale processes introduces a positive bias in the column ozone-neutral altitude that varies between 0.3 up to 1 km.

Finally, previous assessments of the environmental impact of aviation emissions using global climate models have found that coupled chemistry-climate feedback could have a magnifying effect on the response to commercial aircraft emissions. However, the aviation-induced environmental impact estimated with climate models have not been found to be consistent with CTMs [15]. To identify the cause of this discrepancy between climate models and CTMs and to evaluate the relevance of climate feedbacks in the assessment of the environmental response of aviation emissions, I develop a newly-coupled model for climate-chemistry simulations, CESM2-GC, coupling the climate model CESM2 to the model of atmospheric chemistry GEOS-Chem. I then validate CESM2-GC against atmospheric observations and results from the GEOS-Chem CTM and the “native” chemistry option in CESM2, CAM-Chem. Using CESM2-GC, I perform ensemble runs to evaluate the magnitude of the coupled chemistry-climate effects when evaluating aviation’s ozone and particulate matter response. I find that the ensemble mean provides an aviation-attributable population-weighted ozone and particulate matter perturbations of 0.56 ppbv and 0.08 µg/m3 , consistent with previous estimates using the GEOS-Chem CTM. Besides an increase of ~70 mK in tropical and Northern mid latitudes tropospheric temperatures, I observe no statistically significant response in upper-tropospheric meteorology that could indicate that coupled chemistry-climate feedback magnifies the aviation-attributable environmental response.