Direct Sun measurements of NO2 column abundances from

Table Mountain, California:

Retrieval method and intercomparison of low and high

resolution spectrometers

Shuhui Wang, Thomas J. Pongetti, and Stanley P. SanderJet Propulsion Laboratory, California Institute of Technology, Pasadena, California

Elena Spinei and George Mount Washington State University, Pullman, Washington

Alexander Cede University of Maryland, Earth System Science Interdisciplinary Center, College Park, Maryland

Jay HermanNASA/Goddard Space Flight Center, Greenbelt, Maryland

JPL

JPL

• An improved NO2 total column retrieval method.

• Intercomparison of NO2 column measurements

using high and low resolution spectrometers.

Objective Approach

• NO2 column abundance has been measured

with a high resolution Fourier Transform Ultraviolet Spectrometer (FTUVS) instrument at NASA/JPL’s Table Mountain Facility (TMF).

• With a high resolution, FTUVS measures the absolute NO2 columns. The solar lines are

removed using Doppler shifted spectra from the east and west limbs of the rotating Sun.

• Instrument intercomparison campaign at TMF during July, 2007

• MF-DOAS and PANDORA use a high Sun reference spectrum which contains an unknown NO2 column (SCref).

• The high resolution FTUVS measurements can be used as an absolute calibration for the two low resolution spectrometers.

rel refabsolute SC SC +SC

Calibration

FTUVS – JPL High resolution (0.001 nm)

MF-DOAS – WSU Low resolution (0.8 nm)

PANDORA– GSFC Low resolution (0.4 nm)

• East and west limb solar spectra are taken alternatively. The solar lines are Doppler-shifted, while the NO2 absorption

features remain at the same wavenumber.

• Solar lines are removed by matching and ratioing east and west spectra.

Solarsignal

Atmospheric NO2 signal

FTUVS Absolute NO2 Column Retrieval Removal of solar lines

• FFT high pass filter is used to further remove the broad-

band solar baseline, leaving only high frequency NO2

features.

FTUVS Absolute NO2 Column Retrieval Reference

• The reference cross sections were measured using FTS at various temperatures at the Kitt Peak National Observatory [Nizkorodov et al., 2004].

• Low pressure cross sections followed by convolution (normalized Lorentz function) to various pressures are used to create references for each atmospheric layer.

• Based on A priori profiles, average NO2 references in the

Stratosphere and the Troposphere are calculated.

• Total reference is obtained assuming a Strat/Trop partition (typically ~3 at TMF but could go to 1~5 in extreme cases).

• The total reference is convolved to 2 atm to reduce the sensitivity to the assumed partition.

20466 20467 20468 20469 20470 20471-3.00E-019

-2.00E-019

-1.00E-019

0.00E+000

1.00E-019

2.00E-019

3.00E-019

Cro

ss S

ect

ion

(cm

2 /mo

lecu

le)

Strat Trop

• A weighted fit is applied to block the spectral regions with solar residuals, leaving numerous micro-windows

with NO2 features but few solar features.

FTUVS Absolute NO2 Column Retrieval Weighted fit

20150 20200 20250 20300 20350 20400 20450 20500

0.2

0.4

0.6

0.8

1.0

Solar spectrum

0.0

0.2

0.4

0.6

0.8

1.0

Weight

Correlation of NO2 Slant Columns from PANDORA / MF-DOAS vs. FTUVS

0.0 0.5 1.0 1.5 2.0 2.5 3.0-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

PANDORA MF-DOAS

PA

N a

nd M

F-D

OA

S N

O2 S

lant

Col

umn

(10

16cm

-2)

FTUVS NO2 Slant Column (1016cm-2)

Linear fit (PANDORA) a = -0.3787 (or -0.14DU.), b=1.14

Linear fit (MF-DOAS) a = -0.3857 (or -0.14DU.), b=1.08

R=0.97

Langley FTUVS

PANDORA 0.12 DU. 0.14 DU.

MF-DOAS 0.12 DU. 0.14 DU.

• The NO2 slant column in the high-

Sun reference (SCref) determined

with FTUVS agrees with that from

the Langley method.

SCref

Comparison of NO2 Vertical Columns from PANDORA, MF-DOAS and FTUVS

Summary

• Direct Sun measurements of NO2 total columns have been conducted using

three instruments at TMF in July, 2007.

• The high-resolution FTUVS measures the absolute NO2 columns, while the low-

resolution instruments, PANDORA and MF-DOAS, use a high Sun spectrum as the reference to retrieve NO2 columns. The FTUVS absolute NO2 measurements

can thus provide a calibration for the two low resolution spectrometers.

• During this campaign, the NO2 slant columns from PANDORA and MF-DOAS

are compared to those from FTUVS to determine the amount of NO2 in the high

Sun reference.

• The (calibrated) total NO2 vertical columns from all three instruments show good

agreement to ~10%.