5.2. Stratospheric Measurements

5.2.1. Aircraft Projects-ACATS-IV

ASHOE/MAESA: Stratospheric and Atmospheric Lifetimes of Source Gases

The environmental impact of the measured anthropogenically-produced source gases depends, among other factors, on the rate at which they break down, releasing ozone-depleting chemicals in the stratosphere, i.e., on their stratospheric lifetimes. For many compounds studied here, there exist no significant tropospheric sinks (e.g., N₂O, CCl₄, CFCs 11, 12, and 113), so the stratospheric lifetime, defined here as the atmospheric lifetime with respect to stratospheric loss, is identical to the atmospheric lifetime for these species. A long atmospheric lifetime means a greater ozone depletion potential and a greater global warming potential than similar source gases with shorter lifetimes.

Plumb and Ko [1992] showed that in the “global diffuser” model, stratospheric transport can be described by a simple one-dimensional flux-gradient relationship. If the mixing ratios of two long-lived tracers, s₁ and s₂, are in steady state, then the slope of their correlation in the lower stratosphere equals the ratio of their stratospheric removal rates. Thus,

(1)

where B_i is the total atmospheric burden for species i, and t_i is its steady-state stratospheric lifetime, equal to the steady-state atmospheric lifetime for species without tropospheric sinks. Volk et al., [1997] showed that under the same conditions, steady-state stratospheric lifetimes may be derived from the gradient of the steady-state tracer mixing ratio, s, with respect to the mean age of stratospheric air [Hall and Plumb, 1994] , G, in the measured air parcels:

(2)

where M_u is the total number of molecules above the tropopause, and the tracer gradient with respect to age, ds/dG, needs to be evaluated at G = 0 which is assumed to be the extratropical tropopause.

If the tropospheric mixing ratio of the tracer is changing with time, part of the tracer gradient at the tropopause is due to accumulation in the troposphere and 1/t_i in either equation (1) or (2) has to be replaced with t_i^-1 + B_u¢/B, where t_i is the instantaneous lifetime and B_u¢ is the total accumulation rate above the tropopause. Volk et al. [1997] proposed an alternative method of accounting for tropospheric growth and non-steady-state mixing ratios, c, to obtain steady-state lifetimes. In this method, one deduces the correction factors C(c_i) for each species using the tracer gradient with respect to age at the tropopause, dc_i/dG, the time series of tropospheric mixing ratios of the respective species during a 5-year period prior to the stratospheric observations, and estimates of the width of the stratospheric age spectrum from three-dimensional transport models [Hall and Plumb, 1994] .

Volk et al. [1997] showed that corrected steady-state correlation slopes defined as:

(3)

can be used in equation (1) to derive steady-state lifetimes based on a given CFC-11 reference lifetime. Mixing ratios of N₂O from the Airborne Tunable Laser Absorption Spectrometer (ATLAS) [Loewenstein et al., 1989] and ACATS CFC-113 plotted against ACATS-IV CFC-11 (Figure 5.16a, b) are used to calculate the observed gradient (dc_i/dcCFC-11) in Table 5.6. ATLAS N₂O measurements were calibrated from CMDL N₂O standards and agreed with onboard ACATS N₂O measurements to within ±2%. ATLAS N₂O data are used here to increase the number of measurements used by a factor of 2. The steady-state stratospheric lifetime for a source gas using the reference lifetime for CFC-11 is:

(4)

where is the mean atmospheric mixing ratio with the gradient calculated at the tropopause. From Figure 5.16 and Table 5.6, the steady-state atmospheric and stratospheric lifetime for N₂O is 122 ± 22 years and for CFC-113 is 100 ± 32 years using a CFC-11 reference lifetime of 45 ± 7 years using equation (4).

Fig. 5.16. Two methods used to calculate the stratospheric lifetimes from aircraft data. The mixing ratio and its gradient of (a) ATLAS N₂O and (b) ACATS CFC-113 against a lifetime reference molecule such as CFC-1 and the mixing ratio and its gradient of (c) ATLAS N₂O and (d) ACATS CFC-113 against the mean age of the air mass calculated from airborne SF₆ measurements. Observations (pluses, where symbol size indicates uncertainty, left axes), normalized local correlation slopes (triangles with error bars, right axes), and quadratic fits with uncertainty envelopes (lines, right axes) are shown.

TABLE 5.6. Stratospheric Steady-State Lifetimes From Volk et al. [1997] Compared to Current Reference Values

*Uncertainty of CFC-11 lifetime is not included in uncertainty estimate.

† [WMO, 1995] , Table 13-1.

‡[IPCC, 1995] , Table 2.2.

§[WMO, 1992] , Table 6.2, scaled to t_CFC-11= 45 years.

The steady-state gradient, ds_i/dG defined as (observed dci/dG) times C(ci), is used in equation (2) to obtain the steady-state lifetime using the "mean age" technique. The steady-state stratospheric lifetime for a source gas based on mean age becomes

(5)

where M_a is the total atmospheric mass (5.13 Ž 10¹⁸ kg) and M_u is the total atmospheric mass in the upper atmosphere above the tropospause (1.1 Ž 10¹⁸ kg). From Figure 5.16 the steady-state atmospheric and stratospheric lifetime for N₂O is 124 ± 49 years and for CFC-113 is 89 ± 35 years based on “mean age” calculated using equation (5) (Table 5.6). The uncertainties on the lifetimes using mean age are less precise than those calculated from the reference lifetime method because SF₆ was only measured during the last quarter of the flights during ASHOE/MAESA.

Lifetime results from the two methods presented in Volk et al., [1997] are consistent with each other (see Table 5.6). In most cases the calculated stratospheric lifetimes from observations are shorter than the World Meteorological Organization (WMO) or the Intergovernmental Panel on Climate Change (IPCC) reference lifetimes derived from photochemical models. Since the derived stratospheric lifetimes are identical to the atmospheric lifetimes for many of the source gases in Table 5.6, the shorter lifetimes also would imply a faster-than-predicted recovery of the ozone layer following the complete phase out of industrial halocarbons.

STRAT Campaign

The primary goal of the Stratospheric Tracers of Atmospheric Transport (STRAT) mission was to measure the morphology and dynamic properties of long-lived tracers as functions of altitude, latitude, and season to help determine the rates for global-scale transport and future stratospheric distributions of high-speed civil transport (HSCT) exhaust. ACATS-IV participated in four out of six STRAT deployments and was flown on a total of 31 flights that spanned a range of latitudes (2.1°S to 59.1°N), predominantly at 15 to 21 km altitudes (potential temperature, q = 360 to 510 K). These flights included four southbound survey flights into the tropics from Barbers Point, Hawaii (22°N), four northbound survey flights to nearly 60°N from Moffett Field, California (38°N), and 23 midlatitude flights from both locations.

Sulfur hexafluoride has no known sinks below the stratopause (t = 3200 years, [Ravishankara et al., 1993] and has exhibited a steady rate of growth of 6.7% yr^-1 in recent years [Geller et al., 1997] . It is used to study atmospheric transport processes and the age of air masses in the upper atmosphere [Geller et al., 1997; Volk et al., 1997; Wamsley et al., 1998]. The age of air masses at the tropical tropopause is defined as zero. Tropospheric air masses that have not yet reached the tropopause are associated with negative ages. ACATS-IV measurements of SF₆ in the northern midlatitude upper troposphere during November 1995 and December 1996 show that mixing ratios increased by approximately 0.28 ppt in 13 months, which is close to the 0.26 ppt expected from the documented growth rate (Figure 5.17). Figure 5.18 shows the convergence of the November 1995 and December 1996 data sets above q = 450 K that are due to the increased effects of stratospheric mixing with age of the air.

Fig. 5.17. Mixing ratios of SF₆ measured by ACATS-IV during November 1995 and December 1996. The difference of mixing ratios observed in the upper troposphere (0.28 ppt) is close to the expected difference of 0.26 ppt calculated from the SF₆ growth of 6.7% yr^-1 in the troposphere.

Fig. 5.18. STRAT tropical (2°S to 20°N) vertical profiles of CFC-11, showing significant intrusion of midlatitude air into the tropics between q = 410 and 450 K and around 480 K. The solid black curve is a purely advective “unmixed” profile for CFC-11 in the tropics calculated by Volk et al. [1996] . The dashed black curve is a proportionally mixed profile, and the red curve is a least squares fit to all midlatitude data. Thin green lines encompass 95% of midlatitude data. The fork observed during the November 5, 1995, flight is denoted by open circles.

In the tropics (2°S to 20°N) vertical profiles of the mixing ratios of trace gases with different lifetimes conform to the results of Volk et al. [1996] (that is, entrainment into a “leaky tropical pipe”). This entrainment of midlatitude air results in “proportionally mixed” tropical profiles, which for CFC-11 are shifted about 15% from the unmixed tropical model line of Volk et al., [1996] shown in Figure 5.18. In the altitude range important for HSCT (16-20 km), ACATS-IV tropical data show several intrusions of midlatitude air into the tropics, denoted by several measurements that lie far to the left of the proportionally mixed tropical profiles (q = 410-450 K and at 480 K, Figure 5.19). The trace gas mixing ratios of the intruding air masses are about 50% lower than the proportionally mixed profiles and are more typical of midlatitude air from much further aloft (q >440 K; the midlatitude data are shown by a red line and a 95% prediction band on Figure 5.18). Similar, but weaker intrusions of midlatitude air at the same q can also be seen in other tropical data [Elkins et al., 1996a; Volk et al., 1996].

The entrainment of midlatitude air into the tropics is illustrated further by looking at the age of the air in the tropical region. The age measured during the flight of December 11, 1996, reaches 1.8-2.3 years at both 410-450 and 480 K surfaces in the latitude range from 12°N to 20°N. Younger air at 440 K (1.4 years) but up to 2.7 years at the 480 K surface was observed during all other tropical flights. The 1.4-year old air masses are about 1 year older than what is seen in this region in proportionally mixed profiles. Especially illustrative is the southbound survey flight of November 5, 1995, which crossed both proportionally mixed tropical air with age of about 0.4 years and midlatitude air with the age of 2.4 years which is mixing in at 480 K on the edge of the tropical ascent region at 17°N (see the “fork” for this flight shown by open circles on Figure 5.18).

Most of the flights show typical midlatitude vertical tracer profiles represented by the red line (mean) and prediction band on Figure 5.18. Only one flight (February 1, 1996) actually crossed a filament of the polar vortex between 52 and 52.5°N, with the age of the air exceeding 5.9 years. Typically, the mean age of the air in midlatitudes for STRAT at altitudes from 19 to 20 km averages 2.5-3 years and does not exceed 4.4 years, reaching this maximum on four flights (February 2 and 15, 1996; December 9 and 13, 1996).

ACATS-IV measures organic chlorinated compounds that account for 78% of total organic chlorine and halon-1211 that contains about 17% of the total organic bromine in the atmosphere. Values of total chlorine and bromine, along with their organic and inorganic components based on ACATS-IV measurements during STRAT, are shown in Figure 5.19. Techniques from Woodbridge et al. [1995] are used for total Cl and Wamsley et al. [1998] for total Br. Also shown are the total chlorine and bromine calculated from balloonborne measurements by the Lightweight Airborne Chromatograph Experiment (LACE) instrument (section 5.2.2).

Fig. 5.19. (a) Total chlorine and (b) total bromine for STRAT mission. ACATS-IV measures compounds that represent 80% of organic chlorine and halon-1211 containing 16% of organic bromine. The remaining unmeasured organic Cl is calculated using tracer-tracer correlations [Wamsley et al., 1998] . LACE data from a midlatitude balloon flight is comparable to ACATS-IV midlatitude measurements and extends the data set to 32 km into the stratosphere where total organic Cl approaches zero. See section 5.2.2 for further discussion of the LACE data.

POLARIS Campaign

The objective of the 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign was to understand the behavior of polar stratospheric ozone as it changes from very high concentrations in spring to very low concentrations in autumn. Missions were flown from Fairbanks, Alaska (65°N), Moffett Field, California, and Barbers Point, Hawaii. ACATS-IV participated in 26 successful flights from April through September 1997.

For POLARIS, ACATS-IV chromatography was accelerated to decrease analysis time from 180 to 125 seconds. Spatial resolution of measurements was increased by using the folded-back chromatography (Figure 5.20).

Fig. 5.20. Comparison of (a) “straight” and (b) “folded back” chromatographs. In (a) the air peak is removed (Elkins et al. [1996b] and CHCl₃ peak is nearly indistinguishable; injections are made every 180 seconds. In (b) CH₃CCl₃ and CCl₄ peaks from the previous injection appear in front of the air peak; these peaks from the current injection will appear in the beginning of the following injection. Injections are made every 125 seconds.

Chloroform (CHCl₃) was first measured during POLARIS with a relative precision of 22%. Chloroform is the shortest-lived (t = 0.5 year) compound measured by ACATS-IV. Figure 5.21 shows vertical profiles of chloroform and CFC-11 measured by ACATS-IV. Comparison of ACATS-IV chloroform measurements to the results of the Whole Air Sampler (WAS; E. Atlas, National Center for Atmospheric Research, personal communication, 1997) shows that ACATS-IV results are on average about 1 ppt higher than WAS, with a greater difference in the troposphere. The cause of this discordance between ACATS and WAS CHCl₃ data is being investigated.

Fig. 5.21. Chloroform and CFC-11 versus pressure. CFC-11 approaches 10 ppt at 26 km and zero at about 30 km in midlatitudes.

Several POLARIS flights from Fairbanks encountered air masses influenced by the polar vortex. Mixtures of vortex and midlatitude air were sampled as is clearly shown by tracer-tracer correlations of species with significantly different lifetimes (Figure 5.22). During the flight of June 29, 1997, at 21 km altitude at 62-78°N, ACATS-IV sampled photochemically-processed vortex air characterized by very low trace gas mixing ratios (green points on Figure 5.23). The balloon GC, LACE, also observed vortex remnants during its flight from Fairbanks on June 30 (Figure 5.19). Older mean ages of the air were observed during the summer (6.7 years) than during the spring (5.4 years) because of the late breakup of the polar vortex. The two-dimensional distribution of air mass ages during POLARIS (Figure 5.23), calculated from ACATS-IV SF₆ measurements, demonstrates that the tropical lower stratosphere is characterized by young air (0.3 to 0.8 year average) and that the lines of stable tracer mixing ratios (isopleths) typically follow q surfaces. Air masses older than 4 to 5 years are generally found poleward of 50°N, but in some cases were observed around 40°N. The oldest air masses encountered during POLARIS were 6.7 years at about 60-65°N in late June 1997.

Fig. 5.22. Tracer-tracer correlations, POLARIS mission. (a) Medium-lived CFC-113 (t = 85 years) shows some separation in the part of the plot depicting mixing; (b) Longer-lived CFC-12 (t = 102 years.) shows several better separated lines of mixing of air parcels with different age and, subsequently, compounds ratio determined by difference in photolysis rate.

Fig. 5.23. Two-dimensional profiles of altitude and the age of the air based on POLARIS data. The oldest air masses (green) are found at high altitude in the latitude range 60-70°N; younger air is seen at the lower altitudes and latitudes. An age of zero is defined for air at the tropical tropopause, hence negative ages are associated with tropospheric data. Dotted lines show potential temperature surfaces which typically define isopleths (equal mixing ratios).

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