Secondary aerosol formation from photochemical aging of aircraft exhaust in a smog chamber
Field experiments were performed to investigate the effects of\nphoto-oxidation on fine particle emissions from an in-use CFM56-2B gas\nturbine engine mounted on a KC-135 Stratotanker airframe. Emissions were\nsampled into a portable smog chamber from a rake inlet installed\none-meter downstream of the engine exit plane of a parked and chocked\naircraft. The chamber was then exposed to sunlight and/or UV lights to\ninitiate photo-oxidation. Separate tests were performed at different\nengine loads (4, 7, 30, 85%). Photo-oxidation created substantial\nsecondary particulate matter (PM), greatly exceeding the direct PM\nemissions at each engine load after an hour or less of aging at typical\nsummertime conditions. After several hours of photo-oxidation, the ratio\nof secondary-to-primary PM mass was on average 35 +/- 4.1, 17 +/- 2.5,\n60 +/- 2.2, and 2.7 +/- 1.1 for the 4, 7, 30, and 85% load experiments,\nrespectively. The composition of secondary PM formed strongly depended\non load. At 4% load, secondary PM was dominated by secondary organic\naerosol (SOA). At higher loads, the secondary PM was mainly secondary\nsulfate. A traditional SOA model that accounts for SOA formation from\nsingle-ring aromatics and other volatile organic compounds underpredicts\nthe measured SOA formation by similar to 60% at 4% load and similar to\n40% at 85% load. Large amounts of lower-volatility organic vapors were\nmeasured in the exhaust; they represent a significant pool of SOA\nprecursors that are not included in traditional SOA models. These\nresults underscore the importance of accounting for atmospheric\nprocessing when assessing the influence of aircraft emissions on ambient\nPM levels. Models that do not account for this processing will likely\nunderpredict the contribution of aircraft emissions to local and\nregional air pollution.