Abstract
Abstract. Presently only limited sets of tropospheric ammonia (NH3) measurements in the Earth's atmosphere have been reported from satellite and surface station measurements, despite the well-documented negative impact of NH3 on the environment and human health. Presented here is a detailed description of the satellite retrieval strategy and analysis for the Tropospheric Emission Spectrometer (TES) using simulations and measurements. These results show that: (i) the level of detectability for a representative boundary layer TES NH3 mixing ratio value is ~0.4 ppbv, which typically corresponds to a profile that contains a maximum level value of ~1 ppbv; (ii) TES NH3 retrievals generally provide at most one degree of freedom for signal (DOFS), with peak sensitivity between 700 and 900 mbar; (iii) TES NH3 retrievals show significant spatial and seasonal variability of NH3 globally; (iv) initial comparisons of TES observations with GEOS-CHEM estimates show TES values being higher overall. Important differences and similarities between modeled and observed seasonal and spatial trends are noted, with discrepancies indicating areas where the timing and magnitude of modeled NH3 emissions from agricultural sources, and to lesser extent biomass burning sources, need further study.
Highlights
Global high-spectral resolution nadir measurements from the Tropospheric Emissions Spectrometer (TES) on National Aeronautics and Space Administration (NASA)’s Aura platform enable the simultaneous retrieval of a number of tropospheric pollutants and minor trace gases in addition to standard operationally retrieved products
In the atmosphere NH3 can combine with sulfates and nitric acid to form ammonium nitrate and ammonium sulfate, which constitute a substantial fraction of fine particulate matter (PM2.5) (e.g. Seinfeld and Pandis, 1988)
The TES NH3 retrieval is based on an optimal estimation approach that minimizes the difference between the observed spectral radiances and a nonlinear radiative transfer model driven by the atmospheric state, subject to the constraint that the estimated state must be consistent with an a priori probability distribution for that state (Bowman et al, 2006)
Summary
Global high-spectral resolution nadir measurements from the Tropospheric Emissions Spectrometer (TES) on NASA’s Aura platform enable the simultaneous retrieval of a number of tropospheric pollutants and minor trace gases in addition to standard operationally retrieved products (temperature, water vapor, ozone, carbon monoxide, and methane). The smaller footprint of TES (5 × 8 km) allows for the potential to detect localized NH3 sources These TES sensor characteristics and a sophisticated global retrieval algorithm provide the capability to obtain a more detailed estimate of NH3 not previously available over most of the globe. NH3 observations provided by Beer et al (2008) and include: (i) detailed description of the TES NH3 retrieval strategy including error characterization; (ii) estimation of the TES level of detectability of NH3 under various conditions based directly on the SNR; (iii) evaluation of the TES NH3 retrieval performance using simulations; (iv) TES NH3 observation examples showing the spatial and seasonal variability of NH3 globally; (v) initial comparison results of TES observations with GEOS-Chem model output globally and over twelve distinct regions
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