| Tropospheric NO2 measured by satellites|
10-12 Sept 2007
KNMI, De Bilt, The Netherlands
Programme for the Tropospheric NO2 workshop
Monday 10 September
|Session Retrieval - Chair: James Gleason|
- INVITED TALK: Tropospheric NO2 from space : retrieval issues and perspectives for the future
Michel Van Roozendael
Tropospheric NO2 columns have been derived for more than 10 years based on spectroscopic measurements from GOME on ERS-2, SCIAMACHY on ENVISAT and OMI on EOS-AURA. Owing to its strong and highly structured absorption features in the UV and visible regions, NO2 can be measured with high signal to noise ratio so that localized emissions can be easily detected providing a wealth of information on atmospheric events, processes and their evolution with time. Key applications of the recorded data sets include assimilation in air quality models, attribution of source by inverse modeling techniques, identification of long-range transport events and global or regional trend studies. For such quantitative applications, the assessment of the accuracy, stability and cross-platform consistency of the NO2 data sets are of major importance. In this presentation, I will review some of the main issues related to NO2 retrieval from space, based on the experience from GOME, SCIAMACHY and OMI. This will cover spectral fitting using DOAS, elimination of stratospheric background, evaluation of tropospheric AMFs and how to deal with clouds. Approaches currently adopted by different retrieval groups will be contrasted and discussed in terms of their respective strengths and weaknesses. Finally challenges for future tropospheric NO2 retrievals will be addressed in relation with the need for improved spatial and temporal resolutions.
- Direct fitting of NO2
GOME-1, GOME-2, SCIAMACHY, and OMI
Kelly Chance, T.P. Kurosu, R.V. Martin, T. Beck, S. Kondragunta
We have implemented direct fitting of radiance spectra from the GOME-1,
SCIAMACHY, and OMI satellite spectrometers. We are using these instruments in a study to attempt to analyze the spectra in as identical a fashion as
possible, with the most
complete treatment of underlying algorithm physics, in order to separate instrumental, algorithmic, and temporal differences in satellite
measurements. Initial efforts, presented
here, concentrate on NO2.
- Quantitative retrievals of
NO2 from GOME
Lara Gunn, Martyn Chipperfield, Richard
Siddans, Brian Kerridge
The direct measurement of tropospheric trace gases from space is difficult
and so it is
advantangeous to combine models and data to fully exploit
potential of observations. We present a new procedure for the quantitative
tropospheric total column amounts of trace gases from
satellites such as the Global Ozone Monitoring Experiment
(GOME) on-board the European
Research Satellite 2 (ERS-2). The procedure
constrains the stratosphere by
assimilating chemical observations from the Halogen Occultation Experiment (HALOE),
which flew on the Upper Atmosphere Research Satellite (UARS), into the SLIMCAT/TOMCAT
chemical transport model (CTM). The chemical
data assimilation method is
performed using the sequential sub-optimal Kalman filter scheme and here we
assimilate HALOE CH4, H2O, O3 and HCl. The
assimilation scheme preserves tracer correlations and the overall effect is to produce a more
realistic stratosphere in the
stratospheric constraints from the CTM we then calculate tropospheric residuals. In the first instance we have applied this method to GOME observations of
Retrieved slant columns are usually converted to vertical column by an air mass factor. The calculated air mass factor is sensitive to surface albedo, cloud
height and aerosol properties. Our method uses data on cloud and aerosol from the Global
Retrieval of ATSR cloud Parameters and Evaluation
(GRAPE) project and
is retrieved from GOME. This data in conjunction with a
multiple-scattering radiative transfer model, derived from
enable a more accurate air mass factor
We will show quantitative results from the GOME period of derived tropospheric
abundance of NO2. We will discuss the advantages of this scheme, and the
improvements obtained by including the model stratosphere and retrieved aerosol and
- Impact of
clouds on tropospheric NO2 retrieval
Ping Wang, P. Stammes, R. van der A
In general clouds have shielding, albedo and in-cloud absorption effects on trace
retrievals. To correct for these effects the cloud fraction and cloud pressure are two important parameters, which have to be known.Thereto we have developed
in recent years the
O2 A-band cloud algorithms FRESCO and FRESCO+.
The effective cloud fraction and cloud pressure retrieved from the O2 A-band using
FRESCO(+) have been used in
corrections of ozone and NO2 retrievals from GOME, SCIAMACHY and GOME-2, by many users, e.g. in the TEMIS
According to retrieval simulations using the radiative
transfer model DAK we found that a Lambertian cloud with high albedo is a good assumption
for cloud corrections of ozone and NO2.
In FRESCO(+) the effective cloud fraction is
the cloud fraction of a Lambertan surface with albedo 0.8 yielding the
same radiance at the top of
atmosphere (TOA) as the clouds in the scene.
The tropospheric NO2
air mass factor differences between a scattering (Mie)
cloud and a Lambertian cloud of albedo 0.8 are within 10% if the geometric cloud fraction is smaller than 0.2. If geometric
cloud fraction is 0.5, the
tropospheric NO2 air mass factor
difference is within 20% except for the SZA larger than 70 degree and SZA equal to 0.
Recently, FRESCO was
by adding the single Rayleigh scattering. This new version is called FRESCO+. The difference in cloud pressure between FRESCO and FRESCO+ is significant for the
cloudy scenes, which are especially important for tropospheric NO2 retrieval. In this presentation we will show some improvements of using the FRESCO+
product in tropospheric NO2
|Session Retrieval (cont.) - Chair: James Gleason|
- INVITED TALK: Reducing errors in using tropospheric NO2 columns observed from space
In this presentation I will try to give a balanced discussion of the state-of-science with respect to the retrieval and use of tropospheric NO2 columns from nadir UV/VIS sensors. First I will give an overview of the basic retrieval method that is common amongst retrieval groups. Then I will discuss the different choices and assumptions made by various groups and show how these lead to considerable differences between the retrieved columns.
Subsequently I will focus on a number of important issues for the future:
(1) How can we minimize the errors in the various retrievals? Is it simply a question of optimizing the forward model parameters, or
(2) Should we be moving towards a common retrieval approach? Such a convergence requires --amongst others-- an extensive validation dataset that is representative for a wide range of situations (spatially and temporally) thereby serving as an arbiter to find the best possible retrieval approach.
(3) With the new generation of high-resolution sensors SCIAMACHY, OMI, and GOME-2, new types of retrieval and interpretation errors are becoming more important. These are resolution-related; for instance albedo maps and a priori profile shapes that are currently used in retrievals are available at spatial resolutions much coarser than the retrieval resolution.
I will conclude the presentation by showing how tropospheric NO2 data sets from different instruments with different horizontal resolutions and overpass times can be combined.
- A new approach to eliminate the
broadband absorption in DOAS spectra
Milagros Ródenas, E. Soria, J.D. Martín
DOAS data analysis is based on the separation of the broadband structure overlapped with the high
frequency absorptions of the compounds to study. This way, the wavelength-varying contribution of Mie and Rayleigh scattering, as well as the low frequency of
the compound itself, are removed. It also accounts for the effect of the detector etalon, inaccuracies in the instrument calibration or deficiencies in the
knowledge of broadband absorption spectra (eg. O4).
Software for analysis of DOAS data carry out a polynomial to model and remove these undesired
broadband features. The process is mathematically equivalent to a high-pass filtering. In particular, a global polynomial models the low frequencies within all
the evaluation range.
In practice, the order of the polynomial is chosen depending on the width of the fitting window and on the width of the widest trace
gas absorption feature to be fitted. Nevertheless, it doesn’t exist a general solution for choosing the right polynomial degree, and even though it posses a
small effect in data retrieval, mean errors of 8% have been found (Aliwell et al., 2002).
This study proposes a generalization of the classic approach
mentioned above. Polynomial modelings are used to eliminate local broadband components. In other words, the proposal is basically a polynomial fit based on
windows with a certain length. In the particular case of considering the whole length of interest, our algorithm becomes the usual procedure for DOAS analysis.
We have benchmarked our approach with the classic one. A case study on a real problem, such as, the measurement of NO2 is presented.
Aliwell, S.R. et. al. (2002). Analysis for BrO in zenith-sky spectra: An intercomparison exercise for analysis improvement J. Geophys.
Research, V-107, NO-D14.
- Tropospheric NO2 derived by Limb-Nadir-Matching
Andreas Heckel, A. Richter, A. Rozanov, J.P. Burrows
One step in
the retrieval of tropospheric NO2 from satellite measurements is to remove the stratospheric part of the total column. Good estimations of the
contribution are an important and critical point especially for conditions with lower tropospheric columns. Current algorithms use Nadir measurements plus either
priori assumptions on the horizontal stratospheric NO2 distribution or assimilation into stratospheric models. Depending on atmospheric conditions the
uncertainties of the
tropospheric vertical column introduced by the removal of the stratospheric column are in the order of 0.5 to 1e15 molec/cm2. In polluted
regions with high tropospheric columns
this error is negligible compared to the uncertainties introduced by the tropospheric AMF. However in regions with lower
tropospheric NO2 values the accuracy of the removal of the
stratospheric column becomes the dominating factor in the error budget.
The SCIAMACHY instrument
with the combined Limb and Nadir measurements offers the possibility to derive
the stratospheric component independently. The Limb measurements are analyzed to
derive a stratospheric profile of NO2. Integrating this profile gives the stratospheric column.
The tropospheric column is then derived from the difference
between the total column (Nadir) and the stratospheric column (Limb). This study will concentrate on the tropospheric
NO2 columns derived from the SCIAMACHY data
of 2005. The introduction of the method and comparisons to currently available tropospheric NO2 products and aircraft measurements will
Improvements and limitations of this Limb-Nadir-Matching approach will be discussed based on different time scales such as single orbits, daily, monthly or
- A combined
retrieval, modelling and assimilation approach to estimate tropospheric NO2 from OMI measurements
Henk Eskes, Ruud Dirksen, Ronald van der A, Folkert Boersma
imaging spectrometers like OMI, SCIAMACHY, and GOME, provide on a regular basis high quality reflectance spectra of the Earth\'s atmosphere which allows for
the detection of various species of atmospheric trace gases. This paper presents a method to retrieve troposheric nitrogen dioxide (NO2) concentrations from
space. The retrieval consists of two steps; the first step uses the DOAS (Differential Optical Absorption Spectroscopy) approach to compute the total absorption
optical thickness along the light path (the slant column). For OMI the DOAS was implemented in a KNMI/NASA collaboration. The second retrieval step, which was
developed at KNMI, estimates the stratospheric column based on data assimilation and vertical profile estimates from the TM chemistry-transport model. In
combination with the retrieved slant column, cloud fraction and cloud top height the tropospheric vertical column of NO2 is estimated. Because of its wide swath
(115 degrees FOV) and high spectral resolution (13x24km^2 ground pixel size at nadir), the OMI instrument observes NO2 concentrations on an urban scale with
global daily coverage. Our presentation will focus on aspects of the OMI retrieval, comparison with the operational NO2 product of OMI, and on the results of the
latest OMI reprocessing of summer 2007.
|17:50||End day 1|
|19:00||Conference dinner at
Address: Janskerkhof 9, Utrecht, Tel:030-2343406
All busses to and from
KNMI (#50, #52, #53, #74) pass the restaurant, take stop "Janskerkhof". Click for a map
Tuesday 11 September
|Session Retrieval (cont.) - Chair: Eric Bucsela|
- Synergistic use of multiple sensors for tropospheric NO2 measurements
Andreas Richter, A. K. Heckel, J. Leitao, J. P. Burrows
columns of NO2 can
be retrieved from measurements of backscattered UV/vis light taken from satellite, e.g. from the GOME, SCIAMACHY and OMI instruments. These data have
been used to investigate NOx emissions and their spatial and temporal variability in a number of studies. In the retrieval process, a priori information
on profile shape,
stratospheric NO2, aerosol loading and other parameters is used that results in a relatively large uncertainty of the product derived.
Therefore, for the use of data from multiple
sensors e.g. for long-term time series one has to make sure that consistent retrieval assumptions are being used to
avoid artificial changes in the time series. At the same time,
measurements from different platforms taken at different times also enable a check on the
consistency of the different time series and to a lesser extent also on the assumptions
In this study, tropospheric NO2 data from SCIAMACHY, OMI,
and GOME-2 is compared and analysed for their consistency and possible systematic differences.
- Development of Operational GOME-2
Tropospheric NO2 Product for Air Quality Applications
Trevor Beck, Shobha Kondragunta, Kelly Chance, Thomas Kurosu, Lawrence E. Flynn
NOAA/NESDIS is developing an operational
tropospheric NO2 product from Metop-A GOME-2 instrument, flying in the morning orbit
with a 9:30 AM equator crossing time, to meet air quality monitoring and forecasting
requirements in the United States (U.S.). Initial focus is on the
development and characterization of GOME-2 NO2 product with the end goal being assimilation of the data into the
National Weather Service (NWS) air quality model
to constrain NOx emissions and improve forecasting. Algorithm development work will be consistent with Aura/Ozone Mapping
Instrument (OMI) flying in the
afternoon orbit with 1:38 PM equator crossing time. Methodologies to convert slant column amounts to vertical column densities and to extract
amount from total column amount will be similar to OMI, so diurnal changes in NO2 can be tracked. NO2 product status and preliminary comparisons between
NO2 and NWS operational Community Multiscale Air Quality (CMAQ) predictions of NO2 on different spatial and temporal scales will be
- First results on tropospheric NO2 from the GOME-2 instrument on MetOp
Pieter Valks, N. Hao, D. Loyola, W. Zimmer
The Global Ozone Monitoring Experiment-2 (GOME-2) is one of the new-generation European instruments carried on MetOp, which has been jointly established by ESA and EUMETSAT. GOME-2 will continue the long-term monitoring of atmospheric ozone and minor trace gases, started by GOME on ERS-2 and SCIAMACHY on Envisat. GOME-2 is a scanning spectrometer that measures the Earth’s backscattered radiance and extraterrestrial solar irradiance in the ultraviolet and visible part of the spectrum (240-790 nm), and contains two Polarisation Measurement Devices (PMDs). The advanced GOME-2 observes four times smaller ground pixels (80 x 40 km) than GOME on ERS-2, and provides a global coverage within about one day. The ozone and minor trace gas column retrieval algorithms for GOME-2 have been developed by DLR, in the framework of EUMETSAT´s Satellite Application Facility on Ozone and Atmospheric Chemistry Monitoring (O3M-SAF).
In this contribution, we present the first results of the GOME-2 NO2 products, derived with the Differential Optical Absorption Spectroscopy (DOAS) method. The operational NO2 retrieval algorithm for GOME-2 is based on the GOME Data Processor (GDP) version 4.0, and includes a new algorithm for the retrieval of the tropospheric column density of NO2 for polluted conditions. For the calculation of the tropospheric NO2 column, an Air Mass Factor based on an assumed tropospheric NO2 profile is used, which has been derived from a MOZART-2 NO2 climatology. We present maps of total and tropospheric NO2 column densities. In addition, comparisons with other satellite data products of NO2, such as derived from GOME-1, SCIAMACHY and OMI will be shown.
|Session validation - Chair: Pieternel Levelt|
- A comparison of OMI with vertical columns constructed from in-situ measurements in Switzerland
Dominik Brunner, Brigitte Buchmann
tropospheric NO2 columns retrieved by OMI over Switzerland is assessed by comparison with in-situ observations of the Swiss air quality monitoring network NABEL.
each OMI pixel sampled above one of the regional NABEL stations Tänikon and Payerne a concurrent in-situ NO2 column is constructed by combining the
measurements of stations
located at altitudes between about 400 and 3500 m above sea level and taking into account actual PBL heights. Because NO2 is (mostly)
measured with Molybdenum converters, known
interferences with PAN and HNO3 are taken into account based on results of extended parallel measurements with both
photolytic and Molybdenum converters. Furthermore, the ability
of OMI to resolve regional air pollution structures in Switzerland is assessed by comparison with
a high-resolution NOx emissions inventory and NO2 immission maps generated by a
simple air pollution dispersion
- Relationship between ground-level NO2 concentrations and OMI NO2 column
Lok Lamsal, Randall V. Martin, Edward
Dunlea, Martin Steinbacher, Edward
Recent studies have demonstrated that tropospheric NO2 columns are highly sensitive to lower tropospheric pollution. We present an
infer ground-level NO2 concentrations from tropospheric NO2 columns retrieved from OMI. The derived surface NO2 concentrations are compared with in situ NO2 data
279 monitoring sites from the AQS/NAPS networks in the United States and Canada. The in situ NO2 measurements from commercial chemiluminescent NO2 analyzers
molybdenum converter are corrected for interference from other reactive nitrogen species. The OMI derived ground-level NO2 data exhibit significant
agreement with the in situ
measurements after accounting for the interference. This comparison serves as an indirect validation of the tropospheric NO2 columns,
and demonstrates the capability of monitoring
surface air quality from
- NO2 lidar profiles
measured during the DANDELIONS validation
Hester Volten, Ellen Brinksma, Stijn Berkhout, Daan Swart, René van der Hoff, Hans Bergwerff, Pieternel Levelt, Gaia
Pinardi, Michel Van Roozendael
DANDELIONS (Dutch Aerosol and Nitrogen Dioxide Experiments
for validation of OMI and SCIAMACHY) is a project that encompasses
validation of NO2 measurements by the Ozone Monitoring Instrument (OMI) and SCIAMACHY (Scanning Imaging
Absorption SpectroMeter for Atmospheric CartographY), and
of aerosol measurements by OMI and the Advanced Along-Track Scanning Radiometer (AATSR), using an extensive set of
ground-based and balloon measurements over the
polluted area of the Netherlands.
Validation measurements were performed during two campaigns at the Cabauw Experimental Site
for Atmospheric Research
(CESAR, located in the centre of the Netherlands) which ran from May 8-July 14, 2005, and September 1-September 30, 2006, respectively. The campaign
were aimed at validation of the satellite products total and tropospheric nitrogen dioxide and ozone, and aerosol optical thickness.
An extensive dataset
the obtained ground-based, balloon and satellite data on NO2, aerosols, and ozone, will be available for validation of Aura instruments through the Aura
Validation Data Center
(AVDC). Here we focus on the NO2 lidar data. For thorough validation in polluted areas, measurements of the NO2 profiles are required,
since the profile shape directly influences
the conversion from slant column to total NO2, through the air mass factor. There are only very few measurements of
NO2 profiles available. The results of a novel instrument,
namely the NO2 lidar developed at RIVM that is able to measure lower tropospheric profiles will be
presented. The lidar system uses the DIAL technique to measure atmospheric trace
gases. It is housed in a fully self-supporting mobile laboratory, so that
measurements can be taken anywhere. The measured profiles that range from near ground level to about 2500
m will be compared with in-situ values obtained with
in-situ NO2 monitors located on surface-level and at 200 m. In addition, we will use boundary layer measurements and MAXDOAS
data obtained at surface level and
at 200 m to interpret our findings. The presence of NO2 within 200 m is highlighted comparing the MAXDOAS results for downward and upward
viewing directions from
the top of the tower, and a differential column within the tower height is retrieved comparing the columns retrieved at both levels.
In addition to
NO2 profiles, the measured lidar data contain unique information on the spatial homogeneity and the vertical and temporal variability of NO2. We did detailed
the time-resolved three dimensional NO2 distributions. We will investigate the significance of the variability for the retrieval of NO2 columns
- Ground-based measurements of
Folkard Wittrock, Hester Volten, Ellen Brinksma, Hilke Oetjen, Anja Schönhardt, Daan Swart, Andreas
Richter, John P. Burrows
The major topic of this study is the description of
tropospheric NO2 profiles derived from
MAX-DOAS observations during the Dutch
Aerosol and Nitrogen Dioxide Experiments
for vaLIdation of OMI and SCIAMACHY (DANDELIONS).
days with good weather conditions these profiles
have been compared to
measurements from the novel RIVM NO2 lidar instrument (see Volten et al.) and
situ observations close to the ground and on
top of a 200 m mast.
All measurements were performed during two periods at the Cabauw
Experimental Site for Atmospheric Research
4.93°E 0.70 m below mean sea level) which ran from
May 8 - July 14, 2005, and September 1 - September 30, 2006, respectively.
passive UV/vis remote sensing was the development from
ground-based zenith sky observations to multi-axis (MAX)-DOAS measurements,
enabled us to better validate
findings from satellite observations and
study the behaviour of important trace gases in the troposphere on a local
Recently, an automated optimal estimation based
algorithm (BREAM) was developed for the MAX-DOAS measurements. The method first
determines appropriate aerosol settings using measurements of the O4
columns and then inverts the profile of the absorber of interest from the trace gas
columns. Now DANDELIONS has provided an unique opportunity to validate the retrieval
by comparison with independent measurements of the nitrogen
|Session Modelling - Chair: Thomas Wagner|
- INVITED TALK: Tropospheric NO_2 columns as a Top-down Constraint on NO_x Emission Inventories
Tropospheric NO_2 columns are closely related to
surface NO_x emissions. Factors affecting this relationship will be discussed in the context of providing top-down information on NO_x emissions. Examples will
include emissions from fossil fuel combustion, biomass burning,
soil microbial activity, and lightning. Sources of error will be examined. Validation needs
and areas for further algorithmic development will be identified. Accuracy objectives will be proposed for the retrieval of tropospheric NO_2 with respect to
the goal of emissions constraints.
- Distribution and trends of NOx sources
inferred by inversion of one decade
NO2 satellite columns
Jenny Stavrakou, J.-F. Muller, F. Boersma, I. De Smedt, R. van der A
tropospheric NO2 columns retrieved by the GOME and SCIAMACHY
satellite instruments between
January 1997 and December 2006 are used together with the IMAGES CTM and its adjoint to
construct a top-down inventory for anthropogenic, pyrogenic
and natural NOx emissions. The
influence of the emission updates on the chemical NOx lifetime is taken into account, and
found to have a significant impact on the results.
Anthropogenic emission trends are
inferred over industrialized regions of the Northern Hemisphere. The largest emission
increases are found over eastern China, and in particular
in the Beijing area, whereas
important emission decreases are calculated over the U.S., and to a lesser extent over
Europe. The emission changes result in significant trends in
surface ozone, amounting to
15%/decade over large parts of China in
- The use of satellite
measurements for estimation of multi-annual changes in NOx emissions
Igor Konovalov, M. Beekmann, J.P. Burrows, A. Richter
It has been
suggested in several recent studies that the available
NO2 time series from satellite measurements can be used to study long-term changes in anthropogenic
emissions of nitrogen oxides (NOx). It seems reasonable to expect that the most
accurate quantitative estimates of the emission changes can be obtained when the
measurement data are combined with model calculations which can account for the atmospheric
transport and changes in meteorology. We present the first inverse
modelling study of interannual changes of NOx emissions in Europe, the Mediterranean and Middle East. We combine
the data for tropospheric NO2 columns derived
from the long-term measurements performed by GOME and SCIAMACHY satellite instruments between 1996 and 2005 with calculations of the
CHIMERE chemistry transport
model performed on a 10 by 10 degree grid. From this data set, we estimate separately a linear trend of emissions in each grid cell and the
variability superimposed over the linear trend. In contrast to a more common inverse modelling approach, where the improved emission estimates are searched for
deviations from “expert” estimates of emissions, our original method yields measurement-based estimates in each grid cell that are subject to only some
Therefore, our estimates can be regarded as a measurement-based alternative to trends derived from emission cadastres.
of the results is performed by comparing
CHIMERE calculations with independent data for near-surface NOx concentrations from the UK national monitoring network
and ozone concentrations from the EMEP network. It is found
that the use of our emission estimates in CHIMERE improves the agreement between the model
calculations and measurement data in both cases.
Our results confirm the
generally negative trend of NOx emissions in Europe during the past decade.
However, some considerable differences between the trends in the measurement-based and EMEP emission
data are found for several countries, especially outside of
Western Europe. It is found also that the NOx emission trends in several cities and regions (such as Moscow, Madrid,
Eastern Ukraine) are strongly different from
those averaged over a corresponding country. Among interesting features of interannual variations of NOx emissions is their strong
decrease in Iraq in 2003.
|Session Modelling (cont.) - Chair: Thomas Wagner|
- Evaluation of satellite NO2
columns over U. S. power plants using a regional atmospheric chemistry
Si-Wan Kim, A. Heckel, G. Frost, A. Richter, M.K. Trainer, J. Burrows, S. McKeen, E.-Y.
The western U.S. has many isolated point sources of NOx emissions, which have been observed
by various satellites such as GOME, SCIAMACHY, and OMI. Among
these, Four Corners and San Juan Power Plants in New Mexico provide a good site to evaluate the satellite-retrieved
NO2 columns. These are two of the highest
NOx-emitting power plants in the U. S. They produce strong satellite NO2 column signals and are isolated from big cities. Because NOx
emissions are independently
monitored directly at each power plant, there is much less uncertainty in their known emissions than in the emissions from roads and urban areas.
differences between satellite-retrieved and model-calculated columns for these plants will depend on other factors besides errors in emission inventories.
carried out simulations with the Weather Research and Forecasting-Chemistry model (WRF-Chem) for the western US domain during the summer of 2005. Initial
comparisons of the model
and SCIAMACHY NO2 columns give good correlations between the two methods over the model domain, indicating that the model reproduces the
relative spatial and temporal patterns of
emissions across the entire domain. In the Four Corners and San Juan region (2° longitude x 1° latitude), however,
summertime-average model NO2 columns are 30% larger than
satellite NO2 columns. These discrepancies are bigger than the model NO2 variability resulting from the
use of various chemical mechanisms. Some possible reasons for the
satellite-model discrepancies will be discussed.
|15:55||Presentation of posters - Chair: Thomas Wagner|
- Strategies for tropospheric
trace gas retrievals from satellite and comparison to model results
Thomas Wagner, Steffen Beirle, Michael Grzegorski, Ulrich Platt
retrieval of tropospheric trace gas
products from UV/vis satellite observations is a great challenge, because the sensitivity of such observations strongly dependens on several
height profile, surface albedo, and cloud and aerosol properties.
Since usually not all of these quantities can be directly derived from the
observation itself, a-priori or model information is typically used in the retrieval of tropospheric trace gas products (e.g. tropospheric vertical
column density or average
concentration). This mixing of information from measurement and other sources, however, often complicates the interpretation of the
derived trace gas products.
Here we discuss
different strategies to extract information on tropospheric trace gases from satellite information using different
degrees of additional (e.g. model) information. We also discuss
different ways for the comparison of measured trace gas data with model
- Impact of clouds on tropospheric trace gas retrievals
Steffen Beirle, Tim Deutschmann, Michael Grzegorski, Ulrich
Platt, Thomas Wagner
Spectroscopic measurements from
nadir-viewing satellite platforms allow the retrieval of column densities of several atmospheric trace
gases. The retrieval of tropospheric columns is thereby strongly affected by
clouds: Clouds shield boundary layer and lower tropospheric trace gases, leading to
an underestimation of the actual column. On the other hand, the high albedo of clouds, as well
as multiple scattering within the cloud, increase the visibility
of trace gases at and above the cloud top.
Cloud parameters like cloud fraction, cloud top height or cloud
heterogeneity can also be directly deduced from
satellite measurements, using intensity measurements and spectral absorption features of O2, O4 or the so-called
Here we analyze the dependency
of tropospheric NO2 columns on several cloud parameters. This empirical study is complemented by theoretical radiative transfer
modelling studies using the
3D-Monte-Carlo Model TRACY-2, that is in particular capable of modelling radiative transfer in clouds.
With these investigations we check and
understanding on the different cloud effects on radiative transfer (shielding, path-length enhancement and albedo increase). Improved knowledge on the impact of
on trace gas columns allows to interpret clouded pixels, that are currently discarded in most analyses. Temporal or spatial variations of the observed
dependencies of NO2 columns
on cloud parameters hold additional information on e.g. the NO2
- OMI and SCIAMACHY NO2 validation
by MultiAxis and DirectSun DOAS observations during the DANDELIONS campaigns
Gaia Pinardi, Michel Van Roozendael, Francois Hendrick, Caroline Fayt, Christian Hermans, Alexis Merlaud, Martine De Mazière
The DANDELIONS (Dutch Aerosol and Nitrogen Dioxide Experiments for
vaLIdation of OMI and SCIAMACHY) project has been set up in the Netherlands with the aim to contribute to the validation of OMI (Ozone Monitoring Instrument),
SCIAMACHY (Scanning Imaging Absorption SpectroMeter for Atmospheric CartographY) and AATSR (Advanced Along-Track Scanning Radiometer) measurements of aerosols
and nitrogen dioxide (NO2). Two measurement campaigns took place in Cabauw (52° N, 5° E), first from May to July 2005, and second in September 2006. These
gathered several types of complementary ground-based measurement techniques, such as in-situ samplers, LIDAR, MAXDOAS and Direct sun instruments.
work, total and tropospheric NO2 columns derived from a combination of ground-based Multi-Axis DOAS and Direct Sun DOAS measurements, are compared to OMI and
SCIAMACHY NO2 products obtained during both campaigns.
The Multi-Axis DOAS (MAXDOAS) technique rely on UV-visible scattered sunlight observations, whereby
NO2 absorption can be quantified using the Differential Optical Absorption Spectroscopy (DOAS) technique. By scanning viewing angles successively from zenith to
the horizon, atmospheric light paths of increasing length into the lower troposphere are sampled, so that the measured NO2 columns can be vertically resolved,
providing independent information on the tropospheric and stratospheric contents. Complementary to MAXDOAS observations, Direct Sun measurements are
characterized by a well defined geometrical path and therefore provide accurate total NO2 columns.
The aim of the study is to assess the overall
agreement between ground-based and satellite data sets, and to investigate the relative performance of various OMI and SCIAMACHY NO2 retrieval schemes. Analysis
are carried out with respect to several issues, among them the sensitivity of satellite measurements to clouds being present in the field of view (including the
role of the ghost column), the importance of co-location mismatch effects and, for OMI, the swath angle dependency.
- Multi-model ensemble simulations of tropospheric NO2 compared with GOME retrievals for the year 2000
Twan van Noije, H.J. Eskes, F.J. Dentener, D.S.
Stevenson, K. Ellingsen, M.G. Schultz, O. Wild, et al.
We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with
three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from
IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997–2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model
output is analyzed at 10:30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases
due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal
time scale give rise to a positive bias of 30–50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial
regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions (between –25% and
+10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column
densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual
models and retrievals are high, typically in the range 0.81–0.93 after smoothing the data to a common resolution. On average the models underestimate the
retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions
dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to
the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the
seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the
retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the
amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals
during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals
cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal
reduction of the NO2 abundance at 10:30 local time (by 2.5–4.1% over Europe). Overall, there are significant differences among the various models and, in
particular, among the three retrievals. The discrepancies among the retrievals (10–50% in the annual mean over polluted regions) indicate that the previously
estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of
tropospheric NO2 are strongly dependent on the choice of model and retrieval.
- Using MERIS data to
calculate NO2 airmass factors
Alexandre, A. Richter, A. Heckel, J.P.
Burrows, W. von Hoyningen-Huene, A. Kokhanovsky, T. Dinter
Nowadays there are a handful of
satellite instruments (GOME, GOME-2, SCHIAMACHY, OMI) providing measurements that can
be inverted to retrieve trace gas distributions in the atmosphere. These
measurements are of great importance to investigate the global distribution of pollutants as is the case
of ozone (O3), nitrogen dioxide (NO2) and several
others. With these global fields one can identify emission sources and analyse long-term trends of pollutant concentrations.
As in all remote sensing
techniques, the retrieval of tropospheric columns of NO2 from the satellite measurements is based on several assumptions that in one way or
another contribute to
the uncertainty in the final retrieval. The improvement of the a priori assumptions used as well as the characterisation of the uncertainties is a main
to obtain the correct values of NO2 present in the troposphere.
To obtain the vertical column of NO2 one must divide the slant column obtained from
measurement by an airmass factor (AMF). This AMF is dependent on many aspects such as: geometry and wavelength of measurement, vertical distribution of
the species, surface
albedo, aerosol loading and clouds. While some of these factors are well known others are highly uncertain and variable.
poster, we investigate the possibility to
use data from the MERIS instrument to provide measured values for aerosol optical thickness, spectral surface direct
reflectance and possibly also cloud cover to be used in the
computation of airmass factors for SCIAMACHY retrievals. Like SCIAMACHY, MERIS is also operating on
ENVISAT and provides data which are collocated with SCIAMACHY measurements in
space and time. This approach has the potential to replace current values which are
either from climatologies or low resolution atmospheric models by measured values and therefore
could provide more realistic retrievals, in particular over
regions where rapid changes of aerosols are frequently verified.
As this study is still work in progress, the
presentation will focus on sensitivity
studies and selected examples.
- the effect of updating reaction rate data on the tropospheric NO2 column simulated by TM4 as compared t GOME retreivals for the year 2000
Jason Williams, Twan van Noije, Henk Eskes, Ronald van der A
Here we present a systematic comparison of tropospheric NO2 columns calculated using the global chemistry transport model TM4 with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. Since the original study of van Noije et al (2006) the reaction rate parameters in TM4 have been updated using the latest recommendations (Atkinson et al, 2004, 2006; Sander et al, 2006). Here we investigate the differences introduced into the annual mean tropospheric NO2 columns by adopting these updated chemical rates. In general, there are decreases in the NO2 column density in regions influenced by biomass burning activity and increases in the NO2 column density for regions influenced by industrial NOx emissions. These changes are of the order of ~10% of the total tropospheric NO2 column. The reason for these differences is the increase in the reservoir species ORGNIT and a substantial modification in [HO2], respectively. Seasonal decompositions reveal that for regions influenced by a biomass burning season, this improves the agreement between NO2 columns retrieved by GOME and those calculated by TM4. However, for regions dominated by strong NOx emissions the improvements do not reconcile the large differences which occur between the model and the GOME retrievals.
- Comparison between tropospheric NO2 vertical columns by GOME and
surface NO2 mixing ratio by the air-monitoring network over Japan
Katsuyuki Noguchi, H. Itoh, T. Shibasaki, S. Hayashida,
I. Uno, A. Richter, J. P. Burrows
We compared the tropospheric NO2 vertical columns observed by GOME
[Richter et al., 2005] and the surface NO2 mixing ratio by the
air-pollution monitoring network composed of more than 1000 stations
over Japan during 1996-2003. The comparison of GOME and surface
measurements showed almost no long-term trends of NO2 over Japan.
The comparison also showed the similar seasonal variations of NO2,
which were asymmetric with a rapid increase in fall and a slow
decrease in spring. These consistencies suggest that the GOME
successfully observed the NO2 behavior in the lower troposphere
over Japan. We also examined how we should manipulate the data of
GOME and surface measurements. Due to a large coverage of one pixel
of GOME swath data, we should carefully select the swath pixels
which correspond to the surface measurements when we average the
GOME-NO2 from those pixels. We will discuss the effect of the GOME
swath pixel selection on the averages of GOME-NO2.
|16:25||Poster session & drinks|
|18:00||End day 2|
Wednesday 12 September
|Session Applications - Chair: Ernest Hilsenrath|
- INVITED TALK: Urban and Agricultural NOx Emissions
Ronald Cohen, T.H.
Analyses have yet to tap
the full potential of the high space and time resolution of SCIAMACHY, OMI or GOME-II data. Using example from OMI and
SCIAMACHY we describe observations of agricultural NOx
emissions with time resolution of individual rain driven pulses. We use the daily coverage of OMI to
describe day-of-week patterns in urban plumes that show both the well known
decrease in weekend emissions but also a memory effect such that Monday, a weekday
followed by a weekend is different than Wednesday, a weekday followed by a weekday. We will
also discuss a NOx feedback on the NOx lifetime via control over OH
in urban and agricultural
- Monitoring long range transport of tropospheric
NO2 with OMI
Bas Mijling, Ruud Dirksen, Ronald van der
Under favorable meteorological conditions, tropospheric NO2 can be transported fast enough (regarding its life
time) to cross the Northern Atlantic Ocean, and affect overseas
air quality. Due to predominant west winds in this region, long range transport usually occurs
from the industrialized East coast of North America towards the West coast of
To monitor the continental outflow of tropospheric NO2, we use data
of the OMI instrument, taking advantage of its daily global coverage and its relatively high
resolution (13 by 24 km at nadir). Retrievals of the slant column
are done with the DOAS technique. From this, the vertical tropospheric column is derived using a combined
modeling / retrieval / assimilation
The tropospheric NO2 data product of OMI, retrieved within the DOMINO project, includes a modeled ghost column to compensate
for the shielding
effect of clouds. For our purposes, we remove this ghost column to avoid that satellite observations done above cloudy scenes are contaminated
model results. Using this method we obtain an archive of daily images in which long range transport events of NO2 can clearly be distinguished.
order to quantify these
events and to detect the more subtle structures in the column densities of the transported NO2, the quality of the DOMINO-data needs to
be improved. With the newly reprocessed OMI
dataset, collection 3, several artifact in the data are removed, such as the cross track gradient (causing a
difference between eastern pixels and western pixels up to 0.8 1015
molecules/cm2) and clusters of unrealistically high NO2 values (caused by instrument
saturation due to optically dense clouds). Further improvement can be made by applying a new
destriping algorithm, which smoothes out the different offsets of
the pixels in the detector
generation of a
temporally consistent NO2 data record for ocean color work
Wayne Robinson, Ziauddin Ahmad, Charles R. McClain
Accurate ocean color retrievals depend on the accounting for
absorption affecting the visible radiances from 412 nm. up to the Near-IR.
Recently, the Ocean Biology Processing Group at NASA Goddard Space
been working on making an NO2 absorption correction
for the visible bands with GOME, SCIAMACHY, and OMI NO2 as the data
Differences in the NO2 from GOME and
SCIAMACHY to OMI have been seen to
cause discontinuities in the retrived water-leaving radiance
chlorophyll-a products. A correction has been developed to make a
consistent set of NO2 measurements so that these discontinuities are
The development of this correction and the results of the
|Session Applications (cont.) - Chair: Ernest Hilsenrath|
- Sources and fate of tropospheric NOx: What can we learn from satellite observations?
Satellite observations provide several years of tropospheric NO2 data on global scale. Characteristic spatial and temporal patterns, as well as correlations with other quantities like fire or flash counts, have been successfully used to identify and quantify different sources of nitrogen oxides. Furthermore, satellite measurements also provide information on the evolution of the emitted NOx. By studying transport, information on lifetime can be gathered.
This talk will give an overview on different approaches to the identification and quantification of different NOx sources, discussing their difficulties, limitations and results. In addition, the potential of studies on the NO2 transport for lifetime estimates will be demonstrated.
- OMI Tropospheric NO2 from Lightning in
Observed Convective Events
Kenneth Pickering, E. Bucsela, J. Gleason, P. Levelt
Lightning is responsible for an estimated 10-20% of NOx
emissions in the troposphere. In this
study, we present evidence of lightning-generated NO2 (LNO2) using data from the Ozone Monitoring Instrument (OMI), which has observed
tropospheric NO2 since
its launch in 2004. Although LNO2 has been also reported in previous satellite studies from the Global Ozone Monitoring Experiment (GOME) and SCIAMACHY,
better suited for such measurements by virtue of its higher resolution and daily global coverage. The LNO2 signal is clearly seen in OMI data on two days over
of convective systems in the US Midwest in 2006. We also present an analysis of OMI data over northern Australia during the SCOUT-O3/ACTIVE field
campaigns in November and
December 2005. Both single- and multi-day averages are presented to examine possible LNO2 signals from individual diurnally recurrent
convective events. In these events we
compare the OMI signals with aircraft observations from the storm
- Monitoring air quality in an urban area using remote
sensing techniques and in situ measurements.
Louisa Kramer, Roland. J. Leigh, John. J. Remedios, Paul. S. Monks
Monitoring urban air quality is an important issue and
remotely sensed data from ground and space based instruments are being extensively used to study air pollution problems over urban and regional areas.
The Leicester based UV/VIS Concurrent Multi-Axis Differential Optical Absorption Spectroscopy (CMAX-DOAS) system uses a remote sensing technique based on
the concept of observing several viewing geometries simultaneously. It utilises an instrument that images from multiple viewing angles using a single CCD, giving
an instrument that offers temporal resolution on every viewing angle of a minute or less.
Global NO2 data have been retrieved from measurements performed
by the Ozone Monitoring Instrument (OMI), launched onboard the NASA satellite Aura in July 2004. The relatively high spatial resolution of OMI makes it suitable
for measurements of air quality on an urban scale. However, the comparison of near-surface NO2 measurements from chemiluminescence detectors situated in and
around Leicester city centre with tropospheric NO2 columns from OMI and CMAX-DOAS demonstrates that NO2 emissions from a polluted urban area cannot be
simplistically linked to the column measurement by a satellite instrument. Here we demonstrate a FOV-weighted correction to obtain a relationship between column
measurements of NO2 from satellites to those at street level.
|12:30||End workshop & lunch|