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Abstract
Lab-evaluation of Nitrogen Dioxide Spectral Analysis of the
Environmental trace gases Monitoring Instrument
Chengxin Zhang1, Cheng Liu1, Yang Wang2, Wenjing Su1, Thomas Wagner2, Steffen Beirle2, Ka Lok Chan3
1) University of Science and Technology of China, Hefei, China
2) Max Planck Institute for Chemistry, Mainz, Germany
3) Ludwig-Maximilians-Universität München, Germany
contact: zcx2011@mail.ustc.edu.cn
Selected reference
Zhao, M., Si, F., Liu, C., Lu, Y., Wang, Y., Wang, S., Zeng, Y., Jiang, Y., Zhou, H. and Liu, W., 2015. Effect of AO/UV/RD exposure on spaceborne
diffusers: a comparative experiment. Applied optics, 54(31), pp.9157-9166.
Platt, U. and Stutz, J., 2008. Differential Optical Absorption Spectroscopy.
Dobber, M.R., Dirksen, R.J., Levelt, P.F., van den Oord, G.H., Voors, R.H., Kleipool, Q., Jaross, G., Kowalewski, M., Hilsenrath, E., Leppelmeier, G.W.
and de Vries, J., 2006. Ozone monitoring instrument calibration. IEEE Transactions on Geoscience and remote Sensing, 44(5), pp.1209-1238.
Levelt, P.F., van den Oord, G.H., Dobber, M.R., Malkki, A., Visser, H., de Vries, J., Stammes, P., Lundell, J.O. and Saari, H., 2006. The ozone
monitoring instrument. IEEE Transactions on geoscience and remote sensing, 44(5), pp.1093-1101.
The Environmental trace gases Monitoring Instrument (EMI) onboard Chinese high-resolution
remote sensing satellite GaoFen-5 is an UV-Vis imaging spectrometer, built by the Anhui
Institute of Optics and Fine Mechanics (AIOFM), and planed to be launched in autumn of 2017.
The EMI is tasked with quantitatively measuring global distribution of tropospheric and
stratospheric trace gases such as NO2, O3, and SO2. The prelaunch calibration phase is
essential to acquire necessary knowledge on the properties and performance of the EMI
instrument as well as support data processing and retrieval. This work highlights nitrogen
dioxide retrieval from on-ground measurements of gas absorption cell from scattered sunlight,
and an evaluation of the performance of the EMI instrument from the retrieval.
In this study, we shows that NO2 retrieval from scattered sunlight measurements for adjacent
view angles from UV and Vis wavelength range are quite consistent with calculated column
density in the gas cell. Furthermore, the viewing-angle-dependent NO2 retrieval variability are
analyzed and discussed for EMI, due to the well-known problems such as “stripes-pattern” and
“row anomaly” shown in OMI instrument. Finally, the in-orbit signal-to-noise ratios of EMI are
estimated on the basis of on-ground scattered sunlight measurements and radiative
simulations, and EMI in-orbit performance is evaluated.
Result and discussion
The EMI and experimental set-up
Summary
In order to investigate the radiances differences and
evaluate on the EMI in-orbit SNR, radiative simulations are
performed for various satellite geometries and the
scattered sunlight measurements by using the VLIDORT
model (Spurr, 2006). According to that random noise
decrease with the square root of measured radiance (UV-
VIS spectrometers are typically photon-shot noise
dominated), the in-orbit SNR could be simply estimated
using the following equation.
For these radiance simulations with VLIDORT, ozone
profile from OMI and an aerosol optical depth (AOD) of
0.5 at 550nm from MODIS are used for 𝑅𝑔𝑏 simulation;
geometry and albedo information at different latitudes
and seasons from OMI L1b data serve as input
parameters for 𝑅𝑠𝑎𝑡 simulation
The derived 𝑆𝑁𝑅𝑠𝑎𝑡 in three mid-latitude cities vary from
~800 to ~1400 in UV2 NO2 fitting window and from
~600 to ~1000 in VIS1 NO2 fitting window.
Sensitivity tests show radiance variability is less than
2% for the elevation angle vary from 0.01° to 1°, which
are inconsistent with typical horizon scan measurement
by MAX-DOAS. Generally spectral intensity are almost
40% lower at 0° elevation angle than at 1° (from CINDI-
2 campaign), leading to a 30% underestimation of EMI
in-orbit SNR.
The EMI is a nadir-viewing push broom
spectrometer, will measure earthshine radiances
and solar irradiances with the wavelength range
from 240 to 710nm at moderate spectral resolution
(0.3-0.5nm) at nadir. The EMI adopts Offner imaging
spectrometer with four spectral channels, and each
channel uses a two-dimensional (spectral and
spatial) CCD detector. The in-orbit integration time of
EMI is 2s for UV channels (including UV1 and UV2
channel), and 1s for VIS channels (including VIS1
and VIS channel). And the binning of spatial rows is
applied to EMI in order to increase signal-to-noise
ratio (SNR), i.e. 43 binned rows for UV channels,
and 48 binned rows for VIS channels. Thus, the
resulting ground pixel sizes for UV and VIS channels
are 43km × 13km (swath direction × flight direction)
and 37km × 13km respectively. (see Tab. 1)
Tab. 1. Instrument Properties of the EMI
To comprehensively evaluate the derived instrumental parameters and calibrated spectra
through the prelaunch calibration phase, on-ground gas cell measurements of NO2 from
scattered sunlight was performed with the EMI instrument and the results are discussed.
The 8-cm-long quartz gas absorption cell was continuously flushed with NO2 or N2, and the
measured N2 spectrum was served as reference for NO2 spectrum. The NO2 gas flushing
into the cell was taken from a commercial gas bottle with an approximate mixing ratio of 710
parts per million (ppm) NO2 in N2 and at a stable flow rate of 7.5L/min. Under these given
conditions, the calculated NO2 SCD in the gas cell equals to 1.40 × 1017 molecules/ cm2,
with an uncertainty of 6% (mainly due to the cell length and gas mixing ratio). The light
beam from scattered sunlight passed through the quartz cell and went into the entrance slit
of EMI earth port. Due to the large IFOV of EMI, 24° EMI FOV approximately was
illuminated homogeneously for the scattered sunlight measurements. The experiment was
performed in the optical laboratory (latitude 31.91° north, longitude 117.16° east, at an
altitude of 20m above the ground) of AIOFM, on 13 February 2017 from 11:00 am to 11:30
am. (see Fig. 1)
Fig. 1. Experimental set-up for gas cell measurements from scattered sunlight.
Spectral range UV1: 240-315nm
UV2: 311-403nm
VIS1: 401-550nm
VIS2: 545-710nm
Spectral resolution 0.3-0.5nm
Telescope swath
IFOV
114 degrees
Telescope flight IFOV 0.5 degrees
CCD detectors UV1/2: 1024 × 1024 (spectral × spatial)pixels
VIS1/2: 1254 × 576 (spectral × spatial)pixels
Ground pixel size 48km × 13km
Mass 95kg
Size 50cm × 36cm × 65cm
Orbit Polar, sun-synchronous
Average altitude: 705km
Orbit period: 98 minutes 53 seconds
Ascending node local time: 13:30 PM
Anticipated lifetime 8 years
Similar with retrieval algorithm of the published OMI level 2 products (e.g. NO2, SO2, and
HCHO), for the EMI instrument solar irradiance spectrum measured in-orbit can be served
as reference spectrum to retrieve slant column densities (SCDs) of the trace gases from the
earthshine radiance spectrum using the differential optical absorption spectroscopy (DOAS)
fitting technique. In order to derive the instrumental parameters which are needed to
implement the DOAS retrievals, a high quality prelaunch calibration phase is essential and
has been done for EMI instrument, e.g. pixel response non-uniformity calibration, radiometric
calibration, slit function characterization, stray light suppression, diffusers bidirectional
reflectance distribution function measurement.
Fitting window UV2: 338-370nm
VIS1: 435-490nm
Cross sections NO2 cross-section, Vandaele et
al. (1998), 298 K, I0-corrected*
(1017 molecules cm-2 )
Ring spectrum calculated based
on SAO2010 solar atlas and Ring
scaled with (λ/354nm)4 (Wagner
et al., 2009)
Polynomial term 5th-order
Intensity offset None
Reference N2 spectrum(averaged by 1min)
Wavelength calibration Lab-calibrated wavelength and slit
function parameters
Tab. 2. NO2 DOAS retrieval settings of scattered sunlight measurements
The obtained EMI raw spectra are analyzed
with the QDOAS software package (Danckaert
et al., 2015). The logarithm of the ratio of
measured NO2 spectrum and N2 spectrum, are
fitted to NO2 and Ring cross-sections
convolved with the calibrated instrument slit
function and a 5th-order polynomial term using
the following DOAS equation. (see Tab. 2)
ln(𝐼𝑁2 𝜆
𝐼𝑁𝑂2 𝜆) =
𝑗=1
𝑛
𝑆𝑗 𝜆 ∗ 𝑐𝑗 + 𝑃 𝜆
Fig. 2. NO2 SCDs retrieval of adjacent spatial rows
(viewing angles) from UV2 and VIS1 channel, for
scattered sunlight measurements at a selected time
during the steady-state of the NO2 gas flushing
process. Top: viewing-angle-dependent NO2 SCDs
retrieval with error bar for UV2 (red curve) and VIS1
(blue curve) channel. Bottom: the calculated relative
deviation from the median value of the top panel plot.
Fig. 3. Standard deviation of NO2 fitting residuals
from scattered sunlight measurements in UV2 (top
panel) and VIS1 (bottom panel) channel. For every
spatial rows of UV2 and VIS1 channel
corresponding to Fig. 2, standard deviation of
residual is shown with a unique color.
Theoretically, NO2 retrieval SCDs from adjacent
spatial rows of UV2 or VIS1 channel should have
good agreements within reasonable differences
originated from effective light path length.
Similar effect could be found for typical two-
dimensional CCD instruments such as OMI,
which affects several trace gases retrieval and
shows a pattern of “stripes” for each orbital track,
as described in Veihelmann and Kleipool (2006)
Viewing-angle-dependent variability of the EMI
NO2 retrieval is evaluated to be rather small (up
to 2%, 2.8 × 1015 molecules/cm2) based on the
scattered sunlight measurements. Similar size
classes of viewing-angle-dependent NO2 stripes
(within 1.5 × 1015 molecules/cm2) were observed
for OMI NO2 products based on improved
calibrated L1b data and applied to raw SCDs de-
striping algorithm, as described in Boersma et al
(2011).
Fig. 3 shows the calculated standard deviation
(i.e. random noises) of DOAS fitting residuals
from 3 mins measurements during steady-state of
gas flushing process, and the SNR equals to the
inverse of the standard deviation.
The SNR of scattered sunlight measurements is
quite stable in two NO2 fitting windows, within the
range of ~500-800 and with the mean value of
~625 for all illuminated viewing angles. Note that
measurements performed here was at an
elevation angle of 0° with low signal due to strong
aerosol optical extinction and obscuration by
horizon line.
𝑆𝑁𝑅𝑠𝑎𝑡 = 𝑆𝑁𝑅𝑔𝑏 ∗𝑅𝑠𝑎𝑡
𝑅𝑔𝑏
Fig. 8. Estimation of in-orbit EMI SNR for
measurements of three Chinese cities, based on
calculated SNR in Fig. 7 and VLIDORT radiance
simulations using OMI geometries and albedo
information. The three cities are Guangzhou
(latitude 22.6°N), Hefei (latitude 31.5°N), and
Beijing (latitude 39.5°N) in August (top plot) and
February (bottom plot).
From the scattered sunlight measurements by the EMI instruments, we perform NO2 retrieval and evaluation
on the EMI performance. Generally, good agreements between known NO2 concentration and NO2 retrieval
SCDs for adjacent spatial rows from both UV2 and VIS1 channel. On the basis of analysis of the fitting
residual and radiative simulations, in-orbit EMI SNR is presented.
Based on the experimental results and discussion, EMI is expected to be capable of measuring global
distribution of tropospheric and stratospheric trace gases within expected accuracies.
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