Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Ice as a substrate in heterogeneous reactions has not received as much attention in laboratory studies as reactions occurring on other stratospherically relevant
substrates such as supercooled and frozen sulfuric acid (H 2 SO 4 ) hydrates, as well as type Ia and Ib polar stratospheric cloud (PSC) substrates. N 2 O 5 undergoes a
reactive uptake on tropospherically important substrates such as H 2 SO 4 /H 2 O (Hanson and Ravishankara, 1991; Fried et al., 1994) and water ice (Quinlan et al., 1990;
Hanson and Ravishankara, 1992), which results in HNO 3 formation. HNO 3 undergoes non-reactive uptake only on the aforementioned substrates (i.e., condensation)
(Hanson, 1992; IUPAC, 1997a; JPL, 1997). N 2 O 5 also undergoes a bimolecular reaction with hydrogen chloride (HCl) on ice, resulting in ClNO 2 in competition with the
well-documented hydrolysis reaction leading to HNO 3 (IUPAC, 1997a; JPL, 1997; Seisel et al., 1998):
N 2 O 5 + HCl ClNO 2 + HNO 3 (23)
Reaction 23 occurs faster on ice than on H 2 SO 4 because of limitations of HCl solubility in H 2 SO 4 . The increase in the rate of heterogeneous reactions on ice compared
to acidic surfaces generally occurs for all reactions involving hydrohalic acids (IUPAC, 1997a; JPL, 1997). Reaction 23 competes with heterogeneous hydrolysis of
N 2 O 5 , similar to reaction 20a. At temperatures near 200 K and at the limit of high HCl concentrations, 65% of the reaction proceeds via ClNO 2 formation; the rest
proceeds through N 2 O 5 hydrolysis.
NO, NO 2 , and ozone do not interact with H 2 SO 4 at concentrations encountered in the UT, or with water ice. Similarly, HONO does not interact with water ice down to
temperatures of 180 K (Fenter and Rossi, 1996). However, HONO is taken up by H 2 SO 4 /H 2 O binary mixtures (weight of H 2 SO 4 > 65%) from ambient temperatures
down to 180 K and eventually yields nitrosylsulfuric acid (NSA) after protonation (Becker et al., 1996; Fenter and Rossi, 1996; Zhang et al., 1996; Kleffmann et al.,
1998; Longfellow et al., 1998):
HONO + H 2 SO 4 NO+HSO4- + H 2 O (24)
NSA has the ability to activate chlorine by reacting with HCl (reaction 25). The resulting NOCl product rapidly photolyzes in the UT into NO and Cl.
NO+HSO4- + HCl NOCl + H 2 SO 4 (25)
An important finding regarding halogen activation by reactions 24 and 25 is the fact that these reactions occur approximately 20 times faster on pure ice, bypassing
NSA as an intermediate species altogether because of its instability on ice (Fenter and Rossi, 1996). The propensity of many bimolecular heterogeneous reactions to
occur more efficiently on ice surfaces compared to sulfuric acid aerosols is considered particularly relevant with regard to chemical processes occurring on natural
cirrus clouds in the UT and on those seeded by particles emitted from aircraft engines, owing to a change in the reaction mechanism. The relevant water-ice
mechanism corresponds to a simple ionic displacement of the form
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Cl- + HONO NOCl + OH- (26)