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Figure 7.8 Measured Cl/C!v and UC1/CI, ratios versus simultaneously measured temperature. The data were obtained at high southern latitudes in the lower stratosphere (potential temperatures around 450 K, about 20 km altitude). On this flight, the measured temperature was the minimum temperature the air had encountered within the previous 10 days. (After Kawa etal. [248], Figure 1.)

At CIO abundances of 1-2 ppbv the time-scale to convert all of the CIO to dimer is tens of minutes to several hours. The CIO dimer is lost through photolysis with a lifetime ranging from tens of minutes to several hours at solar zenith angles (SZAs) typical of the early spring polar lower stratosphere:

Thermal decomposition of the dimer is also an important loss pathway at temperatures above -200 K:

Figure 7.9 plots the fraction of Cl( in the form of the dimer as a function of temperature. The solid lines in Figure 7.9 are for 3 ppbv of CI, At a SZA of 95°, the air parcel line is in darkness, and formation of ClOOCl (reaction (7.6)) is balanced by thermal decomposition (reaction (7.8)). Below about 200 K, virtually all of the Clt is in the form of dimer. As the temperature rises, thermal decomposition becomes

Figure 7.9 Fraction of active chlorine CI, (CiO + 2 x ClOOCl) in the form ClOOCl determined from a steady-state calculation. Pressure and [MJ are held fixed at 46.4 hPa and 1.67 x 10ls molecules cm ', respectively; the temperature variation affects only the values of the rate constants. Lines are labeled with the SZA of calculation (unlabeled lines are 5" increments from surrounding lines). Solid lines are for .3 ppbv of Cl„ and the dotted line is for 100 pptv of CI,. Rate constants are from DeMore et al. [5]; photolysis calculations are from the photolysis routine from the Goddard three-dimensional chemical transport model [28].

Temperature (K)

Figure 7.9 Fraction of active chlorine CI, (CiO + 2 x ClOOCl) in the form ClOOCl determined from a steady-state calculation. Pressure and [MJ are held fixed at 46.4 hPa and 1.67 x 10ls molecules cm ', respectively; the temperature variation affects only the values of the rate constants. Lines are labeled with the SZA of calculation (unlabeled lines are 5" increments from surrounding lines). Solid lines are for .3 ppbv of Cl„ and the dotted line is for 100 pptv of CI,. Rate constants are from DeMore et al. [5]; photolysis calculations are from the photolysis routine from the Goddard three-dimensional chemical transport model [28].

increasingly important, and [CIOOCl]/lCl,| decreases. Under sunlight conditions (SZAs less than or equal to about 90°), formation of ClOOCl (reaction (7.6)) is balanced by the combination of photolysis (reaction (7.7)) and thermal decomposition (reaction (7.8)). As the sun moves higher in the sky (i.e. to lower SZAs) the rate of photolysis increases rapidly and in general dominates thermal decomposition, except when temperatures are warm (> 220 K). For typical high-Cl, polar sunlit conditions the dimer makes up 40-60% of CI,.

The rate of formation of the CIO dimer is &*|C10|[C10], which is proportional to the square of the CIO abundance. This causes the formation rate of the dimer to drop off rapidly with decreasing CIO concentrations. As a result, the dimer is an unimportant component of CI, at typical mid-latitude CI, abundances. The dotted line in Figure 7.9 is a calculation at 90° SZA for 100 pptv of Cl„ typical of the mid-latitude lower stratosphere. A comparison between this line and the 90° line calculated for 3 ppbv of CI, (solid line) demonstrates the effect of the quadratic dependence of the rate of CIO dimer formation. At the low C1A abundance the dimer makes up only a few percent of C1A, even at the very low stratospheric temperature of 190 K.

Thus, in the polar regions CI, comprises CI, CIO, and ClOOCl, with CI being a negligible component of CI,. Together, CIO and ClOOCl make up the vast majority of CI,, with their relative abundances set by the temperature, SZA, and total abundance of Clv.

Denitrification and dehydration PSCs are composed of HNO, and H20, so their formation and growth leads to the removal of these species from the gas phase, a process known as "denitrification" and "dehydration", respectively. How much of each of these species is removed depends on the composition of the PSC particles. Type I PSCs contain HNO, and H20 in approximately comparable abundances. Because gas phase H20 is initially about 10' times more abundant than HNO,, the growth of type 1 PSCs can deplete HN'O, while leaving H20 essentially unchanged. And because almost all of the NO, in the lower stratospheric polar vortex is in the form of HNO, [244], removal of HNO, is tantamount to removal of NO,.. Thus, formation and growth of type I PSCs leads to denitrification without accompanying dehydration.

Type II PSCs are composed of II20 ice, and formation of these particles depletes gas phase H20. Additionally, it is thought that type II PSCs also incorporate NO, [251]—and therefore remove the NO, from an air mass. Thus, formation and growth of type II PSCs lead to both dehydration and denitrification.

Measurements of type I PSCs reveal characteristic radii of ~l pm [241[. Such small particles have small settling velocities, and therefore sedimentation of these particles is negligible. When the air parcel warms up, the HNO, in the particle is released back into the gas phase. In this case, the denitrification is only temporary. It is possible, however, that growth of a much smaller number of type I PSC will result in larger particles and subsequent sedimentation of type I PSCs [243], In this case the HNO, is physically removed from the air mass, and the denitrification is irreversible. Irreversible denitrification by type I PSCs is consistent with the observation of air masses that have experienced significant irreversible denitrification but not dehydration [252], Type II PSCs are predicted to have characteristic radii of 5-20 pm or so [242,243], and so can experience significant sedimentation during their lifetimes. For example, at 20 km altitude, a particle with radius of 10 pm falls about a kilometer in a day [253]. As these type II particles sediment, they carry H,0 and NO,, to lower altitudes, leading to both irreversible denitrification and irreversible dehydration of the region where the particles were formed [252,254|.

Eventually, the sedimenting PSC particles fall into warmer air and sublimate, forming to layers of enhanced water vapor and oxides of nitrogen [255]. In situ measurements of the lower stratosphere from the NASA ER-2 aircraft show that as much as 90% of the NO,; and 50-70% of the H20 has been removed from the Antarctic lower stratosphere [252,256], Satellite measurements [254,257] are consistent with these results, and show that this denitrification covers virtually the entire vortex.

Figure 7.10 shows a time series of lower stratospheric HNO, abundance, the dominant component of NO, in the lower stratosphere. HNO, begins in late May at between 7 and 13 ppbv, with higher values at higher latitudes. At 60°S, HNO,

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