amt-9-3527-2016.pdf

Atmos. Meas. Tech., 9, 3527–3546, 2016

www.atmos-meas-tech.net/9/3527/2016/

doi:10.5194/amt-9-3527-2016

Assessment of errors and biases in retrievals of X

, X

and X

from a 0.5 cm

−

resolution solar-viewing spectrometer

Jacob K. Hedelius

, Camille Viatte

, Debra Wunch

2,a

, Coleen M. Roehl

, Geoffrey C. Toon

3,2

, Jia Chen

4,b

Taylor Jones

, Steven C. Wofsy

, Jonathan E. Franklin

5,c

, Harrison Parker

, Manvendra K. Dubey

, and

Paul O. Wennberg

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences,

Harvard University, Cambridge, MA, USA

Department of Physics & Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada

Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM, USA

now at: Department of Physics, University of Toronto, Toronto, Ontario, Canada

now at: Electrical and Computer Engineering, Technische Universität München, Munich, Germany

now at: School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences,

Harvard University, Cambridge, MA, USA

Correspondence to:

Jacob K. Hedelius (jhedeliu@caltech.edu)

Received: 5 February 2016 – Published in Atmos. Meas. Tech. Discuss.: 4 March 2016

Revised: 24 June 2016 – Accepted: 28 June 2016 – Published: 3 August 2016

Abstract.

Bruker

™

EM27/SUN instruments are commer-

cial mobile solar-viewing near-IR spectrometers. They show

promise for expanding the global density of atmospheric col-

umn measurements of greenhouse gases and are being mar-

keted for such applications. They have been shown to mea-

sure the same variations of atmospheric gases within a day as

the high-resolution spectrometers of the Total Carbon Col-

umn Observing Network (TCCON). However, there is little

known about the long-term precision and uncertainty budgets

of EM27/SUN measurements. In this study, which includes

a comparison of 186 measurement days spanning 11 months,

we note that atmospheric variations of X

gas

within a single

day are well captured by these low-resolution instruments,

but over several months, the measurements drift noticeably.

We present comparisons between EM27/SUN instruments

and the TCCON using GGG as the retrieval algorithm. In ad-

dition, we perform several tests to evaluate the robustness of

the performance and determine the largest sources of errors

from these spectrometers. We include comparisons of X

, X

, and X

. Specifically we note EM27/SUN bi-

ases for January 2015 of 0.03, 0.75,

−

0.12, and 2.43 % for

, X

, and X

respectively, with 1

running

precisions of 0.08 and 0.06 % for X

and X

from mea-

surements in Pasadena. We also identify significant error

caused by nonlinear sensitivity when using an extended spec-

tral range detector used to measure CO and N

1 Introduction

Measurements of atmospheric mixing ratios of greenhouse

gases (GHGs), including CO

and CH

, are needed to aid

in estimating fluxes and flux changes, and to ensure inter-

national treaties to reduce emissions are fulfilled. The To-

tal Carbon Column Observing Network (TCCON) makes

daytime column measurements of these gases. The Orbit-

ing Carbon Observatory2 (OCO-2) and Greenhouse Gases

Observing Satellite (GOSAT) missions enable column GHG

measurements with global coverage. These GHG monitoring

satellites make measurements at one time of day and, there-

fore, lack the temporal resolution that a dedicated ground site

provides.

Published by Copernicus Publications on behalf of the European Geosciences Union.

3528

J. K. Hedelius et al.: Assessment of errors in Xgas from a 0.5 cm

−

spectrometer

Due to cost, lack of infrastructure, and stringent network

requirements, there are limited ground sites on a global scale;

e.g., there are no TCCON sites currently in operation in con-

tinental Africa, South America, or central Asia (Wunch et al.,

2015), and there currently is no urban area with more than

one TCCON site. Cheaper, portable, solar-viewing Fourier

transform spectrometers (FTSs) can make contributions in

these settings provided they have long-term precision. The

Bruker Optics

™

EM27/SUN, with the “SUN” indicating a

built-in solar tracker, is a transportable FTS that may supple-

ment global GHG measurements made by current networks

(Gisi et al., 2012). This unit is small and stable enough to eas-

ily be transported for field campaign measurements, includ-

ing measurements at multiple locations in 1 day. Column-

averaged dry-air mole fractions (DMFs) of gases (X

gas

)

are

retrieved from the EM27/SUN measurement, like the TC-

CON. X

gas

is calculated from (Wunch et al., 2010):

gas

column

gas

column

dry air

2095

column

gas

column

(1)

where the 0.2095 factor is the fraction of dry air that is oxy-

gen.

Retrieved X

gas

has been compared with a co-located TC-

CON site in Karlsruhe, Germany, in past work for 26 days

of X

retrievals from one EM27/SUN instrument (Gisi et

al., 2012), and 6 days of both X

and X

retrievals from

five EM27/SUN instruments (Frey et al., 2015).

Operators of these instruments have different end goals

to better understand the carbon cycle. X

and X

re-

trievals from these instruments have been compared with

satellite measurements in areas without a TCCON site (Klap-

penbach et al., 2015) as well as with satellite measurements

in highly polluted areas (Shiomi et al., 2015). Emission

flux estimates from the Berlin area (< 30

30 km

)

were

made by combining upwind/downwind measurements from

five spectrometers and were compared with a simulation

(Hase et al., 2015). Chen et al. (2016) have assessed gra-

dient strengths around a large dairy farm (

∼

100 000 cows)

in Chino, California (< 12

12 km

)

, using measurements

from upwind/downwind spectrometers. Weather Research

and Forecast Large-Eddy Simulations (WRF-LES, 4 km res-

olution) were used in combination with four simultaneous

measurements to estimate fluxes from specific grid boxes in

a subregion of the Chino dairy farm area, which is within a

larger urban area (Viatte et al., 2016).

The column measurements used in these studies provide

some advantages over in situ measurements, including less

sensitivity to vertical exchange, surface dynamics, and small-

scale emissions (McKain et al., 2012), which are difficult to

model. Though column measurements can depend on mixed

layer height in highly polluted areas, generally, column mea-

surements depend primarily on regional-scale meteorology,

and regional fluxes (Wunch et al., 2011b; McKain et al.,

2012). For example, Lindenmaier et al. (2014) used obser-

vations from a single TCCON site to verify 1 day of emis-

sions from coal power plants of about 2000 MW each at

∼

and 12 km away. Because of their large spatial sensitivity,

column measurements are well suited for estimation of net

emissions, model comparison, and satellite validation. A sin-

gle site has been used to estimate Los Angeles, California

(L.A.), emissions based on a sufficiently accurate emissions

inventory and the observation that X

gas

anomalies within

L.A. are highly correlated (Wunch et al., 2009, 2016). Gener-

ally though, a single column measurement site is insufficient

to estimate emissions from an entire urban region (Kort et

al., 2013). However, multiple column measurements can be

combined to characterize part or all of an urban area (Hase et

al., 2015; Chen et al., 2016; Viatte et al., 2016).

The main goal of this work is to quantitatively evaluate

the robustness of EM27/SUN retrievals over a long period

of time. This is accomplished by comparing retrievals from

the EM27/SUN with a co-located standard (TCCON site)

at Caltech, in Pasadena, California, United States. TCCON

spectrometers make the same type of measurements (direct

solar near-infrared) at high spectral resolution. Here we re-

port X

, X

, and X

comparison measurements

from an EM27/SUN. The X

and X

measurements were

made possible by a detector with an extended spectral range

provided by Bruker

™

. The EM27/SUN X

and X

TCCON comparison is the longest to date, 186 measure-

ment days spanning 11 months. In part of January 2015,

an additional three EM27/SUN instruments were at Caltech

for 9 to 12 days of X

and X

comparisons to assess

their relative biases. In Sect. 2 we briefly describe differences

in instruments and the data acquisition process. In Sect. 3

we describe the retrieval software. In Sect. 4 we describe

the inherent properties of EM27/SUNs such as instrument

line shapes (ILSs), frequency shifts, ghosts, detector linear-

ity, and external mirror degradation. Section 5 focuses on bi-

ases and sounding precision of different gases compared with

the TCCON. Section 6 describes sources of instrumental er-

ror. We conclude with general recommendations of tests to

perform on any new type of direct solar near-infrared (IR)

instrument used to retrieve abundances of atmospheric con-

stituents.

2 Instrumentation

2.1 TCCON IFS 125HR

All TCCON sites employ the high-resolution Bruker

Optics

™

IR FT spectrometer (IFS) 125HR that has been

described in detail elsewhere (Washenfelder et al., 2006;

Wunch et al., 2011b). For the Caltech TCCON site

(34.1362

◦

N, 118.1269

◦

W, 237 m a.s.l.), the IFS 125HR uses

an extended InGaAs (indium gallium arsenide) detector,

covering 3800–11 000 cm

−

for detection and retrieval of

all gases relevant to this study (O

, CO

, CH

, CO, and

O). Figure 1 has example spectra from IFS 125HR and

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J. K. Hedelius et al.: Assessment of errors in Xgas from a 0.5 cm

−

spectrometer

3529

4000

6000

8000

10 000

Frequency (cm

-1

)

Signal intensity (A.U.)

6216

6220

Frequency cm

-1

0.5

Transmission (%)

TCCON InGaAs

ext. InGaAs

stnd. InGaAs

Figure 1.

Example of scaled spectra from three different detector

types, with retrieval windows highlighted. The spectrum from the

EM27/SUN extended InGaAs detector was scaled 10 times more

than the spectrum from the standard InGaAs detector.

EM27/SUN instruments, with the spectral regions where in-

dividual gases are retrieved highlighted. Oxygen (O

)

abun-

dance is useful in calculating the DMF because it represents

the column of dry air and is combined with the column of the

gas of interest to yield the DMF (Wunch et al., 2010).

The Caltech IFS 125HR uses a resolution of approxi-

mately 0.02 cm

−

(with a maximum optical path difference

(MOPD) of 45 cm). It takes about 170 s to complete one for-

ward/backward scan pair. TCCON sites have single sound-

ing 2

uncertainties of 0.8 ppm (X

)

, 7 ppb (X

)

, 4 ppb

)

, and 3 ppb (X

)

(Wunch et al., 2010). TCCON data

are tied to the World Meteorological Organization (WMO)

in situ trace gas measurement scale through extensive com-

parisons with in situ profiles obtained from aircraft and bal-

loon flights. We use the TCCON as a standard against which

to compare the EM27/SUN instruments. TCCON data from

this study are publicly available from the Carbon Dioxide In-

formation Analysis Center (Wennberg et al., 2014).

2.2 Caltech EM27/SUN

EM27/SUN spectrometers have been described elsewhere

(Gisi et al., 2012; Frey et al., 2015; Klappenbach et al., 2015)

so we focus on differences in setup and acquisition here. The

standard EM27/SUN configuration uses an InGaAs detector

sensitive to the spectral range spanning 5500–12 000 cm

−

which permits detection of O

, CO

, CH

, and H

O (Frey

et al., 2015). For this study, the Caltech EM27/SUN was

delivered with an extended-band InGaAs detector sensitive

to 4000–12 000 cm

−

, which allowed for additional mea-

surements of CO and N

O (Fig. 1). All EM27/SUN spec-

trometers used in this study (Sects. 2.2, 2.3) used the typ-

ical MOPD of 1.8 cm, corresponding to a spectral reso-

lution of 0.5 cm

−

. Interferograms (ifgs) were acquired in

direct-current-coupled mode to allow post-acquisition low-

pass filtering of brightness fluctuations to reduce the impact

of variable aerosol and cloud cover effects (Keppel-Aleks

et al., 2007). Ghosts were reduced as data were acquired

by employing the interpolated sampling option provided by

Bruker

™

(see also Sect. 4.3). A 10 KHz laser fringe rate is

used to reduce scanner velocity deviations, and each for-

ward/backward scan took 11.6 s, or 5.8 s per individual mea-

surement.

To be more consistent with the TCCON measurements,

no spectrum averaging or interferogram apodization was ap-

plied before retrieving DMFs. We recommend averaging

only after retrievals if disc storage and processor speeds are

sufficient, so spurious data can be filtered. To test the pre- vs.

post-averaging effect we used 9 retrieval days with 26 000

forward/backward measurements and used Bruker

™

OPUS

software to create spectra from ifgs. We compared retrievals

from using five combined backward/forward measurements

averaged pre-retrieval with those averaged post-retrieval. We

also compared combined forward/backward measurements

using a medium Norton–Beer apodization with those using

no special apodization. Results are in Table 1 and suggest

that different averaging methods cause only small inconsis-

tencies, under

∼

0.02 % for X

and X

The EM27/SUN was placed within 5 m of the Cal-

tech TCCON solar tracker mirrors on the roof of the

Linde

Robinson building (Hale, 1935). Measurements

started on 2 June 2014 and, for this study, we include

186 measurement days that end on 4 May 2015. About

800 000 individual EM27/SUN measurements and 40 000 in-

dividual TCCON measurements were acquired over this

period. Of these, about 580 000 and 15 000 were consid-

ered coincident and were not screened out by our qual-

ity control filters (QCFs). Our QCFs were conservative,

and they required signal > 30 (Sect. 4.4), solar zenith angle

(SZA) < 82

◦

, 370 ppm < X

< 430 ppm, X

error

< 5 ppm,

error

< 20 ppb, and X

error

< 0.1 ppm. Other users may

consider stricter QCFs. After averaging data into 10 min bins,

there were about 6500 binned comparison points.

2.3 LANL and Harvard EM27/SUN instruments

Three additional EM27/SUN instruments were compared

with the Caltech TCCON site in January 2015 – one owned

by Los Alamos National Laboratory (LANL) and two owned

by Harvard University (HU). To be consistent, all the acqui-

sition and retrieval settings were the same as for the Caltech

EM27/SUN. As opposed to the Caltech EM27/SUN (also

abbr. cn), the LANL (abbr. pl) and HU instruments (abbr.

ha and hb) used the original InGaAs detector type sensitive

over 5500–12 000 cm

−

(Frey et al., 2015). The LANL in-

strument, however, has a different high-pass filter, allowing

it to measure up to 14 500 cm

−

. This different filter is nei-

ther beneficial nor disadvantageous to this instrument as no

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J. K. Hedelius et al.: Assessment of errors in Xgas from a 0.5 cm

−

spectrometer

Table 1.

Pre-averaging and apodization effects on EM27/SUN retrievals.

% error

Md.

5 fwd/bwd pre-avgd.

< 0.01

0.01

−

0.02

0.01

0.36

0.13

< 0.01

0.15

0.30

0.12

NB med. apodz.

0.29

0.09

−

0.07

0.10

0.35

0.23

−

1.01

0.58

−

1.36

0.55

Measurement compared over 1–10 July 2014. Md denotes the median. NB denotes the medium Norton–Beer apodization.

As compared to retrievals from 1

fwd/bwd averaged non-apodized measurement averaged over same time post-retrieval.

Same apodization as standard.

Same pre-averaging as standard.

gas column amounts are retrieved in that region. The LANL

instrument was first used in January 2014 and has been com-

pared with multiple TCCON sites in the United States, in-

cluding sites at Four Corners, LANL, NASA Armstrong, La-

mont, Park Falls, and multiple Caltech comparisons (Parker

et al., 2015). The HU instruments have been operational

since May 2014 and were compared against each other at

Harvard before traveling over 4100 km to Caltech. As noted

by Gisi et al. (2012) and Chen et al. (2016), the ILS of these

instruments is remarkably stable considering the long dis-

tances they traveled.

3 Retrieval software

SFIT (Pougatchev et al., 1995), PROFFIT (“PROFile fit”,

Hase et al., 2004), and GGG (Wunch et al., 2015) are

the three widely used retrieval algorithms to fit direct so-

lar spectra and obtain column abundances of atmospheric

gases. PROFFIT is maintained by the Karlsruhe Institute

of Technology (KIT) and has been used to obtain DMFs

from EM27/SUN instruments as well as NDACC-IRWG

sites (Gisi et al., 2012; Frey et al., 2015; Hase et al., 2015).

GGG is maintained by the Jet Propulsion Laboratory (JPL)

and has been used to obtain DMFs from other low-resolution

instrument measurements (e.g., an IFS 66, see Petri et al.,

2012), in addition to being used to retrieve DMFs from the

MkIV spectrometer in balloon-borne measurements (Toon,

1991) and for the Atmospheric Trace Molecule Spectroscopy

Experiment (ATMOS) flown on the space shuttle (Irion et al.,

2002). GGG is the retrieval algorithm used by the TCCON

(Wunch et al., 2011b). We chose to use GGG for our anal-

ysis because (1) we want to be consistent with the TCCON

for comparison and (2) the GGG software suite containing

GFIT is open-source allowing us to adapt routines if needed.

We used the GGG2014 version for retrievals (Wunch et al.,

2015).

All retrievals used the same pTz and H

O modeled pro-

files as well as the same a priori profiles (Wunch et al.,

2015). We also used the same meteorological surface data

for retrievals from all five instruments. All retrievals also

used the same 0.2 hPa surface pressure offset. This offset

was determined by comparing measurements from the stan-

dard barometer with a calibrated Paroscientific Inc. 765–16B

Barometric Pressure Standard that has a stated accuracy of

better than 0.1 hPa.

3.1 Interferogram-to-spectrum – double-sided

TCCON uses an interferogram-to-spectrum subroutine part

of GGG to perform fast Fourier transforms (FFTs) to create

spectra from ifgs (Wunch et al., 2015). Though the Bruker

™

OPUS software used to operate the spectrometer can also

perform FFTs, we again chose to use GGG to maintain con-

sistency. A developmental version of GGG was used, which

was adapted to also allow FFT processing on EM27/SUN

interferograms. GGG splits a raw forward/backward ifg into

two different double-sided ifgs which are then FFTed to yield

two spectra. GGG also corrects source brightness fluctua-

tions (Keppel-Aleks et al., 2007).

3.2 EM27/SUN GGG and interferogram processing

suite (EGI)

To make GGG retrievals simpler for new EM27/SUN users,

an add-in software suite (EGI) was developed at Caltech

to create correctly formatted input files. This suite is open-

source and can be obtained through correspondence to the

email address listed. EGI can be run using MATLAB or

Python. EGI runs in UNIX, Mac OS, and Linux environ-

ments and runs GGG on multiple processors. EGI central-

izes settings for paths to read and write files, it coordinates

separately acquired ground weather station and GPS data

with EM27/SUN ifgs, and it optimizes processing order. It

also provides some ancillary calculations such as a spectral

signal-to-noise ratio (SNR) calculation. EGI provides a sim-

ple way to turn on and off saving of ancillary retrieval files

(i.e., spectral fits and averaging kernels). EGI can run for in-

struments employing one or two detectors, such as the type

described by Hase et al. (2016). Like the GGG software suite,

EGI also includes benchmark spectra acquired under differ-

ent conditions to run simple tests on. EGI is automated, re-

ducing the learning time as well as the amount of user time

needed to retrieve DMFs. After an initial setup, EGI will run

from ifgs to retrieved X

gas

with two commands. On a com-

puter with 1400 MHz processors the code takes

∼

30 s per

CPU to process each interferogram from the EM27/SUN ex-

tended InGaAs detector.

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J. K. Hedelius et al.: Assessment of errors in Xgas from a 0.5 cm

−

spectrometer

3531

Table 2.

ILS of EM27/SUN instruments.

Instrument

num – ID

9 January 2015

ME, PE (mrad)

28 January 2015

ME, PE (mrad)

Caltech (42 – cn)

0.986,

4.88

0.979

3.58

LANL (34 – pl)

0.999,

−

1.34

Harvard 1 (45 – ha)

0.973

−

1.99

Harvard 2 (46 – hb)

0.991

4.18

0.991,

4.00

Missing values indicate ILS not characterized on that day.

Phase error values are

italicized.

After realigning this instrument the ME was as high as 0.994.

As reported by Chen et al. (2016).

4 Instrument characterizations and performance

4.1 Instrument line shape

Knowledge of the instrument line shape (ILS), or the ob-

served shape of a spectral line from a monochromatic input,

is crucial in assessing instrument performance and avoiding

unknown biases in retrievals. Two parameters in the the LIN-

EFIT algorithm (Hase et al., 1999) are used to characterize

the ILS in relation to an ideal instrument, namely the modu-

lation efficiency (ME) and phase error (PE). ME and PE both

describe the interferogram and vary with OPD (Hase et al.,

1999; Frey et al., 2015). PE is the angle between the real and

imaginary parts of the FT of the ILS (Wunch et al., 2007).

PE has an ideal value of 0 radians, and indicates the degree

of asymmetry in spectral lines. ME is a measure of the nor-

malized observed interferogram signal compared with that

of a nominal instrument with an ideal value of 1 (unitless)

(Hase, 2012). At maximum OPD (MOPD), an ME < 1 causes

a broadening of the measured spectral lines, while an ME > 1

at MOPD causes a narrowing. The ILS can be calculated by

analyzing absorption lines measured through a low-pressure

gas cell, and varies with OPD (Hase et al., 1999). Here, we

use only single ME and PE values at the MOPD (Frey et

al., 2015) to describe the ILS. We characterized the ILS for

the EM27/SUN instruments using the method described else-

where (Frey et al., 2015; Klappenbach et al., 2015). This

method is able to characterize ME to within 0.15 % using

the LINEFIT algorithm (Hase et al., 1999), with supplemen-

tal MATLAB scripts for automation purposes (Chen et al.,

2016). ILS can affect retrieved column values. We note that

the ME at MOPD of the cn and ha instruments in Table 2 are

significantly lower than those reported by KIT on campus of

∼

0.997 (Frey et al., 2015), and post-campaign of

∼

0.996

(Klappenbach et al., 2015).

For this study, the ILS is used to help explain biases, to

demonstrate the stability of the instruments, and gives insight

into how well the EM27/SUN instruments are aligned and

their optical aberrations. Though GGG2014 retrievals do not

account for non-ideal ILS, future versions of GGG will. For

the current study, we assume that ILS impacts using PROF-

FIT will be similar to impacts using GGG. This assumption

Ambient temperature °C

-120

-100

-80

-60

-40

-20

6220 cm

-1

FS (ppm)

CIT EM27

LANL EM27

HU EM27 (1)

HU EM27 (2)

Figure 2.

Frequency shifts (FS) of all four instruments vary with

temperature because the lasers are not frequency-stabilized. FS for

the CO

6220 cm

−

window are shown. FS of the Caltech (CIT)

instrument are far from zero, so an empirical correction is made to

correct the sample spacing number. Only every 300th CIT point and

every 20th LANL point is plotted for clarity. HU EM27 1 and 2 are

also referred to as ha and hb respectively by Chen et al. (2016).

will need to be tested when GGG also can account for a non-

ideal ILS. Because future GGG retrievals will be revised us-

ing historical ILS measurements, a need remains to monitor

the ILS both for future retrievals and as an indicator if re-

alignment is necessary.

4.2 Frequency shifts

EM27/SUN units contain a HeNe 633 nm (15 798 cm

−

)

metrology laser to sample the IR signal accurately as a func-

tion of the OPD. The laser is not frequency-stabilized (Gisi

et al., 2012). This causes apparent spectral frequency to

change with temperature as is shown in Fig. 2. Frequency

shifts are affected by changes in the input laser wavenum-

ber, laser alignment, and IR beam alignment. The input laser

wavenumber will affect the spacing between spectral points.

Since the frequency shift is furthest from zero for the Caltech

EM27/SUN (on order of

−

100 ppm, in red Fig. 2), the spec-

tral spacing is empirically corrected in the EGI suite based on

the CO

6220 cm

−

window frequency shifts. This made lit-

tle difference for the primary gases of interest affecting X

by 0.015 % and X

−

0.005 %, though it did affect X

by 4 %.

4.3 Ghosts

Ghosts are artificial spectral features linked to the aliasing

of true spectral lines that arise in FTS spectra (Learner et

al., 1996). The InGaAs detectors are optically sensitive at

wavenumbers greater than half the HeNe metrology laser

frequency (7899 cm

−

)

. To fulfill the Nyquist criterion and

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J. K. Hedelius et al.: Assessment of errors in Xgas from a 0.5 cm

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spectrometer

prevent aliasing, the IR interferogram is sampled twice each

laser interferogram cycle, on the rising and falling edge.

However, if the laser sampling is asymmetric – for exam-

ple from a faulty electronics board – aliasing can still occur,

folded across the half laser frequency (Messerschmidt et al.,

2010). Because the asymmetry is typically small, the aliased

signal, or ghost spectrum, is small compared with the true

spectrum (Dohe et al., 2013; Wunch et al., 2015).

In EM27/SUN instruments the laser sampling error (LSE)

can be minimized as data are collected by employing the in-

terpolated sampling option provided by Bruker

™

. This re-

sampling mode uses only the rising edge of the laser interfer-

ogram and assumes constant velocity in between the rising

edges to interpolate the sampling (Gisi, 2014). We use a nar-

row band-pass filter (3 dB band width 5820–6150 cm

−

)

the Caltech EM27/SUN to test for LSE ghosts at 9800 cm

−

The ghost to parent ratio is 1.73

−

at a 10 kHz acqui-

sition rate without the interpolated sampling activated. This

ghost is eliminated with the interpolated sampling turned on.

In actual solar tests, turning the interpolated sampling on

and off had no noticeable effect on the DMF retrievals for

the Caltech EM27/SUN; however this may not hold true for

all instruments. The LSE ghost also disappeared at an ac-

quisition frequency of 20 kHz, and returned at higher acqui-

sition frequencies. We opted for the recommended 10 kHz

acquisition rate with the interpolated sampling on for all

EM27/SUNs in this analysis because other instruments may

be more significantly affected by LSE ghosts. A double-

frequency ghost remains at

∼

11 900 cm

−

from radiation

passing through the interferometer twice that is much larger

than the LSE ghost, but is not in a region that will affect re-

trievals.

4.4 Mirror degradation and detector linearity

Solar tracking mirrors provided with the EM27/SUN instru-

ments are gold with a protective coating. Gold is used be-

cause of its excellent reflectance in the near-IR and low re-

flectance in the visible region (Bennett and Ashley, 1965),

which allows a high signal while reducing excess heating of

the field stop and other optics. Through extended tests, we

noted the first two mirrors (gold on plated aluminum, with

a coating) degrade over time, with an e-folding degradation

time of

∼

90 days as is shown in Fig. 3. Arbitrary units (AUs)

for signal are the maximum ordinate values of the unmodi-

fied interferograms multiplied by 6450. The AUs of signal

happen to be close to the spectral SNR – a scaling factor of

1.3 applied to the arbitrary signal has an

of 0.63 relative

to the SNR. Cleaning helped restore some signal, but never to

the original values. The mirror change may not have restored

full signal because the rest of the optics were not cleaned at

the time of the mirror change. Below the blue 150 AU line in

Fig. 3 the fitted O

root mean square (rms) as a percentage of

the continuum level dropped 26 times faster with signal in-

tensity than above it. The instrument did come with an extra

07-14

10-14

01-15

04-15

Month-year

200

400

600

800

1000

ifm

#

6450 (A.U.)

A

Mirror change

100

200

300

400

500

600

Hours of sun observations

Figure 3.

Interferograms from EM27/SUN instruments are nega-

tive, with the most negative ordinate values at ZPD and saturation

occurring at

−

1. Here the interferogram maximums (ifm) refer to

the maximum (least negative) ordinate values of the raw interfer-

ograms. They were normalized so the maximum is 1000 and are

plotted with time showing the loss of signal. These values are af-

fected by clouds, which are the cause for much of the scatter. They

are also affected by SZA which explains some apparent interme-

diate increases. Only every 50th point is plotted for clarity. Mirror

cleaning (thin black lines) helped restore some signal, but never to

original values. The 150 AU line is in blue.

set of mirrors, but because mirrors are consumable parts, it

adds recurring cost and effort to maintain these instruments

long-term. After 1 year of use, the third mirror (gold coated

glass) still remains completely intact. Feist et al. (2016) had

success using steel mirrors under the very harsh conditions at

the Ascension Island TCCON site, though at a cost of 35 %

reflectivity per mirror. The JPL TCCON sites near Caltech

noted no degradation on the external gold mirrors over more

than 1 year of measurements. The lack of degradation on the

third external mirror and the JPL TCCON mirrors is likely

due to differences in how the mirrors were manufactured, in-

cluding how the gold is applied to the substrate and the coat-

ings used. Mirror degradation has likely not been a widely re-

ported problem for most of the EM27/SUN community, per-

haps because these instruments typically are stored indoors

and only used for a few days for campaigns (for example,

Frey et al., 2015). However, this problem may affect mirrors

on other EM27/SUN instruments when mirrors are exposed

outside for extended periods of time.

With signal loss, we would anticipate that gas measure-

ments would become noisier but remain unbiased. However,

with time, the Caltech EM27/SUN X

and X

DMFs

decreased relative to the TCCON DMFs as mirror reflectance

decreased, and X

and X

increased when the mirrors

were replaced. The TCCON IFS 125HR InGaAs detectors

are already known to be sufficiently linear that no correc-

tion is required (Wunch et al., 2011b). We also performed a

simple test repeatedly adding mesh screens in front of the en-

Atmos. Meas. Tech., 9, 3527–3546, 2016