The University of Miami (UM) deployed a sequential two-photon laser-induced
fluorescence (2P-LIF) instrument for the in situ measurement of gaseous
elemental mercury, Hg(0), during the Reno Atmospheric Mercury Intercomparison
Experiment (RAMIX) campaign. A number of extended sampling experiments,
typically lasting 6–8 h but on one occasion extending to
The environmental and
health impacts of mercury pollution are well recognized with impacts on human
health and broader environmental concerns (US EPA, 2000; UNEP, 2013; Mergler
et al., 2007; Díez, 2009; Scheuhammer et al., 2007). There have been
extensive reviews of global emissions, measurements and biogeochemical
cycling of mercury (Mason, 2009; Streets et al., 2011; Pirrone et al., 2009;
Lindberg et al., 2007; Ebinghaus et al., 2009; Sprovieri et al., 2016; Selin,
2009). The concerns associated with the mercury problem have resulted in
attempts to regulate and control emissions at both national and international
levels. The latest attempt in the United States is incorporated in the
Mercury and Air Toxics Standards (Houyoux and Strum, 2011; US EPA, 2013) and
international efforts by the United Nations Environment Program have led to
the Minamata Convention on Mercury, a global legally binding treaty on
mercury controls (UNEP, 2008, 2013, 2014). There is a reasonable consensus on
typical background concentrations of atmospheric mercury, which are extremely
low. Typical concentrations range from 1.2–1.4 ng m compare ambient measurements of gaseous elemental mercury, Hg(0),
gaseous oxidized mercury (RGM) and particulate bound mercury (PBM) by
multiple groups for 4 weeks; examine the response of all systems to spikes of Hg(0) and HgBr examine the response of all systems to Hg(0) in the presence of the
potentially interfering compounds of ozone and water vapor; and analyze the data to quantify the level of agreement and the results of
interference and calibration tests for each measurement system.
In practice, the instrument operated by UH only measured Hg(0) for the first
week of the campaign, and the cavity ring down spectroscopy (CRDS) instrument
deployed by DRI did not produce any data. Hence, RAMIX was primarily an
intercomparison of the UM 2P-LIF instrument, the UW Detector for Oxidized Hg
Species (DOHGS) that is based on two Tekran 2537 instruments, and a Tekran
2537 and two 2537/1130/1135 speciation systems deployed by UNR. Under these
circumstances, we were not able to compare 2P-LIF measurements made at high
temporal resolution with the CRDS instrument. It did allow us to compare the
2P-LIF sensor with independently operated instruments that use
preconcentration on gold, coupled with analysis by cold vapor atomic fluorescence spectroscopic (CVAFS), and to examine
potential interference effects. Our focus here is to compare the short-term
variation in GEM on the timescale that the CVAFS instruments operate,
A detailed description of the RAMIX location and the local meteorology was provided by Gustin et al. (2013). The original RAMIX proposal included participation from the Tekran Corporation to build and test a field-deployed, high-flow-sampling manifold that could be reliably spiked with 10–100 ppq of RGM. Tekran proposed to supply both RGM and Hg(0) spiking using independent generators that were traceable to NIST standards and would be independent of the detection systems being evaluated. However, due to time constraints, Tekran believed that it was unlikely that the manifold and ultra-trace spiking system could be manufactured and fully tested to their standards, so they declined to participate in RAMIX (E. C. Prestbo, personal communication, 2015). Instead, the UW group stepped in to supply and operate the sampling manifold and spiking system, and the details of its characterization are provided in Finley et al. (2013). During the RAMIX campaign, the 2P-LIF instrument sampled on 18 days, typically sampling for between 4 and 6 h. The longest period of continuous sampling lasted for 26 h and occurred on 1 and 2 September. Over this 18-day period, we sampled from the RAMIX manifold and, in addition, at the end of the campaign, we sampled ambient air independently and also attempted to measure TOM by pyrolyzing the sample air and measuring the difference between Hg(0) and TM. We also sampled RGM using KCl-coated annular denuders using LIF for real-time analysis.
Bauer et al. (2002, 2003, 2014) provide a description of the operating
principles of the 2P-LIF instrument. Bauer et al. (2014) provide a detailed
description of the 2P-LIF instrument deployed at RAMIX, including the
sampling configurations, data processing, calibration and linearity tests
together with examples of experimental data. In summary, the system uses
sequential two-photon excitation of two atomic transitions in Hg(0) followed
by detection of blue-shifted LIF. The instrumental configuration at RAMIX
utilized an initial excitation of the Hg 6
We attempted to use the 2P-LIF instrument to measure TM, and hence TOM by
difference. Although we have routinely used this approach to convert
HgCl
RAMIX manual KCl denuder sampling.
We conducted manual denuder sampling on seven afternoons during the RAMIX
campaign to attempt to quantify total RGM. We sampled using both KCl-coated
annular denuders and uncoated tubular denuders that were then analyzed using
programmable thermal dissociation (Ernest et al., 2014). In both cases, we
monitored the Hg(0) that evolved during RGM decomposition, in real time,
using single-photon LIF. Only the annular denuder results are presented here.
The use of denuder sampling coupled with thermal dissociation has been
described by Landis et al. (2002) and is used in the Tekran model 1130
mercury speciation units deployed during RAMIX. Air is pulled through a
KCl-coated annular denuder which captures RGM but transmits elemental and
particulate mercury. After a period of sampling, typically 1 h, the denuder
is flushed with zero-grade air and the denuder is heated to 500
As noted above, the RAMIX manifold had to be constructed and tested by the UW
group under tight time constraints and details of its characterization are
provided in Finley et al. (2013). A critique of the manifold performance has
been presented by Prestbo (2016) and we detail some key issues here. The
manifold deployed at RAMIX was a different size than the prototype tested in
the laboratory. The laboratory manifold showed very large variation in
calculated transmission efficiencies of Hg(0) after spiking with a permeation
source. Finley et al. reported recoveries of 71–101 % for short-term
spikes. The authors speculate that this was associated with rapid changes in
ambient Hg(0) but provide no measurements to support this. The Hg(0) source
used for spiking was gravimetrically calibrated by the manufacturer but was
not used at the calibration temperature requiring the output to be calibrated
by a Tekran 2537B. After the equipment was moved to the RAMIX site, the
permeation tube output increased. The authors also acknowledge a significant
uncertainty (
In fact, we find that several independent measurements of Hg(0) spikes differ
by as much as 30 % from the value calculated by the manifold operators,
suggesting that (
In evaluating the first week of the UM RAMIX measurements, it became clear
that there was some nonlinearity in the relative responses of the 2P-LIF and
UM Tekran systems and that better agreement was obtained by referencing the
Hg(0) concentration to the UNR Tekran. Gustin et al. (2013) concluded that
the UNR Tekran, based on the inlet configuration, only measured Hg(0), and
they suggested that the UM system, due to the long sampling line, was
measuring total gaseous mercury (TGM). We compared the manifold Hg(0)
readings from the UM and UNR Tekrans over the first 260 h in which we took
measurements. The absolute concentration difference relative to the UNR
instrument is shown in Fig. 1. Hour 0 corresponds to 09:00 PDT on 26 August
when we started measurements and hour 260 corresponds to midnight on
5 September. Over the first 24 h, the UM Tekran is offset by
Comparison of Hg(0) readings from the UM, UW and UNR Tekrans over the first 260 h of UM measurements. The absolute concentration difference relative to the UNR instrument is shown in black for the UM Tekran and in red for the DOHGS (UW) Tekran.
The absolute Hg(0) concentrations reported for the 2P-LIF measurements typically use a single 10 min section of Tekran concentration data to calibrate the 2P-LIF signal and place it on an absolute concentration scale. The complete time series of measurements then gives a long-term comparison of the 2P-LIF and Tekran instrumentation with the absolute 2P-LIF concentrations based on the single 10 min calibration point.
This was the first occasion on which the three independent Tekran 2537
instruments and the 2P-LIF system reported simultaneous measurements. The
2P-LIF system sampled from the RAMIX manifold for approximately 6.5 h from
A 7 h measurement period from 5 September. The percent difference of the UNR (black line) and UW (red line) Tekrans and the UM 2P-LIF (green line) measurements relative to the UM Tekran is shown.
The UM and UNR systems sampled simultaneously for a 22 h period, offering an
opportunity to compare the instruments over an extended sampling period. This
sampling also occurred prior to any of the manifold spikes that introduced
substantial concentrations of HgBr
A 22 h sampling period from 1 and 2 September. Comparison of the UM (red line) and UNR (green line) Tekrans with the UM 2P-LIF (black line) concentrations. The concentrations for each instrument are scaled to force agreement during the second manifold spike at hour 33. These are the data from Fig. S3 with the concentration scale expanded to show only ambient data.
A section of the 22 h sampling period from 1 and 2 September. Comparison of the UM (red line) and UNR (green line) Tekrans with the UM 2P-LIF (black line) concentrations. The concentrations for each instrument are scaled to force agreement during the second manifold spike at hour 33. These are the data from Fig. S3 with the concentration scale expanded to show only ambient data between hours 29 and 32.
Almost all of the measurements of atmospheric concentrations of Hg(0) have been made with CVAFS instrumentation and the majority of those measurements have utilized the Tekran 2537. This work provides the first extensive comparison of the Tekran 2537 with an instrument that is capable of fast in situ detection of Hg(0) using a completely different measurement technique. Measurements over two extended sampling periods show substantial agreement between the 2P-LIF and Tekran measurements and suggest that all the instruments are primarily measuring the same species. Intercomparison precision of better than 25 % was achievable over an extended sampling period and precision of better than 10 % was achieved for subsets of the sampling period. As we discuss below, it is difficult to determine the extent to which interferences from RGM contribute to the differences observed.
As noted above, one component of the initial RAMIX proposal was an
examination of the response of the various sensors to potential interfering
compounds HgBr
On 7 September, an ozone interference test was conducted by simultaneously
spiking the sampling manifold with high concentrations of Hg(0) and ozone.
The spike in Hg(0) lasted from 09:00 to 19:30 PDT and there were two ozone
spikes, each 2 h in duration. A comparison of the UM, UW and UNR Tekrans and
the 2P-LIF signal is shown in Fig. 6. The UW Tekran only measured for a
portion of this period but agrees reasonably well with the other Tekrans. The
2P-LIF signal is calibrated by the UM Tekran reading during the initial Hg(0)
spike at 09:30 PDT. The 2P-LIF signal was online for 6 min at the beginning of the
first ozone spike and then went offline for
An ozone interference test on 7 September. A comparison of the UM, UW and UNR Tekrans and the UM-2P-LIF measurements. The “expected” concentration calculated from the ambient Hg(0) concentration prior to the spike plus the calculated spike concentration is also shown.
The changes in the Hg(0) concentration measurements shown in Fig. 6 track the predicted changes in calculated spike concentration. However, the calculated spike concentrations, which are also shown, are 20–40 % higher than the actual measurements made by the Tekrans.
We made attempts to use the 2P-LIF instrument to measure TM, and hence TOM by difference, by sampling through two manifold lines. A pyrolyzer was located at the manifold on one of the sampling lines to measure TM. The other sampling line measured ambient Hg(0). TOM was calculated from the difference in the TM and Hg(0) concentrations, and in this sampling configuration the limit of detection for TOM depends on the short-term variability in ambient Hg(0), which is significant and shows a diurnal variation. The pyrolysis system was set up and tested on 12 September. Manifold sampling was conducted on the 13th and 14th, and sampling from the trailer roof occurred on the 15th. We calculated the means of the pyrolysis and ambient channel concentrations and the difference which gives the TOM concentration. We also calculated the standard deviations and standard errors (SEs) and used these errors to calculate in quadrature the 2 SE uncertainty in the derived TOM concentration. However, as discussed below, the errors in the means do not appear to capture the full variability in Hg(0), particularly at shorter sampling times.
Our most extensive sampling took place on the 14th when we were able to
sample for three
Measurements from 14 September, hours 17–19 (17:00–19:00 PDT). The background-subtracted 2P-LIF signals from the ambient (black) and pyrolyzed (red) sampling lines are shown. The gaps correspond to times when the laser was blocked to check power and background. The means and 1 standard deviation of each sample are shown. The absolute Hg(0) concentrations are obtained by scaling the ambient Hg(0) signal to the absolute Hg(0) concentration reported by the UNR Tekran during the Hg(0) manifold spike.
The third sampling period, which included a large HgBr
The 14 September measurements, hours 17–19. The means of the ambient channel (black) and pyrolyzed channel (red) are shown. The error bars show both 2 standard errors (thicker line) and 2 standard deviations.
TOM concentrations calculated from the difference between the
pyrolyzed and ambient sample concentrations together with 2 SEs in the TOM
concentrations. The reported HgBr
Figure S4 shows the 7 s average of the 2P-LIF signal from the ambient and
pyrolysis sample lines for the first sampling period at 08:00–10:27 PDT together with the mean
and 1 standard deviation (1
KCl manual denuder measurements from 16 September. The raw data for the TDPs for the denuder D1 are shown.
Figures S7–S9 show the corresponding plots for the second sampling period
from
Figures S10 and S11 show the averages of the TOM concentrations from the 2P-LIF system together with the measurements from the UNR speciation systems, the reported spike concentrations and 5 min DOHGS concentrations. During this sampling period, Spec1 sampled from the RAMIX manifold while Spec2 sampled ambient air outside the manifold. Gustin et al. (2013) detailed problems with the response of the Spec2 system and applied a 70 % correction that is also shown as “Spec2 corrected”. Because both the DOHGS and 2P-LIF pyrolysis systems are expected to measure the sum of gaseous (RGM) and particulate (PBM) oxidized mercury, we have plotted the sum of the RGM and PBM concentrations from the speciation systems. They are plotted at the midpoint of the 1 h sampling period.
Over most of the measurement period, the 2P-LIF pyrolysis and Spec1 measurements are consistent and lower than the DOHGS measurements. The exception is the large spike in TOM seen by the 2P-LIF system at hour 18. The spike occurred during the initial portion of Spec1 sampling and, although it measures an increase in RGM relative to Spec2, the magnitude is not consistent with the 2P-LIF pyrolysis observations.
13 September was the first day we were able to sample with the pyrolysis
system and we sampled over a period of 5 h. The only manifold spike during
this period was an O
On 15 September, we sampled from the trailer roof using the same sampling
lines and again alternating between the pyrolyzed and unpyrolyzed channels.
Figure S13 shows the averages of the 2P-LIF signal from the ambient and
pyrolysis channels together with the concentrations measured by the Spec2
system that was sampling ambient air outside the manifold. The concentration
obtained from the UM denuder samples described below are also shown. The UW
DOHGS and Spec1 systems were sampling from the RAMIX manifold with continuous
HgBr
As we have noted above, our limit of detection of TOM depends on the
short-term variability in the ambient Hg(0) concentration because we use a
single fluorescence cell and switch between pyrolysis and ambient channels.
We have attempted to give an estimate of the uncertainty by taking 2 SEs of
the means and combining the errors in quadrature to get an estimate of the
uncertainty in the TOM concentration. If the mean of the ambient Hg(0)
concentration is not fluctuating significantly on the timescale of channel
switching, this approach should give an accurate estimate of the uncertainty
in TOM. In fact, our Hg(0) observations show that the fluctuations in the
Hg(0) concentration show a significant diurnal variation, with large
fluctuations at night, decreasing over the course of morning hours and being
smallest in the afternoon. This can be seen in the long-term sampling from 1
and 2 September and in the observations from 14 September. The observation of
statistically significant but physically unrealistic negative TOM
concentrations on 13 September may be explained by this. Such an artifact
could be produced by contamination in the Teflon valve-switching system that
alternates the flow to the fluorescence cell. This type of contamination
should produce a constant bias that is not actually observed. It appears that
the short-term variability in Hg(0) concentration produces a small bias in
some cases that is not averaged out by switching between the ambient and
pyrolyzed channels. For example, on 13 September the initial sample period of
1 h and 12 min gives an RGM concentration of
0.06
As we describe above, our use of manual denuders was similar to that
described by Landis et al. (2002) with the exception that we did not
incorporate the integrated elutriator/acceleration jet and impactor/coupler
on the denuder inlet and the denuders were not heated. Landis et al. (2002)
suggest that HgCl
KCl manual denuder measurements from 16 September. The calibrated TDPs for the tandem denuder pair, D1 and D3, are shown.
We think a realistic assessment of the RAMIX results is imperative because the interpretation of the RAMIX data and the conclusions presented by Gustin et al. (2013) and Ambrose et al. (2013) have enormous implications for both our understanding of current experimental approaches to atmospheric sampling of mercury species and for the chemistry itself. Speciation systems using KCl denuder sampling are widely used in mercury monitoring networks worldwide to measure RGM concentrations and the Gustin et al. (2013) and Ambrose et al. (2013) papers suggest these results greatly underestimate RGM concentrations with no clear way to assess the degree of bias.
The assessment of the Hg(0) measurements is a little different in the two
papers with Ambrose et al. (2013), noting that “comparisons between the
DOHGS and participating Hg instruments demonstrate good agreement for GEM”
where GEM refers to Hg(0), and they found a mean spike recovery of 86 %
for the DOHGS measurements of Hg(0), based on comparisons between measured
and calculated spike concentrations. Gustin et al. (2013) suggest that the UM
Tekran agreed well with measurements of TM reported by the DOHGS system and
they “hypothesize that the long exposed Teflon line connected to the UM
Tekran unit provided a setting that promoted conversion of RM to GEM, or that
RM was transported efficiently through this line and quantified by the Tekran
system. The latter seems unlikely given the system configuration…”,
where RM refers to reactive mercury. As we note above, we believe that the
best explanation for discrepancies between the UM and UNR Tekrans is an
experimental issue with the UM Tekran response during the initial period of
sampling. We would suggest that data from 5 September, one of the few
occasions when data from multiple instruments agreed over an extended period,
are not compatible with either transmission or inline reduction of RGM in our
sampling line. What is also significant from these data is the very large
discrepancy between the spike concentrations as measured independently by
three different Tekran systems and confirmed by the relative response of the
2P-LIF measurements and the calculated spike concentration. The discrepancy,
on the order of 25–30 %, is larger than the manifold uncertainties
suggested by Finley et al. (2013). We note other examples of the measured
Hg(0) spikes being significantly lower than the calculated concentrations. In
prior work, we have shown that both the Tekran and 2P-LIF systems show
excellent agreement over more than 3 orders of magnitude in concentration
when monitoring the variation in Hg(0) in an N
A more difficult issue is the question of resolving the differences in the
temporal variation of ambient Hg(0) at the 5 min timescale as captured by
the different instruments. The Tekran systems should be in agreement with a
precision of better than 5 % and the 2P-LIF system, with a much faster
temporal resolution and detection limit, should be capable of matching this.
The differences here are not consistently associated with a single
instrument, for example, with the 2P-LIF having some systematic offset with respect to
the CVAFS systems. The extent to which the larger (i.e., larger than 5 %)
observed discrepancy which ranged from 10 to 25 % is a result of
interferences or simply a reflection of instrument precision is difficult to
assess. We note again that the UM instruments had to sample through a very
long sampling line and we expect that oxidized mercury is deposited on the
sampling line. However, it is not possible to assess the extent to which
oxidized mercury is reduced back to its elemental form introducing small
artifacts. As we suggest below, an intercomparison of instrument response to
variation in Hg(0) concentrations in a pure N
The ozone concentration and absolute humidity for a 35 h sampling period on 13 and 14 September that included two ozone spikes and only sampled ambient TOM.
To the best of our knowledge, RAMIX is the only experiment that has measured
ambient TOM using multiple independent techniques. It should again be
emphasized that the TOM measurements using pyrolysis with 2P-LIF detection
were the first attempt to perform such measurements and the use of a single-channel
detection system introduced large uncertainties into the
measurements. The very large discrepancies between the measurements of TOM
reported by the DOHGS system, the Tekran speciation systems and the limited
number of 2P-LIF pyrolyzer measurements are the most problematic aspect of
the RAMIX measurement suite. Work prior to RAMIX suggested a potential ozone
and/or humidity interference in the operation of KCl-coated annular denuders
and a number of studies since have also reported such an effect (Lyman et
al., 2010; McClure et al., 2014). Typically, however, the differences between
the RAMIX measurements are large and are not germane to the differences
between the DOHGS and 2P-LIF pyrolyzer measurements. The Supplement figures give an
example of the differences between the DOHGS measurements and the denuder and
2P-LIF measurements. Ambrose et al. (2013) note that the DOHGS measurements
were, on average, 3.5 times larger than those reported by the Spec1 system
and summarize the comparison with denuder measurements as follows: “These
comparisons demonstrate that the DOHGS instrument usually measured RM
concentrations that were much higher than, and weakly correlated with those
measured by the Tekran Hg speciation systems, both in ambient air and during
HgBr
Expected denuder recovery based on the formula determined by McClure
et al. which varies between a typical value of
We suggest that the ability of the 2P-LIF pyrolysis system to monitor large
spike concentrations is shown by the measurements during the 14 September
HgBr
It is also reasonable to question the extent to which the Tekran speciation
systems operated at RAMIX reflect the performance of these systems when
normally operated under recommended protocols. As noted above, the operation
of the RAMIX manifold and the Tekran speciation systems has been questioned
by Prestbo (2016). In our view, the two most significant issues are the
performance of the two 2537 mercury analyzers associated with each speciation
system and the reduced sampling rate. The performance of the two 2537 units
is detailed in Gustin et al. (2013) and, as they noted, there was a
significant response in each instrument. Examination of Fig. S6 of Gustin et
al. (2013) shows the relative responses of the two instruments, and, using
concentrations up to 25 ng m
The oxidized mercury concentrations presented by Ambrose et al. (2013) for
the RAMIX measurements suggest a well-defined diurnal profile that peaks at
night. It is important to note that the error bars on this profile (Fig. 3 of
Ambrose et al., 2013) are 1 standard error rather than 1 standard
deviation. The standard deviations, which actually give an indication of the
range of concentrations measured show much larger errors indicating
significant day-to-day variation in these profiles. Nevertheless, the
measurements show much larger oxidized mercury concentrations than the
speciation systems and the very limited number of 2P-LIF measurements. As we
note below, there is no known or hypothesized chemistry that can reasonably
explain the large RGM concentrations seen by the DOHGS instrument. Both
Gustin et al. (2013) and Ambrose et al. (2013) draw some conclusions about
the chemistry of mercury that have significant implications for atmospheric
cycling. Gustin et al. (2013) suggest in their abstract that “On the basis
of collective assessment of the data, we hypothesize that reactions forming
RM were occurring in the manifold.” Later, in a section on “Implications”,
they conclude “The lack of recovery of the HgBr
Ambrose et al. (2013) also suggest that the observations of very high RGM
concentrations indicate multiple forms of RGM and that the concentrations can
be explained by oxidation of Hg(0), with O
The discrepancies that are discussed above suggest a need for a careful
independent evaluation of mercury measurement techniques. The approaches used
during the evaluation of instrumentation for the NASA Global Tropospheric
Experiment (GTE) and the Gas-Phase Sulfur Intercomparison Experiment (GASIE)
evaluation offer good models for such an evaluation. The Chemical Instrument
and Testing Experiments (CITE 1–3) (Beck et al., 1987; Hoell et al., 1990,
1993) were a major component of GTE establishing the validity of the airborne
measurement techniques used in the campaign. The GASIE experiment (Luther and
Stecher III, 1997; Stecher III et al., 1997) was a ground-based
intercomparison of SO
We deployed a 2P-LIF instrument for the measurement of Hg(0) and RGM during the RAMIX campaign. The Hg(0) measurements agreed reasonably well with instruments using gold amalgamation sampling coupled with CVAFS analysis of Hg(0). Measurements agreed to within 10–25 % on the short-term variability in Hg(0) concentrations based on a 5 min temporal resolution. Our results also suggest that the operation of the RAMIX manifold and spiking systems were not as well characterized as Finley et al. (2013) suggest. We find that the calculated concentration spikes consistently overestimated the amount of Hg(0) introduced into the RAMIX manifold by as much as 30 %. This suggests a systematic error in concentration calculations rather than random uncertainties that should not produce a high or low bias.
We made measurements of TM, and hence TOM by difference, by using pyrolysis
to convert TOM to Hg(0) and switching between pyrolyzed and ambient samples.
The short-term variation in ambient Hg(0) concentrations is a significant
limitation on detection sensitivity and suggests that a two-channel
detection system, monitoring both the pyrolyzed and ambient channels
simultaneously, is necessary for ambient TOM measurements. Our TOM
measurements were normally consistent, within the large uncertainty, with
KCl denuder measurements obtained with two Tekran speciation systems and
with our own manual KCl denuder measurements. The ability of the pyrolysis
system to measure higher RGM concentrations was demonstrated during one of
the manifold HgBr
Data are available from the corresponding author (ahynes@rsmas.miami.edu).
This work was supported by NSF grant (no. AGS-1101965), a National Science Foundation Major Instrumental grant (no. MRI-0821174) and by the Electric Power Research Institute. We thank Mae Gustin and her research group and Robert Novak for their hospitality, assistance and use of laboratory facilities during the RAMIX intercomparison. We thank Mae Gustin and Dan Jaffe for the use of their RAMIX data for comparison with our results. We thank Eric Prestbo for helpful comments on the manuscript. Edited by: J. B. Burkholder Reviewed by: two anonymous referees