Tuesday, December 13, 2016

Abstract-Terahertz and far-infrared windows opened at Dome A in Antarctica

We directly measured the zenith atmospheric radiation at Dome A throughout the H2O pure rotation band, that is, the terahertz and far-infrared (FIR) band. This study was made possible by China’s ongoing development of Dome A as a scientific base including facilities for astronomy. Such broadband measurements have not been made before at this site and are needed because narrowband radiometry combined with modelling is insufficient for the accurate evaluation of the broadband transmission, primarily due to limited understanding of the H2O continuum absorption, especially at the low temperatures typical of Dome A. Ground-based measurements are needed because satellite measurements have inadequate sensitivity in the lower troposphere. Our results were obtained with a remotely operated Fourier transform spectrometer (FTS), which was deployed to Dome A by the 26th Chinese Antarctic Research Expedition (CHINARE) team during the 2009/2010 traverse. The FTS ran for 19 months from January 2010 until August 2011, measuring the downwelling zenith sky radiance from 0.75 THz to 15 THz with a spectral resolution of 13.8 GHz. Technical details regarding the instrument and calibration methods are given in Methods. As discussed there, to achieve sufficient signal-to-noise ratios, the spectra were averaged into 6-hour bins, and a good calibration for radiance was achieved at frequencies above 1 THz.
For the site survey statistics, we derived transmittance spectra from the measured radiance spectra using the isothermal radiative transfer approximation described in Methods. The fractional accuracy of the transmittance thus obtained corresponds to the fractional change in the absolute atmospheric temperature across the scale height of the water vapour column, which is approximately 5%. Transmittance errors associated with strong water lines terminating within the instrument or in the surface boundary layer can be recognized by their correspondence to the well-known positions of these lines. The key advantage of the isothermal transmittance approximation, in contrast to the more detailed spectroscopic study below, is that it requires no assumptions regarding the vertical structure of the atmosphere.
The transmittance statistics are presented in Fig. 1, where we show quartile transmittance spectra for the entire year (Fig. 1a,c), and for winter (April to September) only (Fig. 1b,d). To correct for uneven sampling over the calendar year, when compiling percentile statistics we applied a de-biasing procedure whereby spectra obtained in calendar periods that were sampled only once during the two years of operation were duplicated, and brief gaps in sampling were covered by interpolating between temporally adjacent spectra. Sufficiently high transmittance to support astronomical observation (~20%) is consistently observed in winter in the three windows at 1.03, 1.3 and 1.5 THz, which encompass the astrophysically important spectral lines indicated in Fig. 1. Much higher FIR transmittance (>40%) is found in a number of windows starting at 7.1 THz. Although our spectra are limited to frequencies above 1 THz, they are consistent with Dome A being an excellent year-round observing site in the submillimetre windows below 1 THz. Moreover, the extremely dry conditions that open the windows above 1 THz at Dome A occur during a much greater time fraction than at other sites where observations in these windows can be attempted  .

Figure 1: Zenith atmospheric transmittance spectra measured at Dome A, Antarctica, during 2010–2011.

ad, Spectra are shown for the entire year (a,c) and for winter (April–September) only (b,d). Quartile statistics for each frequency channel were compiled independently. Solid lines indicate the median transmittances, and the shaded regions show the interquartile ranges. To account for irregular time sampling throughout the year, data were de-biased as described in the text. Frequencies of several astrophysically important spectral lines are indicated; note that some of these lines are observable only at certain non-zero redshift values z.

Given the plans for future observatories and scientific facilities at Dome A, the exposed terahertz and FIR windows present a unique opportunity for ground-based astronomy. For example, unique terahertz spectral line transitions from atomic and molecular species such as N+ at 1.46 THz (205 μm), H2D+ at 1.37 THz (219 μm), high-J CO lines and C+ at a moderate redshift from its rest frequency of 1.90 THz (158 μm) can be observed to trace the lifecycle of stars and interstellar matter, as well as chains of chemical reactions that ultimately shape the chemical composition of planetary systems like our own. In addition, spectral lines of species such as O3+ at frequencies greater than 7 THz allow the exploration of the energy balance in the interstellar medium. As a ground-based site, Dome A could support larger facilities, either single telescopes with a large aperture or multiple telescopes phased up as interferometers, with more rapid and agile development cycles than space-based or aircraft-based platforms.
Our measurements also allow us to address an issue in atmospheric science relating to uncertainties in the absorption spectrum of water vapour. These uncertainties are associated with poorly understood collisional effects that give rise to smoothly varying continuum absorption , which has a significant impact on atmospheric radiation models . The importance of measuring the continuum absorption has motivated several recent field experiments, including the Radiative Heating in Underexplored Bands Campaigns , Earth Cooling by Water Vapor Radiation  and Continuum Absorption of Visible and IR radiation and its Atmospheric Relevance  . However, the full atmospheric temperature range has not been adequately explored. The water vapour continuum absorption includes a component that is associated with homogeneous H2O–H2O collisions (known as the self-continuum) that dominates in the warm and humid lower troposphere, and a component associated with heterogeneous collisions (known as the foreign continuum) that dominates in the cold and dry upper troposphere. The measurements we report here from Dome A provide new constraints on the H2O foreign continuum towards the low range of atmospheric temperatures. In addition, the low H2O column density over Dome A gives unprecedented access to wavelengths in the core of the H2O rotation band where previous measurements have lacked sensitivity.
We studied the water vapour absorption spectrum by combining our broadband spectra with auxiliary data on the atmospheric state over Dome A. Given the atmospheric state in the form of vertical profiles of temperature and water vapour concentration over the site, a radiative transfer model can be used to predict the observed radiance spectrum and thereby test the spectral absorption data used in the model. With the exception of the H2O foreign continuum absorption, the spectral absorption data used in the radiative transfer model are well constrained by laboratory measurements. Therefore, the residuals between the measured and modelled spectra can be interpreted in terms of an implied correction to the continuum absorption coefficient. The radiative transfer model employed the MT_CKD (v. 2.5.2) water vapour continuum  . We chose MT_CKD as the reference continuum model for this study because it has been extensively validated in laboratory and field experiments, albeit at higher temperatures than those accessed in this study, and because it is widely used in the atmospheric radiation codes incorporated into climate models.
Our starting point for estimating the atmospheric state was the NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis  interpolated over Dome A. MERRA provides an estimate of the global atmospheric state constrained by satellite, surface, and upper air measurements with a 6-hour resolution matching the temporal averaging bins of our spectra. The ranges of MERRA-derived vertical temperature and water vapour profiles during the period of this study (August 2010) are shown in Fig. 2. The modest interquartile variation observed in these profiles, together with relatively high data quality in our spectra, are the reasons we decided to focus on this period. Recent dropsonde validations  of the satellite-derived temperature profiles over Antarctica give us high confidence in the data that constrain the MERRA temperature profiles. Moreover, as expected, the lower part of the MERRA water vapour profile closely tracks the ice saturation vapour pressure profile. As indicated in Fig. 2, the temperature range across the water vapour column was small during the study period.

Figure 2: Atmospheric profiles over Dome A, Antarctica, during the August 2010 study period.

a,b, Median vertical profiles of temperature (a) and H2O volume mixing ratio (b) during August 2010, derived from the NASA MERRA reanalysis, are plotted against pressure. The temperature point at the surface, associated with the strong winter surface inversion, is from our own instrument. The H2O volume mixing ratio is relative to the total gas density. The shading around the median profiles indicates the interquartile range of variation during the month. For comparison with the MERRA water vapour profile, the red profile in b shows the saturated mixing ratio over ice that corresponds to the MERRA temperature profile and its interquartile variation. The horizontal dotted lines indicate pressures below which the median MERRA H2O profile contains 90% and 50% of the total H2O column density, indicating the small range in temperature across most of the H2O column. The column density-weighted mean H2O temperature for the study period was 218 K.

MERRA does not resolve the strong winter Antarctic surface inversion , and the absolute accuracy of the MERRA water vapour profile is uncertain. We therefore used an analysis method that involved dividing the measured spectral interval into two parts: the band-edge frequencies (f < 3.6 THz and f > 12.5 THz) that comprise spectral channels where the observed radiance is insensitive to the continuum, and the mid-band frequencies (3.6 THz < f < 12.5 THz) where the radiance in the transmission windows includes a significant continuum contribution. The band-edge channels were used with a radiative transfer model  to fit a two-parameter adjustment to the initial MERRA-derived profiles. These two parameters were a scaling factor on the MERRA water vapour profile in the troposphere and the base temperature of a surface layer modelling the surface inversion.
An example of one of these fits is shown in Fig. 3a. With the atmospheric state anchored to the band-edge channels, we found that the radiances in the mid-band windows were consistently underestimated. Using the channel-by-channel derivative of the model radiance with respect to the foreign continuum absorption coefficient, the residuals in the windows were used to derive an implied adjustment to the absorption coefficient. Figure 3b compares the MT_CKD absorption coefficient with the quartile statistics of the adjusted values found for all of the spectra included in this study, which cover a range of water vapour column densities from 70 μm to 220 μm precipitable water vapour. The consistent value found for the adjusted foreign continuum absorption coefficient across this wide range of water vapour column densities, particularly in the higher signal-to-noise windows from 5 THz to 9 THz, is strong evidence that the residuals are indeed associated with water vapour absorption as opposed to systematic calibration errors, errors in the dry air spectroscopy or absorption by other atmospheric constituents such as hydrometeors. The smooth trend in the adjusted continuum, as well as the generally good fit near the centre of unsaturated water lines, would also appear to rule out errors in the H2O line-by-line spectroscopy as the origin of the radiance residuals. The implied adjustment to the MT_CKD foreign continuum absorption coefficient at the column density-weighted mean H2O temperature of 218 K over these spectra was as high as a factor of 2.5 in the mid-band windows just below 9 THz.

Figure 3: H2O foreign continuum correction derived from spectral residuals.

a, An example of a spectral fit from the August 2010 data set. The measured radiance spectrum is shown in red, and the model spectrum computed from the scaled MERRA profiles is shown in blue. Note the significant residuals in the spectral windows within the H2O rotation band; the grey band plotted along with the residuals indicates the noise level in the measured spectrum (measured 8 August 2010, 0–6 UT)b, Quartile statistics (symbols) of the adjusted H2O foreign continuum absorption coefficient (Cf) derived from the radiance residuals for all August 2010 spectra at 218 K compared with the MT_CKD v. 2.5.2 water vapour continuum model (solid line). The channels used in this analysis were screened using the criteria described in Methods. The units of Cf are those customarily used in the literature16,25,26, and refer to the continuum absorption at a reference temperature T0 = 296 K and foreign gas partial pressure P0 = 1,013 mbar. The H2O foreign continuum contribution τf to the optical depth through a column of water vapour with a column densityNH2O(molecule cm−2) at temperature T and foreign gas pressure P isτf=CfNH2O(P/P0)(T0/T)×vtanh(hcν/2kT), where the frequency v is in wavenumber units (cm−1). RU corresponds to mW m−2 (cm−1)−1 sr−1.

By reaching new extremes of low temperature and accessing windows in the core of the H2O pure rotation band, our measurements at Dome A have provided new constraints on the spectral absorption of water vapour that are important for modelling radiative processes in the cold upper troposphere and for retrieving atmospheric properties from outbound spectral radiance measurements. Taken together with the transmittance statistics for the astronomical observing windows discussed above, our measurements demonstrate the value of this unique site to both astronomy and atmospheric science.

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