Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
distribution and composition and may lead to efficient cloud condensation nuclei production
(Yu and Turco, 1998b).
3.2.2.2. Observations and Modeling of Volatile Particles and Sulfur
Conversion
Volatile particle abundances observed in situ in the plumes (mostly young, < 100 s) of
subsonic and supersonic aircraft are summarized in Figure 3-3a. The data have been
compiled from various field studies (Fahey et al., 1995a,b; Schumann et al., 1997; Anderson
et al., 1998a,b; Schröder et al., 1998a). The results show EIs for ultrafine volatile aerosol
particles (nominal radii > 2 to 3 nm) in the range of 1015 to 1016/kg fuel for low to average
fuel sulfur content values and exceeding 1017/kg fuel for high-sulfur fuel. Besides the
obvious dependence on fuel sulfur content, the spread in EI values may be explained by
differences in the emission characteristics of the engines, variations in the lower size
detection limits of the particle counters, and differences in plume ages at the time of the
observations. The increase in ultrafine particle abundance with increasing fuel sulfur content
for the Advanced Technology Testing Aircraft System (ATTAS), T-38, and B757 aircraft
strongly suggests an important role for fuel sulfur in the growth of volatiles from molecular
clusters to detectable particles.
Figure 3-2: Size distribution of various aerosol types present in
young jet aircraft exhaust plumes (adapted from Kärcher, 1998a).
Only a few observations have been analyzed using detailed microphysical simulation
models (Brown et al., 1996a; Danilin et al., 1997; Kärcher and Fahey, 1997; Yu and Turco,
1997, 1998a; Andronache and Chameides, 1998; Kärcher et al., 1998a,b). Simulations show better agreement between calculated and observed particle
concentrations when ion effects are taken into account. More important, the description of plume microphysics using binary homogeneous nucleation failed to explain a
field measurement (Yu et al., 1998). In two cases, condensation nucleus observations in the exhaust of the ATTAS and the Concorde (Figure 3-3a) have been
explained in detail with a model that includes CI emissions on the order of 1017/kg fuel. The observable (ion mode) particles have mean radii of about 2 to 4 nm in the
young plume, for EI(S) ranging from average to high values. For decreasing levels of available H 2 SO 4 , the ion mode particles decrease in size. The number of
detected particles falls below 1017/kg fuel when the mean radius of their size distribution becomes smaller than the detection limit of the particle counters.
The extent of conversion of fuel sulfur to S(VI) necessary to explain the observed mass of
volatile aerosol in young plumes seems to be variable. Direct measurements of H 2 SO 4 have
provided a lower bound of ~0.4% (for high-sulfur fuel, 2.7 g/kg fuel) and an upper bound of
~2.5% (for low-sulfur fuel, 0.02 g/kg fuel) for the conversion fraction in one case (Curtius et
al., 1998), consistent with calculated SO 3 emission levels (Brown et al., 1996a,c). For the
low fuel-sulfur case, it has been demonstrated that the observed volatile particles cannot be
mainly composed of H 2 SO 4 (Kärcher et al., 1998b). In other cases, conversion fractions
have been indirectly inferred from mass balance arguments involving observed or inferred
particle size and number distributions, available sulfur as measured in fuel samples, and
assumptions of aerosol composition. The dependence of the conversion fraction on EI(S)
differs in the few studies performed to date (Fahey et al., 1995a; Schumann et al., 1996;
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