Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
3.3.3. Observations of Aircraft-Produced Aerosol and Sulfate Aerosol Changes
Observations of aircraft-induced aerosols have increased substantially in recent years (see Section 3.2). Concentrations of aerosol particles and aerosol precursor
gases well above background values have been observed in the exhaust plumes of aircraft operating in the upper troposphere and lower stratosphere. Although
aircraft emissions are quickly diluted by mixing with ambient air to near background values, the accumulation of emissions in flight corridors used in the routing of
commercial air traffic has the potential to cause notable atmospheric changes.
Estimated changes from aircraft emissions (Schumann, 1994; WMO, 1995) are small
compared with natural variability, hence are not always apparent in observational data sets.
However, regional enhancements in concentrations of aircraft-produced aerosol have been
observed near air traffic corridors. During measurement flights across the North Atlantic
flight corridor over the eastern Atlantic, signatures of NO x , SO 2 , and condensation nuclei
(CN) were clearly evident in the exhaust plumes of 22 aircraft that passed the corridor at this
altitude in the preceding 3 h, with values exceeding background ambient levels by 30, 5,
and 3 times, respectively (Schlager et al., 1997). A mean CN/NO x abundance ratio of 300
cm -3 ppbv-1 was measured. This ratio corresponds to a mean particle emission index of
about 1016 kg -1 and implies CN increases of 30 cm -3 in corridor regions where aircraft
increase NO x by 0.1 ppbv (cf. Chapter 2). The regional perturbation was found to be
detectable at scales of more than 1,000 km under special meteorological conditions within a
long-lasting stagnant anticyclone (Schlager et al., 1996). In an analysis of 25 years of
balloon measurements in Wyoming in the western United States of America, subsonic
aircraft are estimated to contribute about 5-13% of the CN concentration at 8-13 km,
depending on the season (Hofmann et al., 1998). This estimate provides only a lower bound
of the aircraft contribution because smaller aircraft-produced particles (radius < 10 nm) are
not detected. Additionally, regular lidar measurements have been made of aerosol optical
depth at aircraft altitudes (10-13 km) in an area of heavy air traffic in southern Germany
(Jäger et al., 1998). Large optical depths on the order of 0.1 that could be attributed to the
accumulation of aircraft aerosol were observed very rarely at this location.
Global changes in sulfate aerosol properties at subsonic air traffic altitudes were small over
the last few decades. The examination of long-term changes in aerosol parameters
suggests that aircraft operations up to the present time have not substantially changed the
background aerosol mass. Multiyear observations for the upper troposphere and lower
stratosphere are available from satellite and balloon platforms and ground-based lidar
systems. Long-term variations of the optical depth in the upper troposphere from the SAGE
satellite (Figure 3-6 and Table 3-3) have been analyzed with periods of volcanic influence
excluded (Kent et al., 1998). Data indicate that changes are less than about 1% yr-1 between 1979 and 1998, when observations are averaged over either hemisphere. A
significant change in aerosol amounts is also not found in the 15- to 30-km region examined
with lidar over Mauna Loa (20°N) for the period 1979 to 1996 (Barnes and Hofmann, 1997).
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Figure 3-7: Aerosol-mass column between 15- and 20-km altitude,
derived from backscatter measurements made by lidar at Garmisch-
Partenkirchen, Germany, between 1976 and end of 1998.