Stratospheric HCl observations are an important diagnostic for the evaluation of catalytic processes that impact the ozone layer. We report here in situ balloon-borne observations of HCl employing an off-axis integrated cavity output spectrometer (OA-ICOS) fitted with a reinjection mirror. Laboratory assessments demonstrated that the spectrometer has a 90 % response time of 10 s to changes in HCl and a 30 s precision of 26 pptv. The instrument was deployed alongside an ozone instrument in August 2018 on a balloon-borne descent between 20–80 hPa (29–18 km altitude). The observations agreed with nearby satellite measurements made by the Earth Observing System Microwave Limb Sounder within 10 % on average. This is the first time that stratospheric measurements of HCl have been made with ICOS and the first time any cavity-enhanced HCl instrument has been tested in flight.
Most of the hydrogen chloride (HCl) in the stratosphere exists because of the cumulative emissions of chlorofluorocarbons (CFCs), whose widespread use as nontoxic refrigerants began in the 1950s (WMO, 2018). CFCs have been largely phased out globally since the Montreal Protocol in 1987 and subsequent London and Copenhagen amendments because their degradation via photolysis in the stratosphere leads to the catalytic destruction of the ozone layer, a layer that protects life from the Sun's destructive ultraviolet radiation (Molina and Rowland, 1974). Though some emissions of CFC-11 are still being detected in the Northern Hemisphere, atmospheric CFC levels have been declining for decades (Montzka et al., 2018). Despite this, elevated chlorine levels persist in the stratosphere (Mahieu et al., 2014). In 2016, approximately 80 % of the chlorine input from the troposphere to the stratosphere still came from substances controlled under the Montreal Protocol (WMO, 2018).
Stratospheric HCl is considered an inorganic chlorine reservoir because the bonded hydrogen renders the chlorine atom unreactive to ozone. When environmental conditions are favorable, however, HCl can react, liberating chlorine from its reservoir into a catalytically active form. Three stratospheric perturbations can increase the rate at which HCl is converted to reactive chlorine species on the surface of liquid or solid aerosols: (1) lower temperatures, (2) enhanced water vapor levels, and (3) enhanced aerosol levels (Anderson et al., 2012; Solomon, 1999). Deep convective storm systems have been observed over the Great Plains of the United States that can enhance stratospheric water vapor levels (Hanisco et al., 2007; Smith et al., 2017). These water vapor enhancements can persist in the lower stratosphere over the United States because they are trapped in the North American monsoon anticyclone for periods of a few days to over a week (Weinstock et al., 2007; Clapp et al., 2019). Alternatively, sulfate aerosol levels can be enhanced by volcanic eruptions or by solar geoengineering strategies, which propose to inject sulfate particles into the stratosphere as a way to mitigate climate change by reflecting solar radiation back to space (Solomon et al., 1999; Keith, 2000; Tilmes et al., 2009). These environmental changes all increase the probability of transient ozone depletion over the United States and other parts of the world.
With increased forcing of the climate by increasing greenhouse gas (GHG) levels, severe summer storms that lead to deep convective injection in the United States may increase in number or intensity, while stratospheric temperatures are expected to decrease (Diffenbaugh et al., 2013). Furthermore, as climate change gets more severe, the consideration for solar geoengineering strategies to complement GHG emission reductions will likely be taken more seriously; the need for research into the effects of sulfate aerosol injection on stratospheric chemistry will become correspondingly greater. Therefore, an established strategy for monitoring the stratosphere is important for understanding the extent to which the expected stratospheric perturbations over the next century translate to perturbations in its chemical composition.
HCl monitoring is an essential component of a platform that seeks to evaluate the effects of stratospheric perturbations. HCl is quick to respond to perturbations in temperature, water vapor, and aerosol levels (for details into these reaction mechanisms, see Anderson et al., 2012, 2017). The heterogeneous catalytic conversion from HCl to the radical chlorine monoxide (ClO) may occur only where there is enhanced water vapor from convective injection. Therefore, observations of HCl with high spatial resolution are of particular importance. In situ HCl measurements are also of interest because the molecule can serve as a tracer for stratospheric air masses and ozone in the lower stratosphere (Marcy et al., 2004), and it can provide better understanding of the total stratospheric chlorine budget (Bonne et al., 2000). Satellite monitoring systems do currently exist and provide invaluable information about the stratosphere's chemical composition, including HCl. The main example is the Earth Observing System (EOS) Microwave Limb Sounder (MLS), which is located on NASA's Aura satellite (Waters et al., 2006). MLS, however, has limited spatial and temporal resolution and a set trajectory that limits the number of local environmental phenomena that can be observed. In situ, airborne measurements that can attain higher horizontal and vertical resolution are essential for understanding how perturbations of the future stratosphere may alter the delicate balance established by the chemicals that compose it. Previous in situ measurements of stratospheric HCl have used either multi-pass cells (ALIAS-I, Webster et al., 1994; ALIAS-II, Scott et al., 1999 and Christensen et al., 2007) or chemical ionization mass spectrometry (CIMS, Marcy et al., 2005). While measurements made via the CIMS technique have excellent precision, the technique has greater mass and volume requirements compared to optical approaches, and it requires constant in-flight calibration (Roberts et al., 2010).
Recently, several HCl instruments based on cavity-enhanced spectroscopy have
been developed which have the advantage of longer effective path lengths
(
We report here a flight-tested OA-ICOS instrument for measuring HCl that can contribute to a future airborne platform for in situ stratospheric monitoring. The instrument was fitted with a third mirror, called a reinjection mirror (RIM), which amplifies the light intensity within the optical cavity, thus increasing the amount of light delivered to the detector (Leen and O'Keefe, 2014). This HCl instrument was deployed along with an ozone instrument on a balloon-borne NASA campaign, called the Harvard University Stratospheric Chemistry Experiment (HUSCE). The instrument platform was launched from the Columbia Scientific Balloon Facility at Fort Sumner, New Mexico, on 24 August 2018. This is the first reported in-flight demonstration (1) of a RIM-ICOS instrument and (2) of any cavity-enhanced instrument that measures HCl. We discuss here the details of this instrument, its performance in the laboratory, and comparison of the August 2018 test flight results with other stratospheric measurements of HCl.
The molecular absorption line that the RIM-ICOS instrument engages to
measure HCl is at 2963 cm
Line intensities of fundamental transition of HCl. The
relatively smaller line that accompanies each larger line represents
the transition of H
To be a useful absorption feature for measurement, the HCl transition must
be spectroscopically isolated from transitions by other molecules that occur
in the stratosphere. Transitions from other molecules can obfuscate the
signal, limiting both the accuracy and precision of the HCl measurement.
When selecting the line, the spectral interference from several common
stratospheric molecules was considered, including CH
Theoretical spectrum in the vicinity of the HCl feature.
Parameters are
The photon source used for the HCl instrument is a continuous wave (cw)
interband cascade laser (ICL) custom-made by the Microdevices Laboratory at
NASA's Jet Propulsion Laboratory (Borgentun et al., 2015) (full instrument
schematic shown in Fig. 3). The laser emission is centered at 2963 cm
Diagram of the HCl instrument layout. The schematic is intended to clarify the path of laser light and gas flow path in a lab setting (optical setup is identical in flight): (1) ICL and laser TEC; (2) optical isolator; (3) beam splitter; (4) etalon; (5) etalon MCT detector; (6) RIM; (7) focusing lenses; (8) ICOS detector; (9) tank of ultra-zero air; (10) tank of 5 ppmv HCl; (11) gas deck, which includes the intermediate gas bottle; and (12) vacuum pumps. Grey arrows indicate gas flow path. The main text describes each component in more detail.
The laser output beam is collimated within the laser housing, a standard
TO-3 can that serves to protect the laser from ambient air (most crucially
from water vapor, which can condense on the cooled laser). A thermoelectric
cooler (TEC) within the housing is used to regulate the temperature to
around 275 K. The laser wavelength is tuned by changing the current, which
is controlled using a custom-designed laser current driver (Sayres et al.,
2009). The driver is a field-programmable gate array (FPGA)-based waveform generator that achieves highly repeatable, precise, and linear current ramps. During flight,
the laser completed a scan every 5 ms, which includes 10 % laser
off time used to establish the dark voltage signal from the detector. The
in-flight laser scan covered a range of 3 cm
Light emitted from the laser TO-3 can passes through a polarizer (ISP Optics, POL-3-5-SI-25) and a quarter-wave plate (ISP Optics, WP-Z-Q-3500). The polarizer and quarter-wave plate (shown collectively as (2) in Fig. 3) serve as an optical isolator that prevents laser light from ultimately reflecting back into the laser housing, which is known to cause unwanted feedback. With the optical isolator, any laser light reflected back from the primary ICOS cavity mirror or other optics is blocked. The effectiveness of the optical isolator was demonstrated separately with the laser on the optical bench before being integrated into the full instrument.
The laser beam then passes through a telescope that reduces the beam
diameter to 2 mm and then through a CaF
The majority of the laser beam passes through the beam splitter and is reflected by two gold-plated steering mirrors (Thorlabs, PF10-03-P01-10). The steering mirrors direct the beam through a hole drilled in the reinjection mirror (RIM), located 2 cm from its center. The RIM is a 7.6 cm diameter gold-plated mirror with a radius of curvature (ROC) of 100 cm (Thorlabs, CM750-500-M01). The addition of the RIM is a relatively recent innovation that seeks to mitigate a major disadvantage of cavity-enhanced absorption techniques: a large loss of laser power resulting from the high reflectivity of the cavity mirrors (Leen and O'Keefe, 2014). In the usual OA-ICOS setup, more than 99.98 % of the laser light is reflected away by the first ICOS mirror before entering the cavity. This loss in photon flux was considered necessary to obtain the large effective path length within the cell, leading to a dramatic increase in the percent absorbance associated with molecular transitions occurring in the cavity (Ouyang et al., 2012). With the RIM present, when 99.98 % of the photon flux is reflected away from the cavity mirror, it hits the RIM but is shifted spatially with respect to the entry hole. The laser beam then reflects off the cavity mirror and RIM 10–15 times before finally passing back through the hole in the RIM, essentially forming a Herriott cell between the RIM and the first cavity mirror. The result is that more of the light enters into the cavity with each pass – hence why it is referred to as a “reinjection” mirror – and ultimately increases the laser power that impinges on the detector.
The HCl ICOS cavity mirrors (LohnStar Optics, custom) are separated by
47.37 cm. Each mirror is made of zinc selenide (ZnSe) and has a highly
reflective coating on the concave side facing inward on the cell, forming
the high-finesse cavity. Each mirror also has an antireflective coating on
the plano side facing away from the cell. The light is reflected multiple
times between the cell mirrors of an ICOS cavity, creating an effective
average path length that is a factor of
Upon exiting the cavity, the laser beam passes through two positive meniscus
lenses (ISP Optics, ZC-PM-50-76 and GE-PM-12-25-C-2) that focus the light
onto the detector. The first focusing lens immediately follows the
detector-side ICOS mirror. This lens is 5.1 cm (2 in.) in diameter with a
focal length of 7.6 cm. The second focusing lens has a 1.3 cm (0.5 in.)
diameter with a focal length of 2.5 cm. This lens is placed about 1 cm away
from the detector. During the HUSCE balloon flight, the detector used was a
four-stage thermoelectrically cooled MCT detector that, coupled with a
preamplifier, had a bandwidth of 1 MHz (Vigo, PVI-4TEMXPXX-F). The TEC
cooled the detector to around 193 K, and the detector itself was 1 mm in
diameter. The responsivity at this wavelength is 2.5 A W
An electronically controlled pinch valve was used to regulate pressure in the cell both in lab and during flight. Unless otherwise specified, cell pressure in the laboratory was regulated to 53 hPa. Air inlet and outlet ports for the cell are both perpendicular to the cell's axis to improve thorough mixing within the cavity. Furthermore, the inlet and outlet are close to the ends of the cell to prevent the formation of dead space near the mirrors. Air is pulled through the system by two scroll pumps each drawing up to 50 standard liters per minute (Scroll Labs, SVF-50P). These pumps were chosen for their compactness and low weight (two important considerations for airborne campaigns). The net flush rate for the cell is 0.83 times per second under normal operation.
HCl is a particularly challenging molecule to measure, primarily due to how easily it adsorbs to surfaces. In addition, HCl reacts chemically with surfaces and can be quite corrosive. Thus, the most important aspect of the material that composes the cell and gas flow tubing is that minimal HCl adsorbs to its surface. To address this, the cell, the intermediate gas bottle, the pressure gauge in the gas deck, and nearly all of the plumbing was treated with in an inert silicon coating (SilcoNert2000 coating by SilcoTek), a coating that is considered to have excellent compatibility with HCl specifically (SilcoTek, 2020). The coating is applied via chemical vapor deposition, a process that requires the treated material to be heated to extremely high temperatures. Aluminum consequently could not be used to make the cell and plumbing, despite its advantages of being lighter weight and possessing higher thermal conductivity than stainless steel, the material chosen for this instrument.
Pressure and temperature in the cell are both measured 4 times per second.
The cell pressure is measured by an Omega pressure sensor (PX409-005A5V),
which was calibrated before flight. The recorded cell temperature is the
average reading of three 100 k
Laboratory assessments of the instrumental performance, which are discussed
further in Sect. 2.3, were based on flowing diluted HCl through the
instrument. Stratospheric HCl typically does not exceed 3 parts per billion by volume (ppbv), but due to
HCl's corrosive nature and ease with which it adsorbs to surfaces, it is not
possible to purchase gas cylinders with HCl mixing ratios this low. The
cylinder used for laboratory assessments of the HCl instrument is 4.6
The mixing ratio of HCl in the cylinder is far too high to directly feed
into the instrument for analysis. Therefore, air from this primary HCl
cylinder is diluted with air from an ultra-zero air cylinder
(N
Extensive tests were carried out in the laboratory to evaluate and characterize the instrument performance. First, the time response of the flight instrument to precipitous changes in HCl was determined. The instrument reaches a 90 % settling time in 10 s, referred to as the 90 % response time. Figure 4 shows the decrease in HCl after 15.5 ppbv HCl flow was shut off and replaced with flow of ultra-zero air. Increase in HCl had a similar response time. The response time is comparable to Webster et al. (1994) and Hagen et al. (2014), who both took similar steps to ensure their instruments were suitable for HCl measurements (Hagen et al., 2014, made their cell out of Teflon instead of coating a stainless-steel cell with an inert silicon layer). Importantly, the response time is significantly faster than instruments that did not take these precautions. Even in cases where instruments incorporated short cavities made of material less resistant to HCl adsorption (specifically nonporous fluoropolymer), 90 % response times have been reported to be greater than 90 s (Roberts et al., 2010).
Response time of the HCl instrument when 15.5 ppbv HCl was replaced with ultra-zero air flow. The observed mixing ratio is 10 % of the input value after 10 s. The light grey line is 1 Hz sampling. The black line is a 3 s smoothing average. The cell was heated to 315 K, and cell pressure was 50.7 hPa.
The instrument was also assessed for linearity in its response to different
amounts of HCl. This was done by first filling the intermediate gas bottle
with gas from the primary HCl cylinder and ultra-zero air as described. At
this point, a stepwise dilution was performed. At each step, around 40 %
of the air was passed through the cell (typically around 800 psia). The
intermediate gas bottle's pressure was recorded for 1–2 min and then
refilled back to
The HCl mixing ratio for the very first dilution was determined based upon
measurements with the instrument using the published spectral parameters
from the HITRAN database, which have a combined uncertainty of less than 5 % (Gordon et al., 2017). The mixing ratio of every subsequent dilution was
determined using the initial concentration and the ratio of the gas bottle
final pressures prior to and after refilling. The results in Fig. 5 are
the combined data of two separate executions of the dilution strategy
described above. The vertical error bars are 1
Observed HCl mixing ratio after sequential dilution of
original HCl cylinder to illustrate that the instrument responds linearly to
different input mixing ratios. Each point represents an approximately 1 min average of 1 Hz spectra. The cell was heated to 315 K, and the cell
pressure was around 24 hPa. Vertical bars represent 1
A distinctive feature of the HCl instrument described here is the coupling of OA-ICOS with the RIM. Past literature has credited the RIM with increasing power impinged on the detector by a factor of 22.5 (Leen and O'Keefe, 2014). It was confirmed that a similar increase in power was possible for this HCl instrument. However, this did not result in the lowest fractional noise. The laser trajectory was instead aligned to optimize noise reduction. The power increase associated with this alignment was a factor of 9.
It was found that the presence of the RIM did not just increase laser power impinged on the detector; it also reduced the fractional noise by half (i.e. doubling the signal-to-noise ratio, or SNR). This was true across an array of laser scan rates (Fig. 6). That fractional noise is not reduced by the same factor that power is increased is due to the fact that optical noise tends to scale linearly with power, while electronic noise tends to remain constant. Figure 6 shows that not all of the noise is scaled up, so RIM leads to a direct improvement in the instrument's SNR. Figure 6 also shows that as the laser scans more quickly across roughly the same wavelength range, the fractional noise is reduced. This is due to the quenching of optical noise created by standing waves in the cavity, whose constructive and destructive interferences translate into oscillations in the signal. Scanning too fast can begin to compromise the absorption features as well, though, so a balance must be met to achieve optimal an signal-to-noise ratio (Witinski et al., 2010).
Fractional noise as a function of scan rate of the laser. The number of samples per scan was held constant, so a larger number of samples observed by millisecond (ms) indicates a higher scan rate. Two important trends are illustrated: total fractional noise is lower with faster scan rates, and fractional noise is lower when the RIM is incorporated into the optical layout.
Following the HUSCE campaign, the TEC MCT detector was replaced with a
Stirling-cooled indium antimonide (InSb) detector (Teledyne Judson
Technologies, J10D-J508-R02M-60). The InSb detector has a peak responsivity
of 3.68 A W
One downside is that increased detector size tends to increase electronic
noise. The InSb detector is 4 times larger than the in-flight MCT detector.
As electronic noise tends to scale up with the square root of the active
area, the InSb detector should have around twice as much electronic noise
(Hamamatsu, 2011). The electronic noise of the two detectors can be compared
by evaluating the standard deviation in current (A) they produce when the
laser is off (i.e. no light impinged on the detector and ignoring their
respective gains, V A
With the laser on, the increased electronic noise is more than offset by increased power captured by the detector. To illustrate this, fractional electronic noise was used to compare the two detectors. The fractional electronic noise was determined by calculating the standard deviation of the 1 Hz detector signal when the laser was off and dividing that number by the mean signal when the laser was on. The improvement offered by the current configuration was evaluated by comparing the average fractional electronic noise achieved in the laboratory with the new InSb detector with that obtained with the MCT detector in flight. The fractional electronic noise with the InSb detector was found to be 4.5 times lower than that of the MCT detector (0.02 % vs 0.09 %), indicating future campaigns with the instrument should expect improvement in SNR from fractional electronic noise reduction alone.
The 30 s precision for the HCl instrument with the InSb detector is 26 pptv
(Fig. 7). For the 1 h sample period, the improvement in the SNR follows
the theoretical white noise limit (
Allan variance plot of the HCl instrument after
Comparison of uncertainties among HCl instruments. “n/a” means not applicable.
On the morning of 24 August 2018, a 7 million cubic feet (
Figure 8 shows balloon altitude as a function of time for the HUSCE flight.
The full length of the flight, from initial launch to release of the
instrumental platform from the balloon, was
The full altitude profile of the HUSCE flight is shown in grey. The black segment of the profile is when the HCl instrument was on and recording measurements. The tropopause is determined by the observed reversal in lapse rate from the ambient temperature sensor on the gondola.
The descent rate of the balloon was adjusted in real time, averaging 2 m s
During flight, a pinch valve was used to regulate cell pressure at 53 hPa. At ambient pressures below 60 hPa, the pinch valve remained completely open to maximize flow through the cell. The cell pressure gradually rose from a minimum of 16 hPa at maximum altitude (29.5 km) to the point where the pinch valve started regulating to maintain 53 hPa (19.5 km).
Ambient pressure was measured during flight by an Omega pressure sensor (PX409-015A5V) attached to a port on the side of the pressure vessel. The sensor was calibrated before flight and also verified against ambient pressure readings collected by the Columbia Scientific Balloon Facility at Fort Sumner. The zero offset of the onboard pressure sensor was adjusted as a result of this comparison. The NASA facility also measured ambient temperature as well as altitude via GPS. These measurements were used solely for describing the flight profile but were not used in the formal analysis of the dataset obtained from the HUSCE flight.
Water vapor was also measured by the HCl instrument during flight. The laser
scan range includes a strong water vapor absorption feature at 2961.7126 cm
While the average laser power at the detector remained stable throughout the flight, intermittent power oscillations with a frequency of 7.9 kHz and variable amplitude were observed in nearly half of the recorded spectra. Laboratory assessments after the campaign suggest the origin of the oscillation was electronic and likely due to a grounding issue on the gondola itself.
In the following analysis, the 1 s spectra were fit and then averaged in 30 s
bins. The baseline was fit with the absorption feature using a least-squares
fitting algorithm described in detail in Sayres et al. (2009) and Allen (2020). Briefly, the laser power curve and electrical oscillations (if
present), along with minor optical etalons that formed in the cavity, are
modeled using a fourth-order polynomial and three sine and cosine waves. The
use of sine and cosine waves for each frequency allows the algorithm to
account for the phase of the oscillation. The absorption feature is fit
using a Voigt line-shape function. The spectrum in Fig. 9b is
a 30 s averaged spectrum that corresponds to an ambient mixing ratio of 1.19 ppbv HCl, measured at an atmospheric pressure of 26.3 hPa. The fit residual is
shown in Fig. 9a. The absorption feature has an asymmetric skew due to the
cavity time constant being comparable to the tuning rate of the laser. This
skew, characteristic of all ICOS spectra, is taken into account when fitting
the HCl absorption feature. The ICOS spectra were fit using HITRAN
spectroscopic parameters for the HCl transition described in Sect. 2.1 and
converted to mixing ratio using measurements of cell temperature and
pressure. The fitting algorithm itself was written in C
Example of a 30 s spectrum from flight, focused on the region that was used to determine HCl mixing
ratios. The spectrum corresponds to 1.19 ppbv HCl at a cell pressure of 13.6 hPa.
The 30 s average spectra had variable precision throughout the flight, in
large part due to the sporadic electrical interference which was less
present later in the flight. The noise-equivalent absorption, used here for
30 s precision, varied from 40–70 pptv HCl. This range was determined by
calculating the relative standard deviations of cell pressure, cell
temperature, and the fit residual for each of the 30 s spectra taken
during the flight (less than 10 % of the noise-equivalent absorption is
due to uncertainty in the cell pressure and cell temperature). All
observations were greater than 3
The HCl profile obtained during the HUSCE campaign (Fig. 10, grey circles) is consistent with other reported measurements of stratospheric HCl over the United States (Froidevaux et al., 2008). Ranges from MLS observations of HCl made on the same day and nearby geographic region are shown as well, demonstrating the agreement between HUSCE campaign observations and satellite measurements. The comparison with MLS is discussed further in Sect. 4.1. The pump speed was changed during small portions of the flight to evaluate in-flight HCl adsorption on the inlet and cell. We observed a low bias up to 20 % for HCl mixing ratios during the lowest pump speeds (one-fourth the normal operating pump speed), though atmospheric variability may account for some of this change. Periods where pump speed was lowered like this are excluded from the profile shown in Fig. 10.
While the HCl profile is more variable in the mid-stratosphere than in the lower stratosphere, instrument diagnostics do not suggest any operational cause for the increased variability. There is evidence that balloon interference may have impacted portions of the mid-stratospheric descent, based on anomalous readings from the diagnostic water vapor measurement and the ambient temperature measurement (for more detailed discussion of balloon interference, see Kräuchi et al., 2016). However, neither the temperature profile nor the points of elevated water vapor correlate with the observed HCl mixing ratios.
Observations from the HUSCE campaign were compared with measurements from MLS, which is used as the primary instrument for comparison because it provides continuous and reliable global measurements of stratospheric HCl for the full altitude range of the HUSCE flight.
MLS measures HCl by observing the molecule's rotational transition band at 640 GHz, with an estimated single-profile precision of 0.2 ppbv in the pressure range encountered during the HUSCE flight (Livesey et al., 2020). From stratospheric pressures 100 to 0.15 hPa, MLS agrees well with ACE-FTS (within 5 % on average) and with previously made in situ balloon-borne observations launched from Fort Sumner, New Mexico (Froidevaux et al., 2008). For comparison with HUSCE, 11 MLS observations were selected from 24 August 2018 (Fig. 11). The range of these observations, including error bars, is shown as green shading in Fig. 10, with their average profile shown in dark red. In this case, and in all cases where MLS data are used for illustration or comparison in this paper, the recommended quality control was applied (Livesey et al., 2020). For more details on MLS, see Waters et al. (2006).
MLS observations of HCl made near HUSCE on 24 August 2018. Error bars indicate MLS-reported uncertainty. Dashed lines are for readability; they do not indicate known trends between the observations shown as colored triangles. Inset: locations of each MLS observation (colors are coordinated with the main panel).
The MLS data between 60 and 20 hPa illustrate that the HCl profile does not have a monotonic increase with altitude. The dashed lines that connect each MLS profile are meant to simply help guide the eye. They should not be understood as an accurate or even reasonable interpolation. Considering the non-monotonic variability in all of the profiles, it is much more likely that HCl mixing ratios at intermediate pressures would have more variation than the linear interpolations suggest. Finally, while the MLS observations closest to HUSCE were chosen for this comparison, the spatial and temporal overlap is not perfect, and some disagreement is expected. Nonetheless, the HUSCE observations of HCl agree well with MLS, with all of the 5 hPa averaged observations being within range of MLS observations that day and more than 97 % of the 30 s averaged in situ observations being within the MLS error bar range (Fig. 10a).
Reported MLS values are the result of averaging mixing ratios at various
pressure levels described by the MLS averaging kernel. For HCl, however,
more than 98 % of the weighting at the three pressure levels shown in Fig. 10b is at those exact pressure levels (Livesey et al.,
2020). For calculating the percent difference then, all 1 s HUSCE
observations within
A separate instrument was used to measure ozone during the flight. The
instrument, a multi-pass white cell with a UV LED light source, detected
ozone by measuring the absorption of UV light at 255 nm. Sample gas was
alternately directed into the detection cell to measure the absorption
signal or through a MnO
Ozone profile from HUSCE (grey circles) overlaid onto 11 MLS observations in the same way as in Fig. 10. The 11 MLS observations are at the same locations as those shown in Fig. 11. The light blue circles are observations from a NOAA ozone sonde in Boulder, CO, on the evening of 23 August 2018.
The difference in behavior between the ozone and HCl profiles observed
during HUSCE may be explained by examining global stratospheric profiles of
these two gases. To illustrate this, zonally and meridionally averaged
profiles for the month of August 2018 were determined for MLS ozone and HCl
from 100 to 5 hPa. The resulting contour plots of MLS HCl mixing ratio as a
function of latitude and longitude are shown in Figs. 14 and 15,
respectively. The HCl mixing ratios are based on the monthly means for
August 2018 for each pressure–latitude coordinate (or pressure–longitude in
Fig. 14). The HCl mixing ratios are provided by MLS as averages in the
following pressure bins: 100, 68, 46, 32, 22, 15, 10, 7, and 5 hPa. The
trajectory of HUSCE balloon descent is represented by a white dotted line in
the figures. Temperatures and pressures used to determine the zonally
(meridionally) averaged potential temperature surfaces for August 2018 were
obtained from the European Center for Medium-Range Weather Forecasts
ERA-Interim reanalysis (ECMWF, 2011). The black lines shown in Figs. 14
and 15 represent the derived isentropic contours. The calculated potential
temperatures ranged between 365 and 630 K at 100 and 20 hPa, respectively.
Though not shown, isentropic contours derived from MLS temperature profiles
yielded similar results. The above analysis was also performed for methyl
chloride (CH
Contour plot of zonally averaged MLS HCl profiles as a function of latitude for the month of August 2018. The white dashed line indicates the HUSCE descent. The black lines represent zonally averaged isentropes for August 2018.
Same as for Fig. 13 except the meridionally averaged HCl profile is shown as a function of longitude instead.
Same conditions as for Fig. 13 except the profile illustrated is CH
Same conditions as for Fig. 13 except the profile illustrated is ozone. Note that ozone has very little latitude dependence at the altitudes and latitude at which HUSCE observations were made, especially compared to HCl.
The general trend for HCl in the stratosphere is consistent with the
degradation via photolysis and oxidation via OH of the organic chlorine
compounds (e.g., CFCs and CH
Figure 15 is the same as 13 except the molecule plotted is CH
Figure 16 shows the global profile across latitudes for ozone. Ozone also
has some latitude dependence. However, its change with latitude is
significantly less pronounced than that of HCl and CH
In the lower stratosphere, observations made with a chemical ionization mass spectrometer (CIMS) that was flown aboard NASA's WB-57 aircraft demonstrated that HCl has a tight correlation with ozone (Marcy et al., 2004). CIMS was deployed to measure HCl in the lower stratosphere as part of the Cirrus Regional Study of Tropical Anvils and Cirrus Layers – Florida Area Cirrus Experiment (CRYSTAL-FACE) mission out of Key West, FL, in the summer of 2002, and again as a part of the Aura Validation Experiment (AVE) out of Houston, TX, in 2005. (data available at NASA, 2005; results published in Froidevaux et al., 2008).
Marcy et al. (2004) established that a tight correlation exists between HCl
and O
Plot of HUSCE O
In this paper, we report on a new RIM-ICOS instrument that measures
atmospheric HCl by probing the molecule's fundamental ro-vibrational
transition at 3.37
We also evaluated the instrument performance during the HUSCE campaign, a
balloon-borne descent made over the central United States during August 2018. HUSCE's
HCl observations agree well with MLS observations made that same day.
Vertically averaged values are 8 % different from MLS on average, all
within MLS-reported uncertainty. Finally, HUSCE's HCl measurements and
O
Since the HUSCE campaign, an updated detector has been installed in the HCl instrument, and the wavelength range has been narrowed to focus on the HCl absorption feature. Future campaigns that include the instrument will likely house a separate instrument that measures water vapor. With these improvements, we find the lab-based 30 s precision is 26 pptv.
This is the first time that stratospheric measurements of HCl have been made with ICOS, where even the in-flight 30 s precision of 70 pptv compares favorably with most other measurement techniques for HCl. This is also the first time any cavity-enhanced HCl instrument has been tested in flight (in contrast to the ground-based field test performed by Hagen et al., 2014). Finally, all known previous research discussions of RIM-ICOS were either strictly theoretical or coupled with lab-based demonstrations. The HUSCE campaign demonstrates that RIM-ICOS is feasible for making in situ measurements in balloon-borne field deployments. Cavity-enhanced techniques, such as ICOS and CRDS, offer the ability to obtain a higher level of precision in a small cell than instruments that rely on direct absorption from multi-pass cells of the same length. Furthermore, the ability of the RIM to enhance the signal-to-noise ratio and its stability during flight support integration of this additional mirror for further gains in precision for in situ instrumentation.
The data generated and analyzed for the current study are
available at
DSS, JBS, JW, and JGA designed the study. MR, MG, NA, TM, JW, and DSS developed the HCl instrument. JBS developed the ozone instrument that flew alongside the HCl instrument and processed its data. All authors contributed to data collection and field work during the HUSCE campaign. JW, DSS, and NA contributed to data processing, lab testing, and uncertainty analysis for the HCl instrument. JW was mainly responsible for interpreting the results and writing the manuscript. All authors participated in editing the manuscript.
The authors declare that they have no conflict of interest.
We thank the Columbia Scientific Balloon Facility for providing the resources to evaluate the instrument performance in the stratosphere.
This work was funded by NASA (grant no. NNX16AI72A).
This paper was edited by Keding Lu and reviewed by two anonymous referees.