Global distributions, time series and error characterization of atmospheric ammonia (NH<sub>3</sub>) from IASI satellite observations

Abstract

Abstract. Ammonia (NH3) emissions in the atmosphere have increased substantially over the past decades, largely because of intensive livestock production and use of fertilizers. As a short-lived species, NH3 is highly variable in the atmosphere and its concentration is generally small, except near local sources. While ground-based measurements are possible, they are challenging and sparse. Advanced infrared sounders in orbit have recently demonstrated their capability to measure NH3, offering a new tool to refine global and regional budgets. In this paper we describe an improved retrieval scheme of NH3 total columns from the measurements of the Infrared Atmospheric Sounding Interferometer (IASI). It exploits the hyperspectral character of this instrument by using an extended spectral range (800–1200 cm−1) where NH3 is optically active. This scheme consists of the calculation of a dimensionless spectral index from the IASI level1C radiances, which is subsequently converted to a total NH3 column using look-up tables built from forward radiative transfer model simulations. We show how to retrieve the NH3 total columns from IASI quasi-globally and twice daily above both land and sea without large computational resources and with an improved detection limit. The retrieval also includes error characterization of the retrieved columns. Five years of IASI measurements (1 November 2007 to 31 October 2012) have been processed to acquire the first global and multiple-year data set of NH3 total columns, which are evaluated and compared to similar products from other retrieval methods. Spatial distributions from the five years data set are provided and analyzed at global and regional scales. In particular, we show the ability of this method to identify smaller emission sources than those previously reported, as well as transport patterns over the ocean. The five-year time series is further examined in terms of seasonality and interannual variability (in particular as a function of fire activity) separately for the Northern and Southern Hemispheres.

Validation of the CrIS fast physical NH<sub>3</sub> retrieval with ground-based FTIR

“…A recent study by Dammers et al (2016a) explored the use of Fourier transform infrared (FTIR-NH 3 , Dammers et al, 2015) observations to evaluate the uncertainty of the IASI-NH 3 total column product. The study showed the good performance of the IASI-LUT (look-up table; Van Damme et al, 2014a) retrieval with a high correlation (r ∼ 0.8), but indicated an underestimation of around 30 % due to potential assumptions of the shape of the vertical profile (Whitburn et al, 2016;IASI-NN, neural network), uncertainty in spectral line parameters and assumptions on the distributions of interfering species. The study showed the potential of using FTIR observations to validate satellite observations of NH 3 , but also stressed the challenges of validating retrievals that do not provide the vertical measurement sensitivity, such as the IASI-LUT retrieval.…”

“…Both the IASI-LUT (Van Damme et al, 2014a) and the IASI-NN (Whitburn et al, 2016) retrievals from observations by the IASI instrument aboard MetOp-A will be used. A short description of both IASI retrievals is provided here; for a more in-depth description see the respective publications by Van Damme et al (2014a) and Whitburn et al (2016). The IASI instrument on board the MetOp-A platform is in a sun-synchronous orbit and has a daytime overpass at around 09:30 LST (local solar time) and a night-time overpass at around 21:30 LST.…”

“…Toon, http://mark4sun.jpl.nasa.gov/pseudo.html). The NH 3 a priori profiles are based on balloon measurements (Toon et al, 1999) and refitted to match the local surface concentrations (depending on the station either measured or estimated by model results). For the interfering species a priori profiles we use the Whole Atmosphere Community Climate Model (WACCM, Chang et al, 2008, v3548).…”

“…Recent studies show the global distribution of NH 3 measured at a twice daily scale (Van Damme et al, 2014a, 2015a can reveal seasonal cycles and distributions for regions where measurements were unavailable until now. Comparisons of these observations to surface observations and model simulations show underestimations of the modelled NH 3 concentration levels, pointing to underestimated regional and national emissions Shephard et al, 2011;Heald et al, 2012;Nowak et al, 2012;Zhu et al, 2013;Van Damme et al, 2014b;Lonsdale et al, 2017;Schiferl et al, 2014Schiferl et al, , 2016.…”

“…Over the last decade the development of satellite observations of NH 3 from instruments such as the Cross-track Infrared Sounder (CrIS, , the Infrared Atmospheric Sounding Interferometer (IASI, Clarisse et al, 2009;Coheur et al, 2009;Van Damme et al, 2014a), the Atmospheric Infrared Sounder (AIRS, Warner et al, 2016) and the Tropospheric Emission Spectrometer (TES, Beer et al, 2008;Shephard et al, 2011) have shown the potential to improve our understanding of NH 3 distribution. Recent studies show the global distribution of NH 3 measured at a twice daily scale (Van Damme et al, 2014a, 2015a can reveal seasonal cycles and distributions for regions where measurements were unavailable until now.…”