Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Burtscher, 1992). In the jet regime, some recent models indicate that CIs effectively promote formation and growth of electrically charged droplets containing H 2 SO 4
and H 2 O (Yu and Turco, 1997). In addition, CIs may contribute to the activation of exhaust soot. Positive ions include H3O+ and organic molecules like CHO+, C3H3+,
and larger molecules (Calcote, 1983), whereas the free electrons rapidly attach to other molecules to form negative ions with sulfate and nitrate cores. Measurements
of positive CIs in exhaust plumes are not available, and only very few in situ measurements of negative CIs are available to date.
Arnold et al. (1998a) measured a total negative CI concentration of 3 x 10 7 cm -3 (about 3 x 1015/kg fuel) at plume ages of around 10 ms in the exhaust of a jet engine
on the ground, consistent with approximately 10 9 cm -3 at a plume age of 1 ms (Yu et al., 1998).
That concentration represents a lower bound from diffusion losses of these particles within sampling devices prior to detection and the limited detection range of the
employed mass spectrometer. The fact that these measurements yielded only a fraction of CIs in the plume has been partially confirmed by in-flight measurements
(Arnold et al., 1998b) showing smaller total CI count rates for high-sulfur fuel compared with low-sulfur fuel. Therefore, current CI data are consistent with a CI
emission index of about 2-4 x 1017/kg, corresponding to a concentration of about 2 x 10 9 cm -3 at the engine exit. This value has been estimated numerically based on
coupled ion-ion recombination kinetics and plume mixing (Yu and Turco, 1997; Kärcher et al., 1998b; Yu et al.,1998). Although not directly comparable, CIs have been
observed in hydrocarbon flames at concentrations of about 10 8 to 10 11 cm -3 (Keil et al., 1984), supporting the estimated concentration of negative CIs.
3.2.1.4 Nitrogen Species
The primary nitrogen emission from aircraft is in the form of NOx (Chapter 2). In reactions of NOx with OH in the plume, gaseous HNO2 and HNO3 are formed. Despite
larger reaction rates, less HNO3 is formed than HNO2 because the ratio of NO2 to NO at the engine exit is small (< 0.2) (e.g., Schulte et al., 1997) (see Chapter 7).
HNO3 can also form in the plume even in the absence of NO2 emissions (Kärcher et al., 1996a). In situ measurements in young plumes revealed both HNO2 and
HNO3 concentrations above background levels (Arnold et al., 1992, 1994; Tremmel et al., 1998). HNO3 can be more abundant in plumes than H2SO4 , especially for
low EI(S) values. These acids (especially HNO3 ) are important because they can be taken up by water-soluble exhaust particles and form stable condensed phases
such as nitric acid trihydrate (NAT = HNO3•3H2O) and liquid ternary (H2O/H2SO4 /HNO3 ) solutions under cold and humid plume conditions (Arnold et al., 1992;
Kärcher, 1996).
3.2.1.5. Hydrocarbons
Aircraft engines emit non-methane hydrocarbons (NMHCs) as a result of incomplete fuel combustion. These species include alkenes (mostly ethene), aldehydes
(mostly formaldehyde), alkines (mostly ethine), and a few aromates. A few (8 to 10) species were found to account for up to 80% of NMHC emissions (Spicer et al.,
1992, 1994). High levels of carbonyl compound emissions (on the order of 0.2 ppmv) also have been observed in a combustor (Wahl et al., 1997). Some in-flight data
indicate that NMHCs with up to 8 carbon atoms have EIs in the range 0.05 to 0.2 g C/kg fuel and represent approximately 70% of total NMHC emissions (Slemr et al.,
1998). However, the current database on NMHC emissions and on partitioning between individual compounds is small and perhaps not representative for all engine
types. Some emitted NMHCs might act as aerosol-forming agents in nascent plumes and may be adsorbed or dissolved in plume particles, thereby possibly
contributing to the total amount of volatile aerosol found in plumes (Kärcher et al., 1998b). In addition, the presence of trace NMHCs amounts may facilitate nucleation
(e.g., Katz et al., 1977) and alter the hygroscopic behavior and growth rates of particles (Saxena et al., 1995; Cruz and Pandis, 1997). Engines also may emit volatile
particles containing engine oils or other lubricants, but this effect has not been quantified.
http://www.ipcc.ch/ipccreports/sres/aviation/034.htm (3 von 9)08.05.2008 02:41:56