AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-2291-2016Atmospheric mercury measurements onboard the CARIBIC passenger
aircraftSlemrFranzfranz.slemr@mpic.deWeigeltAndreasEbinghausRalfKockHans H.BödewadtJanBrenninkmeijerCarl A. M.Rauthe-SchöchArminhttps://orcid.org/0000-0001-5738-8112WeberStefanHermannMarkushttps://orcid.org/0000-0002-5124-1571BeckerJuliaZahnAndreasMartinssonBengtMax-Planck-Institut für Chemie (MPIC), Hahn-Meitner-Weg 1, 55128
Mainz, GermanyHelmhotz-Zentrum Geesthacht, Institut für Küstenforschung,
Max-Planck-Straße 1, 21502 Geesthacht, GermanyLeibniz-Institut für Troposphärenforschung, Permoserstrasse
15, 04318 Leipzig, GermanyInstitut für Meteorologie und Klimaforschung (IMK-ASF), Karlsruhe
Institut für Technologie, Hermann-von-Helmholtz-Platz 1, 76344
Leopoldshafen, GermanyUniversity of Lund, Division of Nuclear Physics, P.O. Box 118,
22100, Lund, Swedennow at: Bundesamt für Seeschifffahrt und Hydrographie (BSH),
Wüstland 2, 22589 Hamburg, Germanynow at: Hessisches Landesamt für Umwelt und Geologie (HLUG),
Rheingaustrasse 186, 65203 Wiesbaden, GermanyFranz Slemr (franz.slemr@mpic.de)24May201695229123023December201518January20166April20165May2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/2291/2016/amt-9-2291-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/2291/2016/amt-9-2291-2016.pdf
Goal of the project CARIBIC (Civil Aircraft for the Regular Investigation of
the atmosphere Based on an Instrumented Container) is to carry out regular
and detailed observations of atmospheric composition (particles and gases)
at cruising altitudes of passenger aircraft, i.e. at 9–12 km. Mercury has
been measured since May 2005 by a modified Tekran instrument
(Tekran Model 2537 A analyser, Tekran Inc., Toronto, Canada) during monthly
intercontinental flights between Europe and South and North America, Africa,
and Asia. Here we describe the instrument modifications, the post-flight
processing of the raw instrument signal, and the fractionation experiments.
Introduction
The biogeochemical cycle of mercury has attracted much interest because of
the bioaccumulation of the highly neurotoxic methyl mercury in the aquatic
nutritional chain to concentrations harmful for humans and animals (e.g.
Mergler et al., 2007; Scheuhammer et al., 2007; Lindberg et al., 2007, and
references therein). The concern about adverse environmental impacts of
mercury prompted the United Nations Environment Programme (UNEP) to undertake
a global assessment of mercury and its compounds in the environment, which
resulted in the Minamata Convention on Mercury
(www.mercuryconvention.org) in 2013.
The atmospheric mercury cycle, responsible for the worldwide transport of
mercury and its deposition, is still not well understood despite more than 30
years of intensive research (e.g. Lin et al., 2006; Lindberg et al., 2007;
Slemr et al., 2011). Mercury is released into the atmosphere by natural
processes, such as emissions from volcanoes, and anthropogenic processes,
such as coal burning and ore processing (Pirrone et al., 2010; Song et al.,
2015). It is emitted as elemental vapour (gaseous elemental mercury, GEM) or
as gaseous or particulate Hg2+ mercury compounds (gaseous oxidized
mercury, GOM, and particle bound mercury, PM). While estimates of global
anthropogenic emissions claim a relatively modest uncertainty of 30 % or
less, the estimates of natural emissions and reemissions from a legacy of
historical anthropogenic mercury pollution are much less certain (Pirrone et
al., 2009; Mason, 2009). GOM has a short atmospheric lifetime of a few days
due to its low vapour pressure and high solubility and is thus supposed to be
deposited mostly near its sources (Lindberg et al., 2007). Particles carrying
PM have also a short lifetime of several days. GEM, however, is
almost insoluble and has a relatively high vapour pressure. Measurements of
its worldwide tropospheric distribution, with a pronounced interhemispheric
gradient and small gradients within the hemispheres, suggest that its
atmospheric residence time is of the order of 1 year (Slemr et al., 1985).
GEM thus has to be oxidized to GOM and PM for removal from the atmosphere.
Three mechanisms to oxidize elemental mercury to Hg2+ compounds have
been proposed (reactions with O3, OH, and Br), but their relative
contributions are still not well known (Lin et al., 2006; Lindberg et al.,
2007, Hynes et al., 2009).
Due to its rather long atmospheric residence time GEM will reach the
stratosphere. The information on the behaviour of mercury in the upper
troposphere and lower stratosphere (UT–LS) is tenuous because of lack of
measurements due to instrumental limitations. Only recently has the progress in
measurement techniques enabled extensive but short-term aircraft
measurements of mercury distribution in the troposphere and lower
stratosphere (Ebinghaus and Slemr, 2000; Friedli et al., 2003a, b, 2004;
Banic et al., 2003; Ebinghaus et al., 2007; Radke et al., 2007; Talbot et
al., 2007, 2008; Swartzendruber et al., 2008, 2009a; Slemr et al., 2009,
2014; Lyman and Jaffe, 2012; Brooks et al., 2014; Ambrose et al., 2015; Shah
et al., 2016; Weigelt et al., 2016). All observations have so far shown a
pronounced decrease of gaseous mercury (GEM + GOM) concentrations in the
lower stratosphere (Ebinghaus et al., 2007; Radke et al., 2007; Talbot et
al., 2007; Slemr et al., 2009; Lyman and Jaffe, 2012), which implies a
conversion to PM. This implication is supported by observations of high PM
concentrations in the lower stratosphere but not in the upper troposphere
(Murphy et al., 1998, 2006). However, the mechanism of this conversion and
its importance for the atmospheric mercury cycle is not known.
Since May 2005 mercury has been measured during monthly CARIBIC (Civil
Aircraft for the Regular Investigation of the Atmosphere Based on an
Instrumented Container; Brenninkmeijer et al., 2007) flights. The objective
of these measurements is to gain information on the worldwide distribution of
mercury in the UT–LS (Slemr et al., 2009) and on mercury emissions from
biomass burning and other sources (Ebinghaus et al., 2007; Slemr et al.,
2014). Here we describe the mercury instrumentation, present a method for
post-flight data processing, and discuss the results of several fractionation
experiments.
The CARIBIC container
Since December 2004 a new CARIBIC container (Brenninkmeijer et al., 2007;
www.caribic-atmospheric.com) onboard a Lufthansa Airbus A340-600 has
been flown monthly on intercontinental flights. The routes of the flights
starting in Frankfurt or Munich with destinations in North and South America,
Africa, and East and South Asia can be found at
www.caribic-atmospheric.com. Typically, a sequence of four individual
flights is flown every month. A modified freight container holds automated
analysers for gaseous mercury, CO, O3, NO, NOy, CO2, total and
gaseous water vapour, oxygenated organic compounds, and fine particles (three
counters for particles with diameters > 4, > 12,
and > 18 nm), as well as one optical particle size spectrometer
for particles with diameters > 140 nm (Hermann et al., 2016). In addition, air and
aerosol particle samples are taken and analysed after the flight for
greenhouse gases, halocarbons, hydrocarbons, and particle elemental
composition and morphology (Brenninkmeijer et al., 2007). Between the end of
2009 and spring of 2010 several instruments were replaced by improved ones
and some new instruments were added. In the context of this paper the most
important changes are the replacement of the malfunctioning optical particle
size spectrometer (OPSS) and the addition of a whole-air sampler with a
capacity of 88 samples. The improved and extended instrumentation has been in use
since May 2010. In summer 2014, a single-particle soot photometer (SP2)
instrument was added.
The inlet system with four separate inlets for aerosols, trace gases, total
water, and gaseous water and the container plumbing are described in detail
by Brenninkmeijer et al. (2007) and Slemr et al. (2009). Briefly, the trace
gas probe consists of a 30 mm inner diameter diffuser tube with a
forward-facing inlet orifice of 14 mm diameter and outlet orifice of 12 mm, diameter
providing an effective ram pressure of about 90–170 hPa depending on
cruising altitude and speed. This ram pressure forces about
100 L min-1 of ambient air through PFA tubing (3 m long, 16 mm ID
PFA-lined tube) connecting the inlet and the container and 1.5 m long 16 mm
ID PFA tubing within the container to the instrument manifold, all heated to
∼40∘C. The air sample for the mercury analyser is taken at a
flow rate of 0.5 L (STP) min-1 (STP: p=1013.25 hPa, T=273.15 K) from the manifold using the 4 mm ID PFA tubing heated by the
energy dissipated in the container to about 30 ∘C. The arrangement
is similar to that described by Talbot et al. (2008) and was optimized to
transmit highly sticky HNO3 (Neuman et al., 1999). The large flow
through the trace gas diffuser tube of more than 2000 L min-1 and
perpendicular sampling at much smaller flow rates of about 100 L min-1
discriminate against particles larger than about 1 µm diameter
(50 % aspiration efficiency; Baron and Willeke, 2001). But particles
smaller than 0.5 µm will pass into the manifold in the container.
The separate aerosol inlet is designed to obtain representative aerosol
sampling characteristics at cruising speeds of nominally 250 m s-1 and
is described in detail by Brenninkmeijer et al. (2007). It essentially
consists of a heated shroud, which makes the aerosol sampling characteristics
largely independent of flight conditions (angle of attack), and of an aerosol
diffuser tube, which slows down the air flow to velocities comparable to
those in the tubing connecting the aerosol inlet with the aerosol
instruments. Because the sampled air is heated from an ambient temperature of
∼-50 to ∼+30∘C in the container, water and semivolatile
compounds such as ammonium nitrate, organics, and some mercury compounds will
evaporate.
Flow scheme of the modified CARIBIC mercury instrument.
Mercury instrument and its modifications
The mercury instrument is based on an automated dual-channel, single-amalgamation,
cold vapour atomic fluorescence analyser (Tekran Model
2537 A, Tekran Inc., Toronto, Canada), and its flow scheme is shown in Fig. 1.
The instrument features two gold cartridges. While one is adsorbing mercury
during a sampling period, the other is being thermally desorbed using argon
as a carrier gas. Mercury is detected using cold vapour atomic fluorescence
spectroscopy (CVAFS), which responds only to GEM.
However, mercury compounds collected by the gold cartridge were found to be
converted to elemental mercury probably during the thermodesorption and will
thus be detected as well (Slemr et al., 1978, 1979). The functions of the
cartridges are then interchanged, allowing continuous sampling of the
incoming air stream.
After switching on the instrument, the sampling lines are flushed for 240 s
with ambient air made mercury free by an activated charcoal filter (carbon
filter in Fig. 1). After that the instrument changes to measurement mode. The
sample air is directed to the gold cartridges either directly or via a quartz
wool trap (15 cm long and 2 cm diameter quartz glass tube packed with
ca 5 g cleaned quartz wool). Lyman and Jaffe (2012) showed that a
quartz wool trap removes GOM and aerosol particles (i.e. PM) but no GEM. We
make a similar assumption for the removal of GOM in our system and,
therefore, presume that the CARIBIC mercury analyser can measure only GEM
with the trap or GEM + GOM + PM (PM measurement capability will be
discussed in Sect. 6) without it. On flights from Germany to a certain
destination GEM is measured, and on the return flights total mercury
(TM = GEM + GOM + PM). A 45 mm diameter PTFE pre-filter (pore
size 0.2 µm) protects the sampling cartridges against contamination
by particles which pass through the inlet system. To ensure a proper sample
air flow of 0.5 L min-1 (STP) even at a flight level of 12 km
(200 hPa), a second diaphragm pump (Neuberger Model KNF UN89 KTDC) was
installed at the inlet of the instrument until May 2011. It was tested for
contamination and losses of mercury, and none were found. Since June 2011 this
pump has been moved downstream of the original internal Tekran pump as shown
in Fig. 1. The pumping speed of both pumps is controlled by the flow meter. A
buffer volume between the valve and the flow meter reduces the pump-induced
pressure oscillation which might bias the flow meter reading. If the
instrument is switched off, the valve between the volume and the pump is
closed to ensure that no air is pulled through the mercury analyser to the
other CARIBIC instruments. To avoid contamination of the instrument and of
the tubing connecting the sampling manifold with the instrument during
ascents and descents in heavily polluted areas near most of the larger
airports, the sampling pumps are switched on only at ambient pressure below
500 hPa. Consequently, only measurements above an altitude of about 5 km
are available.
With changing flight level the pressure in the aircraft cargo bay and
therefore the pressure in the CVAFS detector cell changes. As the response signal of
the CVAFS is pressure dependent (Ebinghaus and Slemr, 2000; Talbot et al.,
2007), we keep the pressure in the detection cell constant using a pressure
controller (Bronkhorst EL-PRESS P-702CV) downstream of the detection cell.
The pressure controller was set to a constant upstream pressure value of
1013 hPa.
Internal default integration of mercury peaks at mercury loads of
less than 10 pg. The peak of 0.8 pg load can be integrated offline with an
uncertainty of less than 10 %, but it was not detected by the internal
default integration. Abscissa in 0.1 s units, ordinate in relative units.
A data acquisition computer was added to the Tekran instrument to control the
instrument, to record data from the instrument as well as from the
additional sensors (pressure, temperature, valve position), and to
communicate with the CARIBIC master computer which controls the operation of
all instruments in the container. Initially we used a data acquisition
computer (DAQ; ICP COM 7188) which, unfortunately, was too slow to record
both the Tekran internally processed data and so-called “raw data dump”
output (raw detector signal with 10 Hz resolution). In February 2014 we
replaced the computer by a CompactRIO DAQ (National Instruments). Since then
the recorded raw data dump signal can be viewed and processed after the
monthly flight sequence, as described in Sect. 4.
Including all components the modified instrument has a total weight of
36 kg. The power consumption in measurement mode is 300 W. The instrument
meets the DO160-E limits on emitted and conducted electromagnetic radiation
which is required for the certification of the instrument for operation
onboard passenger aircraft.
To reduce the number of high-pressure cylinders in the container, the
instrument was initially operated with a gas mixture of 0.25 % CO2
in argon used also as an operating gas for the CO instrument. As the addition
of CO2 to argon reduced the sensitivity of the fluorescence detector by
quenching by ∼35 %, the instrument was run initially with 15 min
sampling time (corresponding to ∼225 km flying distance) until
March 2006 and with 10 min until June 2007. Since August 2007, CO2 has
been removed from argon by an cartridge filled with X10 molecular sieve. The
corresponding sensitivity gain enabled us to run the instrument with 5 min
sampling (corresponding to ∼75 km flying distance). To improve the
detection limit and the precision of the measurements, we returned to 10 min
sampling in the flights between August 2011 and January 2014. Since February
2014 the instrument has been run again with 5 min resolution. All mercury
concentrations are reported here in ng Hg m-3 (STP, i.e. 1013.25 hPa
and 273.15 K).
The instrument is calibrated after every other monthly flight sequence by
comparing its ambient air measurements in the laboratory with simultaneous
measurements by a carefully calibrated reference Tekran instrument. The
reference instrument has an internal permeation source whose permeation rate
is determined by injections of known mercury amounts every year. Calibration
during the flight using the internal permeation device was not attempted
because unavoidable power and carrier gas flow interruptions and ambient
pressure changes prevent reaching a stable permeation rate.
Post-flight data processing
Even with the additionally installed diaphragm pump only about 2 L (STP) air
samples are collected during 5 min sampling time at cruising altitudes of
10–12 km. The Tekran detector then has to analyse ∼2–3 pg of mercury
in the troposphere and less in the stratosphere. This is far below the
threshold of ∼10 pg needed for the optimal integration of the mercury
peak by the instrument (Swartzendruber et al., 2009b). Figure 2 illustrates
the problem connected with the internal default integration: a 7.3 pg peak
is integrated correctly, whereas the integrals for 2.6 and 1.5 pg are
substantially underestimated, and the peak of 0.8 pg is not detected at all.
Generally, the low signal-to-noise ratio of small peaks causes the internal
default integration program to start to integrate later and to end earlier
than it should, resulting in concentrations that are biased low.
Swartzendruber et al. (2009b) discussed several procedures to alleviate this
problem such as to use a different setting of the internal integration
parameters and offline peak quantification using peak height,
cross-correlation of the peak centre with a calibration peak, and a longer
integration. We have developed a procedure similar to the integration of gas
chromatographic peaks: we display every measurement on the computer screen
and correct the default integration using a cursor if we deem it necessary.
With a Matlab script it takes about 2 h to process data from an
intercontinental flight (∼130 measurements with a resolution of 5 min).
The Tekran raw signal is available only from February 2014, but because of
missing raw signal data for the calibration before February 2014 only the
data from April 2014 onward (i.e. since CARIBIC flight 468) could be processed.
The uncertainty of the offline integration is < 0.1 pg, which can
be also considered as a detection limit. Relative to the median sampling volume
of 1.7 L (STP) with 5 min sampling time during CARIBIC flights 468–532
(April 2014–January 2016) concentrations of around 0.05 ng m-3 can be
detected. This is a substantial improvement against the instrument default
integration with the detection limit of ∼0.5 pg. In more than 6000
offline-processed measurements during flights 468–532 we have not found
a single peak which could not be integrated by our offline procedure,
despite many measurements in the deep stratosphere (i.e. O3 > 300 ppb and potential vorticity of up to 11 PVU) during the
flights to San Francisco, Tokyo, and Beijing. Even the 0.8 pg peak shown in
Fig. 2, which was not detected by the instrument default integration, can be
integrated with an uncertainty of < 0.1 pg. Thus zero
concentrations occurring occasionally in our old stratospheric data and in
reports by others (Talbot et al., 2007) are most likely not real but due to
integration problems.
With a median sample volume of 1.7 L (STP) taken within 5 min sampling time
and a median mercury concentration of 1.2 ng m-3 during flights
468–492 the median loads are about 2 pg. The typical precision of the
offline-processed data is thus ∼5 %, i.e. ∼0.06 ng m-3.
With the precision of ∼3 % of the calibration, the overall precision
is ∼6 %. The precision of the tropospheric measurements for flights
before April 2014 employing the instrument default integration only is
∼25 and ∼13 % with 5 and 10 min sampling time, respectively. The
contribution of the calibration precision for these measurements is
negligible. As mentioned earlier, the instrument was run with
Ar + CO2 mixture until June 2007, which reduced the detector
sensitivity by ∼35 %. The precision of these measurements is thus
∼20 and ∼13 % with sampling times of 10 and 15 min,
respectively. With lower concentrations in the stratosphere, the precision
becomes worse.
Ratio of new (offline processed) to old (internal default
integration) concentrations vs. old mercury load: (a) all individual
data (more than 3000 measurements) from flights 468–492 (April
2014–January 2015), (b) binned data from diagram (a) and a
function used to correct old tropospheric data with old mercury
loads > 1 pg.
Simultaneous default and offline integration during flights 468–492
allow us to assess the data quality for the earlier flights until 467
(February 2014). Figure 3a displays the ratio of offline processed (new) to
default integrated (old) concentrations as a function of the old mercury
loads > 0.5 pg for the measurements during flights
468–492. The ratio is almost always larger than 1 and increases with
decreasing mercury load. That means that the old measurements are mostly
biased low. An alternative plot of the ratio of new to old concentrations vs.
new mercury load (not shown) shows a nearly linear inverse function of the
new, i.e. the real mercury load below ∼8 pg and a ratio of 1 above
∼8 pg. Figure 3b shows the binned data from Fig. 3a and the derived
function which we used for the correction of the bias of old
loads >1 pg in the database until flight 467, i.e. for all
CARIBIC flights between May 2005 and February 2014. A correction function
encompassing even smaller old loads led to unrealistically high correction
for old loads in the range of 5–15 pg which are not biased. In addition,
the larger the correction is, the less credible it becomes, and occasionally
occurring zero old loads cannot be corrected at all. By limiting the
correction to old loads > 1 pg, we essentially remove the bias
of all old tropospheric and some stratospheric measurements near to the
tropopause. Measurements deeper in the stratosphere are irretrievably lost.
We note also that by applying the correction we remove only the average bias
but do not improve the precision of the old data, which we estimate to be
∼0.2–0.3 ng m-3. The corrected old tropospheric data from
flights 468–492 (flights with both new and old data) deviated on average
by 3 % from the corresponding offline-processed concentrations. This
gives us confidence in the correction and homogenization of the old
tropospheric data. A homogenization is needed e.g. for trend investigations
because the varying sampling times and the resulting varying mercury loads
led to varying low biases between May 2005 and February 2014.
We would like to point out that the problem with integration of sub-optimal
sample loads also applies for a large part of GOM and PM measurements
reported in the literature. Most of these measurements are made by an
automated Tekran method (Tekran 2537/1130/1135 system, e.g. Gay et al.,
2013). The system is typically run with 2 h sampling of GOM and PM at a flow
rate of 10 L min-1 (STP) each, yielding sample volumes of 1.2 m3
(STP). Average GOM and PM concentrations of ∼2 and ∼4 pg m-3 (Gay et al., 2013), respectively, provide mercury loads of
∼2.4 and ∼4.8 pg, which are well below 10 pg needed for the
default bias-free integration of the peaks. To the best of our knowledge, the
underestimation of GOM and PM concentrations caused by the integration of the
sub-optimal Hg loads has not been considered in recent discussions of the GOM
and PM measurement accuracy so far (e.g. Gustin et al., 2013, 2015).
Aerosol collection and mercury analysis by PIXE
Aerosols sampled through the aerosol inlet are collected by a multi-channel
aerosol sampler described in detail by Nguyen et al. (2006). The sampler
operates using the impaction technique at a flow rate of 10.4 L min-1
and has 16 sampling channels. Each channel contains two sample types: one is
used for quantitative analysis of aerosol element composition using
particle-induced X-ray emission (PIXE), and the second one for single-particle
morphology investigation using electron microscopy. The sampler is loaded
with sampling substrates to cover both the outbound and the return flights.
Fourteen channels are used for sequential sampling, and the remaining two
channels are sampling during the entire outbound or return flight (integral
samples). The detection limit of PIXE analyses for mercury is
∼0.2 ng m-3.
So far we have not detected any PM above the detection limit of the PIXE
analyses, neither in tropospheric nor in stratospheric aerosol samples
collected during the CARIBIC flights. This appears to be in contradiction
with the observations of Murphy et al. (2006), who report high PM
concentrations in the stratosphere while they could hardly detect any PM in
the troposphere. Based on assumptions about the Hg ionization efficiency of
their Particle Analysis by Laser Mass Spectrometry (PALMS) instrument, which
was not calibrated, they roughly estimate that PM constitutes 5–100 % of
all mercury in the LS. There are two possible explanations for this
contradiction: (1) the concentrations of PM observed by Murphy et al. (2006)
are below the PIXE detection limit or (2) PM compounds are semivolatile and
evaporate in the aerosol sampling inlet (Sect. 2), in the tubing
connecting the inlet and the aerosol sampler, or during the PIXE analysis in
vacuum.
Murphy et al. (2006) could not detect any PM in particles at lower altitudes
in the troposphere (∼7 km below the tropopause, i.e. ∼5 km above
the ground). Consequently, the free-tropospheric PM concentrations at this
altitude represent the lower limit of the PALMS sensitivity. PM vertical
profiles were measured only recently (Brooks et al., 2014). Although
measurements in July, August, and September are missing, the data show a
pronounced seasonal variation with minimum PM concentrations in winter
months. Most of the measurements by Murphy et al. (2006) were made in spring
and summer, and the corresponding PM concentrations reported by Brooks et
al. (2014) for the altitude range of 4–6 km varied between ∼20 and
110 pg m-3. With a sample volume of 300 L the concentrations of
∼20 pg m-3 are probably biased low because of integration
problems described in Sect. 4. Thus the PALMS lower detection limit
represents ∼2–10 % of total mercury concentration of
∼1 ng m-3 at the tropopause, which is consistent with the lower
estimate by Murphy et al. (2006) for the PM fraction of the total mercury
concentration.
Another possibility to assess the PM concentrations measured by Murphy et
al. (2006) is the recent measurements of oxidized mercury in the UT–LS
(Lyman and Jaffe, 2012; Brooks et al., 2014; Shah et al., 2016). Lyman and
Jaffe's (2012) measurements suggest that Hg2+ represents more than
∼90 % of total mercury above the tropopause. Brooks et al. (2014)
report spring and summer GOM concentrations of 30–80 pg m-3 in the
altitude range of 5–7 km above ground, but these measurements might be
biased low by a factor of 3 (Gustin et al., 2013, 2015). Shah et al. (2016)
report unbiased average Hg2+ concentrations of ∼200 pg m-3
in the altitude range of 6–7 km in summer. Taking the bias in measurements
of Brooks et al. (2014) into account, their measurements are consistent with
those of Shah et al. (2016). Hg2+ compounds are semivolatile and will
readily attach to particles at low temperatures near the tropopause and above
it. Rutter and Schauer (2007) measured the gas–particle partitioning
coefficients for HgCl2 as a proxy compound for Hg2+ on different
types of particles within a rather narrow temperature range of ca.
270–303 K, and Amos et al. (2012) derived them from GOM and PM observations
in the planetary boundary layer. If these partitioning coefficients are
extrapolated to a tropopause temperature of ∼230 K and taking into
account a median particle mass concentration during the CARIBIC flights of
0.6 µg m-3 (STP), at least 70 % of GOM should be attached
to particles. Consequently, GOM concentrations of 200 pg m-3 observed
by Shah et al. (2016) at 6–7 km altitude and much higher GOM
concentrations expected at higher altitudes (Lyman and Jaffe, 2012) imply PM
concentrations at and above the PIXE detection limit. Our inability to see
any PM by PIXE thus suggests that PM likely evaporates either during the
sampling or during the PIXE analysis in vacuum. We are aware that
extrapolation of partitioning coefficient from the narrow range of measured
temperatures might be fraught with substantial error. Measurements of
partitioning coefficients over a larger temperature range are needed.
The above considerations also have implications for the GEM and GOM
measurements via the trace gas inlet. The temperature within the container
varies between 30 and 40 ∘C. With GOM–PM equilibria determined by
Rutter and Schauer (2007) and Amos et al. (2012), almost all PM will
evaporate to GOM during the transport from the trace gas inlet to the Tekran
instrument and will be measured as such. Based on the above discussion, our
measurements likely include mercury volatilized from the particles that pass
through the CARIBIC trace gas inlet, in addition to all gaseous compounds. We
calculate that particles with diameter < 0.5 µm will pass
through the trace gas inlet described by Brenninkmeijer et al. (2007),
representing ∼70 % of the particle mass in the UT and LS. Hg2+
formed by photochemical processes will be attached to particles
proportionally to their surface area, which is dominated by smaller particles.
Consequently, 70 % of particulate mercury represents the lower limit for
the mercury on particles which will be co-determined by our system.
Fractionation experiments and their interpretation
One of the reviewers (P. Swartzendruber) of this paper pointed out that the
more commonly used term “speciation” is incorrect because no individual GOM
compounds (i.e. species) have so far been detected despite some evidence that
they exist (Huang et al., 2013). In accordance with IUPAC definitions
(http://goldbook.iupac.org) we use here the more appropriate term
“fractionation”.
The fractionation experiments onboard the CARIBIC container are restricted by
the certification procedures which allow only small internal instrument
modifications without any safety relevance. A dedicated external
fractionation unit is thus not an option. Within these limitations we added a GOM
scrubber upstream of one of the two gold collectors or upstream of both gold
collectors during the outbound flights as shown in Fig. 1. In the initial
experiments we run the instrument alternately in a mode without and with the
scrubber (5 min with and 5 min without). The experiments in March–June
2008 were carried out with the commercially available soda lime trap (Tekran
part no. 90-13310-06). Despite the careful cleaning of the soda lime trap
before the monthly flight sequence, mercury concentrations measured by the
channel with soda lime trap were higher than those measured by the channel
without the trap at the beginning of each individual flight. The difference
disappeared within about 1–2 flight hours. It seems that soda lime contains
traces of mercury which diffuse slowly and continuously from the bulk of the
material to its surface and accumulate in the time before the flight sequence
and between the individual flights when the trap is not flushed by the sample
air. After switching on the pumps, it is slowly flushed away. Because of this
sort of mercury bleeding, the soda lime trap was replaced during the flights
in March and April 2009 by a trap of the same size filled with quartz sand
coated with KCl. Beginning in August 2014 (flight 472) we used a quartz wool trap
described by Lyman and Jaffe (2012). The air is directed through the trap
during the outward flight and bypasses the trap during the return flight (see
Fig. 1 and associated description). All traps were tested for quantitative
transmission of elemental mercury before inserting them into the instrument.
We are aware of the problems frequently encountered with GOM (and PM) traps
such as interference of KCl surface with ozone and humidity (e.g. Lyman et
al., 2010; Gustin et al., 2013; Huang et al., 2013; Huang and Gustin, 2015).
Quartz wool traps are claimed not to be influenced by ozone (Ambrose et al.,
2013, 2015) but can release GOM in humid air. This should not pose a problem
in the UT–LS with very low absolute humidity. Because of these problems we
discuss the results of the trap experiments mostly only in qualitative terms.
After an initial flushing effect described above, the experiments with soda
lime trap showed generally smaller mercury concentrations with the trap than
without it. The difference between the concentrations without (TGM) and with
the trap (GEM if the trap removes GOM quantitatively) tended to be larger in
the stratosphere than in the troposphere. These results are consistent with
the fractionation measurements of Lyman and Jaffe (2012) and demonstrate
qualitatively that at least some GOM is transmitted through the inlet and the
tubing into the instrument.
This conclusion is supported by the experiments with the KCl trap. It was
deployed in March 2009 during the flight sequence Frankfurt–Cape
Town–Frankfurt–Orlando–Frankfurt (CARIBIC flights 262–265) and in April 2009
during the flight sequence Frankfurt–Caracas–Frankfurt–Vancouver–Frankfurt
(flights 266–269). Both flight sequences were of the same duration
(∼36 h), but the stratospheric section of the flights in March (total of
∼9 h) was much smaller than in April (total of ∼21 h). The KCl
trap was analysed for its mercury content after each flight sequence and
50 pg of mercury was found after the flights in March, with an uncertainty
of ∼±10 pg. The total sampling volume during the flights in March
was 500 L (STP), resulting in an average GOM concentration of
∼0.10 ng m-3. A total of 57±10 pg of mercury was found on the KCl
trap after the April 2009 flight sequence. With an overall sampling volume of
450 L (STP) it suggests an average GOM concentration of
∼0.13 ng m-3. This concentration is somewhat larger in accordance
with the longer time spent in the stratosphere although not proportional to
it. Mercury found on the KCl traps again shows that GOM is transmitted
through the inlet and the tubing to the instrument.
Overview of the parameters measured during flight 269 on
24 April 2009, from Vancouver to Frankfurt. The Hg instrument was run with a
resolution of 5 min alternatively with and without a KCl trap. Tentative GOM
concentrations are calculated as a difference between concentrations measured
without the KCl trap (TGM) and those with the KCl trap (presumably GEM). The
mercury data are not corrected (see Sect. 4) because most of the mercury
loads were smaller than 1 pg.
Data from flights 476 and 477 between Munich and San
Francisco on 23 and 24 September 2014. The air sample passed the quartz wool
trap upstream of the instrument during the forward flight from Munich to San
Francisco, and, assuming the quantitative GOM removal, we denote the data as
GEM. The return flight was run without the trap and is denoted as TGM. The
data are offline-processed.
During flight 269 on 24 April 2009 from Vancouver to Frankfurt the
aircraft flew almost always in the deep stratosphere (on average
∼7 PVU, ∼550 ppb O3, CO below 30 ppb – see Fig. 4). The
mercury data were not corrected because most mercury loads were smaller than
1 pg (see Sect. 4). Despite the large uncertainty of the uncorrected data
the TGM concentration remained with ∼0.3 ng m-3 nearly constant,
while the difference between the channel without and with the KCl trap varied
between 0 and 0.3 ng m-3. None of the simultaneously measured
parameters provided a hint as to what this variation might have depended on. But
the same concentrations of total gaseous mercury and its complete removal by
the trap during some sections of the flight suggest that all gaseous mercury
was oxidized. In addition, the constant TGM over large parts of this flight
with GOM varying from nearly zero concentration to concentrations comparable
to TGM suggests that the GOM transmission by the inlet tubing might be nearly
quantitative.
Figure 5 shows mercury concentrations (offline-processed) as a function of
simultaneously measured ozone mixing ratios measured during the outbound
flight from Munich to San Francisco (flight 476) on 23 September 2014
and the return flight on 24 September 2014 (flight 477). Sample air was
directed through the quartz wool trap during the outbound flight, and these
measurements are denoted as GEM. During the return flight the quartz wool
trap was bypassed and the sample air was fed directly into the instrument, and
these measurements are denoted as TGM. A direct GEM vs. TGM comparison is not
possible because of the differences in flight track and flight altitude. In
the troposphere at O3 < 100 ppb GEM and TGM concentration
tend to be comparable, whereas TGM concentrations tend to be larger than GEM
at O3 > 200 ppb, i.e. in the stratosphere.
In summary, experiments with all three trap types demonstrate clearly that
GOM is at least partly transmitted by the inlet tubing to the instrument. The
tendency for higher GOM concentrations in the stratospheric sections of the
flights further supports this evidence. The equal concentrations of TGM and
GOM during some sections of flight 269 suggest that all gaseous mercury
was transformed to GOM and that the transmission might be nearly
quantitative. We note that GOM transmission through PFA tubing is an
unresolved issue with contradictory findings (Temme et al., 2003; Landis and
Stevens, 2003; Swartzendruber, 2009; and the comments by both reviewers of
this paper), and our results thus cannot be generalized. But they are
consistent with those of Temme et al. (2003), who found that GOM is
transmitted quantitatively by PFA tubing at low temperatures and humidity
encountered in Antarctica, conditions similar to those encountered during the
CARIBIC flights at cruising altitude. The CARIBIC trace gas inlet is also very
similar to that described by Lyman and Jaffe (2012) with demonstrated GOM
transmission. Despite all evidence for nearly quantitative transmission of
GOM to the instrument we cannot prove it by measurements. One reason is the
lack of devices producing GOM test mixtures (Lyman and Jaffe, 2012). But even
with recently developed devices for GOM test mixture generation (Huang et
al., 2013; Ambrose et al., 2015) we will not be able to replicate the
sampling conditions during the flight. We have to wait for in-flight
comparison with an aircraft with proven GOM measurement capability.
In summary, we believe that the mercury concentrations measured by the
instrument in the CARIBIC container represent all gaseous elemental and
oxidized mercury, and at least 70 % of particulate mercury depending on
its particle size distribution. In the troposphere, where particulate
mercury constitutes usually less than a few percent of gaseous mercury, our
measurement will approximate total mercury.
Conclusions
The instrument described here has been onboard the CARIBIC container since
May 2005 and provided mercury data for more than 98 % of the flight time.
With this data availability it is one of the most reliable instruments in the
container. Unfortunately, the Tekran raw signal became available only in
April 2014. Using a Matlab script, we demonstrated the necessity of the
post-flight offline integration of the raw signal to get bias-free and more
precise data. In addition, no zero mercury concentrations have been detected
since the implementation of the post-flight integration of the Tekran raw
signal. Using simultaneous Tekran default and post-flight integrated data
from flights made between April 2014 and January 2015, we derived a function
which enabled us to remove the low bias of the old tropospheric data until
February 2014. The larger part of the stratospheric data until February 2014,
however, is lost. We would like to emphasize that the problem with the biased
integration of small mercury loads (<∼10 pg) also applies for
a large part of GOM and PM concentrations reported in the literature. To the
best of our knowledge, the low bias of GOM and PM concentrations caused by
the biased integrations of small mercury amounts has not been discussed so
far (e.g. Gustin et al., 2013, 2015).
Fractionation experiments demonstrated qualitatively that GOM is transmitted
through the inlet system to the instrument and will be measured together
with GEM. However, due to limitations given by the use of a passenger
aircraft the proof of quantitative GOM transmission is feasible only by an
in-flight intercomparison using a research aircraft with proven GOM
measurement capabilities.
Particles are also collected onboard CARIBIC using a separate aerosol inlet
and an impactor sampling device. No PM could be found on aerosol samples by
PIXE analyses with a detection limit of 0.2 ng m-3 for mercury. Our
inability to detect PM by PIXE in the LS despite of high PM concentrations
reported by Murphy et al. (2006) suggests that PM has evaporated during either
the sampling or the PIXE analysis in vacuum. PM–GOM partitioning
coefficients measured within a narrow range around 20 ∘C and
extrapolated to temperatures encountered at the tropopause suggest that most
Hg2+ will be GOM at container temperature and PM at tropopause
temperatures. Evaporation of PM to GOM during the sampling is thus quite
probable.
If there is PM on particles which make it through the trace gas inlet into
the instrument manifold, it will evaporate during the transport from the
inlet to the instrument and will be measured as GOM. At flight conditions
particles with a diameter < 0.5 µ m will pass through
the trace gas inlet representing ∼70 % of the aerosol mass. As GOM
will be preferably attached to smaller particles, this is the lower limit of
particulate mercury which will be measured together with the gaseous mercury
(GEM + GOM). In summary, we believe that the CARIBIC instrument provides
mercury data that approximate total mercury content of the sampled air
including mercury on particles.
Acknowledgements
We would like to thank Lufthansa, Lufthansa Technik, and all members of the CARIBIC team for
their continued effort to keep such a complex project running. We thank
especially Dieter Scharffe, Claus Koeppel, Stefan Weber, and Torsten Gehrlein
for the day-to-day maintenance and operation of the CARIBIC container.
Funding from the European Community within the GMOS (Global Mercury
Observation System) project and from Fraport AG is thankfully acknowledged.
We also thank Philip Swartzendruber and Anthony Hynes for their valuable
comments.
The article processing charges for this open-access publication were covered by the Max Planck Society.Edited by: R. Volkamer
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