Introduction
At present, bromine is estimated to be responsible for roughly one-third of the
photochemical loss in global stratospheric ozone . Past research
has revealed that total stratospheric bromine (Bry) has (in 2013) four
major sources or contributions: (1) CH3Br which is emitted by
natural and anthropogenic sources, with a present contribution of 6.9 ppt to
Bry, (2) four major halons (CClBrF2 or halon-1211; CBrF3
or halon-1301; CBr2F2 or halon-1202; and CBrF2CBrF2 or
halon-2402), all emitted from anthropogenic activities, with a present
contribution of 8 ppt to Bry; (3) so-called very short-lived species
(VSLS); and (4) inorganic bromine transported into the upper troposphere,
e.g., previously released from brominated VSLS and/or sea salt (e.g.,
). This
inorganic bromine is also partly transported into the stratosphere. Together
sources 3 and 4 are assessed to contribute 5 (2–8) ppt to stratospheric
bromine . Previous assessments of total Bry and its
trend revealed [Bry] levels of ≈ 20 ppt (16–23 ppt) in
2011, which has been decreasing at a rate of -0.6 % yr-1 since the peak
levels observed in 2000. This decline is consistent with the decrease in
total organic bromine in the troposphere based on measurements of
CH3Br, and the halons .
Estimates of stratospheric Bry essentially rely on two methods:
first, the so-called organic (Bryorg) method, where all bromine
from organic source gases (SGs) found at the stratospheric entry level is summed
(). Second, total inorganic bromine
(Bryinorg) is inferred from atmospheric measurements (e.g.,
performed from the ground, aircraft, high-flying balloons, or satellites) of
the most abundant Bry species, BrO, assisted by a suitable
correction for the Bryinorg partitioning inferred from
photochemical modeling (e.g., ). Further
constraints on stratospheric Bry (range 20–25 ppt) were obtained
by satellite-borne measurements of BrONO2 in the mid-infrared (IR) spectral
range at nighttime . While the organic method is rather
precise for the measured species (accuracies are several tenths of a ppt), it
suffers from the shortcoming of not accounting for any inorganic bromine
(contribution 4) directly entering the stratosphere. Uncertainties in the
inorganic method arise from uncertainties in measuring BrO as well as
from modeling Bryinorg partitioning, of which the combined error
amounts to ±(2.5–4) ppt, depending on the type of observation and
probed photochemical regime.
Past in situ measurements of Bryorg were performed at different
locations and seasons within the upper troposphere, the tropical tropopause layer (TTL), and stratosphere. In
the present context the most important were measurements performed within the
TTL (for the definition of TTL, see ) over the Pacific
from where most of the stratospheric air is predicted to originate (e.g.,
). These include the
measurements (a) by , who found
[VSLS] = 1.3 ppt (contribution 3) at the tropical tropopause over the
central Pacific (Hawaii) in 1996, (b) by and
, with [VSLS] = 2.25 ± 0.24 ppt (range 1.4–4.6 ppt) and [VSLS] = 1.35 ppt (range 0.7–3.4 ppt) found within the TTL
over northeastern Brazil in June 2005 and June 2008, respectively, and (c)
most recently by , who found [VSLS] = 2.96 ± 0.42 and 3.27 ± 0.49 ppt at 17 km over the tropical eastern and
western Pacific in 2013 and 2014, respectively. Information on contribution
3 was further corroborated by measurements performed in the upper tropical
troposphere by , who found [VSLS] = 3.72 ± 0.60 ppt in
the upper tropical troposphere over Borneo in fall 2011, and by
, who inferred [VSLS] = 3.4 ± 1.5 ppt for the
CARIBIC (Civil Aircraft for the Regular Investigation for the Atmosphere Based on an Instrument Container) flights from Germany to Venezuela and Colombia during 2009–2011, Germany to
South Africa during 2010 and 2011, and Germany to Thailand and Kuala Lumpur,
Malaysia, during 2012 and 2013.
Supporting information on brominated VSLS concentrations typical of the
boundary layer of the western Pacific came from measurements performed during
the TransBrom ship cruise in October 2009 (median 2.23 ppt and range from
1.45–4.14 ppt; ) and the VSLS measurements made around
Borneo during the SHIVA (Stratospheric Ozone: Halogen Impacts in a Varying Atmosphere) project (median 5.7 ppt and range from 3.9 to 10.7 ppt; ). Corroborating model calculations to these field
studies by (a) indicated that from the western Pacific
on average only 0.4 ppt and at a maximum up to 2.3 ppt of the emitted VSLS
bromine may reach the stratosphere, while (b) estimated that
up to 8 ppt of VSLS bromine may enter the base of the TTL at 150 hPa,
whereby the VSLS emissions from the tropical Indian Ocean, the tropical
western Pacific, and off the Pacific coast of Mexico are suspected to be most
relevant, and finally (c) the Community Atmosphere Model with Chemistry (CAM-Chem) modeling performed within the study of
, which indicates that over the eastern and western Pacific
contributions 3 and 4 (called [VSLS + Bryinorg] in the study)
amount to 6.20 ppt (range 3.79–8.61 ppt) and 5.81 ppt (range 5.14–6.48 ppt), respectively.
Using the inorganic method, contributions 3 and 4 have been indirectly
estimated from BrO measured at the ground, high-flying
balloons, or satellites (e.g., ; ; ; ;
; ; ; ; ;
; ; ). All together
these studies pointed to a range between 3 and 8 ppt with a mean of 6 ppt for
contributions 3 and 4. The most direct information on contributions 3 and 4
come from the studies of , , and
. They inferred 1.25 ± 0.16 ppt (very short lived – source gases, VSL-SGs,
contribution 3) +4.0 ± 2.5 ppt (product gases, PGs, contribution 4) = 5.25 ± 2.5 ppt (contributions 3 and 4) and 2.25 ± 0.24 ppt (VSL-SGs,
contribution 3) +1.68 ± 2.5 ppt (PGs, contribution 4) = 3.98 ± 2.5 ppt (contributions 3 and 4) from two balloon-borne soundings performed
in the TTL and stratosphere over northeastern Brazil during the dry season in
2005 and 2008, respectively. The inferred bromine was thus often larger than
[VSLS] inferred using the organic method (contribution 3), indicating that
variable amounts of Bryinorg (i.e., several parts per trillion) are directly
transported from the troposphere into the stratosphere (contribution 4).
Based on these findings, and
provided evidence for the efficiency of short-lived halogens to influence
climate through depletion of lower-stratospheric ozone (for contribution 3)
but without explicitly considering the effect of inorganic bromine readily
transported across the tropical tropopause (i.e., contribution 4). They
concluded that VSLS bromine alone exerts a 3.6 times larger ozone radiative
effect than is due to long-lived halocarbons when normalized to their halogen
content. Moreover the benefit for ozone and UV radiation due to the declining
stratospheric chlorine and bromine since the implementation of the Montreal
protocol was quantified in a recent study by .
Finally, in a recent study pointed out that bromine from
contribution 3 and 4 contributes about 14 % to the formation of the
present Antarctic ozone hole, in particular at its periphery. Further, they
suggests a large influence of biogenic bromine on the future Antarctic ozone
layer.
The present paper reports measurements of BrO (and NO2,
O3, CH4, and the brominated source gases) made during the
ATTREX (Airborne Tropical Tropopause Experiment) deployments of the NASA Global Hawk into the lowermost stratosphere (LS), upper troposphere (UT), and TTL of the
eastern Pacific in early 2013. Corresponding data collected during the
western Pacific deployments in early 2014 will be reported in a forthcoming
paper, primarily since most of the 2014 measurements were performed under TTL
cirrus-affected conditions, for which the interpretation of UV–visible
(UV–vis) spectroscopic measurements is not straightforward (see below). The present
paper further addresses the amount of inorganic bromine found in the TTL and
its transport into the lowermost tropical stratosphere (contribution 4),
together with the implications for ozone.
Our study accompanies those of and . While
discusses the instrumental details and the methods employed
to remotely measure BrO, NO2, and O3, the study of
reports the Global Hawk Whole Air Sampler (GWAS) measurements of CH3Br
(contribution 1), the halons (contribution 2), and the brominated VSLS
(contribution 3) analyzed in whole-air samples, which were simultaneously
taken from aboard the NASA Global Hawk over the eastern and western Pacific
during the 2013 and 2014 deployments, respectively.
The paper is organized as follows. Section 2 briefly describes all key
methods used in the present study. Section 3 discusses the measurements along with some (necessary) data reduction. In Sect. 4, the major
observations are presented and they are compared with previous BrO
measurements and our modeling results, along with their implications for the
amount of inorganic bromine present within the TTL. Further implications of
our measurements for the photochemistry of bromine and ozone within the TTL
and lowermost subtropical stratosphere are discussed. Section 5 concludes the
study.
Methods
The instruments of the NASA-ATTREX package most important for the present
study consist of a fast UV photometer for measurement of ozone
, a gas chromatograph (Unmanned Aircraft System Chromatograph for Atmospheric Trace Species – UCATS; , and ) as well as a Picarro instrument (Harvard University Picarro Cavity Ringdown Spectrometer –
HUPCRS; ) to measure
CH4, CO2, and CO, a whole-air sampler (GWAS;
) to analyze a large suite of
stable trace gases, and a three-channel scanning limb mini-DOAS (differential optical absorption spectroscopy) instrument for
spectroscopic detection of O3, NO2, BrO, OClO,
IO, O4, O2, H2Ovapor, H2Oliquid,
and H2Osolid in the UV–vis–near-IR spectral ranges (e.g.,
).
All instruments, techniques, methods, and tools are briefly described in the
following.
DOAS measurements of O3, NO2, and BrO
The mini-DOAS instrument is a UV–vis–near-IR three-channel optical spectrometer
by which scattered skylight received from limb direction and direct sunlight
can be analyzed for O3, NO2, and BrO (as well as for some
other species; see above). Since the instrument and retrieval methods are
described in detail in the accompanying paper by (for
further details, see Table 2 therein), only some key elements of the data
analysis are described here.
The post-flight analysis of the collected data for the detection of
O3, O4, NO2, and BrO and concentration
retrieval include (a) the spectral retrieval of the targeted gases using the
DOAS method (for the DOAS settings, see Table 4 in
), (b) forward radiative transfer (RT) modeling of each observation using the
Monte Carlo model McArtim (Monte Carlo Atmospheric Radiative Transfer Inversion Model; ; for further details, see
Sect. ), and (c) for the concentration and
profile retrieval either the nonlinear optimal estimation
or the novel x gas scaling technique (for details, see Sect. 4.2. and 4.3 in ). Typical errors are ±5ppb for O3, ±15ppt for NO2, and ±0.5ppt for BrO, to which possible systematic errors in the
individual absorption cross section need to be added. These are for
O3-UV ±1.3 %, O3-vis ±2 %, NO2 ±2 %,
and BrO ±10 % (for more details on the error budget, see
).
In situ measurements of O3
The NOAA-2 polarized O3 photometer is a derivative of
the dual-beam, unpolarized, UV absorption technique described by
. Briefly, the ambient and O3-free air flow is
alternately directed into two identical 60 cm long absorption cells. The
253.7 nm UV light from a mercury lamp is split into two beams that are each
directed into one of the absorption cells. Since O3 strongly absorbs
253.7 nm photons, the UV beam passing through the cell containing ambient
ozone is attenuated more than the beam passing through the cell containing
O3-free air. Knowing the O3 absorption cross section (σ(O3)) and the absorption path length (L), the O3
partial pressure (p(O3)) in the ambient air can be derived using
Beer's law.
The instrument has a fast sampling rate (2 Hz at < 200 hPa, 1 Hz at 200 to
500 hPa, and 0.5 Hz at ≥ 500 hPa), high accuracy (3 % excluding
operation in the 300–450 hPa range, where the accuracy may be degraded to
about 5 %), and excellent precision (1.1×1010 O3 molecules cm-3 at 2 Hz, which corresponds to 3.0 ppb at 200 K
and 100 hPa or 0.41 ppb at 273 K and 1013 hPa). The size (36 L),
weight (18 kg), and power (50–200 W) make the instrument suitable for many
unmanned aerial vehicle systems and other airborne platforms. In-flight and
laboratory intercomparisons with existing O3 instruments have shown
that measurement accuracy (3 %) is maintained in flight.
Overview of the NASA Global Hawk ATTREX flights conducted from
Dryden in 2013. The thickness of the lines corresponds to flight altitudes,
where the thinnest line is for an altitude of around 14 km and the thickest
line for around 18 km.
CH4 measurements by UCATS
The UCATS measures atmospheric methane (CH4) on one gas chromatographic
channel along with hydrogen (H2) and carbon monoxide (CO) once
every 140 s. UCATS has two chromatographic channels with electron
capture detectors (ECDs), two ozone (O3) ultraviolet absorption
spectrometers, and a water vapor (H2O) tunable diode laser absorption
spectrometer (TDLAS). The details of the CH4 chromatography are
similar to those on balloon and airborne instruments described in
and . The addition of ∼ 100 ppm of nitrous
oxide to the make-up line of the ECD enhances the sensitivity to H2,
CO, and CH4 (, and ). The
separation of these gases in air is accomplished with a pre-column of
Unibeads (2 m × 2 mm diameter), and a main column of molecular sieve
5A (0.7 m × 2.2 mm diameter) at ∼ 110 ∘C
. The precision of the CH4 measurement during ATTREX
was ±0.5 % and is calibrated during flight with a secondary standard
after every three ambient air measurements. Instrumental drift is corrected
between the standard injections. UCATS measurements are traceable to the WMO
Central Calibration Laboratory (CCL) and are on the CH4 WMO X2004A
scale (, with updates given at
http://www.esrl.noaa.gov/gmd/ccl/ch4_scale.html).
CH4 measurements by HUPCRS
The HUPCRS consists
of a G2401-m Picarro gas analyzer (Picarro Inc., Santa Clara, CA, USA)
repackaged in a temperature-controlled pressure vessel, a separate
calibration system with two multi-species gas standards, and an external pump
and pressure control assembly designed to allow operation at a wide range of
altitudes. The Picarro analyzer uses wavelength-scanned cavity ringdown
spectroscopy (WS-CRDS) technology to make high-precision measurements of
greenhouse gases ().
HUPCRS reports concentrations of CO2, CH4, and CO every
∼ 2.2 s and the data are averaged to 10 s. In-flight
precision for CH4 is 0.2 ppb in 10 s.
Briefly, the analyzer uses three distributed feedback (DFB) diode lasers in
the spectral region of 1.55 to 1.65 µm. Monochromatic light is
injected into a high-finesse optical cavity with a volume of 35 cm3
and a configuration of three highly reflective mirrors (≥ 99.995 %).
Internal control loops keep the cavity at 140 ± 0.02 Torr and 45 ± 0.0005 ∘C in order to stabilize the spectra. The injected light is
blocked periodically, and when blocked, the exponential decay rate of the
light intensity is measured by a photodetector. The decay rate depends on
loss mechanisms within the cavity such as mirror losses, light scattering,
refraction, and absorption by a specific analyte. A sequence of specific
wavelengths for each molecule is injected into the cavity in order to
reconstruct the absorption spectra. A fit to the spectra is performed in real
time and concentrations are derived based on peak height. High-altitude
sampling (i.e., very low pressure and temperature) necessitated transferring
the core components of the Picarro analyzer to a sealed tubular pressure
vessel, which is maintained at 35 ∘ C and 760 Torr. The analyzer's components are isolated from the pressure vessel to provide vibration damping
and decoupling from deformations in the pressure vessel caused by external
pressure changes.
The sampling strategy for HUPCRS consists of bringing in air through a
rear-facing inlet, filtered by a 2 µm Zefluor membrane, and dehydrating
this air by flowing it through a multi-tube Nafion dryer followed by a
dry-ice cooled trap prior to entering the Picarro analyzer. A choked upstream
Teflon-lined diaphragm pump delivers ambient air to the analyzer at 400 Torr,
regardless of aircraft altitude, via a flow bypass. A similar downstream
pump, with an inlet pressure of 10 Torr, facilitates flow through the
analyzer at high altitude and ensures adequate purging of the Nafion drier.
Measurement accuracy and stability are monitored by replacing ambient air
with air from two NOAA-traceable gas standards (low- and high-span) for a
total of 4 min every 30 min. These standards are contained in
8.4 L carbon fiber wrapped aluminum cylinders and housed in a
temperature-controlled enclosure. The total weight of the package is 97 kg.
The Global Hawk Whole Air Sampler
The GWAS is a modified version of the Whole
Air Sampler used on previous airborne campaigns (). Briefly, the
instrument consists of 90 custom-made Silonite-coated (Entech
Instruments, Simi Valley, CA) canisters of 1.3 L, controlled with Parker Series 99
solenoid valves (Parker-Hannifin, Corp., Hollis, NH). Two metal bellows
compressor pumps (Senior Aerospace, Sharon, MA) allow the flow of ambient air
through a custom inlet at flow rates ranging from 2 to 8 standard L min-1,
depending on altitude. The manifold and canister module temperatures are
controlled to remain within the range of 0–30 ∘C. GWAS is a fully
automated instrument controlled from the ground through an Ethernet
interface. Parameters to fill the canisters, flush the manifold, and control
the temperature are predetermined in the data system module (DSM) inside
the aircraft, to fill the canisters automatically in the case of failure of the
aircraft networks. However, during the entire flight, the parameters are
manually set with the ground laptop computer to improve the sampling
collection at different altitudes. During the ATTREX campaign, the canisters
were filled to ∼ 3 standard atmospheres (40 psi) in about 25 s at
14 km and 90 s at 18 km. The samples are analyzed using a high-performance
gas chromatograph (Agilent Technology 7890A) and mass spectrometer with mass
selective, flame ionization and electron capture detectors (Agilent Technology
5975C). Samples are concentrated on an adsorbent tube at -38 ∘C with
a combination of cryogen-free automation and thermal desorber system (CIA
Advantage plus UNITY 2, Markes International). The oven temperature profile
is -20 ∘C for 3 min, then 10 ∘C min-1 to 200 ∘C, and 200 ∘C for 4 min for a total analysis time of 29 min. Under these
sampling conditions the precision is compound/concentration dependent and
ranged from ≤ 2 to 20 %. Calibration procedures as well as mixing
ratio calculations are described elsewhere .
TOMCAT/SLIMCAT predictions of mixing ratio curtains of CH4
(upper left), O3 (upper right),
NO2 (middle left), BrO (middle right), Bryinorg (bottom left), and Bryorg
(bottom right) for the sunlit part of SF3-2013 (14 February 2013). Note the different color scale ranges. The white
line is the flight trajectory of the Global Hawk. For better visibility, the simulated mixing ratios are shown for
the altitude range 0–25 km, although the TOMCAT/SLIMCAT simulations cover the range of 0–63 km altitude.
During ATTREX 2013 the whole-air sampler measured a variety of organic trace
gases, including non-methane hydrocarbons, CFCs, HCFCs, methyl halides,
solvents, organic nitrates, and selected sulfur species. For this work a
range of long- and short-lived organic bromine gases are measured, including
CH3Br, CH2Br2, CH2BrCl, CHBrCl2,
CHBr2Cl, CHBr3, and halon-1211 (CBrClF2) and halon
2402 (C2Br2F4). Halon-1301 (CBrF3) is not measured and a
constant value of 3.3 ppt is used to account for the bromine content from
this compound.
Radiative transfer modeling
The measured limb radiances of the
mini-DOAS instrument are modeled in spherical 1-D and in selected cases in
3-D, using version 3.5 of the Monte Carlo RT model
McArtim . The model's input is chosen according to the
on-board measured atmospheric temperatures and pressures, including
climatological low-latitude aerosol profiles from Stratospheric Aerosol and Gas Experiment III (SAGE
III) (https://eosweb.larc.nasa.gov/project/sage3/sage3_table), and lower-atmospheric cloud covers as indicated by the cloud physics lidar measurements
made from aboard the Global Hawk (GH; see http://cpl.gsfc.nasa.gov/). In the standard run,
the ground (oceanic) albedo is set to 0.07 in UV and 0.2 in the visible spectral
range. The RT model is further fed with the actual geolocation of the GH,
solar zenith, and azimuth angles as encountered during each measurement, the
telescopes azimuth and elevation angles, as well as the field of view (FOV)
of the mini-DOAS telescopes. Figure 5 in displays one example
of an RT simulation for limb measurements at 18 km altitude. The simulation
indicates that correctly accounting for the Earth's sphericity, the
atmospheric refraction, cloud cover, ground albedo, etc., is relevant for the
interpretation of UV–vis–near-IR limb measurements performed within the
middle atmosphere . Even though the three (UV–vis–near-IR)
mini-DOAS spectrometers are not radiometrically calibrated on a absolute
scale, past comparison exercises of measured and McArtim modeled limb
radiance provide confidence in the quality of the RT simulations (see, e.g., Figs. 5 and 6 in , and Fig. 2 in ).
For the simulations of the trace gas absorptions measured in limb direction,
the RT model is further fed with TOMCAT/SLIMCAT-simulated curtains of the
targeted gases simulated along the GH flight paths (see Sect. ). In the RT simulations [BrO]
is set to 0.5 ppt near the ground, where TOMCAT/SLIMCAT predicts lower
BrO concentrations (see Fig. middle right
panel), in agreement with the findings discussed in and the
recent study of .
Photochemical modeling
For the interpretation of our measurements, we use simulations of the
TOMCAT/SLIMCAT 3-D chemical transport model (CTM; ). More specifically, the simulations are used for
intercomparison with measured photochemical species, for assessment of the
budget of Bryinorg, and for sensitivity studies on the impact of
our measurements on the photochemistry of bromine and ozone in the
subtropical UT–LS, tropical UT, and TTL.
For the present study, the TOMCAT/SLIMCAT model is driven by meteorology from
the ECMWF ERA-interim reanalyses . The reanalyses are used for
large-scale winds and temperatures as well as convective mass fluxes
. The model has a detailed stratospheric chemistry scheme
with kinetic and photochemical data taken from JPL-2011 ,
with recent updates. The model chemical fields are constrained by specified
time-dependent surface mixing ratios. For the brominated species, the
following surface mixing ratios of stratospheric-relevant source gases are
assumed: [CH3Br] = 6.9 ppt; [halons] = 7.99 ppt;
[CHBr3] = 1 ppt; [CH2Br2] = 1 ppt; and
Σ [CHClBr2,CHCl2Br,CH2ClBr,etc.] = 1 ppt of Br.
Organic bromine is thus [Bryorg] = 20.89 ppt at the
surface, in agreement with recent reports (e.g., ).
No other (unknown organic or inorganic) sources of bromine for UT, LS, and
TTL are assumed (e.g., ).
Omitting the release and heterogeneous processing of bromine from sea-salt
aerosols (e.g., ) in the model for the sake of saving
computing time appears justified since (1) even though it is predicted to be
relevant for bromine (∼30 % of the total Bryinorg) in
the free troposphere (), its contribution to BrO
in the TTL is at most of the order of the accuracy (∼0.5 ppt) of our
BrO measurements, (2) its time- and space-dependent sources (as for
the brominated VSLS) are not well constrained, (3) in the modeled troposphere
inorganic bromine only serves as a boundary condition for bromine in the TTL,
and (4) the additional BrO would not affect the
BrO-measurement-based calculation of Bryinorg for the TTL
(see below). Further, the surface concentration of CH4 is specified
based on observations of AGAGE (Advanced Global Atmospheric Gases Experiment; https://agage.mit.edu/) and NOAA, which
reflect recent variations in its growth rate.
Panel (a) shows the time–altitude trajectory of the sunlit part of
the GH flight track (SF1-2013) on 4–5 February 2013 (SF1-2013). Panels
(b)–(e) show intercomparisons of TOMCAT/SLIMCAT-simulated fields with observations
of (b) CH4 (UCATS), (c) O3 (NOAA), (d) NO2
(mini-DOAS), and (e) BrO (mini-DOAS). The grey-shaded error bars of
the mini-DOAS NO2 and BrO measurements include all
significant errors, i.e., the spectral retrieval error, the error due to a
contribution to the slant absorption from above the aircraft and from the
troposphere, and the absorption cross section uncertainty. Panel (f) shows
the SLIMCAT modeled Bry partitioning for the standard run no. 583.
Panel (g) shows a comparison of inferred and modeled Bryinorg,
including the uncertainty as a grey band.
The standard model run (no. 583) is initialized in 1979 and spun-up for 34
years at low horizontal resolution (5.6∘ × 5.6∘) and
with 36 unevenly spaced sigma-pressure vertical levels in the altitude range
0–63 km. Output from 1 January 2013 is interpolated to a high horizontal
resolution (1.2∘ × 1.2∘), and the simulation continued
over the ATTREX campaign period using this resolution. The model output is
sampled online along the Global Hawk flight tracks for direct comparison
with the observations. Two further high-resolution sensitivity experiments
are performed from 1 January 2013 onwards. In run no. 584, the ratio of the
photolysis frequency of BrONO2 and the three-body association rate
reaction coefficient kBrO+NO2 is increased by a factor 1.75 (e.g.,
). In run no. 585 the second-order rate reaction coefficient
kBr+O3 is set to the upper limit of its uncertainty range
().
For all model levels and for the time resolution (∼ 30 s) of the
mini-DOAS measurements, “curtains” of the targeted gases along the flight
track are stored (see, e.g., Fig. 6 in , and
Fig. of our present study).
They are imported into the RT model McArtim for further forward simulations
of the observations, and measurement-versus-model intercomparison studies.
The inclusion of simulated TOMCAT/SLIMCAT curtains in our study is
particularly necessary for (a) the retrieval of absolute concentrations using
the O3-scaling technique (see , Sect. 4.3),
(b) estimate of errors and retrieval sensitivities to various parameters (see
Sect. 4.4 and the Supplement to ), (c) the separation of
dynamical and photochemical processes in the interpretation of our data, (d)
sensitivity tests for the assumed kinetic data, and (e) the assessment of
total Bryinorg (see Sect. ).
Finally, details of how the loss in ozone is calculated is provided in
Appendix Sect. .
Measurements and data reduction
Within the framework of the NASA-ATTREX project, the Global Hawk performed
six flights in the subtropical LS, UT, and TTL over the eastern Pacific in
early 2013 (Fig. ) and another nine
flights over the western Pacific in early 2014. The present paper reports the
2013 flights since the 2014 flights were mostly performed in the cold TTL,
where cirrus clouds mostly prevailed at flight level. Evidently, due to the
multiple scattering of light by the cirrus cloud particles, the
interpretation of our UV–vis limb measurements is not straightforward.
Accordingly, the data collected in 2014 will be reported elsewhere. Details
on the NASA-ATTREX 2013 instrument package, the flights, and some results of
the collected data can be found in , as well as
on the project's website
https://espo.nasa.gov/missions/attrex/content/ATTREX.
In February and March 2013, the NASA-ATTREX flights of the Global Hawk were
strongly biased with respect to the sampled air masses, mostly because the
scientific interest was primarily put on probing the TTL over the eastern
Pacific for aerosols and cirrus cloud particles during the convective season rather than for the photochemistry of bromine in the LS, UT, and TTL (see
Fig. ). Therefore, and due to
operational reasons, typical flight patterns extended from Dryden, California,
in a southerly or southwesterly direction during daytime until a turn-point
was reached, and the back leg to Dryden in a northeasterly direction occurred
during the night when the mini-DOAS instrument could not take measurements.
The dives were mostly performed within the TTL and occasionally within the
subtropical lowermost stratosphere during the return legs at night but not
during the outgoing daytime legs. Finally the landings at Dryden were
scheduled for the early local morning, mostly due to operational constraints.
Therefore, no profiles of the targeted species could be obtained in the
subtropical lowermost stratosphere during the daytime, but a large number were obtained within the
UT and TTL.
Same as Fig. but for the research flight
on 9–10 February 2013 (SF2-2013). The dashed vertical lines in Figs. 4–9
separate different atmospheric regimes: (I) is the extratropical lowermost
stratosphere; (IIa, IIb, etc.) are different mixing regimes of air from the
extratropical lowermost stratosphere; and (III) is from the tropical
tropopause
layer.
Furthermore, the latitudinal definition of the notations “subtropical” LS and
“tropical” TTL need some clarification. According to the definition of
, the latitudinal boundary between the subtropics and
tropics should be where the subtropical jet is located. However, since we do
not infer dynamical parameters (such as the potential vorticity) from our
data, we conveniently define the boundary according to proxies for (a) different air mass ages (i.e., [CH4] concentrations ≤ 1790 ppb
are labeled subtropical and [CH4] ≥ 1790 ppb are labeled
tropical) and (b) photochemical regimes (i.e., [O3] is subtropical
when [O3] ≥ 150 ppb and TTL when [O3] ≤ 150 ppb),
which we find suitable from a visual inspection of our data (see below).
As mentioned above and outlined in detail in the study of ,
the processing of the mini-DOAS data included (a) spectral retrieval of the
targeted gases from the mini-DOAS measurements (Sect. ), (b) forward modeling of the RT for each measured spectrum
(Sect. ), and (c) either applying optimal
estimation or the novel x gas scaling technique (see Sect. 4.1 and 4.2 in
). Comprehensive sensitivity simulations indicated that
optical estimation based on constraints inferred from measured O4
and/or relative radiance would not result in the desired error range
(, Sect. 4.2). Therefore, we decided to apply the x gas
scaling technique (, and ) with x being ozone
measured in situ by the NOAA-2 O3 photometer (see Sect. ).
The O3-scaling technique makes use of the in situ O3 measured
by the NOAA instrument and the limb-measured O3 total
slant column amounts (SCDO3) either monitored in the UV (for the
retrieval of BrO in the 343–355 nm wavelength band) or visible
wavelength range (for the retrieval of NO2 in the 424–460 nm
wavelength band; see Eq. 12 in ). Here the ratio of the
measured slant column and SCDO3/[O3] measured in situ can
be regarded as a proxy for the (horizontal) light path length over which the
absorption is collected. In fact, in the paper of , it is
argued that the so-called α factors account for the fraction of
the absorption of the scaling gas x (e.g., x = O3 in our study)
picked up on the horizontal light paths ahead of the aircraft relative to the
total measured absorption. The sensitivity study on the α factors
presented in (e.g., in the Supplement) indicates that for
the targeted gases, uncertainties in α factor ratios due to
assumptions regarding the RT (for example due to Mie scattering by aerosols
and clouds) mostly cancel out, while uncertainties in the individual profile
shapes of the targeted and scaling gas are most relevant for the errors of
the inferred gas concentrations. Therefore, in the present study, profile
shapes of the targeted and scaling gas predicted by the TOMCAT/SLIMCAT CTM
are used in the RT calculations, aiming at the calculation of the
α factors. The uncertainties in the profile shapes (assumed to be
of the order of the altitude adjustment of the CH4 and O3
curtains, which are typically much smaller than the altitude grid spacing in
the SLIMCAT/TOMCAT simulations) are then carried over to calculate the
overall errors, as discussed in Sect. 4.4 of the study.
It should be noted that for the flight on 21 February 2013 (SF4-2013), the DOAS
retrieval is much less robust than for all the other flights, most likely
because the Fraunhofer reference spectra (taken via a diffuser) are affected
by temporally changing residual structures likely due to ice deposits or some
other residues on the entrance diffuser. Therefore, the data of this flight
are not analyzed in detail, but they are only reported for completeness here.
Finally, in our analysis only those data which are taken at a solar zenith
angle (SZA) ≤ 88∘ are considered because for increasing SZAs
the received skylight radiance requires increasingly longer signal
integration times (longer than the standard integration time, which is 30 s)
and are thus averaged over longer distances ahead of the aircraft. Moreover,
as the SZA increases, the skylight is expected to traverse an increasingly
inhomogeneous curtain of the probed radicals (e.g., see Figs. 5 and 6 in
). As a consequence, the spatial grid of TOMCAT/SLIMCAT (1.2 × 1.2∘) on which the photochemistry is simulated appeared too
coarse for a useful interpretation of our measurements at large SZAs.
Therefore, for a tighter interpretation of our data, a model with higher
spatial resolution than provided by TOMCAT/SLIMCAT would be required. Such an
approach is for example followed in the balloon-borne studies of
, , , and others. However,
since both processes are likely to increase the error of our analysis and
since large SZA (≥ 88∘) measurements only constitute a minor part of
all measurements, we refrain from this much more complicated approach.
Results and discussion
In this section we first discuss how our mini-DOAS measurements of
O3, NO2, and BrO, as well as of CH4 (from
UCATS and HUPCRS), and of the organic brominated source gases (from GWAS)
compare with the model predictions of the TOMCAT/SLIMCAT model (Sects. and ). Then measured BrO is compared
with previous measurements in the UT–TTL–LS (Sect. ) and with the model
predictions (Sect. ). Uncertainties and errors in
the inferred Bryinorg are assessed (Sect. ) before implications of our
measurements for total Bry (Sect. ) and impacts of our measurements on TTL ozone are discussed
(Sect. ).
Comparison with TOMCAT/SLIMCAT predictions
Figures to provide
overviews on the measured data together with the TOMCAT/SLIMCAT modeled
Bryinorg partitioning (panels f) and inferred total
Bryinorg (panels g) as a function of universal time for each
flight. The modeled values are obtained by linear interpolation of the
curtain data (see Fig. ) to the exact altitude of the
GH.
The panels b and c of Figs. to show comparisons of measured and modeled
CH4 and O3 mixing ratios. Here the measured and modeled
species closely agree within the given error bars after the modeled curtains
are altitude-shifted (i.e., interpolated) by the same amount until measured
and modeled O3 agree (for details, see ). It is noteworthy that in most cases the altitude adjustment is less than the grid spacing
of TOMCAT/SLIMCAT (about 1 km in the TTL), thus mostly accounting for the
altitude mismatches of the actual cruise altitude of the Global Hawk and the
model output rather than indicating deficits of the model in properly predicting the
vertical transport. The astonishingly good agreement achieved between
measured and modeled CH4, and O3 lends confidence that the
altitude-adjusted TOMCAT/SLIMCAT model fields reproduce the essential
dynamical and photochemical processes of the probed air masses well. The quality
of the dynamical simulations are further tested by comparing modeled and
measured O3 as a function of CH4 (Fig. ). For all flights the agreement of the observed
and modeled O3 vs. CH4 correlation is reasonably good, except
for flights SF1-2013 and SF2-2013, where the UCATS measured CH4
scatters around the simulated CH4 concentrations. This scatter is
most likely due to precision errors of UCATS rather than reflecting the real
behavior of the atmosphere. Evidence for this conclusion is provided from the
CH4 comparisons for SF3-2013 and SF6-2013, in which the HUPCRS
CH4 data are taken; these data do not show such scatter and compare
reasonably well with the model predictions.
Panels d of Figs. to
compare measured and modeled NO2. Overall, the measured (and modeled)
NO2 concentrations meet the expectations with respect to NO2
partitioning and total NOx (= NO + NO2 +
NO3) abundances in the LS, UT, and TTL over the pristine Pacific.
Elevated NO2 concentrations (range 70 to 170 ppt) are measured
within the subtropical lowermost stratosphere, where aged air masses are
probed, as indicated by depleted CH4 concentrations and elevated
O3 concentrations (and presumably decreased N2O
concentrations). Note that N2O is the primary source for
stratospheric NOx, and in the stratosphere CH4 and N2O
destruction processes closely follow each other (e.g.,
). Very low NO2
concentrations (≤ 30 ppt) are detected within the UT and TTL,
indicating that the analyzed air does not originate from recently polluted or
lightning-affected regions. Further, the modeled NO2 concentrations
(red line in panel d) are found to fall into the given range of errors in the
measured NO2 concentrations. This finding strongly indicates that the
NOx and NOy (= NOx, N2O5, HONO3,
HO2NO2, etc.) budget and photochemistry of the LS, UT, and TTL are
reproduced well in the TOMCAT/SLIMCAT simulations and that overall the
O3-scaling technique works well for NO2.
Panels e in Figs. to
compares measured and modeled BrO. Again, measured and modeled
BrO mixing ratios compare reasonably well for most flight sections, but
sizable discrepancies are also discernible for some flight sections. Possible
reasons for the latter are discussed in the following and may be due to deficits
in the model's assumption regarding the sources of bromine (see Sect. ) and/or deficits in
the adopted photochemistry (see Sect. ).
Same as Fig. but for the research flight
on 14–15 February 2013 (SF3-2013).
Comparison of measured and model organic bromine
Before measured and modeled BrO can be compared quantitatively, it is
necessary to compare the measured amounts of different brominated source
gases with the model predictions (Fig. ). For the
assumed (constant) surface mixing ratios (see Sect. ), measured and modeled CH3Br
(upper left panel), CHBr3 (upper right panel), and all other
halons, for example H1211 (lower right panel), compare well, even if the data are scattered from flight to flight. For CH2Br2, however,
TOMCAT/SLIMCAT run no. 583 underpredicts the observed mixing ratio for high
concentrations (by 0.1 ppt) and overpredicts it by up to 0.2 ppt for low
concentrations (lower left panel). This is most likely due to an assumed a surface concentration that is too low (1 ppt), variable mixing ratios at the surface not
being correctly considered in the model, and/or errors in the atmospheric lifetime by
reactions of CH2Br2 with OH radicals in the model (e.g.,
; ; ).
Same as Fig. but for the research flight
on 21–22 February 2013 (SF4-2013). Note that DOAS analysis of BrO for
SF4-2013 is somewhat uncertain because the Fraunhofer reference spectra
(taken via a diffuser) are affected by temporally changing residual
structures likely due to ice deposits or some other residues on the entrance
diffuser (see text).
Same as Fig. but for the research flight
on 26–27 February 2013 (SF5-2013).
The flight-to-flight and sample-to-sample scatter in CH3Br and
CHBr3 is mostly due to different source regions of the air masses
probed during SF1-2013 to SF6-2013. This implies a spatially (and possibly
time-dependent) varying source strength of the brominated natural source
gases (e.g., ). In the present version of
the TOMCAT/SLIMCAT simulations, this scatter introduces an estimated
uncertainty of ± 0.8 ppt into Bryorg, and potentially into the
inferred Bryinorg available in the TTL. The systematic
underprediction of 0.1 ppt at high CH2Br2 concentrations and its
overly long lifetime in the TTL leading to CH2Br2
concentrations in the model that are too large for old air (by up to 0.2 ppt). Consequently, the model underpredicts Bryinorg
by an additional ≤ 0.4 ppt. Both contributions to the uncertainty in the
Bryorg are considered when comparing measured and modeled
BrO and Bryinorg (see below).
Comparisons of measured BrO with previous studies
Next, we compare our data with previous BrO measurements in the UT and
TTL, i.e., the balloon measurements of and the aircraft
measurements of and during the Tropical Ocean tRoposphere Exchange of Reactive halogen species and Oxygenated VOC
(TORERO)
campaign.
Same as Fig. but for the research flight
on 1–2 March 2013 (SF6-2013).
Overall, the balloon-borne BrO profile measurements of
performed over tropical Brazil during the dry (i.e., the non-convective
season) in June 2005 and June 2008 compare well with the BrO profiles
inferred from our measurements for the UT and TTL (i.e., typically
[BrO] = 0.5–1.0 ppt in the upper UT and base of the TTL, and up
to 5 ppt at the cold-point tropopause, e.g., compare Fig. 1 in
, with Fig. of the present study).
The present study and the BrO profile measurement of
do not, however, confirm the recently reported presence of BrO of up
to 3 ppt in the tropical and subtropical UT and around the bottom of the TTL
at 14 km (; compare Fig. 2, panel a, in
, with left panel of Fig. of the present study). Sensitivity studies using the BrO
profile of Wang et al. (2015) as the a priori of an optimal-estimation
concentration retrieval for the ATTREX measurements result in a kink of
BrO around 12 km (Fig. ). This behavior can be
explained with the disagreement between the observed profiles above 13 km
and the insensitivity of the ATTREX observation to BrO below this altitude.
While the geographical location of the observations by
and those of the GH did not overlap, the ATTREX flights covered a wide
geographic area over which we do not find indications of unexpectedly high or
elevated BrO concentrations in the UT or TTL, either from inspecting
the UT from above (see, e.g., Fig. 15 in ) or when directly
probing the TTL (see Figs. to ).
Several similarities and differences exist between the TORERO measurements
reported by and and our study. Using
NSF/NCAR GV HIAPER (Gulfstream-V High-performance Instrumented Airborne Platform for Environmental Research), probed the UT and the bottom of the
TTL (up to about 14 km) for BrO over an adjacent part of the Pacific,
i.e., mostly off the western coasts of South and Central America, the same
season but in an area more to the south than that probed during the present
study.
It is possible that the TORERO observations of and
off the western coasts of South and Central America,
i.e., further south than the ATTREX region but during the same season,
encountered an unusual meteorological situation that would have caused
downward transport of bromine-rich air from the lower stratosphere to the UT
and the bottom of the TTL (up to about 14 km) or that sea-salt-released
bromine played a role (e.g., ).
Correlation of observed CH4 (UCATS SF1-2013 and SF2-2013;
HUPCRS SF3-2013 to SF6-2013) and O3 (NOAA) for the six NASA-ATTREX
science flights in 2013. Also shown are the corresponding correlations from
the TOMCAT/SLIMCAT simulation.
However, our study has identified possible problems when using an
optimal-estimation technique with constraints based, for example, on measured
O2–O2 for high altitude aircraft limb observations. The RT
below the aircraft and in particular in the lower troposphere plays a crucial
role for the observations due to the much higher O2–O2
concentrations. Also since individual limb measurements already cover an area
of typically ∼200 × 20 km in front of the aircraft (see Fig. 5
in ) and even more crucially when applying optimal
estimation for profile inversion, a series of measurements taken during the
ascent and descent of the GH are jointly inverted. Hence the radiative field
and its time dependence need to be known over a larger footprint (i.e., the
RT is 2-D, or even 3-D plus its time dependence over the period of single
profile measurement).
We did not encounter conditions without (marine stratus cumulus) clouds in
this footprint during any of the ATTREX flights. Therefore, any skylight
analyzed for the O2–O2 absorption in the limb direction may
carry additional or even substantial information on the radiative transfer of
lower-atmospheric layers (see Fig. 7 in ) rather than of
the targeted atmospheric layers. We acknowledge that and
selected “cloud-free” conditions at the location of
their profile measurement, but the cloudiness in the large area ahead of
their aircraft is less clear.
Correlation of GWAS measured and TOMCAT/SLIMCAT modeled major
brominated source gases. Upper left panel for CHBr3, upper right
panel for CHBr3, lower left panel for CH2Br2, and lower
right panel for halon-1211. The concentrations for different flights are
color-coded: SF1-2013 in blue, SF3-2013 in yellow, SF4-2013 in light blue,
SF5-2013 in purple, and SF6-2013 in green.
Another challenge we encountered was that of the overhead BrO column,
which can substantially contribute to the limb BrO signal. The large
concentrations of BrO in the stratosphere during the daytime and its
potential column changes mostly due to a changing tropopause height or
intrusion of tropospheric air (e.g., at the subtropical or polar jet) may
thus mimic the presence of BrO in the limb direction or at the flight
altitude (e.g., , and , and
Fig. 14 in ). We solved this problem by using a highly
resolved stratospheric CTM to study the potential influence of changing
overhead BrO concentrations on our results.
In conclusion, our sensitivity studies have shown a potential problem with
the O2–O2 constrained RT calculations used to retrieve
vertical BrO profiles as well as the need to accurately determine the
stratospheric BrO column. With this in mind and the disagreement
between our upper-troposphere–lower-stratosphere (UTLS) BrO profiles and
TORERO flights 12 and 17 ( and ),
it is clear that future work is needed in reconciling the observations as
well as the different retrieval approaches.
Comparison of the inferred BrO profile for the ascent after
dive no. 2 of the flight on 5–6 February 2013 with
previously published (modeled and measured) BrO profiles. Please note the different altitude ranges in the two panels.
BrO profiles retrieved using the optimal-estimation method are shown in black and those using the O3-scaling
technique are shown by green symbols, error bars, and lines. In the two panels, different a priori information is used to
constrain the optimal-estimation retrieval.
Left panel: TOMCAT/SLIMCAT model predictions are used as a priori (blue). Also shown for comparison is the BrO
profile published by , which was measured over northeastern Brazil in June 2005 (red).
Right panel: the BrO profile of (red) and its extrapolation to 20 km (blue) are used as a
priori in the optimal estimation. The kink in the retrieved BrO profile (black) at about 12 km strongly
indicates that the BrO profile of is neither compatible with the BrO profiles inferred
using the O3-scaling technique (green) nor those obtained from optimal estimation (black) (for further details, see
Sect. ).
Comparison of measured and modeled BrO for the NASA-ATTREX
science flights 1 to 6 in 2013. Black crosses are for model run no. 583, blue
crosses for no. 584, and red crosses for no.
585.
Comparison of measured and modeled BrO
Measured and modeled BrO are displayed in
Figs. to (panel e)
together with the modeled Bryinorg partitioning (panel f) and
inferred Bryinorg (panel g). Elevated BrO concentrations
are measured within the LS (range 3–9 ppt), and lower BrO
concentrations are measured in the TTL (range 0.5–5 ppt), with the smallest
BrO concentrations (0.5–1 ppt) occurring near the bottom of the TTL.
Overall, this behavior is expected from arguments based on the amount and
composition of the brominated organic and inorganic source gases, their
lifetimes, atmospheric transport, and photochemistry (e.g.,
).
In particular, for our daytime measurements, it is observed that
(a) BrO increases with O3 and available Bryinorg
and thus altitude and (b) the predicted
BrO / Bryinorg ratio decreases towards the bottom of
the TTL, where (c) HBr and/or Br atoms may become comparable to
BrO, but HOBr does not play a major role in the
Bryinorg partitioning. While observation (a) is due to the
increased destruction of primarily the short-lived Bryorg
species and the efficient reaction of the released Br atoms with
increasing altitude and increasing ozone concentrations, observations (b) and
(c) are due to reactions of the Br atoms with CH2O (and less
H2O2) into HBr, which is recycled back by reactions with
OH and by variable amounts heterogeneously (depending on the available
surface of aerosols and cloud particles) to Br atoms, as predicted by
, and . The predicted minor role of
HOBr eventually formed by reactions of OH radicals with
heterogeneously produced Br2 or by the reaction HO2+BrO and
photolytic destruction of HOBr in the TTL is also noteworthy. While
the rate of the former reaction is small due to the short photolytic lifetime
of Br2 anyway, the rate of the latter reaction is small due to the
small OH concentration in the TTL as compared to photolysis of
HOBr during the daytime.
Figure compares
measured and modeled BrO. For the majority of all flights (except
flight SF4-2014, for which a DOAS retrieval problem exists which causes a
constant bias of about 2 ppt in inferred BrO), measured and modeled
BrO closely compare for low concentrations (i.e., close to the bottom
to the TTL) or are comparable younger air based on measured CH4. For
larger BrO concentrations (and older air), good agreement between the
measurement and model is found for SF1-2013, SF5-2013, and SF6-2013 when air
of low NO2 concentrations (and predicted low BrONO2
concentrations) is usually probed. For large BrO concentrations as
encountered during flights SF2-2013 and SF3-2013, the measured BrO is
up to 2 ppt or 25 % larger than what the model predicts. This gap could
partly be closed by adjusting the CH2Br2 surface concentration and
atmospheric lifetime or by considering a detailed scheme for the
dehalogenation of sea salt, i.e., bromine activation (e.g.,
). Adjusting
CH2Br2 would add 0.4 ppt of Bryinorg or
∼ 0.3 ppt to BrO, thus removing the flight-to-flight scatter in
source gas concentrations (±0.8 ppt) in Bryinorg. This
could for example be done by a detailed back trajectory and source
appointment analysis, to which a forthcoming study will be devoted. Likewise,
the dehalogenation of sea salt could add another 0.5 ppt to BrO (or
about 0.7 ppt of Bryinorg) in the upper TTL (e.g.,
).
Uncertainties in estimating the inorganic bromine partitioning
Another reason for the gap in measured and modeled BrO may come from
uncertainties in the kinetic constants used and how they affect the
Bryinorg (= Br +2⋅Br2 + BrO
+ BrONO2 + HOBr + HBr + BrCl)
partitioning. Our photochemical modeling, aimed at reproducing measured
O3, NO2, and BrO (see the panels f in
Figs. to ), indicates that
during the daytime HOBr and HBr contribute less than 10 %
to Bryinorg. Therefore, we concentrate on the photochemical
model errors due to the partitioning primarily among BrO, Br,
and BrONO2. In this context, the reactions BrO + NO2
+ M → BrONO2 + M are the most important, followed by
the photolysis of BrONO2 and the reaction Br + O3
→ BrO + O2.
Histogram of Bryinorg occurrence as a function of
potential temperature for [CH4]
≥ 1790 ppb (left panel) and [CH4] ≤ 1790 ppb (right panel). High [CH4] can be
considered as a marker for young air mostly found in the freshly ventilated TTL, while low [CH4] can
be considered as a marker for aged air mostly found in the subtropical lowermost stratosphere. The mean and
the variance of Bryinorg for young air (left panel) are for θ = 350–360 K, 2.63 ± 1.04 ppt;
θ = 360–370 K, 3.1 ± 1.28 ppt; θ = 370–380 K, 3.43 ± 1.25 ppt; θ = 380–390 K,
4.42 ± 1.35 ppt; θ = 390–400 K, 5.1 ± 1.57 ppt, and θ ≥ 400 K, 6.74 ± 1.79 ppt.
Aged air (right panel): for θ = 390–400 K,
4.22 ± 1.37 ppt;
and θ ≥ 400 K, 7.67 ± 2.72 ppt.
Bry as a function of potential temperature (θ) for
all dives during the 2013 NASA-ATTREX flights when joint measurements of
Bryorg and Bryinorg are
available.
How uncertainties of the photolytic destruction (J) and three-body
formation reaction (k; together referred to as J/k) of BrONO2
propagate into BrO is tested in model run no. 584. Here, according to
the finding of , J/k was increased by a factor 1.7
(+0.4/-0.2) as compared to the Jet Propulsion Laboratory (JPL) recommendation (see
the blue crosses in
Fig. ). Evidently,
increasing J/k helps to close the remaining gap in measured versus modeled
BrO, which becomes particularly relevant to reproducing BrO
when NO2 is large, i.e., in the subtropical LS.
Furthermore, estimate the uncertainty in the reaction rate
coefficient kBr+O3 at low temperature (T = 190 K) to be
±40 % (see comment G31). When only considering the two studies which
actually measured rather than extrapolated the reaction rate coefficient into
the relevant temperature range (T = 190–200 K), a smaller uncertainty
(28 %) is indicated (, and ).
Therefore, in the following, an uncertainty of 28 % for kBr+O3 is assumed. Overall, increasing kBr+O3 (model run no. 585)
to the upper limit possible according to the JPL compilation (i.e., by factor
of 1.28) changes the measured vs. modeled correlation for BrO very
little (see the red crosses in
Fig. ). It does,
however, change the Bryinorg partitioning so that [BrO]
is always largely prevalent over [Br] even at the lowest altitudes of
the TTL (see, e.g., panel f in Figs.
to ). Our joint measurement of O3,
NO2, and BrO and the supporting CTM simulations thus indicate
[Br] / [BrO] < 1 for all probed regimes. Our finding is therefore in
contrast to the simulations of and
, who suggest that [Br] / [BrO] may become larger
than unity in the tropical UT and TTL during the daytime. This conclusion is
due mostly to the measured O3 concentrations, which are larger than
those modeled in the study of and ,
and the conclusion is irrespective of what (within the given error bars) is
assumed for kBr+O3.
Gaussian addition of all uncertainties and errors (i.e., the errors of the
retrieved BrO concentrations (in Sect. 4.4 of ),
the cross section error, and the uncertainty in the modeled
[Br] / [BrO] and [BrONO2] / [BrO]
ratios) leads to the Bryinorg error, as indicated in panel f of
Figs. to .
Inferred total Bryinorg
Finally, we discuss the inferred Bryinorg (contribution 4) as a
function of potential temperature in the LS, UT, and TTL over the eastern
Pacific during the 2013 convective season (Fig. ). Here
we discriminate between young-air [CH4] ≥ 1790 ppb, mostly
found within the tropical UT and TTL (Fig. , left
panel), and older-air [CH4] ≤ 1790 ppb
(Fig. , right panel), mostly found in the subtropical
lowermost stratosphere. The different histograms in
Fig. clearly indicate that Bryinorg
increases with increasing potential temperature, i.e., from
2.63 ± 1.04 ppt at θ = 350–360 K (at the bottom of
the TTL) to 4.22 ± 1.37 ppt for θ = 390–400 K (just
above the cold-point tropopause). The inferred Bryinorg thus
brackets the modeled [Bryinorg] = 3.02 ± 1.90 ppt
predicted to exist at 17 km in the TTL well .
The increase in Bryinorg with increasing potential temperature
θ and decreasing CH4 concentration thus reflects the
decrease in concentrations of brominated VSLS (contribution 3). The
correspondence of decreasing Bryorg with increasing
Bryinorg concentrations is also found on a sample-to-sample as
well as on a flight-to-flight basis. This correspondence keeps [Bry]
almost constant within the TTL during an individual flight, but [Bry]
varies from flight to flight in a range of [Bry] = 20.3 ppt to
22.3 ppt (Fig. ).
Bryinorg as a function of the sum of all brominated VSLS
using the same color code as in Fig. . If all
Bryinorg resulted from destroyed VSL bromine of the same air mass
from near the surface, then all data points should follow individual diagonal
lines.
Moreover, it appears that the increase in Bryinorg with
θ mostly corresponds to a decrease in concentrations of the
brominated VSLS if only the same (young) air masses of large CH4
concentrations are probed (Fig. ).
For example for SF1-2013, SF5-2013, and SF6-2013 when mostly air masses of
the TTL are probed, all data points fall into a band of about ±1 ppt in
width, next to a flight-dependent diagonal line (not shown), but this is not
the case for SF3-2013 when air masses of the LS (and thus older air) and TTL
are probed. When extrapolating the data points along lines of constant
[VSLS]+[Bryinorg] bromine (grey dashed lines in
Fig. ) for SF1-2013, SF5-2013, and
SF6-2013 to [Bryinorg] = 0 and assuming no bromine is
effectively lost in the troposphere, then the apparent concentrations of
brominated VSLS at the surface should range between 4 and 8.5 ppt. However,
larger concentrations of brominated VSLS (some 10 ppt) are frequently
measured in the boundary layer of the Pacific (e.g.,
).
Further, bromine released from sea salt also contributes to
Bryinorg in the marine boundary layer and may reach the bottom
of the TTL in variable amounts (e.g.,
). Therefore, effective
loss processes for inorganic bromine, for example by the heterogeneous uptake
of inorganic bromine on aerosol and cloud particles, must be active in the
atmosphere (e.g., ).
Next, when subtracting the almost constant contribution of CH3Br and
the halons to total stratospheric bromine (14.6 ppt in 2013) from the range
(20.3 to 22.3 ppt) of total Bry given, a variable contribution from
VSLS bromine (contribution 3) and Bryinorg (contribution 4) to
total TTL bromine in the range of 5.7 ppt to 7.7 ppt (± 1.5 ppt) is
calculated (Fig. ). We note that this range
falls well into the range assessed in and recently estimated by
(6 ppt; range = 4–9 ppt) for contribution of 3 and
4 to the total stratospheric Bry. It is, however, somewhat (up to
2 ppt) larger than indicated in earlier work, including our balloon-borne
studies (for details, see Sect. ).
Here one may wonder whether (a) this result is significant, (b) some
Bryinorg is actually removed by heterogeneous processes in the
TTL (e.g., ), or (c) TTL Bry shows
some seasonality analogous to the “tape recorder” for
H2O (e.g.,
).
The non-negligible amounts of Bryinorg (2.63 ± 1.04) ppt
are also remarkable and range from 0.5 to 5.25 ppt, which is from close to 0
to 25 % of all TTL bromine inferred for altitudes at the bottom of the
TTL (θ = 350–360 K), of which 40 to 50 % may consist of
BrO. This finding clearly sets a range and an upper limit for the
Bryinorg influx into the TTL due to entrained air masses of
recent tropospheric origin (contribution 4). Again, the latter can most
likely be attributed to different source regions (and thus emission
strengths) of the brominated VSLS and bromine released from sea salt and a
varying degree of photochemical and heterogeneous processing of the air
masses transported from the surface to the TTL. The increase in variance
found for Bryinorg, which increases in absolute terms but
decreases in relative terms (i.e., from 0.4 for θ in the range
350 to 360 K to 0.3 for θ = 390 to 400 K) with increasing
θ, is also noteworthy. This may indicate a subsequent
flattening-out of the air-mass-to-air-mass variability of
Bryinorg in aging air due to the photochemical decay of the
brominated organic source gases and atmospheric mixing processes.
Implications for ozone
The ozone budget in the TOMCAT/SLIMCAT simulation has been analyzed based on
the rate-limiting steps of the catalytic ozone destruction cycles, according
to the concept of . The chemical rates are averaged over
the eastern Pacific region (20∘ S–20∘ N,
170–90∘ W) for the duration of the campaign. Within this domain,
the net rate of ozone change varied from a loss of -0.3 ppbv day-1
at the base of the TTL (θ = 355 K, p = 150
hPa) to a production of +1.8 ppbv day-1 at the top
(θ = 383 K, p = 90 hPa). This increase
in O3 with height is due to the strong vertical gradient in the
production rate of odd oxygen by O2 photolysis. Within the catalytic
ozone loss cycles in the TTL, the model indicates that those containing
bromine contribute between 12 % (base of TTL) and 22 % (top of TTL)
of the total (see Appendix Sect. ). The dominant contribution
to this is through the cycle involving BrO + HO2 to form
HOBr. Overall, the modeled ozone loss cycles which account for the
majority of the destruction in this region are those with the rate-limiting
steps of the reaction HO2 + O3 to form OH +
2O2 and the reaction of HO2 + HO2 to form
H2O2, i.e., cycles involving HOx species. Therefore, increases in
the bromine loading of the TTL caused by possible or expected increases have
the potential to deplete ozone in a region where ozone changes have the
largest impact on radiative forcing .
Quantifying the radiative impact of the O3 changes described above is
the beyond the scope of this study. However, we can note that (i) recent work
has highlighted the efficiency of brominated VSLSs at influencing climate
(through changed O3), owing to their efficient breakdown in the UTLS
, and (ii) a significant increase in Bry in this
region (from VSLS or other sources) could be important for future climate
forcing. The latter could conceivably occur given suggested climate-induced
changes to (1) tropospheric transport (e.g., ),
(2) changes in OH, affecting VSLS lifetimes (), (3) and/or an elevated bromine loading in the UT-LS and TTL due to the expected increase in VSLS emissions from the rapidly growing aquaculture industry .
Conclusions
The subtropical lowermost stratosphere, upper troposphere, and tropopause
layer of the eastern Pacific are probed for inorganic bromine during the
convective season (February and March 2013). The measurements of CH4,
O3, NO2, BrO, and some important organic brominated
source gases are inter-compared with TOMCAT/SLIMCAT simulations. After the
simulated TOMCAT/SLIMCAT curtains of O3 are projected onto the
measured O3 concentrations, measured and modeled CH4 agree
well. This agreement is not surprising since O3, and CH4 are
strongly anticorrelated (see Fig. ). It thus
provides evidence that the relevant dynamical processes are represented well
in the TOMCAT/SLIMCAT simulations. When the simulated curtains of NO2
are adjusted with the same parameters as inferred above, excellent agreement
is again found between measured and modeled NO2, thus providing
further confidence in our measurement technique, in the modeled NOy
photochemistry, and in our overall approach.
The measured and modeled TTL concentrations of CH2Br2 and
CHBr3 are found to compare reasonably well to the surface
concentrations and atmospheric lifetimes of both species adopted in the model
([CHBr3] = 1.4 ppt, [CH2Br2] = 1 ppt at the
surface). Further, the contribution to bromine in the LS, UT, and TTL by some
other VSLS chloro-bromo-hydrocabrons (Σ [CHClBr2,CHCl2Br,CH2ClBr,etc.]) is accounted for by assuming a constant surface
concentration of 1 ppt in the model. Flight-to-flight total organic bromine
inferred from these VSLS species is found to vary by ± 1 ppt in the
TTL over the eastern Pacific in early 2013, which clearly indicates different
origins and possibly atmospheric processing of the investigated air masses.
The measured BrO concentrations range between 3 and 9 ppt in the subtropical
LS. In the TTL they range between 0.5 ± 0.5 ppt at the bottom of the
TTL and about 5 ppt at θ = 400 K, in overall good agreement
with the model simulations and the expectation based on the decay of the
brominated source gases and atmospheric transport. In the TTL, the inferred
Bryinorg is found to increase from a mean of
2.63 ± 1.04 ppt for θ in the range of
350–360 K to 5.11 ± 1.57 ppt for
θ = 390–400 K, whereas in the subtropical LS it
reaches 7.66 ± 2.95 ppt for θs in the range of
390–400 K. The non-negligible Bryinorg found for the
lowest altitudes of the TTL, i.e., 2.63 ± 1.04 ppt with a range from
0.5 to 5.25 ppt (or close to 0 % up to 25 % of all TTL bromine) is
also remarkable. This may indicate a sizable but rather variable influx of
inorganic bromine into the TTL, largely depending on the air mass history,
i.e., source region, and atmospheric transport and processing.
Our findings on LS and TTL Bryinorg are in broad agreement with
past experimental and theoretical studies on the processes and the amount of
bromine injected by source gas and product gases into the TTL and eventually
into the extratropical lowermost stratosphere
(,
and many others). Our study, however, sets tighter limits than previous ones
existing on the amount of Bryinorg and Bryorg, the
influx of brominated source and product gases, and the photochemistry of
bromine in the TTL and LS.
In particular, our study (re-)emphasizes that (a) variable amounts of VSLS
bromine and (b) non-negligible amounts of Bryinorg are also
transported into the TTL. While process (a) may strongly depend on the source
region and season , process (b) may depend on the
efficiency of heterogeneous processing and the removal of some
Bryinorg by atmospheric (ice) clouds and aerosols (e.g.,
). Therefore, it is not surprising that
TTL Bry is rather variable (i.e., 20.3 to 22.3 ppt) in the season
studied.
We also note that the amount of Bry over the eastern Pacific during
the convective season assessed here and in the study of is
somewhat (up to 2 ppt) larger than that presently found on average in the
stratosphere (e.g., ). By assuming that this gap is significant, additional
processes may come into the focus of stratospheric bromine research, i.e., the
seasonality and possibly long-term trend of the bromine transported into the
stratosphere (e.g., ).
It is conceivable that adding some inorganic bromine (from contribution 4) to
TTL bromine would exert an additional impact on ozone. For the eastern
Pacific (170–90∘ W), our model-based assessment indicates a net
loss of ozone of -0.3 ppbv day-1 at the base of the TTL
(θ = 355 K) and a net production of +1.8 ppbv day-1
in the upper part (θ = 383 K). Within the catalytic ozone
loss cycles in the TTL (see Appendix Sect. ), the model
indicates that those containing bromine contribute between 12 % (at the
base of the TTL) and 22 % (at the top of the TTL) of the total.