Infrared emission measurements in the Arctic using a new extended-range AERI

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Introduction
The Canadian Network for the Detection of Atmospheric Change (CANDAC) has equipped the Polar Atmospheric Environment Research Laboratory (PEARL) at Eureka, Nunavut (80 • N, 86 • W) for measurements during International Polar Year (IPY,project 196) and beyond.One of the instruments at PEARL is the Extended-range Atmospheric Emitted Radiance Interferometer (the E-AERI).This instrument measures the absolute downwelling infrared radiation spectrum for studies of the Arctic radiation budget and atmospheric composition.The E-AERI was installed in October 2008 and acquired one full year of measurements at the PEARL Ridge Lab.AERI instruments were developed at the University of Wisconsin Space Science and Engineering Centre (UW-SSEC) from 1989 to 1998 using an MR100 spectroradiometer developed from an MB-120 interferometer designed for industrial applications by ABB Bomem Inc. of Quebec, Canada (Collard et al., 1995).A suite of AERI instruments was then developed for the Atmospheric Radiation Measurement (ARM) (DOE, 1990) Climate Research Facility sites in the Southern Great Plains (Oklahoma), the North Slope of Alaska, and the Tropical Western Pacific (Stokes and Schwartz, 1994;Knuteson et al., 2004a).In addition to the downwelling radiance measurements, measurements of the atmospheric state (e.g.radiosonde profiles) are made at these sites allowing for radiance simulations (Ellingson and Wiscombe, 1996).These AERI systems have collected over a decade of data at the ARM sites and have been used to generate climatologies in downwelling infrared radiance (Turner and Gero, 2011) and to investigate long-term trends (Gero and Turner, 2011).In addition to the ARM AERIs, other AERI instruments include the Marine AERI (M-AERI) (Minnett, 2001), the University of Denver's high-resolution AERI-X deployed at the PEARL Ridge Lab from 1994-2002 (Olson et al., 1996), and the University of Idaho's Polar AERI (P-AERI) (Walden et al., 2005;Rowe et al., 2008).
The newest generation AERI (e.g. the E-AERI installed at PEARL) was developed by ABB under commercial license from UW-SSEC.These AERIs incorporate the newest Introduction

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Full generation of ABB Bomem's Fourier Transform Infrared (FTIR) spectroradiometer, a UW blackbody cavity, the latest signal conditioning units, and a new detector/Stirling cooler with an extended lifetime.The new AERI configuration allows in-field replacement of the Stirling cooler and metrology laser.The software has been updated with a combination of ABB software for data acquisition and the instrument communication/control interface and UW software for post-processing and atmospheric science functions to ease future evolution of hardware and science functions by each party.
The new AERIs also include front-end and back-end enclosures that protect against atmospheric precipitation and temperature variation, respectively.In addition to these modifications, the E-AERI extends the spectral coverage range of a standard AERI (550-3000 cm −1 ) to 400-3000 cm −1 .The thermal emitted radiance measured by the E-AERI is primarily dependent on the water vapour content and temperature of the atmosphere in cloud-free scenes.Therefore, temperature and humidity profiles of the planetary boundary layer can be retrieved from AERI and E-AERI spectra, as demonstrated in Feltz et al. (1998), Smith et al. (1999), andTurner et al. (2000).This allows for high-temporal-resolution records and analyses of temperature and water vapour changes due to mesoscale meteorological features.The sampling interval of approximately seven minutes allows the study of short-term meteorological phenomena in the lower atmosphere, e.g.inversion developments, cloud effects, and front passages.
The spectral range of the E-AERI covers the so-called "dirty window" (around 400 cm −1 , or 20 µm), where most of the infrared cooling currently occurs in the dry air of the Arctic.The importance of the far infra-red (IR) for mid-to-upper tropospheric cooling has been well demonstrated in Clough et al. (1992).Due to climate change, the water vapour content of the Arctic atmosphere is expected to increase, with a corresponding change in downwelling radiance.Such a regime shift should be visible in the E-AERI data record.In addition, the far IR is important for cloud thermodynamic phase determination, as demonstrated by Rathke et al. (2002).A previous extended-range version of the AERI system was deployed at the Surface Heat Budget of the Arctic Introduction

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Full Ocean Ice Station in 1997; this AERI was used to investigate the far infrared H 2 O continuum (Tobin, 1999) and to characterize the phase and microphysical properties of mixed phase Arctic boundary layer clouds (Turner, 2005).Retrievals of total columns of various trace gases (e.g.CH 4 , CO, CO 2 , O 3 , N 2 O) using a prototype version of the retrieval algorithm SFIT2 (Pougatchev et al., 1995;Rinsland et al., 1998) modified to analyze emission features are currently being evaluated.Recently, retrievals of CO with high sensitivity to the lower troposphere have been demonstrated for the AERI systems (Yurganov et al., 2010).In contrast to solar absorption measurements of atmospheric trace gases, which depend on sunlit clearsky conditions, the use of emission spectra allows measurements year-round (except during precipitation events or when clouds are present).This capability allows the E-AERI to provide temporal coverage throughout the four months of polar night, when the PEARL Bruker 125HR solar absorption FTIR spectrometer is not operated (Batchelor et al., 2009).
The installation of the E-AERI at PEARL was preceded by the P-AERI, which was deployed at the Zero-altitude PEARL Auxiliary Laboratory (0PAL) from March 2006 to June 2009.The two instruments are similar, but the P-AERI does not provide spectral coverage below 550 cm −1 .In addition, the E-AERI was housed in the PEARL Ridge lab at an altitude of 610 m, while the P-AERI was operated 15 km away at an altitude of 10 m.Having two AERI instruments close in proximity at these altitudes presents an opportunity for investigation of the lowest 600 m of atmosphere, potentially including differences in trace gases, aerosols, and low-level liquid and ice clouds.Other instrumentation at 0PAL that provides information for cloud studies includes the Millimeter Wave Cloud Radar (MMCR), which measures equivalent radar reflectivity, Doppler velocity, spectral width, and Doppler spectra over 2-s intervals, and UW's Arctic High Spectral Resolution Lidar (AHSRL), which measures aerosol backscatter cross section and particulate circular depolarization ratio (Eloranta, 2005).These can be used to determine cloud heights, thicknesses, composition, internal structure and vertical motions with high vertical and temporal resolution.Introduction

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Full In this paper, we present the E-AERI instrument, detailing its operation, calibration, and noise characteristics, and show that measured spectra are comparable to those of other AERI instruments.Following this is a first look at the spectra recorded by E-AERI at the PEARL Ridge Lab, including examples of clear and cloudy skies, investigation of the lower 600 m making use of the P-AERI measurements for both the full overlap period October 2008 to April 2009 and for an ice cloud case study, and comparisons to simulated radiances.A description of the E-AERI hardware, performance, and software and operational parameters are presented in Sect. 2. Section 3 describes the instrument's performance, including calibration results obtained at UW, side-by-side comparisons between the E-AERI and UW's AERI-07 and AERI-Bago (AERI-03) instruments, and side-by-side comparisons between the E-and P-AERI instruments at sea-level.Section 4 presents clear and cloudy sky E-AERI measurements, comparisons to radiance simulations from a fast line-by-line radiative transfer model, radiance residuals between the E-AERI (at 610 m) and P-AERI (at 10 m), and an analysis of the radiative impact of a low-level ice cloud at the two measurement sites.A summary of conclusions is given in Sect. 5.

Instrument hardware
The E-AERI is composed of three distinct parts: the front-end optics, the back-end electronics and interferometer, and the computer (standard off-the-shelf laptop).The front-end optics consists of two blackbodies and the scene mirror.The scene mirror has a gold reflecting surface and is mounted at 45 • to the motor rotation axis, which is in turn positioned coincident with the interferometer input optical axis.This configuration allows different views: nadir, zenith, Ambient Blackbody (AB) and Hot Blackbody (HB).The E-AERI measures radiance emitted by the atmosphere using an MR-300 series Fourier Transform Spectrometer (FTS) calibrated with the two blackbodies.The Introduction

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Full maximum optical path difference is 1 cm, providing a spectral resolution of 1 cm −1 .
Two IR detecting channels are supported by an extended-range photoconductive mercury cadmium telluride (MCT) detector coupled with a photovoltaic indium antimonide (InSb) detector mounted in a sandwich configuration.The detectors are housed in a dewar and cooled below 70 K by a linear Stirling-cycle cryo-cooler.The back-end components' temperatures are kept stable through a temperature control unit attached to the back-end.Electronic modules are also mounted in the back-end: the signal conditioning electronics, blackbody temperature controller, and Stirling cooler support electronics.
The instrument is designed to be configured as stand-alone or mounted thru-wall.
A protective enclosure protects the instrument against sun, rain, snow, wind, sand, etc.Currently, the instrument is removed from its mobile base for thru-wall installation and is fixed on a mounting platform.The photographs in Fig. 1 were taken after the E-AERI was installed using the thru-wall configuration in a penthouse on the roof of the PEARL Ridge Lab.The back-end, where the interferometer is located, remain inside the penthouse at room temperature.The performance characteristics of the E-AERI are summarized in Table 1.Items with a * indicate performance metrics verified during calibration at UW-SSEC.

Instrument scan sequence
One measurement cycle includes a zenith-sky measurement as well as calibration measurements of the blackbodies to ensure accurately calibrated zenith-sky spectra.
Measurements are independent of sunlight or moonlight and are only interrupted during precipitation events to prevent damage of the optics.The E-AERI is designed to operate on two different repeating scene-mirror schedules in which the downwelling infrared radiation is bracketed by views of the reference blackbodies.Rapid sampling mode is used in order to follow very short-term changes due to variable cloud-cover.
In this mode, the E-AERI scene mirror pivots such that the instrument looks at the Introduction

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Full HB, AB, and then Sky View (SV) in the following order: HB AB SV SV SV SV SV SV SV SV AB HB.This is useful for quick measurements of the sky to detect changes in frontal passages.The E-AERI is usually operated in slow sampling mode, following the pattern: HB AB SV AB HB.This method sandwiches each SV measurement between blackbody measurements, improving the calibration of the spectra.The instrument performs two Michelson mirror sweeps (forward and backward) for each spectrum.Coadding spectra for the blackbodies takes about two minutes, and for the sky view it takes about three minutes, such that 127 co-adds are performed for each blackbody measurement and 248 co-adds are performed for each SV measurement.

Instrument enclosure
A temperature control unit maintains of the back-end enclosure at a constant temperature, this contains all electronics and the FTS, and can operate between −30 • C and +40 • C. The front-end enclosure protects against precipitation (both rain and snow) but is not stabilized in temperature; it is free running near the ambient temperature.It is detachable to enable it to be sealed to the external side of a wall, as is done at PEARL.
A hatch, actuated by precipitation sensors or directly by the user or instrument commanding script, can be closed to protect the optical head, or opened to look at the atmosphere.Two Vaisala DRD11A precipitation sensors (one elevated and one flush with the enclosure) detect any precipitation occurring.In the presence of precipitation, the system will automatically close the hatch and turn itself off in a self-protection mode that does not allow sky measurement.After the precipitation ends, the system returns to normal acquisition mode.Rearranging the placement of the sensors may be required since they failed to detect falling snow several times during the first year of measurements.Temperature, pressure, relative humidity, and sun sensors are also monitored in the enclosure to obtain knowledge of the measurement conditions.Introduction

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Blackbodies
The two calibration blackbodies are of identical construction, each consisting of a thermally isolated cavity which is painted with a high-emissivity diffuse black paint.Each cavity contains a heater, a feedback thermistor and three thermistor sensors.In principle, each blackbody's temperature can be set using the instrument's software.For operation at PEARL, the HB is set at 310 K and the AB operates at the outside temperature.
The E-AERI calibration methodology uses a pair of HB/AB views bracketing the SV measurement.The blackbody temperatures are fit to a linear function of time to account for changes in the instrument temperature during a calibration sequence.Similarly, a linear interpolation to the SV time is performed for each HB and AB measurement.The approach used for radiometric calibration of the E-AERI system is based on that of Revercomb et al. (1988).

Data products
The standard E-AERI data product consists of calibrated downwelling radiance spectra from the two detectors: 400-1800 and 1800-3000 cm −1 .Combined, these form a continuous spectrum from 400-3000 cm −1 .Two data description files and one summary file contain measurement time, instrument parameter metrics, mean radiance, imaginary radiance, corrective offset, responsivity, brightness temperature (sometimes referred to as radiance temperature), blackbody temperature drift, and scene Noise Equivalent Spectral Radiance (NESR) from each detector for each seven-minute sampling interval.The radiance data are automatically processed using instrument-related corrections such as those described in Sect.2.4 as well as in Knuteson et al. (2004b), resulting in calibrated radiances with a standardized spectral scale in real time.Introduction

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System software architecture
The E-AERI software system performs all necessary data calibration in real-time.The software also performs a series of quality control checks in real-time and allows unattended operation of the system.The data is accessible via a digital network with an on-board storage capacity of several days.The monitoring Graphical User Interface (GUI) supports interactive use of the instrument and monitors the state of the instrument.Calibrated spectra and some auxiliary parameters are displayed at the end of each scan.Within each window of the monitoring GUI, the radiance, air temperature, Stirling cooler current, and instrument temperature are displayed.Thirty crucial instrument parameters are also monitored using the GUI.Table 2 lists the variables that are used as real-time quality metrics for the E-AERI.If one of these parameters falls outside of a tolerable range, the operators are alerted.
3 Instrument performance

Performance evaluations
Measurements of the NESR were performed in Quebec City (before shipping to Eureka) and again at Eureka.The root-mean-square of the variation in the calibrated radiances from 284 spectra of the HB determines the NESR.NESR measurements taken at Eureka are shown in Fig. 2. The observed increase in noise at 667 cm −1 and from 2300-2400 cm −1 is due to CO 2 absorption inside the instrument, which reduces responsivity.For both the long wavelength (LW; MCT) and short wavelength (SW; InSb) bands, the NESR is well below the specified requirement; about one-half and one-third of the specification for the LW and SW, respectively.One exception is in the region around 400 cm −1 ; due to the higher NESR at this edge of the LW band, a new detector will be installed in fall 2011.Introduction

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Full The E-AERI was sent to UW-SSEC for calibration and performance evaluation in September 2008.A four-body test was performed using the AB, HB (set to 330 K for these tests), an intermediate blackbody (temperature between AB and HB, around 318 K), and an Ice-Blackbody (IB) at 273 K.The intermediate body and IB are used as external reference sources to verify blackbody temperature knowledge.These external reference sources are calibrated using the same National Institute of Standards and Technology traceable approach as used for the other blackbodies.During the test period, the scene mirror views each blackbody to measure the blackbody temperatures over several hours to ensure stability and reduce the noise level.
Calibration verification test results are shown in Fig. 3 for the intermediate body and IB.Water vapour has strong absorption between 1450 and 1800 cm −1 and CO 2 has strong absorption around 667 and 2380 cm −1 , resulting in the observed discrepancies in these regions.The IB measurement has an offset of 0.2 K; this is comparable to the error of ±0.2 K for other AERI systems (Knuteson et al., 2004a).This error can be due to incorrect placement of the ice-body (too far away or not exactly centred).
A side-by-side comparison with two UW AERI systems, the AERI-07 (with an extended-range detector) and the AERI-Bago (AERI-03), was performed.These instruments have been employed in numerous measurement campaigns and are considered reliable benchmarks for measurement inter-comparison purposes (Knuteson et al., 2004a).Results from these comparisons indicate that agreement between the E-AERI and the other two instruments is comparable to the agreement between the AERI-07 and AERI-Bago, as shown in Fig. 4, verifying the accuracy of the E-AERI radiances (UW-SSEC, 2008).
Three field-of-view tests were performed on the zenith-sky, HB, and AB views and confirmed proper optical alignment of the source and reference blackbody cavities.
Wavenumber calibration was performed by finding an effective laser wavenumber value for the HeNe laser that provides the best spectral match for regions of regular spectral lines.The wavenumber knowledge was found to be 0.0018 cm −1 , which is better than the 0.01 cm −1 requirement.A spectral line stability test was performed by tracking any Introduction

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Full movement in the 1918 cm −1 water line during the ambient blackbody measurements for twenty hours.The wavenumber stability also meets the requirement of <10 ppm.

Side-by-side comparisons with the P-AERI at sea-level
Prior to its installation at the PEARL Ridge Lab, the E-AERI was operated side-by-side with the P-AERI at 0PAL for several days.The P-AERI has been extensively characterized (Walden et al., 2005;Rowe et al., 2008) and the data at Eureka have been quality controlled (Rowe et al., 2011a,b).With exception of the E-AERI's extended range (400-550 cm −1 ), the P-AERI has similar specifications to the E-AERI and a measurement period that overlaps the E-AERI's, providing an opportunity for comparing the two instruments and evaluating the E-AERI spectra.Coincident E-AERI and P-AERI radiances recorded on 20 October 2008 are shown in Fig. 5 for the P-AERI spectral range of 500 to 3000 cm −1 .The radiances agree within ±1 mW/(m 2 sr cm −1 ) for the MCT detector and ±0.2 mW/(m 2 sr cm −1 ) for the InSb detector with the exception of the spectral regions: 1450-1800 cm −1 and 2300-2400 cm −1 .Discrepancies between 1450 and 1800 cm −1 are due to humidity inside the E-AERI.After ten days in the dry Arctic air, the water vapour inside the E-AERI evaporated and such discrepancies ceased to exist.Discrepancies between 2300 and 2400 cm −1 are due to CO 2 absorption in the instrument, which reduces responsivity.These results are comparable to the AERI-07 and AERI-03 comparisons.

Impact of clouds on the radiation budget
The ability of AERI systems to detect the presence of clouds and provide information about cloud properties has been well demonstrated (e.g.Collard et al., 1995;DeSlover et al., 1999;Shaw et al., 2005;Turner, 2005,).Figure 6a shows E-AERI measurements Introduction

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Full The MMCR indicates that on 16 April, early-morning clear skies were followed by a thick cloud layer above Eureka for the remainder of the afternoon and evening (Fig. 7).The top panel of Fig. 7 shows the measured equivalent radar reflectivity (dBZ), the middle panel shows the measured Doppler velocity (m s −1 ), and the bottom panel shows the measured spectral width (m s −1 ).This figure shows slowly descending cirrus clouds.
The cloud is mostly ice until around 09:00 UTC; then the spectral width increases (as indicated by the appearance of green shading), which may be due to the presence of liquid water.The E-AERI radiance increases after 04:00 UTC in the 400-600 cm −1 and 750-1400 cm −1 regions due to emission by cloud particles, correlating with the MMCR's detection of a low-altitude, relatively thick (∼2 km) cirrus cloud that first appeared above Eureka at 04:00 UTC. Figure 6b displays the brightness temperature measured by the E-AERI throughout the day.Periods of increased brightness temperature correlate with increased cloud cover above Eureka.The averaged radiance over 750-1200 cm −1 increases from 4.4 to 21.6 mW/(m 2 sr cm −1 ) and the brightness temperature increases 44 %.Measurements made by an AERI instrument at the Southern Great Plains Cloud and Radiation Testbed show considerably smaller increases (typically an increase from ∼10 to ∼30 mW/(m 2 sr cm −1 ) averaged over 750-1200 cm −1 ) in the presence of similarly thick clouds (Turner et al., 2000), while previous measurements in the high Arctic taken at the Surface Heat Budget of the Arctic Ocean (SHEBA) show increases similar or larger to those made by the E-AERI in Eureka (typically >40 % increase in brightness temperature averaged over 750-1200 cm −1 ) (Turner, 2005).Thus the impact of clouds on the radiation budget is greater in the Arctic than in other more humid regions due to the extremely cold and dry Arctic air and hence to the main atmospheric window being more transparent.This coincides with model results suggesting that the impact of clouds on the radiative budget is most pronounced for the Arctic (Vavrus, 2004).Such large increases in radiance and brightness temperatures in these spectral regions provide a proxy for cloud detection and enable analysis of cloud optical depth, phase, Introduction

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Full and particle size (Turner, 2005).Cloud detection is crucial in the post-processing of E-AERI data since temperature and trace gas retrievals using SFIT2 are significantly more difficult in cloudy scenes.hibit temperature inversions (due to the extremely cold surface) for the first two kilometers above the surface; thus temperatures at the Ridge Lab are typically ∼20 K warmer than at 0PAL depending on the time of year.This is reflected in the 600-800 cm −1 CO 2 band (which is where the largest differences in Fig. 8 occur), corresponding to warmer temperatures at the Ridge Lab and a smaller CO 2 column (due to the 600 m elevation difference) resulting in greater E-AERI radiances.Other periods with large differences between the instruments correspond to the occurrence of ice crystals, high aerosol concentrations, fog or low cloud cover.For instance, the large radiance differences (red bands) in February occur on days for which low-level clouds were present above Eureka.For the low-level clouds that exist below 610 m, only the P-AERI can measure the increased emission from the cloud particles; hence the radiance difference increases for such days.

Comparisons with simulated radiances
Clear-sky comparisons between the E-AERI and P-AERI measurements at 00:08 UTC on 4 April 2009, are shown in Fig. 9a, with the residuals (P-AERI-E-AERI) in Fig. 9d.

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Full correspond to differences >14 % and >50 %, respectively.Such differences are expected due to the large temperature difference at the two sites and the absorption by CO 2 , emphasizing the dependence of the radiative budget on the atmospheric temperature profile.Positive residuals typically occur in more transparent spectral regions where CO 2 absorption does not exist; they are positive due to the additional 600 m of atmospheric emission measured by the P-AERI.The measured radiance differences provide insight into the spectral regions that experience increasing or decreasing downwelling radiance at different altitudes.
The 4 April 2009 radiances were simulated using a Fast Line-By-Line Radiative Transfer Model (FLBLRTM) for an altitude of 10 m (P-AERI) and 610 m (E-AERI).The FLBLRTM is a faster version of a standard Line-By-Line Radiative Transfer Model (LBLRTM), as developed in Clough et al. (2005).This is achieved by replacing the computationally expensive section of the LBLRTM code that calculates absorption coefficients directly from a spectral database with absorption coefficient lookup tables set on a wavenumber, pressure, and temperature grid.Although the absorption coefficient has an absorber amount dependency, it is generally insignificant for atmospheric cases (Turner, 1995).Water vapour is an exception and is treated differently in that two tables are required to represent it instead of one as with other absorbers.In addition to speeding up the calculation, the use of tables also reduces the amount of code and hence enables the code to be relatively easy to tailor to a specific problem.
For this work, absorption coefficient tables for H 2 O, CO 2 , O 3 , N 2 O, CO, CH 4 , O 2 and N 2 were created for the spectral region from 400 to 3000 cm −1 on a grid interval of 0.005 cm −1 .The spectral line data are taken from HITRAN 2008(Rothman et al., 2009).The H 2 O continuum is accounted for by CKD2.4 continuum model (Clough et al., 1989) and in all cases the Voigt line shape is assumed.
The radiances are computed for clear skies with no aerosols.Temperature, pressure, and relative humidity data from Vaisala radiosondes launched near 0PAL were interpolated to 00:08 UTC on 4 April and included in the simulation.Results from the model simulations are shown in Fig. 9b and agree with E-and P-AERI radiances within Introduction

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Full ±20 mW/(m 2 sr cm −1 ).The simulated E-AERI spectrum in the 1600-1800 cm −1 region is significantly smoother than the measured E-AERI spectrum.Emission and absorption features in this spectral region become quickly saturated as altitude increases.Analysis of the simulation indicates that there is large simulated radiance sensitivity for the lower 3 km to changes in temperature and, to a lesser extent, H 2 O.
Radiosonde H 2 O profiles have lower precision (10-20 %) at the low humidities encountered in the Arctic, causing large errors in the radiosonde profile (Schneider et al., 2010).Hence errors in the radiosonde temperature and H 2 O profiles used in the simulation likely caused the observed discrepancy.Differences between the measured radiances (Fig. 9a) and simulated radiances (Fig. 9b) are shown in Fig. 9c and have good agreement with the exception of the wings of the 600-800 cm −1 CO 2 band.Differences between the measured P-AERI-E-AERI radiance differences (Fig. 9d) and the corresponding simulated radiance differences (Fig. 9e) are shown in Fig. 9f and indicate agreement for the majority of the spectrum with slightly larger differences around the 600-800 cm −1 CO 2 band, further indicating small errors in the radiosonde temperature profile.The differences between the simulated and measured spectra indicate the model's reliance on accurate radiosonde temperature profiles in order to accurately describe Arctic radiances.Large errors in the 400-600 cm −1 (>29 %) region also indicate the model's reliance on accurate relative humidity profiles.

Radiative impact of an ice crystal cloud within the first 610 m
A low-level ice crystal cloud was present on the last measurement overlap day shown in Fig. 8 (5 April 2009), as reported throughout the day by a meteorological technician at the Eureka weather station.A 10-min interval (20:30-20:40 UTC) was selected from this day since the ice crystals were concentrated below 610 m at this time.This provides a unique opportunity to assess the impact of ice crystals on the radiative budget at two altitudes; one above the ice crystals (E-AERI) and one below (P-AERI).
Figure 10 shows backscatter cross-section and depolarization ratios measured by the

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Full AHSRL, which was deployed at Eureka in August 2005 and was operated throughout the period of interest.The circular particulate depolarization ratio was >20 % below ∼700 m (indicating ice crystals), and the backscatter cross-section above the ice crystal layer was <10 −6 m −1 sr −1 (indicating that liquid water clouds or precipitation were not associated with the ice crystal event).Both of these conditions pass the criteria used to screen for ice crystals in Lesins et al. (2009).
Ice crystal radiances were compared to clear-sky radiances measured the day before at 00:08-00:18 UTC on 4 April 2009, as shown in Fig. 11.The temperature difference between the PEARL Ridgle Lab and 0PAL during the time of measurement was 2 K, accounting for small differences in the 600-800 cm −1 CO 2 band (radiation emitted by ice crystals on 5 April also contribute to differences in this region, but to a lesser degree).Radiance measurements made during these two time intervals were averaged, and both instruments' differences (ice-clear) are shown.The P-AERI measured the largest increases in radiance since it was located below the ice crystal cloud and received emission from the ice particles.For instance, the largest ice-clear sky difference increase was 104 % for the P-AERI and only 6 % for the E-AERI averaged over 750-1000 cm −1 , indicating the large difference in downwelling radiance at these two altitudes.
Longwave downwelling radiances were converted to longwave downwelling all-sky irradiance using the method of Cox et al. (2011).The longwave downwelling thermal emission from the cloud was obtained by differencing the cloudy-sky and clear-sky irradiances; the clear-sky irradiance was obtained from an LBLRTM calculation using vertical profiles that were interpolated between the two radiosondes that bracket the time of the P-AERI measurement.This is important to avoid inclusion of irradiance contributions in spectral regions associated with the temperature structure and not the cloud since the irradiance calculation integrates over the whole spectrum.A 6 % ±1 % increase in downward irradiance from the ice cloud was measured by the P-AERI.The E-AERI measured a negligible increase, consistent with its location near the top of the ice cloud.A 6 % irradiance increase for this thin cloud (<600 m thick) is in Introduction

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Full agreement with the measured surface forcing of up to 36 % of the downward irradiance from thicker ice clouds (>2 km) found in Lesins et al. (2009).Given the frequency of ice crystal events at Eureka, this can have important consequences for the surface energy balance in the Arctic region.

Conclusions
A new extended-range AERI has been installed in the Canadian high Arctic.The E-AERI was calibrated and passed certification testing at the UW-SSEC in September 2008.Clear-sky inter-comparison tests between the E-AERI and UW's AERI-07 and AERI-Bago indicate comparable agreement between all three instruments.The performance of the E-AERI instrument meets or exceeds all initial design specifications for 1.0 cm −1 resolution infrared spectral observations with high absolute accuracy, with the exception of higher NESR values around 400 cm −1 .The E-AERI was installed at PEARL in October 2008.Side-by-side comparisons with the P-AERI in cloud-free conditions indicate agreement that is comparable to results from the UW-SSEC tests over most of the E-AERI's spectral range.Infrared radiance spectra of the sky and two calibrated blackbodies have been collected continuously approximately every seven minutes year-round, including both clear-sky and cloudy conditions.A FLBLRTM was used to simulate clear-sky radiances at two altitudes and agrees with E-and P-AERI radiances for the majority of the spectrum.The largest differences between measured and simulated radiances occur in spectral regions that are strongly influenced by atmospheric temperature and/or water vapour, illustrating the model's dependence on accurate meteorological data from radiosondes.The difference between the downwelling radiance at two altitudes is shown to be highly variable during the measurement overlap period for different spectral regions due to meteorological changes, such as cloud cover and temperature.AHSRL's detection of clouds above Eureka.This increase is larger for the Arctic than in other more humid regions, indicating that cloud cover plays a crucial role in the Arctic's radiative budget.A 6 % increase in irradiance from a thin (<600 m thick) ice cloud for the P-AERI (below the cloud) and a negligible increase for the E-AERI (above the cloud) were found.Future detailed investigation of periods of large differences in the P-AERI-E-AERI radiances is planned to further investigate the impact of different meteorological events on the radiative budget in the high Arctic.
The implementation of a new emission version of the SFIT2 retrieval code is currently underway so that measurements of trace gases above Eureka can be performed using E-AERI spectra.Obtaining total columns of various trace gases (e.g.O 3 , CO, CH 4 , N 2 O) using SFIT2 will provide insight into the atmospheric composition above Eureka throughout polar night.The E-AERI observations will provide a climatology of the infrared radiation budget and total columns of trace gases.For a comprehensive investigation of the radiative exchange in the Arctic, the E-AERI is supported by the AHSRL and MMCR, as well as an aerosol photometer suite and Baseline Surface Radiation Network (BSRN) instrumentation, all located at PEARL.Introduction

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− 1 *Fig. 1 .
Fig. 1.Exterior (left panel, red arrow) and interior (right panel) views of the E-AERI installed on the roof of the PEARL Ridge Lab using the thru-wall configuration.Left panel: the hatch is closed during precipitation events (Photo: Stephane Lantagne).Right panel: the back-end of the E-AERI instrument with its internal protective enclosure removed, showing the MR-300 series interferometer housing and electronics (Photo: Zen Mariani).

Figure 1 :
Figure 1: Exterior [Left, red arrow] and interior [Right] views of the E-AERI installed on the roof of the PEARL Ridge Lab using the thru-wall configuration.[Left]: The hatch is closed during precipitation events (Photo: Stephane Lantagne).[Right]: The back-end of the E-AERI instrument with its internal protective enclosure removed, showing the MR-300 series interferometer housing and electronics (Photo: Zen Mariani).

Figure 2 :
Figure 2: NESR test for the MCT [Left] and InSb [Right] detectors performed in Eureka.The solid black line indicates the specified requirement for the NESR test.The elevated black line indicates the spectral region where the NESR is expected to increase due to CO 2 in the instrument.

Fig. 2 .Fig. 5 .Figure 5 :
Fig. 2. NESR test for the MCT (Left panel) and InSb (right panel) detectors performed in Eureka.The solid black line indicates the specified requirement for the NESR test.The elevated black line indicates the spectral region where the NESR is expected to increase due to CO 2 in the instrument.