Reactive Chemistry in Aircraft Exhaust Implications for Air Quality

ing exhaust plume is drastically enhanced compared to that of the ambient atmosphere (9). Reaction rates are magnified relative to normal conditions due to concentrated levels of directly emitted reactive species such as volatile organic compounds (VOCs), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen oxides (NOx), all of which are categorized as either a criteria air pollutant or a precursor by the Environmental Protection Agency (10). The species central to the chemical breakdown of these primarily emitted pollutants in the atmosphere are the hydroxyl (OH) and hydroperoxyl (HO2) radicals, collectively known, due to the fast interconversion between themselves, as HOx radicals. HOx species play a key role in the nonlinear, highly coupled dynamics of atmospheric chemistry (see Figure 1). For instance, oxidation of VOCs by HOx lowers the concentration of the parent VOC, but leads to the formation of potentially more harmful secondary organic aerosols and oxygenated VOCs. Increased chemical activity also leads to the formation of tropospheric ozone (O3), another criteria air pollutant and greenhouse gas, as well as a major OH precursor. At the Alternative Aviation Fuels Experiment (AAFEX) conducted in January 2009, mixing ratios were observed of all of the dominant HOx precursors emitted in aircraft exhaust, including nitrous acid (HONO), formaldehyde (HCHO), acetaldehyde (CH3CHO), and O3. Measurements show that photolysis of these HOx precursors during daytime results in plumes that are drastically more reactive than air unperturbed by emissions. Analyses suggest that the enhanced reactivity persists even after plumes have diluted down to ambient levels several kilometers downwind. These results call for the need to account for these species that are driving the chemistry, in order to properly assess impact of airports on downwind air quality.