5.2.1. OVERVIEW

A new four-channel GC, Airborne Chromatograph for Trace Atmospheric Species (ACATS-IV), was deployed for the first time in 1994 as part of the year-long Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) mission. ACATS-IV flew successfully on 24 flights spanning latitudes from 70°S to 60°N during four seasons. During ASHOE/MAESA, ACATS-IV was configured to measure ten different molecules. In addition to CFC-11, CFC-113, and CH₄ (compounds measured previously with a two-channel version of the instrument) ACATS-IV measured CH₃CCl₃, CCl₄, CFC-12, H-1211, N₂O, SF₆, and H₂. ACATS-IV provides an important set of tracer measurements for several different aspects of atmospheric research: (a) Dynamic and chemical models can be constrained by the wide range of these tracer´s lifetimes (4.5-3200 years). (b) Halogens play an important role in stratospheric ozone destruction and ACATS-IV provides in situ stratospheric measurements of 80% of the chlorine containing species and the bromine containing compound, H-1211 which contains about 20% of the total organic tropospheric bromine. (c) Apart from tropical transport of water from the troposphere to the stratosphere, CH₄ oxidation is the largest source of stratospheric water, which has a large global warming potential. Simultaneous CH₄, hydrogen, and water measurements completely constrain the stratospheric hydrogen budget. (d) The age of stratospheric air is an important input to atmospheric models. SF₆ is a purely anthropogenic compound with no known tropospheric sinks, a stratospheric lifetime of 3200 years, and an approximately linear tropospheric growth rate that makes it an excellent indicator of the age of stratospheric air.

A complete instrument description can be found in a recent publication by Elkins et al. [1996]. The GC has been optimized for low ppt work and frequent sampling of 3-6 minutes by using an appropriate choice of separation columns, very sensitive ECDs, and 12-port gas sampling valves that permit heart-cutting the chromatogram (Figure 5.17). The measured tracers have a wide range of lifetimes that can be used to estimate a tropical-midlatitude exchange in the stratosphere as demonstrated in Volk et al. [1996]. The H-1211, SF₆, CFC-11, and CFC-12 measurements from ASHOE/MAESA have also been incorporated into a calculation that indicates the oldest stratospheric air measured by ACATS-IV has 17 ± 3 ppt of bromine and that all of the bromine resides in inorganic form.

Fig 5.17. (a) Schematic of the ACATS-IV instrument showing pressure transducers (P), electron capture detectors (ECD), gas sampling valves (GSV), and the stream selection valve (SSV). Shaded areas are temperature-controlled zones where the temperatures for the GSV, SSV, and flow module are indicated. ECD and sample loop pressure controllers use a valve (MKS Instruments, Inc., Andover, Massachusetts) servo-controlled to a pressure gauge (Micro Gage, Inc., El Monte, California). The GC inlets for the calibration, carrier gases, and air sample have 10m screens to remove particles. (b) The first position of the 12-port GSV permits loading the sample loop, backflushing of the pre-column, and detection of the peaks of interest from the previous sample injection. (c) Turning the rotor of the 12-port GSV allows injection of the sample onto the columns and diversion of the column exhaust away from the ECD.

5.2.2. TRANSPORT IN THE LOWER STRATOSPHERE

ACATS-IV observations in the lower tropical and midlatitude stratosphere during ASHOE/MAESA provide new information about mass exchange between the tropics and midlatitudes. Because of the profound impact of transport on the distribution of long-lived stratospheric constituents, the magnitude of such exchange is critical for prediction of ozone depletion by human activities. The sparse set of previous tropical in situ tracer data [Goldan et al., 1980; Murphy et al., 1993] and satellite observations of tracer and aerosol distributions [Trepte and Hitchman, 1992; Randel et al., 1993; Mote et al., 1996] have provided evidence for a subtropical "barrier" to horizontal exchange. These observations led to the suggestion that the stratosphere might be closer to a "tropical pipe" model [Plumb, 1996], in which tropical air ascends in isolation from midlatitude influence, than a "global diffuser" model [Plumb and Ko, 1992]. For the first time the ASHOE/MAESA campaign provided extensive tropical measurements of many tracers with local lifetimes ranging from less than 1 to 100 years. These data provide a powerful tool for quantifying the amount of transport across the subtropical barrier. A simple tropical tracer model was used to analyze ACATS data for CFC-11, CFC-12, CFC-113, CCl₄, CH₃CCl₃, halon-1211, and CH₄ along with measurements of N₂O, NO_y, and O₃ from three other instruments aboard the ER-2 [Podolske and Loewenstein, 1993; Fahey et al., 1989; Proffitt and McLaughlin, 1983]. The observations during ASHOE/MAESA span latitudes from 60°N to 70°S and altitudes up to 21 km. The model considers the vertical evolution of a tropical tracer, including loss and production resulting from local photochemistry and entrainment of midlatitude air (due to isentropic mixing):

(1)

where  and _mid are the mean tropical and midlatitude mixing ratios;  is potential temperature used as vertical coordinate; Q = d/d is the net diabatic heating rate, equivalent to vertical ascent rate; P is the photochemical production rate;  is the lifetime for photochemical loss;  is the long-term growth rate; and _in is a time scale for import of midlatitude air, the quantity to be determined by this analysis. Tropical ascent rates (Q) were obtained from published calculations [Rosenlof, 1995; Eluszkiewicz et al., 1996]; chemical production and sinks for the species considered were calculated with a radiative transfer model [Minschwaner et al., 1993] and a photochemical model [Salawitch et al., 1994]; and long-term growth rates () were derived from CMDL network data. Midlatitude mixing ratios were constrained from observations between 35° and 55° in both hemispheres. Tropical air was identified as the region equatorward of the sharp meridional gradient in the NO_y/O₃ ratio observed in the subtropics [Murphy et al., 1993].

A qualitative impression of the isolation of the tropical ascent region can be gained by comparison of vertical profiles of tracer mixing ratios observed in the tropics to profiles calculated assuming unmixed ascent (unmixed profiles), i.e., solutions to Eq. (1) with _in =  (Figure 5.18). Observed profiles of the longer-lived species, N₂O and CFC-12, and also of CH₄ and NO_y (not shown) deviate noticeably from unmixed profiles, indicating mixing with photochemically aged midlatitude air. However, for CFC-113, CFC-11, and the shorter-lived species CH₃CCl₃, CCl₄, and halon-1211 (not shown), observed profiles fall within the uncertainty range of values calculated for unmixed ascent because their vertical profiles are controlled primarily by photochemical loss that dominates loss by mixing for these shorter-lived species.

Fig. 5.18. Vertical profiles of mixing ratios of several long-lived trace species in the tropics (light filled circles) and at midlatitudes (dark filled squares) [Volk et al., 1996]. For the midlatitudes the data was binned into 10K increments of potential temperature (); the profiles shown represent the bin averages and the error bars represent the standard deviation within each bin. Calculated tropical profiles are shown for unmixed ascent (_in = ) (-) from  = 380K (the mean tropical tropopause height) along with an uncertainty range induced by a 50% uncertainty in Q (---). Also indicated is the "effective lifetime" T (=1/(^-1+)) at  = 440 K (~19 km altitude) for each of the species.

Quantitative derivation of the rates of transport between the tropics and midlatitudes is best achieved by analyzing correlation diagrams of two species with disparate lifetimes [Volk et al., 1996]. Differences in the slopes of correlations observed at midlatitudes and in the tropics provide a direct measure of exchange between the two regions if horizontal mixing is fast compared to photochemistry for one of the two species (but not both). As an example, for a given mixing ratio of N₂O, the shorter-lived species show lower abundances in the tropics than at midlatitudes because their loss processes are larger near ~20 km (Figure 5.19), whereas N₂O is not destroyed until the air reaches higher altitudes. Because the abundance of N₂O in the tropics is sensitive to isentropic mixing, however (Figure 5.18), the tropical correlations do not match the correlations calculated assuming unmixed ascent.

Fig 5.19. Correlations of mixing ratios for the shorter-lived species versus N₂O in the tropics (dark filled circles) and at midlatitudes (light filled circles) [Volk et al., 1996]. Mean midlatitude correlations used in the model (long dash) were obtained from quadratic fits to the correlations. Calculated tropical correlations are shown for the unmixed case (_in= ) (short dash) and for a constant entrainment time _in that yielded the best agreement (in a least-squares sense) with the observed tropical correlations (long dash). Also indicated is the "effective lifetime" T (= 1/(^-1+g)) at  = 440 K for each of the species.

In order to derive the entrainment time (_in) from the correlation diagrams in Figure 5.19, we consider Eq. (1) for the tropical mixing ratios of two tracers X and Y:

(2)

Eq. (2), constrained by the mixing ratios for midlatitudes from observations and computed photochemical sources and sinks, is solved to calculate the tropical correlation Y(X) of two species; the entrainment time _in is treated as a free (altitude-independent) parameter. For each pair of tracers displayed in Figure 5.18, the value of _in is determined giving best agreement between the calculated tropical correlations (Figure 5.19) and the observations. The same procedure was also applied to correlation diagrams of the longer-lived species, CH₄, N₂O, CFC-12, CFC-113, and NO_y, versus O₃, which is shorter-lived (with a photochemical production time of only ~3.5 months at 19 km). Analysis of each correlation diagram yielded a mean for _in of 13.5 months, with an uncertainty of ~20%. This seasonally and vertically-averaged entrainment time is longer than the time scale for isentropic mixing at midlatitudes of less than 3 months [Boering et al., 1995], confirming that mixing into the tropics is slow compared to mixing within midlatitudes. Because of the variability of the tropical correlations and the limited seasonal coverage, the data do not provide information on the dependence of _in with height.

Entrainment of air into the tropics is not necessarily balanced by poleward detrainment from the tropics. In the annual mean, the net mass flux out of the tropics (detrainment minus entrainment) must be balanced by the mean mass divergence within the tropics (that can be determined from the mean ascent rate):

(3)

where _out  is a time scale for export of air whose inverse is the detrainment rate;  is the air density; z is altitude; and w is the mean vertical velocity. Detrainment rates computed from Eq. (3) for the estimate of _in (13.5 months) and ascent velocities averaged over 24 months, show that over much of the altitude range considered, more air is exported from the tropics than is imported (Figure 5.20a). The corresponding detrainment time (_out) of less than ~6 months below 19 km and the morphology of decreasing detrainment with altitude is consistent with observations of the propagation of the seasonal cycles of CO₂ and H₂O from the tropics to midlatitudes [Boering et al., 1995; McCormick et al., 1993] and with studies of aerosol dispersal from the tropics [Trepte and Hitchman, 1992].

As shown in Figure 5.20b, for an entrainment time of 13.5 months, ~45% of air of extratropical origin accumulates in a tropical air parcel during its ~8 month ascent from the tropopause to 21 km. The large uncertainty range in this result (Figure 5.20b) results from the uncertainty of the ascent velocity. This substantial entrainment of midlatitude air into the tropical ascent region of the lower stratosphere implies that a significant fraction of NO_x (=NO + NO₂) and other effluents emitted from supersonic aircraft at midlatitudes between 16 and 23 km will likely reach the middle and upper stratosphere where enhancements in NO_x are expected to lead to reductions in ozone [Stolarski et al., 1996].

Fig 5.20. (a) Entrainment rate into and detrainment rates out of the tropics versus potential temperature, expressed as % of air within a tropical air volume (at a fixed altitude) entrained/detrained per month [Volk et al., 1996]. Results are for _in =13.5 months and ascent rates from Rosenlof [1995] (short dash) and Eluszkiewicz et al. [1996] (dotted). The disagreement between the detrainment rates based on these two studies reflects differences in the vertical profiles of the ascent rates. (b) Fraction of midlatitude air within the tropics versus potential temperature for nominal (long dash) and extreme (dotted) values of _in and ascent rates w from Rosenlof [1995] as indicated.

While estimating the effects of human activity on ozone remains a task for multi-dimensional models of atmospheric transport and chemistry, the determination of the rates of transport and the fraction of midlatitude air within the tropical ascent region constitutes important tests for the accuracy of such models. Most current 2-D models do not reproduce steep meridional tracer gradients in the sub-tropics such as observed in the NO_y/O₃ ratio [Murphy et al., 1993], suggesting they generally overestimate the magnitude of mixing between the tropics and midlatitudes. Tests with a particular two-dimensional model show that greater reductions of midlatitude ozone are calculated, improving agreement with observed trends, if mixing parameters are modified to simulate restricted exchange across the tropics [M.K.M. Ko, private communication, 1996]. Realistic representation of dynamical coupling between the tropical source and midlatitude sink regions of ozone may thus hold the key to understanding and reliably predicting the response of the stratospheric ozone layer to a variety of anthropogenic as well as natural perturbations.

5.2.3. BROMINE BUDGET

Concern over bromine´s contributions to stratospheric polar ozone loss [McElroy et al., 1986] and potential for midlatitude ozone destruction [Yung et al., 1980] has resulted in an international regulation of halons and methyl bromide. These bromine-containing compounds occur at much lower stratospheric mixing ratios than chlorine-containing compounds, but bromine is 40-100 times more efficient than chlorine at destroying ozone in the lower stratosphere [WMO, 1995]. In an effort to improve the understanding of brominated compounds in the strato-sphere, the first real-time, in situ stratospheric measurements of the purely anthropogenic compound, CBrClF₂ (H­1211) [Elkins et al., 1996] were obtained. Measurements of H-1211 and nine additional tracers were obtained in 1994 at latitudes ranging from 70S to 60N and to altitudes of 20 km as part of the ASHOE/MAESA mission. The complete H-1211 data set for ASHOE/MAESA is shown in Figure 5.21. These measurements were incorporated into a calculation of the total 1994 stratospheric bromine burden. The lower stratosphere was calculated to contain 17 ± 3 ppt of bromine in 1994 and that essentially all of the organic bromine had been converted to inorganic forms.

Fig. 5.21. Latitudinal profile of halon-1211 as a function of pressure (mb). ACATS-IV measurements of halon-1211 in ppt during ASHOE/MAESA.

The total stratospheric bromine burden was calculated by summing the bromine content in the tropospheric organic bromine species with lifetimes long enough to allow their transport to the stratosphere. This approach assumed that the only source of stratospheric bromine is at the earth´s surface. The organic bromine species included in the model are CH₃Br, H-1211, H-1301, CH₂Br₂, H-2402, and CH₂BrCl with November 1994 mixing ratios of 10.1, 3.3, 2.3, 1.1, 0.47, and 0.14 ppt respectively. The CMDL background site monitoring program provided a historical record of H-1211, H­1301, H-2402, and CH₂Br₂. The CH₃Br data were collected on transects of the Atlantic and Pacific Oceans in 1994 by CMDL researchers[Lobert et al., 1995]. The CH₂BrCl data are from Whole Air Sampler (WAS) measurements taken by researchers at NCAR in early 1995. Taking the weighted sum of the mixing ratios of these species yields the following equation for the total bromine contained in these species as a function of time.

where t is time in years measured from 1994. The tropospheric burden of these species provides an upper limit on the concurrent stratospheric bromine burden. ACATS-IV ASHOE/MAESA measurements of the sulfur hexafluoride (SF₆) mixing ratios are used to determine the age of the stratospheric air and thus, the total bromine present in the air being sampled.

Photolysis of the organic bromine species produces inorganic bromine species with the majority of the inorganic bromine in reactive forms. The partitioning of stratospheric bromine was calculated using the equation

Total Bromine = Organic Bromine + Inorganic Bromine (5)

where Total Br is determined as outlined previously. The only organic bromine species that ACATS-IV measured is H-1211, but from whole air sampler measurements from NCAR (Schauffler, private communication, 1996) and the University of California, Irvine (Blake, private communication, 1996), correlations were compiled of the unmeasured species with measured CFCs. These correlations are shown in Figure 5.22. These correlations permit an estimate of total organic bromine (Figure 5.23). The oldest air sampled by ACATS-IV during ASHOE/MAESA indicated that all of the stratospheric bromine was in inorganic forms.

Fig. 5.22. Correlations of bromine-containing species with CFC-11. The longest-lived species, H-1301, is correlated with CFC-12. Each data set was normalized individually and then the complete data set for each species was fit with a function of the form [X_n] = [CFC_n](^1/d), where the subscript, n, represents a normalized quantity. The longest lived species, H-1301, is correlated with CFC-12. ACATS-IV data are shown by circles. NCAR data are denoted by triangles and UC-Irvine data are shown by squares.

Fig. 5.23. Calculated results for the total stratospheric bromine, Br_total. (open triangles), and its partitioning into total organic CBr_y (open circles), and inorganic, Br_y (open diamonds), components. The dashed lines represent standard deviation windows.
 

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