Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
scale weather systems. An estimate of annual flux into the free troposphere from European surface NO x sources, as a fraction of the total surface NO x source, can be
made from European Monitoring and Evaluation Program (EMEP) modeling studies (Tuovinen et al., 1994). For NO x , 52% of the emitted NO x was deposited within the
EMEP area of Europe and 48% was exported out of the model region during 1985-93. Approximately half of this material is vented into the free troposphere as NOy
(1.7 Tg N yr -1 ), with the remainder deposited elsewhere, without reaching the free troposphere.
In North America, a similar picture applies. Model calculations (Brost et al., 1988) have estimated that about 1.8 Tg yr-1 (of total North American emissions of 7.9 Tg N
yr-1 ) is transported east to the Atlantic Ocean between the surface and 5.5-km altitude. That is, about 25% of the North American NOx emissions remained airborne in
the boundary layer or free troposphere as the air left North America. More recent studies (Jacob et al., 1993; Horowitz et al., 1998; Liang et al., 1998) have derived a
lower estimate of the transport out of North America (on the order of 6%).
A detailed model study of advective and convective venting of ozone, NOx , and NOY out of the boundary layer over northwest Europe during July and October-
November 1991 showed that, of surface NOx emissions, 7% were brought to the free troposphere as NOx and 20% as NOy during the summer (Flatoy and Hov, 1996),
with slightly smaller percentages during the fall.
Transport from the surface is therefore undoubtedly an important contributor to background NOx . However, it is difficult to evaluate how well this process is handled in
each of the tropospheric assessment models used for aviation impact calculations.
Lightning is an important NOx source in the UT (Chameides et al., 1987; Lamarque et al., 1996). Because of its sporadic nature and small spatial scale (tens of km), it
is exceedingly difficult to represent quantitatively in even the most complex of tropospheric models. Most model studies include some representation of lightning NOx ,
with global total emissions in the range 1-10 Tg N yr-1 . However, there is no consensus on how to represent this source with time of day, season, altitude, or spatially,
nor how to treat lightning in concert with convection, cloud processing, and wet scavenging.
Stratospheric NOy is a further important source of NOx in the UT and LS, through the photolysis of HNO3 (Murphy et al., 1993). There is a downward flux of NOy from
the stratosphere to the troposphere that globally balances the stratospheric NOx source produced by the reaction of N2O with O(1D). Few model studies of aircraft NOx emissions extend high enough in altitude to include a full treatment of stratospheric NOx and NOy. Moreover, most do not include an explicit representation of
stratospheric chemistry (i.e., halogen chemistry) from which to realistically calculate stratospheric NOx . Instead, most models use a constant ratio of NOy to ozone and
describe the stratospheric NOy source in the same way as the stratosphere-troposphere exchange of ozone. Typical ozone to NOy ratios are assumed to be about
1000:1, giving a stratospheric NOy source in the UT of about 0.5 Tg N yr-1 .
Although the issue of the magnitude of background NO x levels has been clearly identified and much
work has been performed to characterize surface, lightning, and stratospheric sources, there are still
too few measurements of NO x and NOy in the UT and LS with which to assess quantitatively
representations of background NO x in the models summarized in Table 2-1. Recently published data
(Emmons et al., 1997) have begun to be used for evaluation of model performance (Wang et al.,
1998b).
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