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Introduction Nitrogen dioxide
Nitrogen oxides play a central role in tropospheric chemistry, and there are
several reasons why an improved knowledge of the global tropospheric
distribution of NOx (NO+NO2) is important:
Observing NO2 from space
An important step in filling the gap in our knowledge of tropospheric NOx
has been made by the GOME
instrument on ERS-2. The prime advantage of satellites is their capability
of providing a full global mapping of the atmospheric composition. After
cloud filtering, GOME provides global coverage NO2 maps rougly every week.
Column amounts of NO2 can be derived from the detailed spectral
information provided by GOME in the wavelength range 420-450 nm.
Good signal-to-noise ratio's (of about 20) are obtained for NO2
with the Differential Optical Absorption Spectroscopy (DOAS)
retrieval technique. This is related to the absence of strong other
absorbers (e.g. ozone) in this spectral interval. GOME has also
demonstrated the ability to observe boundary layer NO2: on top of
a stratospheric background enhanced column NO2 amounts are
observed that correlate well with known industrialised areas. GOME has also
detected NO2 plumes originating from biomass burning events. Furthermore, there are
signatures of lightning-produced NO2 in the GOME data set.
World map of the averaged tropospheric NO2 column measured by OMI in the period
May 2006 till February 2007.
Uncertainties in retrieval
A major challenge is the derivation of good quality quantitative
tropospheric NO2 column amounts for individual ground pixels based
on the satellite data. The retrieval of tropospheric trace gas species is
characterised by large uncertainties, related to clouds, the surface albedo,
the trace gas profile, the stratospheric column of NO2, and
The largest uncertainties are due to clouds, as they will shield
near-surface NO2 from the view of the satellite. The retrieval
depends very sensitively on the presence of clouds, and even small coud
fractions (between 5 to 20%) have a major impact. High quality observations
of the cloud properties (at least cloud fraction and cloud top height) are
necessary for a quantitative retrieval.
The surface albedo directly influences the sensitivity of GOME for boundary
layer NO2. High quality albedo maps in the relevant spectral range
Profiles of NO2 are characterised by a large range of variability. At
emission areas the NO2 concentration will peak at the surface, while
downstream of such areas the pollution plume will peak at higher altitudes.
The profile of NO2 will be determined by aspects like the distribution of
emission sources, the stability and height of the boudary layer, wet removal
of nitric acid, deep convection and long-range transport by the wind. All
these aspects are strongly varying in time and space.
The NO2 columns measured by GOME consist of comparable stratospheric and
tropospheric contributions. The stratospheric background has to be
quantified carefully in order to derive the tropospheric column. Atmospheric
dynamics is well known to generate significant variability in stratospheric
tracer amounts, consistent with for instance HALOE observations of NO2. A
standard approach applied to GOME is based on the assumption that
stratospheric NO2 is zonally uniform, or at least has only a small
longitudinal variation. Such simplification makes the retrieval of small
tropospheric NO2 columns practically impossible.
Another source of uncertainty are aerosols. Thick aerosol layers influence
the radiation field and the sensitivity of GOME for near-surface NO2.
- Boersma, K.F., H.J. Eskes and E.J. Brinksma, 2004. Error analysis for
troposheric NO2 retrieval from space. J. Geophys. Res., 109, 4311, doi: 10.1029/2003JD003961.
Retrieval of tropospheric NO2
Satellite instruments (such as GOME, SCIAMACHY and OMI) use spectroscopy to retrieve
atmospheric trace gas concentrations in the atmosphere. By comparing the measured
spectrum of the backscattered light from the Earth's atmosphere with a reference
spectrum, the column density of nitrogen dioxide along the light path can be
determined. The NO2 stratospheric column is deduced from a chemistry-transport
model assimilation run of the NO2 column data. Subsequently, the assimilated
stratospheric column is subtracted from the retrieved total column, resulting in
a tropospheric column. Information about the global tropospheric NO2 columns is
publicly available on the TEMIS website http://www.temis.nl. More details about
the satellite observations and the retrieval technique can be found in
NO2 has been monitored by satellite since 1995 with GOME, since 2002 with SCIAMACHY,
and since 2004 with the OMI instrument; the latter two instruments having the advantage
of a high spatial resolution.
- Eskes, H.J. and K.F. Boersma, 2003. Averaging Kernels for DOAS total-column satellite retrievals. Atm. Chem. Phys., 3, 1285 - 1291.
- Boersma, K.F., H.J. Eskes and E.J. Brinksma, 2004. Error analysis for troposheric NO2 retrieval from space. J. Geophys. Res., 109, 4311, doi: 10.1029/2003JD003961.
- Boersma, K.F., 2005. Satellite observations of tropospheric nitrogen dioxide; retrieval interpretation and modelling, Ph.D Thesis, Universiteitsdrukkerij Technische Universiteit Eindhoven, Eindhoven.
In the Figure above the mean tropospheric NO2 is shown as measured by OMI in the period May 2006
till February 2007. Clearly visible are the industrial regions in China, Europe,
South-Africa and the USA. The yearly averaged NO2 column for 2005 measured with SCIAMACHY
zoomed-in over China can be seen in the Figure below. It shows high concentrations of NO2 above
highly populated regions like Beijing, Shanghai, Hong Kong and South Korea. It can also
be seen that the satellite detects the emissions around the Yellow river (Huang He). Over
the sparsely populated western part of China, low NO2 concentrations are observed, except
over the large city Urumqi in the Northwest.
The yearly averaged tropospheric NO2 column measured by SCIAMACHY for 2004 in China.
High values are measured above the major cities. The industrial area around the Yellow
River (Huang He) is also noticeable and highlights the river stream.
Air quality monitoring
In Blond et al. it is shown that SCIAMACHY provides detailed information on the nitrogen
dioxide content in the planetary boundary layer. The cloud free satellite observations
were compared with surface measurements and simulations over Western Europe performed
with the regional air-quality model CHIMERE (shown in Figure below). The model has a
resolution of 50 km similar to the satellite observations. CHIMERE underestimates
surface NO2 concentrations for urban and suburban stations which we mainly attribute
to the low representativeness of point observations. No such bias is found for rural
locations. The yearly-average SCIAMACHY and CHIMERE spatial distributions of NO2 show a
high degree of quantitative agreement over rural and urban sites: a bias of 5% (relative
to the retrievals) and a correlation coefficient of 0.87 (n=2003). The consistency of
both SCIAMACHY and CHIMERE outputs over sites where surface measurements are available
gives confidence in evaluations of the model over large areas not covered by surface
observations. The NO2 columns show a high daily variability. Still, the daily NO2
pollution plumes observed by SCIAMACHY are often well described by CHIMERE both in
extent and in location. This result demonstrates the capabilities of a satellite
instrument such as SCIAMACHY to monitor the NO2 concentrations over large areas on a
regular basis. It provides evidence that present and future satellite missions, in
combination with a regional air quality model and surface data, will contribute to
improve quantitative air quality analyses at a continental scale.
- Blond, N., K.F. Boersma, H.J. Eskes, R.J. van der A, M. Van Roozendael, I. De Smedt, G. Bergametti and R. Vautard, 2007. Intercomparison of SCIAMACHY nitrogen dioxide observations, in-situ measurements, and air quality modeling results over Western Europe. J. Geophys. Res, in press.
Comparisons between annual means of, a) NO2 SCIAMACHY tropospheric columns
(1015molecules cm-2), b) NO2 CHIMERE tropospheric columns obtained by using the
averaging kernels, c) emissions of nitrogen oxides (NOx) over Western Europe for
10h00 UTC for 1998, and d) NO2 CHIMERE tropospheric columns computed without using
the averaging kernels. The emissions are derived from data given by EMEP,
interpolated on the CHIMERE grid domain, unit 1010 molecules cm-2 s-1.
Trends in tropospheric NOx emissions
The combined measurement series of both GOME and SCIAMACHY almost span a decade, which
favours a trend analysis of NO2 concentrations. To do so, the averaged monthly tropospheric
NO2 columns are fitted with a linear model that also includes a sinus to represent the
seasonal variation of NO2. The seasonal variation for anthropogenic NO2 is mainly
determined by the changing day length over the year. In absence of sunlight NO2 has
a longer lifetime in the atmosphere, which explains that the NO2 columns are on average
higher during wintertime. By applying the model to each grid cell a spatial distribution
of the fit parameters is calculated. Furthermore the precision of the trend is calculated.
It can be concluded that the 10 years long NO2 dataset from GOME and SCIAMACHY can be used
for significant trend analysis in most parts of the world. In highly populated and
industrialised areas the trend is large enough to be significant. For instance Shanghai
had a yearly increase of tropospheric NO2 of about 29% since 1996.
The Figure below shows the derived annual growth in the tropospheric NO2 columns from this analysis.
The largest trend is found in east China, where the economic growth is one of the fastest
of the world. The fastest growing city with respect to both economy and tropospheric NO2
is Shanghai. It is interesting to note that the growth in the region around Hong Kong is
less than for other regions with a high economical activity. This is probably due to the
already high level of economic activity in 1996 when our trend study started, and to a
package of measures against air pollution in Hong Kong over the last years. Further results
of this trend study are published in:
- van der A, R.J., D.H.M.U. Peters, H.J. Eskes, K.F. Boersma, M. Van Roozendael, I. De Smedt and H.M. Kelder, 2006. Detection of the trend and seasonal variation in tropospheric NO2 over China. J. Geophys. Res., 111, D12317, doi:10.1029/2005JD006594.
World map of the linear trend per year for tropospheric NO2 in the period 1996 till 2005
derived from satellite observations by GOME and SCIAMACHY. For the light grey areas no
significant trend has been found in the time series. For the dark grey areas not enough
observations were available to construct a time series of tropospheric NO2.
The fast growing emissions in China lead locally to rapidly increasing NO2 concentrations,
which affects the local ozone concentrations. Clearly these large increases will have
severe consequences for the local air quality, but even effects on the global scale can
be expected, because the lifetime of tropospheric ozone is much larger than the lifetime
of NO2. Therefore, ozone can be transported over large distances by the wind. Using a
chemical transport model the change in ozone due to increasing emissions in China can be
calculated. The Figure below shows increasing ozone concentrations in the Northern hemisphere
caused by the growing Chinese emissions in the period 1997-2005. In this period of eight
years the global averaged tropospheric ozone column has increased with 0.54 %. The
largest growth in tropospheric ozone we find in a plume reaching from China to the East
along the direction of the prevailing winds. From the Figure we conclude that the
tropospheric ozone concentrations in the entire Northern hemisphere are increased due
to the growing emissions in China. These increases seem small, but are still important.
In Europe, the air pollution has been increased as a result of intercontinental transport.
In addition, since ozone is a strong greenhouse gas, the effects on climate change cannot
- Kuenen, J.J.P., 2006. Anthropogenic NOx emission estimates for China based on satellite measurements and chemistry-transport modeling, KNMI, Technical Report TR-288, 62p.
The difference in ground-level ozone caused by the increase of Chinese NOx emissions
between 1997 and 2005.