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.