Preparation for Sentinel S5P and Air Quality 3

Formic acid (HCOOH) is one among the most abundant volatile organic compounds (VOCs) present in the atmosphere. HCOOH sources include emissions from vegetation, soil and biomass burning. To a lesser extent, it is also produced by motor vehicles. It is mainly a secondary product from organic precursors.

HCOOH plays a role in the oxidizing capacity of the troposphere and on the global budget of tropospheric ozone (O₃), and it is also a source of rain acidity in remote areas. There are however large uncertainties on sources and sinks and HCOOH is misrepresented in the global emissions inventories.

In this work, we retrieve concentrations from spectra recorded by the IASI (Infrared Atmospheric Sounding Interferometer) instrument launched onboard the MetOp-A satellite in 2006. IASI is a nadir looking Fourier transform spectrometer, sounding the atmosphere with a global coverage twice per day. The HCOOH global distributions are derived using a new retrieval approach, based on conversion factors between brightness temperature differences and representative retrieved total columns.

We present global distributions and comparisons with FTIR measurements obtained at La Reunion, Wollongong and Jungfraujoch from 2008 to 2013. The IASI instrument provides a 6-year record for different regions, highlighting the signatures from biomass burning events and allowing the study of seasonal and interannual variations.

In 2016, the Sentinel 5 Precursor mission will be launched with the TROPOMI instrument as its single payload. It will deliver daily global measurements of the atmospheric composition for air quality and climate application as part of the Copernicus atmospheric services.  In this presentation, we focus on the measurements of the shortwave infrared (SWIR) spectral range providing global distributions of CH_4, CO, H₂O and its isotope HDO. Starting with the status of the SWIR instrument module and its calibration, we discuss the operational data processing of the SWIR trace gases including an estimate of the data quality. The main challenge is to account for scattering by water clouds, cirrus and tropospheric aerosols without exceeding the computational constraints of the processing facility.  To demonstrate the maturity of the algorithms, we show applications of the algorithms to GOSAT and SCIAMACHY measurements and the verification with on-ground measurements.

Nitrogen dioxide is one of the key atmospheric trace gases which are detectable by UV/visible remote sensing from satellite. It plays an important role in stratospheric ozone chemistry, both by acting as ozone depleting catalyst and by removing reactive halogen oxides into less reactive reservoir substances. In the troposphere, NO2 is a pollutant which adversely affects human health, leads to photochemical ozone smog formation and contributes to acidification of rain and surface waters.

Tropospheric nitrogen dioxide columns have been retrieved from measurements of the GOME, SCIAMACHY, and GOME-2 instruments, as well as from OMI observations and more recently OMPS data. For the upcoming TROPOMI instrument on S5P, two nitrogen dioxide retrieval algorithms are being developed, the prototype which will be used in the operational processor and the verification algorithm, which combines different approaches to the retrieval of tropospheric, stratospheric and total columns of NO₂.

Here, we report on work performed on improvements of the spectral retrieval including analysis of effects found at very large NO₂ absorptions and attempts to use the spectral information to derive a rough estimate of the vertical distribution of NO₂. The basic ideas of the selected approaches will be presented, sensitivity studies on synthetic data be reported and first applications on real GOME-2 data be shown.

Glyoxal is, like formaldehyde, a short-lived intermediate in the oxidation of non-methane volatile organic compounds (NMVOC) emitted by plants, vegetation fires and anthropogenic activities. It is also identified as a precursor of secondary organic aerosols (SOA). Both compounds absorb in the UV-visible spectral region and have been measured by the SCIAMACHY satellite sensor since 2003, and more recently, by OMI and GOME-2. Previous modelling studies using SCIAMACHY data have pointed to the existence of large additional sources, in particular over forests (Stavrakou et al. 2009), and more recently over Eastern China, most likely due to aromatic hydrocarbons (Liu et al. 2012), suggesting that glyoxal can serve as an indirect estimator of urban VOC sources.

The current study is motivated by (i) recent advances in our understanding of chemical pathways leading to glyoxal formation, in particular from the oxidation of isoprene, the most largely emitted NMVOC, (ii) the existence of numerous in situ concentration measurements for the key anthropogenic glyoxal precursors (e.g. actylene, aromatics) over industrialized areas, which can be used to narrow down the anthropogenic emission estimates in these regions, and  (iii) substantial improvements in retrieval algorithms for glyoxal columns from UV-visible satellite instruments, which has led to an significant reductions of the number of unphysical negative columns over the oceans as well as to generally lower glyoxal columns over continents.

In this study, the chemical mechanism and NMVOC emission inventories of the global CTM IMAGESv2 are revised based on recent investigations. The relative importance and possible uncertainties of different chemical pathways leading to glyoxal formation in the oxidation of isoprene are determined by box model simulations. Next, GOME-2 and OMI glyoxal and formaldehyde data are used to constrain the emissions of biogenic, pyrogenic and anthropogenic VOCs. To that effect, the inverse modelling technique using the adjoint model of IMAGESv2 is used. The role of model uncertainties is explored through a number of sensitivity studies. The model results are evaluated against selected ground-based observations. The consequences for e.g. SOA and O3 formation in polluted areas are also briefly discussed.

Biomass burning strongly affects the atmospheric composition. Also, biomass burning is strongly related to climate. Traditional methods to estimate biomass burning emissions are based on satellite measurements of burned area and fire radiative power. An alternative approach is the observation of trace gases from space. Carbon Monoxide (CO) is arguably the most interesting species in this respect. Its atmospheric lifetime of about 1-2 months ensures good tracebility of biomass burning plumes in the atmosphere.

In the recent years, we developed a data assimilation system to ingest large amounts of IASI CO column measurements with the aim to improve biomass burning emission estimates. The system is based on the TM5 atmospheric chemistry transport model, coupled to a 4DVAR data assimilation system. We applied the system, zooming in on strong biomass burning areas in the tropics, viz. South America and Africa. We find that our system is able to capture the biomass burning CO plumes very well. However, we also find that the emissions from the GFED4 bottom up inventories have to be modified. In South America and South Africa, the GFED4 CO emissions in the early burning season appear to be too high. In contrast, emissions seem to be too low near the end of the biomass burning season. In Northern Africa, GFED4 emissions appear to be too high also.

In the presentation I will discuss the system, the results, and some uncertainties related to other sources of CO. Possible future applications of the system will also be discussed.

The ESA project „SEOM-Improved Atmospheric Spectroscopy Databases (IAS)“ will improve the spectroscopic database for retrieval of the data products CO, CH₄, O₃ and SO₂ column amounts measured by the TROPOMI instrument (TROPospheric Monitoring Instrument) aboard the Sentinel 5 precursor. The project was launched in February 2014 with 3 years duration.  The spectroscopy of CO, CH₄ and O₃ in the 2.3 µm region is covered in the first 2 years, while UV measurements of SO₂ and UV/FIR/IR measurements of ozone will be carried out in the last year. User requirements for the spectroscopic database were obtained by retrieval simulations, indicating the need to take line mixing into account in case of CH₄, to measure HDO in case of water and even to measure line broadening of CH₄ lines by water vapor. A dedicated line model will be used to represent the laboratory spectra since the Voigt routine was found to be not sufficient to obtain the needed accuracy. Details on the retrieval simulations will be given together with the user requirements and the laboratory measurement plan. Measurements of pure water and ambient temperature air broadened measurements of water were finalized in March 2015 utilizing a high resolution Bruker IFS 125HR Fourier-Transform spectrometer at DLR. Complementary, continuous wave cavity ring-down measurements of pure water were also finalized in Grenoble, France. First quality checks and intercomparison of the data will be shown. Furthermore, the latest laboratory results will be presented.