5.2.2. High Altitude GC Tracer Measurements Project: OMS/STRAT/POLARIS
LACE is a relatively new, three-channel GC instrument (Figure 5.24). It was constructed to extend real time GC measurements of atmospheric tracers such as those measured with the ACATS instrument, to higher altitudes of up to 32 km where, particularly in the tropics, a larger portion of ozone production and loss takes place. The LACE instrument has also made improvements on the spatial and temporal resolution by speeding up the chromatography to obtain a sampling period of 70 seconds. Many of these improvements have now been incorporated into the ACATS instrument. During 1997 LACE obtained high-quality data from four balloon flights onboard the Observations of the Middle Stratosphere (OMS) gondola. As part of the STRAT campaign, LACE flew one midlatitude flight out of New Mexico at 35°N and two tropical flights out of Brazil at 7°S. There was also one polar flight out of Fairbanks at 65°N as part of the POLARIS campaign.
Fig. 5.24. The LACE GC is constructed in three main layers: an oven layer, a GC plumbing layer, and an electronics layer. The GC is pressurized to maintain an even redistribution of heat through convection. This is especially important at the operating altitude of 32 km. Forced air is circulated through the instrument and along the pressurized shell via air ducts to help dissipate 200 watts of heat.
The OMS package is currently configured to make measurements pertinent to stratospheric transport issues. Toward this end, LACE has made in situ measurements from the surface to the middle stratosphere of the long lived tracers H-1211, CFC-11, -113, -12, N2O, and SF6 with a typical precision of between 1% and 4%. The stratospheric lifetimes of these halocarbons and nitrous oxide are dominated by simple photolysis. Their global atmospheric lifetimes span several orders of magnitude. The local photolytic lifetimes of some gases are reduced by 4 orders of magnitude from the tropopause to 32 km. This range in lifetimes covers the dynamic time scales of stratospheric transport. Gas mixing ratios are extremely sensitive to this transport with a strong dependency on vertical flow. Sulfur hexafluoride, however, does not have this altitude-dependent photo-disassociation and has a global atmospheric lifetime greater than 3200 years. Spatial and temporal gradients in the mixing ratio of SF6 are, therefore, driven by surface emissions which are predominantly in the northern hemisphere. This leads to a large interhemispheric surface gradient and, in this instance, a nearly linear surface growth rate [Geller et al., 1997]. Because there is no known stratospheric sink for SF6, its sensitivity to transport is driven entirely by the mean transit time from this growing tropospheric source. Unlike the halocarbons, the measured gradients in the value of SF6 in the stratosphere have only a passive sensitivity to altitude, yet maintain a strong dependency on the time scales of transport [Volk et al., 1997]. Our current precision of SF6 in the stratosphere is 2%, which translates to 3.5 months of growth.
Although this 3.5 month resolution is adequate to track stratospheric dynamics by defining a mean age of the air parcel since entering the stratosphere, it is not adequate to track transport within the free troposphere other than interhemispheric exchange. Nonetheless, tropospheric vertical gradients with both temporal and latitudinal dependence appear to persist. The interhemispheric surface gradient is believed to be transported into the free troposphere between 30°N and 30°S in a time scale comparable to or faster than this 3.5-month resolution. A measurement of these vertical gradients in the SF6 mixing ratio below the 380 K isotherm coupled with the existing measured surface gradients, can be used to quantify tropospheric transport and stratosphere-troposphere exchange. This may also be used to improve the connection between the CMDL global mean value of SF6 and stratospheric age of the air (e.g., Figure 5.23).
Finally the CFCs and H-1211 mixing ratios represent a significant fraction of the total chlorine and bromine entering the stratosphere. As with the ACATS instrument, an estimate of total chlorine and bromine and their organic-inorganic partitioning can be made from LACE measurements.
Stratospheric Dynamics
Major strides in understanding the interactions between pollutants, production to loss of ozone, and climate forcing have been made in the last decade. Today, uncertainties in transport appear to be the limiting parameter in predictive three-dimensional models. The quality of the upcoming assessment on the environmental impact of existing aircraft and the proposed High Speed Civilian Transport (HSCT) fleet is, therefore, limited by our ability to quantify stratospheric transport. In evaluating LACE data, estimates have been made of entrainment of air from midlatitudes into the tropics, the mean age of an air parcel after crossing the tropical tropopause, and mean flow. Breakdown of the arctic vortex and the resulting cross theta mixing surface was also observed in the LACE measurements.
As discussed by Plumb and Ko [1992], tracer-tracer correlations between simple photochemical species are robust as long as quasi-horizontal mixing dominates over vertical advection, as in the global diffuser model. In the tropical pipe model of Plumb [1996] the concept of a tight tracer-tracer correlation is also valid in the midlatitudes and, to a large degree, in the tropics on the other side of the tropical barrier. Mixing across this tropical barrier, the so-called leaky pipe, connects these two regions. This mixing is a major mechanism for transport of midlatitude, lower stratospheric air into the middle and upper stratosphere. This midlatitude mixing into the tropics is also a key uncertainty in the HSCT assessment and has been a primary focus of the OMS platform.
A modified Volk et al. [1996] analysis, which quantifies the mixing of air from the extratropical stratosphere into the tropical pipe, has been extended to 32 km by using the new LACE CFC and SF6 profiles. Two approaches have proven fruitful. The first approach relies on mean vertical advection rates (Q). Local chemical losses of the CFCs are dominated by simple photolysis; therefore, assuming no midlatitude influence, isolated tropical profiles can be calculated given the advection rate, photochemical lifetimes (t), tropospheric growth rates (g), and mixing ratios (c) of the CFCs entering the tropical tropopause (Figure 5.25a,b; dotted black line). These isolated tropical profiles can then be evolved a second time to incorporate mixing from midlatitudes along constant potential temperature (q) surfaces. This is done with the entrainment time (tin) assumed to be constant. The tracer continuity is governed by equation (6).
(6)
These profiles have also been corrected for weak O(1D) chemistry and photochemical production (P) (Figure 5.25a,b).
Fig. 5.25. Panels (a) and (b) show least square fits to SF6 and CFC-12 using equation (6) over three altitude ranges. They are sensitive to uncertainties in vertical advection rates. Panels (c) and (d) show least square fits using equation (2) for CFC-11 versus. SF6 and CFC-11 versus CFC-12 over the same three altitude ranges. They are, therefore, independent of vertical advection. Panel (e) shows a compilation of entrainment from the four fits in each of the three altitude ranges.
To evaluate whether tin varies with height, a least squares fit of this second profile to the measured profile is then performed over three altitude ranges. The entrainment time (tin) is varied as the free fit parameter and is held constant over each given altitude range to stabilize the fits.
One problem that limits this approach is the large uncertainty in the tropical advection rate. Fortunately, the advection rate can be eliminated from the calculation by using tracer-tracer correlation between molecules of differing atmospheric life times. This can be seen by taking the ratio of equation (1) for the mixing ratio Y of one molecule, to equation (1) for the mixing ratio c of a second molecule.
(7)
Because this advection rate Q is common for all molecules in the same air mass moving up the tropical pipe, it drops out of equation (7) (Figure 5.25c,d).
LACE data are consistent with a constant entrainment time of 1.5 
months over the entire range up to 32 km (Figure 5.25e). By 32 km, 90% of the
air in the tropical upwelling region is of midlatitude origin because of the
total integrated entrainment. A comparison of the first approach (equation 6),
which is sensitive to Q, with the second approach (equation 7), which is independent
of Q, may help to constrain mean flow in the tropics. This work is consistent
with the Volk et al, [1996] earlier analysis and is expected to generate
a more complete picture of midlatitude intrusions into the tropics when finalized.
Data taken recently in the tropics also revealed air masses that were of midlatitude origin showing the characteristic signature of lower mixing ratios. Surprisingly, the ozone profile did not show the same midlatitude signature and remained representative of tropical air. The chemical equilibrium time for ozone at these locations is fast (weeks to a month), apparently much faster than the mixing time over the spatial scale of these midlatitude intrusions during this time of weak upwelling. These data and the chemical equilibrium time of ozone can, therefore, set lower limits on mixing within the tropical upwelling region.
Sulfur hexafloride has proven to be useful for evaluating transport in the lower and middle stratosphere. Measurements of stratospheric SF6 permit an accurate determination of the mean age of an air parcel after it crosses the tropical tropopause. Mean age estimates are shown in Figure 5.26 for all LACE flights and three-dimensional model estimates. In general, these models underestimate the age of air as defined by SF6 distributions. Although increased mixing from mid-latitudes could account for the older measured age in the tropics, the excessively young midlatitude model estimates imply that mean flow in these models is too high.
Fig. 5.26. Profiles of mean age of the air from zonally-averaged models (supplied by D. Waugh [private communication, 1998]). This modeled age is plotted with (a) ACATS measured age (triangles) against latitude, and LACE measured age (diamonds) against altitude for (b) the tropics at -5° latitude, (c) the midlatitudes at 35° latitude, and (d) the polar regions at 65° latitude. The shaded region indicates the dominant range of model results. Mean age is taken to be zero at the equator at 20.0 ± 0.5 km.
One important lesson from our OMS balloon launch from POLARIS in Fairbanks was the observation of unmixed and mixed remnants of the polar vortex from the earlier March 1997 arctic ozone depression. Because the remnants of this vortex lasted as late as July, these measurements show one reason why we should continue to sample the arctic. As shown in Figure 5.27, this mixed air is clearly not representative of a typical midlatitude distribution and must continually be monitored to quantify both the chemistry and dynamics that are taking place at the pole.
Fig. 5.27. Tracer-tracer correlation plot of CFC-12 versus CFC-11 from LACE on the OMS gondola for flights on September 21, 1996 (midlatitudes), February 14, 1997, and November 11, 1998 (tropics), and June 30, 1997 (POLARIS, polar regions). LACE data are compared against the ACATS data from the POLARIS flight of the ER-2 aircraft on June 30, 1997. The blue line indicates tropical data isolated from the mid and high latitudes by the leaky pipe, dashed red lines represent a nominal midlatitude and/or polar profile, and the dashed gray line indicates conservative mixing across isopleths due to the anomalously late break down of the polar vortex. This mixing line can only be observed in a tracer-tracer plot over regions that have curvature in the nominal midlatitude profile.
Atmospheric Dynamics Below the 380 K Isentropic Surface
LACE has taken highly precise measurements of several trace gases in the lowermost stratosphere and upper troposphere, regions for which very little tracer data exist. Measurements of SF6 and CFC-11, in particular, reveal many interesting features in these regions and their vertical gradients offer constraints on transport time scales. Simulation of these tracer gradients by models is important for an accurate assessment of tracer transport in the lower stratosphere and troposphere where the impact of aircraft exhaust and the transport of greenhouse gases play a key role.
The lowermost stratosphere is that part of the stratosphere that lies between the tropopause and the 380 K potential temperature surface. The lowermost stratosphere is a unique part of the stratosphere, since air can be exchanged isentropically between the stratosphere and troposphere. Thus the lowermost stratosphere contains a mixture of older stratospheric air that has been advected downward by the mean meridional circulation and tropospheric air that has been transported isentropically. The relative importance of downward advection and isentropic transport largely depends on season and location.
Figure 5.28 shows profiles of SF6 and CFC-11 from the Ft. Sumner flight on September 21, 1996. The 380 K and tropopause heights are indicated on the figure and lines are drawn through the data in each height region. The two profiles have different vertical gradients in the lowermost stratosphere. CFC-11 has almost no vertical gradient below the 380 K surface, while SF6 has a large vertical gradient in the lowermost stratosphere and a small but still noticeable gradient in the upper troposphere. The constant CFC-11 mixing ratio suggests that the gradients seen in the SF6 profiles are of tropospheric origin. Downward advection would have a larger effect on CFC-11 because of its more rapid decrease above the 380 K surface. Weak downward flow across the 380 K surface at the time of our flight is consistent with the seasonality of the mean meridional circulation in the stratosphere and the midlatitude location.
Fig. 5.28. Profiles of SF6 and CFC-11 from the September 21, 1996, Ft. Sumner, New Mexico, flight. The tropopause and 380 K surfaces are indicated on each plot and rough linear fits to the data are included in each region.
The vertical gradient in SF6 from the Ft. Sumner flight is, therefore, assumed to be due to an interplay between isentropic mixing from the tropical upper troposphere and residence time of air in the lowermost stratosphere. Were it the result of growth, the vertical gradient in SF6 would represent a 9-month age difference over the depth of the lowermost stratosphere. The time scale of isentropic mixing between the tropopause and the northern midlatitudes is on the order of a month or less and the flushing time of the northern hemisphere lowermost stratosphere is thought to be roughly 4 or 5 months. Therefore, some of the vertical gradient in the lowermost stratosphere could be caused by transport of the interhemispheric gradient of surface SF6 to the upper troposphere. Some portion of the interhemispheric gradient in SF6 is likely to be mapped along the tropopause from the tropics to the midlatitudes. As tropospheric air is isentropically mixed into the midlatitude lowermost stratosphere, the latitudinal tropopause gradient will contribute to the vertical gradient of SF6 in the lowermost stratosphere.
This gradient in SF6 along the tropopause may have a strong dependence upon seasonal variability of tropospheric transport, coupled with the interhemispheric gradient of SF6 at the surface. Profiles of SF6, which have been normalized to remove the growth rate for the two Brazil flights, are shown in Figure 5.29. The February Brazil flight has SF6 mixing ratios in the upper troposphere that are close to the global mean surface value and decrease toward the southern hemisphere surface average with decreasing height (Figure 5.29). This profile is consistent with a significant amount of northern hemispheric surface air entering the tropical upper troposphere during February. The monthly mean position of the Hadley circulation was estimated by Oort and Yienger [1996]. In January and February the northern hemisphere Hadley cell is shifted south and is also consistent with a significant amount of northern hemispheric surface air being transported into the tropical upper troposphere as suggested by our data.
Fig. 5.29. Profiles of SF6 from the two Brazil flights and the Ft. Sumner, New Mexico, flight normalized to remove the growth rate. Normalized northern and southern hemispheric mean surface mixing ratios from the CMDL network are also included at the bottom of the graph.
The November flight has a nearly constant mixing ratio in the upper troposphere that is close to the southern hemisphere surface average. In October and November the northern hemisphere Hadley cell is weak and shifted north of the equator so the tropical upper troposphere should be dominated by southern hemispheric air at a 7°S, again consistent with our data.
These tropical profiles suggest that the seasonal cycle of the Hadley circulation causes a seasonal cycle in the SF6 mixing ratios in the tropical upper troposphere. This seasonal cycle will likely cause a seasonal cycle in the latitudinal gradient of SF6 mixing ratios along the tropopause, since the mean Hadley circulation flow in the upper troposphere is poleward. For the northern hemispheric tropopause we would expect a large gradient during summer, when more southern hemispheric surface air is transported into the tropical upper troposphere, and a weak gradient during winter when mostly northern hemispheric surface air is transported into the tropical upper troposphere. Our midlatitude flight was in September at the beginning of fall when a remnant of the tropopause gradient caused by the summer tropospheric transport could have been present. This also suggests that a small seasonal correction to the connection between stratospheric age of air and the CMDL global average may be needed.
The effects of significant mean downward motion across the 380 K surface on trace gas profiles in the lowermost stratosphere can be seen in Figure 5.30. CFC-12, -11, H-1211, and SF6 measurements from the June 30, 1997, flight over Fairbanks are shown in this figure along with the heights of the 380 K and tropopause surfaces. Mixing ratios of all four tracers decrease above the tropopause.
Fig. 5.30. Profiles of CFC-12, CFC-11, H-1211, and SF6 from the Fairbanks, Alaska, flight. The tropopause and 380 K surfaces are indicated. The three distinct regions, the middle stratosphere (above 380 K), the lowermost stratosphere, and the troposphere, are clearly defined by both the gradients and spread in the data. Relationships between the time scales of mean flow and mixing can be inferred.
This decrease and the vertical gradient in the lowermost stratosphere are largest in the shortest lived tracer, H-1211, which is consistent with the expected effect of downward advection. Subsequently smaller decreases above the tropopause and vertical gradients in the lowermost stratosphere are seen in CFC-11 and CFC-12 which have longer photochemical lifetimes. SF6 also has a large decrease above the tropopause due not to photolysis but to its growth rate. Air advected down across the 380 K surface in the high latitudes is 2 years older on average, than northern tropospheric air. Therefore the sharp decrease in SF6 above the tropopause is a result of the growth of SF6 in the northern high-latitude troposphere during the time air was transported to the high-latitude lowermost stratosphere.
Even though June is a time of relatively weak mean downward flow in the northern hemisphere high latitudes, the tracer data suggest that this flow is the dominant transport into this part of the lowermost stratosphere. It is interesting to note the relative compactness of the profiles above and below the 380 K surface. The lowermost stratosphere appears to be relatively well mixed compared with the stratosphere above the 380 K surface.