Abstract
Presented is a detailed description of the TES (Tropospheric Emission Spectrometer)-Aura satellite formic acid (HCOOH) retrieval algorithm and initial results quantifying the global distribution of tropospheric HCOOH. The retrieval strategy, including the optimal estimation methodology, spectral microwindows, a priori constraints, and initial guess information, are provided. A comprehensive error and sensitivity analysis is performed in order to characterize the retrieval performance, degrees of freedom for signal, vertical resolution, and limits of detection. These results show that the TES HCOOH retrievals (i) typically provide at best 1.0 pieces of information; (ii) have the most vertical sensitivity in the range from 900 to 600 hPa with ~2 km vertical resolution; (iii) require at least 0.5 ppbv (parts per billion by volume) of HCOOH for detection if thermal contrast is greater than 5 K, and higher concentrations as thermal contrast decreases; and (iv) based on an ensemble of simulated retrievals, are unbiased with a standard deviation of ±0.4 ppbv. The relative spatial distribution of tropospheric HCOOH derived from TES and its associated seasonality are broadly correlated with predictions from a state-of-the-science chemical transport model (GEOS-Chem CTM). However, TES HCOOH is generally higher than is predicted by GEOS-Chem, and this is in agreement with recent work pointing to a large missing source of atmospheric HCOOH. The model bias is especially pronounced in summertime and over biomass burning regions, implicating biogenic emissions and fires as key sources of the missing atmospheric HCOOH in the model.
Highlights
Formic acid (HCOOH) plays an important role in tropospheric chemistry
We carried out a series of simulated retrievals to test the performance of the Tropospheric Emission Spectrometer (TES) HCOOH measurement in a situation www.atmos-meas-tech.net/7/2297/2014/
Due to the low signal-to-noise ratio (SNR) over the oceans, we include here only retrievals over land
Summary
Formic acid (HCOOH) plays an important role in tropospheric chemistry. It regulates pH-dependent reactions in clouds (Keene and Galloway, 1988) and is one of the main sinks of in-cloud OH radicals (Jacob, 1986). The ACE-FTS makes solar occultation measurements using limbviewing geometry, and the resulting data are restricted to the upper troposphere and lower stratosphere Another limb instrument, the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding)-Envisat emission spectrometer, was used to examine the regional and seasonal variability of HCOOH at 10 km, and revealed very strong seasonal cycles over southern Africa and Indonesia–Australia (Grutter et al, 2010). The present paper will describe the retrieval of HCOOH from global, high spectral resolution nadir Tropospheric Emission Spectrometer (TES) measurements, an FTS on NASA’s Aura platform (Beer et al, 2001). The retrieval performance (i.e., sensitivity, error estimates) is evaluated in a simulation environment, and initial global TES retrievals (year 2009) are provided and compared against model output for the same year, in order to assess the degree to which the observed spatiotemporal variability in HCOOH is consistent or at odds with present scientific understanding of its sources and sinks
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