Validation of a Satellite Retrieval of Tropospheric Nitrogen Dioxide

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Validation of a Satellite Retrieval of
Tropospheric Nitrogen Dioxide
Kelly ChanceChris SiorisSmithsonian
Astrophysical Observatory
Randall MartinDalhousie University Smithsonian
Astrophysical Observatory
David ParrishNational Oceanic and Atmospheric
Administration
Daniel JacobElisabeth SturgesHarvard University
Comparison
INTRODUCTION Nitrogen oxide radicals (NOx NO
NO2) originating from combustion, lightning, and
soils largely control tropospheric ozone
production. Tropospheric ozone plays a key role
in determining the oxidizing power of the
atmosphere, is an important greenhouse gas, and
is toxic to biota. Previous comparison of
tropospheric nitrogen dioxide (NO2) retrieved
from the Global Ozone Monitoring Experiment
(GOME) Burrows et al., 1999 and simulated with
a global 3-D model of tropospheric chemistry
(GEOS-CHEM model Bey et al., 2001 Martin et
al., 2003a) showed that retrieved tropospheric
NO2 columns over Houston are half of simulated
columns during summer Martin et al., 2003b.
Here we examine the consistency of tropospheric
NO2 retrieved from GOME with in situ measurements
over Houston.
Relating GOME NO2 with in situ measurements from
TexAQS requires accounting for the spatial
sampling characteristics of both measurements. We
achieve more uniform spatial weighting of the in
situ measurements by calculating tropospheric NO2
columns at 0.5o by 0.5o resolution and
subsequently averaging the cells. The resolution
of 0.5o by 0.5o is chosen as a spatial scale over
which the in situ measurement is representative
given a NOx lifetime of a few hours and nominal
wind speeds of 10 m s-1.
Texas Air Quality Study
The Texas Air Quality Study (TexAQS) measured
tropospheric NO2 Ryerson et al., 2000 over
Houston on 14 separate flights over August 16 -
September 13, 2000. More than 15 of the in situ
measurements were taken in one grid cell directly
over Houston. The spatial coverage of aircraft
flights during the campaign is comparable to the
scale of a GOME footprint. The measured profile
shows that tropospheric NO2 is largely confined
within the lowest kilometer.
Tropospheric NO2 column determined from in situ
measurements. The column is integrated in 500 m
increments from the surface to 6 km. The 17th
percentile of all in situ measurements within a
500 m slab is assumed for slabs without any in
situ measurements for each column calculation.
The 17th percentile is chosen to reduce sampling
biases from the mission focus on plume studies.
The GEOS-CHEM model value of 5x1014 molecules
cm-2 was added to all columns to represent
tropospheric NO2 above 6 km.
The relative vertical distribution of NO2 (shape
factors Palmer et al., 2001) over Houston (29o
to 31o lat by -96.25o to -93.75o lon). The blue
line shows the shape factors calculated with the
GEOS-CHEM model as used in the AMF calculation.
The red line shows the shape factors determined
from in situ measurements as part of TexAQS.
SCIAMACHY
Number of in situ measurements taken within a
tropospheric column of 0.5o x 0.5o resolution
during TexAQS. White areas indicate regions
without measurements. The white lines show an
example GOME footprint.
Measured NO2 as part of TexAQS. The yellow dots
show each 1-second measurement. The green line
is the mean. The median is represented by the
red line, with error bars showing the 17th and
83rd percentiles respectively.
Retrieval of Tropospheric NO2
Comparison of GOME and in situ measurements. The
blue lines show the average GOME measurement
during TexAQS that falls within a 4.5o range
(-97.25o to -92.75o lon), plotted as a function
of the latitude of the center of the GOME
footprint. The red line is the average over
-97.25o to -92.75o lon of the in situ column as a
function of latitude. Blue error bars are the
GOME retrieval error Martin et al., 2003b. Red
lower bounds use the 5th percentile instead of
the 17th percentile in the in situ column
calculation. Red upper bounds average over 3.5o
(-96.75o to -93.25o lon).
We retrieve tropospheric NO2 from GOME using the
algorithms described in Martin et al. 2002
2003b. This includes the air mass factor (AMF)
formulation of Palmer et al. 2001 which
computes the AMF as the integral over the
tropospheric column of the relative vertical
distribution of NO2 (shape factor) weighted by
the local sensitivity to NO2 of the solar
radiation backscattered to space (scattering
weights). The shape factor is specified from a
global 3-D model simulation (GEOS-CHEM model).
Tropospheric NO2 determined from SCIAMACHY by
limb-nadir matching. Validation of measurements
from higher resolution instruments will be
facilitated by smaller pixel size, but sub-grid
scale heterogeneity may remain an issue.
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Tropospheric NO2 retrieved from GOME observations
during the TexAQS campaign.