Global distributions of atmospheric ammonia (NH
Humankind has increased the global emissions of reactive nitrogen to an
unprecedented level (Holland et al., 1999; Rockström et al., 2009). The
current global emissions of reactive nitrogen are estimated to be a factor
of 4 larger than pre-industrial levels (Fowler et al., 2013). Consequently,
atmospheric deposition of reactive nitrogen to ecosystems has substantially
increased as well (Rodhe et al., 2002; Dentener et al., 2006). Ammonia
(NH
Despite the fact that NH
Advanced IR sounders such as the Infrared Atmospheric Sounding Interferometer
(IASI), the Tropospheric Emission Spectrometer (TES), and the Cross-track
Infrared Sounder (CrIS) enable retrievals of atmospheric NH
Only a few studies have explored the quality of the IASI-NH
First, we concisely describe the ground-based FTIR retrieval and
IASI-NH
Mean IASI-NH
The first global NH
The FTIR-NH
FTIR-retrieved NH
FTIR stations used in the analysis. The location, longitude,
latitude, and altitude are given for each station as well as the instrument
used for the measurements. Typical emission sources are mentioned in the
station specifics tab. The topography describes the geography of the region
surrounding the site.
An effort has been made to gather observations from most of the station part
of the Network for the Detection of Atmospheric Composition Change (NDACC),
which have obtained relevant solar spectra between 1 January 2008 and
31 December 2014. We excluded stations which have only retrieved or are
believed to have NH
The Bremen site was operated on the university campus by the University
of Bremen in the northern part of the city (Velazco et al., 2007). Bremen is
located in the northwest of Germany, which is characterized by intensive
agriculture. It is most suitable for comparisons with IASI given the very
high observed concentrations (Fig. 2, blue) and flat geography surrounding
the station. NH
The Toronto site (Wiacek et al., 2007) is located on the campus of
the University of Toronto, Canada. The city is next to Lake Ontario with few
sources to the south. NH
The Boulder observation site is located at the NCAR Foothills Lab in Boulder, Colorado, United States of America, about 60 km northwest of the large metropolitan Denver area. It is located at 1.6 km a.s.l. on the generally dry Colorado Plateau. Directly to the west are the foothills of the Rocky Mountains and to the east are rural grasslands, as well as farming and ranching facilities. Among them are large cattle feed lots to the northeast near Greeley approximately 90 km away. The area is subject to occasional seasonal local forest fires and also occasionally sees plumes from fires as distant as Washington or California. The retrieved columns (Fig. 2, grey) show the largest increase during summers.
The Tsukuba site (Ohyama et al., 2009) is located at the National
Institute for Environmental Studies (NIES) in Japan. The region is a mixture
of residential and rural zones with mountains to the north. NH
The Pasadena site lies on the northern edge of the Los Angeles
conurbation in the United States of America, at the foot of the San Gabriel
Mountains which rise steeply to the north to over 1.5 km altitude within
5 km distance. Local sources of NH
The Mexico City site is located on the campus of the National
Autonomous University of Mexico (UNAM) at 2280 m a.s.l., south of the
metropolitan area. Surface NH
The measurement site on the university campus of Saint Denis (Senten
et al., 2008) is located on the remote Réunion Island in the Indian Ocean.
Observed NH
The Wollongong site is located on the campus of the University of
Wollongong. The city of Wollongong is on the southeast coast of Australia
with the university only about 2.5 km from the ocean. The measurement site
is also influenced by a 400 m escarpment 1 km to the west and the city of
Sydney 60 km to the north. NH
The Lauder (Morgenstern et al., 2012) National Institute of Water
and Atmospheric Research (NIWA) station in Central Otago, New Zealand, is
located in a hilly region with NH
NH
Applied data filters to the IASI-NH
Any hill or mountain range located between the satellite pixel and the FTIR station may inhibit transport and decrease their comparability. To account for the topography we only used observations that have at maximum an altitude difference of 300 m (in) between the location of the FTIR and the IASI pixel position. The 300 m criterion was chosen based on tests using the FTIR and satellite observations from Lauder. For the calculation of the height differences we used the Space Shuttle Radar Topography Mission Global product at 3 arcsec resolution (SRTMGL3, Farr et al., 2007).
NH
The error of the IASI-NH
The lowest detectable total column of the retrieval depends on the thermal
contrast of the atmosphere (Van Damme et al., 2014a). For example, the
retrieval has a minimum detectable NH
Correlation
The complete list of selection criteria is summarized in Table 2.
No filters were applied to maximize the number of observations usable in the
comparison. The resolution and detection limit of the FTIR instruments is
usually better than that of the IASI instrument, leading to retrieved columns
with, in principle, less uncertainty. Overall the FTIR retrievals show an
error of
When performing a direct comparison between two remote sensing retrievals,
one should take into account the vertical sensitivity and the influence of a
priori profiles of both methods. One method to remove the influence of the a
priori profile and the vertical sensitivity is the application of the
averaging kernels of both retrievals to the retrieved profiles of both
products. The IASI-NH
Time series of NH
Following the approach of Irie et al. (2012) we will first show the
correlation
For most stations an increasing
Overall the highest correlations are seen at the Bremen site, which can
partially be explained by the overall high number of observations with high
concentrations (more than 15–20
From the correlation analysis as a function of spatial coincidence, we
conclude that an
Observations from multiple years are used to show the coincident seasonal
variability of the FTIR and IASI-NH
Correlations between the FTIR and IASI total columns with filters
thermal contrast
Figures 5 and 6 show a direct comparison of the FTIR and IASI NH
Summarized results of the comparison between FTIR-NH
Correlations between the FTIR and IASI total columns with filters
thermal contrast
Recent satellite products enable the global monitoring of atmospheric
concentrations of NH
In this study, we collected FTIR measurements from nine locations around the
world and followed the retrieval described by Dammers et al. (2015). The
resulting datasets were used to quantify the bias and evaluate the seasonal
variability in the IASI-NH
To optimally compare the satellite product to the FTIR observations it is
best to reduce the spatial collocation criterion to the size of the satellite
instrument's footprint and allow for a time difference as short as possible.
These considerations are to reduce effects of transport, chemistry, and
boundary layer growth but limit the number of coinciding observations
significantly. We have shown that the spatial distance between the IASI
observations and the FTIR measurement site is of importance: the larger the
distance in space, the lower the correlation. When there is no exact match in
the position of both observations the variations in the spatial separation
lead to correlation coefficients that can greatly change even when changing
the spatial criteria (
Overall we see a broad consistency between the IASI and FTIR observations.
The seasonal variations of both datasets look similar for most stations.
Increased column values are observed for both IASI and FTIR during summers as
the result of higher temperatures, with some sites showing an increase in
concentrations due to manure application and fertilization events in spring
(Bremen, Toronto). In general our comparison shows that IASI underestimates
the NH
In comparison to ground-based in situ systems, the FTIR observations have the
big advantage to provide coarse vertical profiles, from which a column can be
derived, which are more similar to what the satellite measures and therefore
more useful for validation. Dedicated NH
The direct comparison of the IASI and FTIR columns is an addition to earlier
efforts by Van Damme et al. (2015a) to validate IASI column observations with
surface in situ and airborne observations. Our results presented here
indicate that the product performs better than the previous upper-bound
estimate of a factor of 2 (i.e.
Although the FTIR observations offer some vertical information, studies
combining this technique with tower or airborne observations are needed to
further improve knowledge and sensitivity of the FTIR and satellite
observations to the vertical distribution of NH
The IASI-NH3 product is freely available at
This section further covers the other stations, in addition to the sites covered by Sect. 3.1.
The results for Mexico City show an overall constant correlation coefficient
except for small criteria
Correlation
This work is part of the research programme GO/12-36, which is financed by the Netherlands Organisation for Scientific Research (NWO). The Lauder NIWA FTIR programme is funded through the New Zealand government's core research grant framework from the Ministry of Business, Innovation and Employment. We thank the Lauder FTIR team for their contribution. We acknowledge the Université de La Réunion and CNRS (LACy-UMR8105 and UMS3365) for their support of the Réunion Island measurements. The Réunion Island data analysis has mainly been supported by the A3C project (PRODEX Programme of the Belgian Science Policy Office, BELSPO, Brussels). The University of Toronto's NDACC contribution has been supported by the CAFTON project, funded by the Canadian Space Agency's FAST programme. Measurements were made at the University of Toronto Atmospheric Observatory (TAO), which has been supported by CFCAS, ABB Bomem, CFI, CSA, EC, NSERC, ORDCF, PREA, and the University of Toronto. Part of this research was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. IASI has been developed and built under the responsibility of the “Centre national d'études spatiales” (CNES, France). It is flown on-board the Metop satellites as part of the EUMETSAT Polar System. The IASI L1 data were received through the EUMETCast near-real-time data distribution service.
The IASI-related activities in Belgium were funded by Belgian Science Policy Office through the IASI Flow Prodex arrangement (2014–2018). Pierre.-F. Coheur, Lieven Clarisse, and Martin Van Damme also thank the FRS-FNRS for financial support. Lieven Clarisse is a research associate with the Belgian F.R.S-FNRS. Cathy Clerbaux is grateful to CNES for scientific collaboration and financial support. The National Center for Atmospheric Research is supported by the National Science Foundation. The Boulder observation programme is supported in part by the Atmospheric Chemistry Observations & Modeling Division of NCAR. The measurement programme and NDACC site at Wollongong have been supported by the Australian Research Council for many years, most recently by grants DP110101948 and LE0668470. The Mexico City site was funded through projects UNAM-DGAPA (109914) and CONACYT (249374, 239618). A. Bezanilla, J. Baylón, and E. Plaza are acknowledged for their participation in the measurements and analysis. We would like to thank David Griffith, Clare Murphy, and Voltaire Velazco at the School of Chemistry, University of Wollongong, for maintaining Fourier transform spectroscopy (FTS) instrumentation and conducting FTS measurements. We are grateful to the many colleagues who have contributed to FTIR data acquisition at the various sites. Edited by: R. Müller Reviewed by: two anonymous referees