Preparation for Sentinel S5P and Air Quality 4

Bromine monoxide (BrO) is an important catalyst in the depletion of tropospheric and stratospheric ozone (O₃). In the troposphere, reactive bromine can be released from sea ice, volcanoes, sea-salt aerosol or salt lakes and high concentrations of BrO are associated with the autocatalytic ‘bromine explosion’ cycle. For all of these natural sources enhanced BrO vertical column densities (VCDs) have been successfully observed from ground using Differential Optical Absorption Spectroscopy (DOAS). Until now, satellite observations were only reported for polar regions during springtime and volcanic emissions (mostly for major eruptions).

We present the first satellite observations of enhanced monthly mean BrO VCDs over a salt marsh, the Rann of Kutch (India/Pakistan), during 2004-2014 as seen by the Ozone Monitoring Instrument (OMI). The Rann of Kutch is a so-called ‘seasonal’ salt marsh. During India's summer monsoon (June/July – September/October), the flat desert of salty clay and mudflats, which average 15 meters above sea level, fills with standing rain and sea water. With more than 7500 km² it is the largest salt desert in the world and additionally one of the hottest areas of India with summer temperatures around 50 °C and winter temperatures decreasing below 0 °C. Probably due to these rather extreme conditions, the Rann of Kutch has not been yet investigated for atmospheric composition measurements by ground-based instruments. Satellite observations, however, provide the unique possibility to investigate the entire area remotely over a long-time period.

The OMI data reveals recurring maximum BrO VCDs during April/May, but no enhanced column densities during the monsoon season while the area is flooded. In the following months the signal only recovers slowly while the salty surface dries up. We discuss the possible effects of temperature, precipitation and relative humidity on the release of enhanced reactive bromine concentrations. In order to investigate a possible diurnal cycle of the BrO concentration, the OMI results (at a local overflight time around ~13:30) are compared to corresponding results from the Global Ozone Monitoring Instrument (GOME-2, local overflight time at ~9:30).

Sun-induced chlorophyll fluorescence (SIF) is an electromagnetic signal emitted in the 650-850 nm spectral range by the chlorophyll-a of assimilating plants. The monitoring of SIF from space can provide critical information about the photosynthetic functioning of terrestrial ecosystems. In fact, global retrievals of SIF from space have recently been achieved from a number of spaceborne spectrometers (namely, GOSAT, GOME-2, SCIAMACHY and OCO-2) originally intended for atmospheric research. Although not designed for land applications, those atmospheric instruments have turned out to provide the necessary spectral and radiometric sensitivity for SIF retrieval from space.

Despite these achievements, the exploitation of SIF measurements for most applications is strongly hampered by the coarse spatial resolution and low number of observations provided by existing instruments: most terrestrial ecosystems are highly heterogeneous at spatial scales larger than 5-10 km, whereas existing global SIF products are typically gridded in cell boxes with size ranging from 0.5 to 2º. This issue will be greatly improved with the advent of the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor.

TROPOMI is a push broom grating spectrometer combining a wide swath (2600 km) with high spatial resolution (7x7 km2 at nadir) and daily global coverage. TROPOMI will perform nadir observations in the 675-775 nm spectral window with a spectral resolution of 0.5 nm. Because of the similar spectral characteristics, approaches for SIF retrieval from TROPOMI can be based on methods developed for GOME-2 and SCIAMACHY. However, TROPOMI's much finer spatial resolution promises to improve substantially the information content of the SIF data with respect to existing data sets, for example over fragmented agricultural areas and over tropical rainforest regions, which will especially benefit from the much higher frequency of clear-sky observations. It can be shown that TROPOMI can reduce global uncertainties in SIF mapping by more than a factor 2 with respect to GOME-2, which comes together with an about 5-fold improvement in spatial sampling. In addition, TROPOMI will be the first imaging spectrometer ever to deliver global data with a moderate spatial resolution and a continuous spectral sampling of the red and near-infrared spectral regions (the so-called red-edge). Red-edge reflectance measurements by TROPOMI can be used to monitor a number of vegetation geophysical parameters of great value to interpret the SIF measurements and to convert them into quantitative estimates of photosynthetic fluxes.

In this contribution, we will provide a short review of the state of the art of global terrestrial fluorescence monitoring and will present a sensitivity analysis illustrating the breakthrough in the field that we can expect from TROPOMI.

Air quality is a major health concern in most megacities and important industrial regions. It is therefore important to properly quantify the emissions of the main air pollutants and to understand their transport pathways and their reactivity in the atmosphere. In recent years, there have been important progresses to monitor near-surface atmospheric composition from satellites, which offer the possibility to draw spatial distributions from local to global scale as well as time evolutions for a series of regulated air quality species. Among the sounders currently in orbit, those operating in the thermal infrared have usually their maximum sensitivity in the mid-troposphere but have been shown to be able to monitor air surface pollution when the temperature contrast between the ground and the air above it is sufficiently large.

In this work, we will present a series of results demonstrating the capability of infrared sounders to measure surface air composition. We will focus the presentation on the measurements of sulfur dioxide (SO₂) and carbon monoxide (CO) from the IASI instrument. More specifically, the development of a new product allowing the retrieval of SO₂ from IASI observations at global scale will be briefly presented. Global distributions of anthropogenic SO₂ surface pollution will be shown, focusing on the identification of the principal hotspots and of exceptional pollution events. In addition, we will provide results of CO pollution from IASI and show how in favorable conditions, the surface CO concentration can be de-correlated from the free tropospheric abundance. We will put the results in the perspective of current and future missions, which, by combining infrared and UV sounding missions, offer improved possibilities for the global surveillance of air quality.

We present global observations of tropospheric bromine monoxide (BrO) derived from the Ozone Monitoring Instrument (OMI) on the EOS-Aura satellite, based on cloud-slicing techniques.

BrO is a halogen oxide present mostly in the lower stratosphere, where it catalytically destroys ozone with about 25 times the efficiency of ClO. BrO also has a tropospheric component, where it is released from sea surfaces, at the interface of ocean water and sea ice in the polar spring, in volcanic plumes, and in the vicinity of salt lakes. Tropospheric BrO has been linked to mercury (Hg) deposition through BrO-induced conversion of gaseous Hg to reactive Hg, which is then deposited on the surface and enters the food chain, ultimately affecting human health.

As part of NASA's Aura Science Team, we are developing an OMI Tropospheric BrO data product that provides a unique global data set on BrO spatial and vertical distribution in the troposphere and stratosphere. Information of this kind is currently unavailable from any of the past and present bromine-monitoring instruments. We present first results of spatially and vertically resolved tropospheric BrO loading in equatorial to mid-latitudinal regions.

The vertical columns of formaldehyde (HCHO) retrieved from two satellite instruments, the Global Ozone Monitoring Instrument-2 (GOME-2) on Metop-A and the Ozone Monitoring Instrument (OMI) on Aura, are used to constrain global emissions of HCHO precursors from open fires, vegetation and human activities in 2010. To this end, the emissions are varied and optimized using the adjoint model technique in the IMAGESv2 global CTM on a monthly basis and at the model resolution. Given the different local overpass times of GOME-2 (9h30) and OMI (13h30), the simulated diurnal cycle of HCHO columns is investigated and evaluated against ground-based optical measurements at 7 sites in Europe, China and Africa. The agreement between simulated and ground-based columns is found to be generally better in summer (with a clear afternoon maximum at mid-latitude sites) than in winter, and the annually averaged ratio of afternoon to morning columns is slightly higher in the model than in the ground-based measurements.

Both optimizations infer reductions of the global biogenic and pyrogenic flux estimates compared to their a priori values and show a high degree of consistency. A reduction of the global annual biogenic emissions of isoprene is derived, by 9% and by 13% according to GOME-2 and OMI, respectively, compared to the a priori estimate of 363 Tg in 2010. The reduction is largest (up to 25-40%) in the Southeastern US, in accordance with earlier studies. The GOME-2 and OMI satellite columns suggest a global pyrogenic flux decrease by 36% and 33%, respectively, compared to the GFEDv3 inventory. This decrease is especially pronounced over tropical forests such as Amazonia and Thailand/Burma, and is supported by comparisons with IASI CO observations. In contrast to these flux reductions, the emissions due to harvest waste burning are strongly enhanced in the Northeastern China plain in June, as well as over Indochina in March. Sensitivity inversions are conducted in order to explore the possible impact of uncertainties associated to the priori errors on the emission fluxes, the cloud fraction filter applied to the satellite data, and the isoprene oxidation mechanism on the inferred estimates.

Emission inventories are developed for use in scientific applications and as input in urban, regional, continental or global scale air quality models. Furthermore, emission estimates are used by policy makers in order to evaluate progress towards emission abatement measures and to decide on future strategies.
Within the GlobEmission project (part of the Data User Element programme of ESA) emission estimates are developed from satellite observations of air constituents. The main advantage of space-based emission estimates are the spatial consistency, high temporal resolution and the rapid availability of these estimates to the user. The emission estimates are developed for specific applications of users that are involved in the project. A few examples of these applications are air quality modeling over China, monitoring emissions of the oil industry in the Middle East, isoprene emissions from the biosphere and the impact of fires. In this presentation examples will be given of those applications.

During the discussion, seed questions proposed by the chairs will be discussed with the audience.”