Cross-track Infrared Sounder (CrIS) satellite observations of tropospheric ammonia

Abstract

Abstract.Observations of atmospheric ammonia are important in understanding and modelling the impact of ammonia on both human health and the natural environment. We present a detailed description of a robust retrieval algorithm that demonstrates the capabilities of utilizing Crosstrack Infrared Sounder (CrIS) satellite observations to globally retrieval ammonia concentrations. Initial ammonia retrieval results using both simulated and real observations show that (i) CrIS is sensitive to ammonia in the boundary layer with peak vertical sensitivity typically around ∼ 850-750 hPa (∼ 1.5 to 2.5 km), which can dip down close to the surface (∼ 900 hPa) under ideal conditions, (ii) it has a minimum detection limit of ∼ 1 ppbv (peak profile value typically at the surface), and (iii) the information content can vary significantly with maximum values of ∼ 1 degree-of-freedom for signal. Comparisons of the retrieval with simulated "true" profiles show a small positive retrieval bias of 6 % with a standard deviation of ∼ ±20 % (ranging from ±12 to ±30 % over the vertical profile). Note that these uncertainty estimates are considered as lower bound values as no potential systematic errors are included in the simulations. The CrIS NH 3 retrieval applied over the Central Valley in CA, USA, demonstrates that CrIS correlates well with the spatial variability of the boundary layer ammonia concentrations seen by the nearby Quantum Cascade-Laser (QCL) in situ surface and the Tropospheric Emission Spectrometer (TES) satellite observations as part of the DISCOVER-AQ campaign. The CrIS and TES ammonia observations show quantitatively similar retrieved boundary layer values that are often within the uncertainty of the two observations. Also demonstrated is CrIS's ability to capture the expected spatial distribution in the ammonia concentrations, from elevated values in the Central Valley from anthropogenic agriculture emissions, to much lower values in the unpolluted or clean surrounding mountainous regions. These initial results demonstrate the capabilities of the CrIS satellite to measure ammonia.

NH<sub>3</sub> emissions from large point sources derived from CrIS and IASI satellite observations

“…This larger relative bias can occur for observations close to the detection limit of the CrIS instrument, which conservatively is about 0.9 ppb as reported by Shephard and Cady-Pereira (2015), but under ideal conditions reaches down to ∼0.3 ppb (Kharol et al, 2018) (Assuming that 1 ppb is equal to a total column of 2±1x10 15 molecules cm −2 this leads to a detection limit of 5 1.8±0.9x10 15 molecules cm −2 ). Note, Version 1.5 of the CrIS NH 3 retrievals only contains values that have a detectable NH 3 spectral signal, thus in regions where there is a significant fraction of the values below the detection limit the mean values can be biased towards values at or above the detection limit (see Shephard et al (2019) for more details). For this study only daytime observations between Jan 2013 and December 2018 are used.…”

“…The instrument 5 covers a wide swath of up to 2200 km width with circular pixels that have a nadir diameter of 14km at nadir. In this study we use version 1.5 data of the CrIS-Fast Physical Retrieval (FPR)-NH 3 product (see Shephard and Cady-Pereira (2015) and Shephard et al (2019) for more details). The CrIS-FPR retrieval is a physical retrieval based on the optimal estimation method (Rodgers, 2000) that uses the fast Optimal Spectral Sampling(OSS) OSS-CrIS (Moncet et al, 2008) forward model to minimize the residual between the measured and simulated spectra.…”

A flexible and robust neural network IASI‐NH3 retrieval algorithm

“…The vertical evolution of NH 3 emitted from the ground can be observed in the first atmospheric layers with surface instruments [e.g., Erisman et al, 1988], but the majority of the measured profiles useful for satellite retrievals are acquired by airborne instruments. These cover a larger part of the tropospheric column, provide useful probing of the vertical distribution inside the column, and allow consistent validation of satellite quantities [e.g., Sun et al, 2015;Shephard et al, 2015;Van Damme et al, 2015a]. Nevertheless, the spatial and temporal coverage of such data sets is very heterogeneous, as the majority of observations are acquired during dedicated campaigns.…”

First detection of ammonia (NH<sub>3</sub>) in the Asian summer monsoon upper troposphere

“…Subsequently, various papers have been published describing retrieval, validation and interpretation of NH 3 derived from the nadir sounders TES (Clarisse et al, 2010;Shephard et al, 2011), IASI (Infrared Atmospheric Sounding Interferometer) Clarisse et al, 2009Clarisse et al, , 2010Van Damme et al, 2014), CrIS (Cross-track Infrared Sounder) (Shephard and Cady-Pereira, 2015), and AIRS (Atmospheric Infrared Sounder) (Warner et al, 2016). The vertical sensitivity of these satellite retrievals is mainly limited to the lower troposphere up to about 3-4 km, and no altitude resolution is achieved (e.g., Clarisse et al, 2010;Shephard and Cady-Pereira, 2015). Recently, retrievals of NH 3 vertical column amounts from ground-based Fourier transform infrared (FTIR) solar observations located at various sites have been presented by Dammers et al (2015) and are being used for the quantitative validation of spaceborne nadir-viewing datasets (Dammers et al, 2016).…”