Air Quality

The TROPOspheric Monitoring Instrument (TROPOMI) will be launched in 2016 onboard the ESA Sentinel-5 Precursor (S-5P) platform and will provide global observations of atmospheric trace gases, with unprecedented spatial resolution. Sulfur dioxide (SO₂) is a key atmospheric constituent and measurements from TROPOMI will improve the monitoring capability of SO₂ from anthropogenic and volcanic emissions, and will extend the long-term datasets from past and existing UV sensors (TOMS, GOME, SCIAMACHY, OMI, GOME-2, OMPS).

In this presentation, we give an overview on the work done on SO₂ retrievals as part of the S-5P level-2 processor development. Within the S-5P project, BIRA-IASB is in charge of the development of the prototype algorithm for the retrieval of SO₂, while MPIC and DLR are responsible for the verification algorithms, developed in parallel to test and challenge the prototype algorithm. We will describe the different algorithms, their main features and discuss the results of the inter-comparison of the SO₂ algorithms applied to synthetic and measured (OMI) spectra. We will also show results of the prototype algorithm applied to OMI with a focus on anthropogenic SO₂, for strongly polluted regions (including validation results in China) and weak sources (only detectable in long-term averaged data). 

Volcanic effects and their consequences have been observed in Europe originating either from European (Icelandic, Italy) or from distant large volcanic eruptions (e.g. Kasatochi in the Aleutians and Africa). The interference of the volcanic plumes with air traffic corridors have been noticed and studied thoroughly in the case of 2010 eruptions of Eyafallajökull. There have been similar eruptions that have not interfered with air traffic in the past decade such as the recent Bárðarbunga (September 2014) whose forward trajectories where below 6000m. The present study aims at looking for evidence of columnar SO2 amounts that have followed excursions from Icelandic and volcanic eruptions of importance to Europe in general. Columnar SO2 records from remote sensing spectrophotometers over Europe and from space as well as simulated by models will be compared. The columnar SO2 measurements are also compared with ground based SO2 monitors from the Airbase dataset. Finally the impact of the above mentioned volcanic eruptions in traffic will be assessed.

Satellite sulfur dioxide (SO2) measurements from the Ozone Monitoring Instrument (OMI) satellite sensor processed with the new Principal Component Analysis (PCA) algorithm, averaged over a period of several years, are used to detect large point emission sources. Roughly 200 continuously emitting point sources releasing from about 100 kT to 5000 kT of SO2 per year have been identified and grouped according to the source origin: volcanic, coal-burning power plants, smelters, and sources related to the oil and gas industry.  In addition, nitrogen dioxide (NO2) measurements by OMI are used to better differentiate between the source types. Examples of SO2 emission sources located in various regions of the world are given.

To estimate the emission levels from these sources, a new method has been developed. It is based on fitting satellite SO₂ vertical column density to a three-dimensional parameterization as a function of the coordinates and wind speed. An effective lifetime (or, more accurately, decay time) and emission rate are then determined from the parameters of the fit. The method has been validated using OMI data in the vicinity of approximately 50 large US near-point sources. The obtained results are then compared with available emissions inventories.  The correlation between the estimated and reported emissions is about 0.91 with the estimated lifetimes between 4 and 12 hours. It is demonstrated that individual sources with annual SO2 emissions as low as 30 kt y^-1 can produce a statistically significant signal in OMI data.

The obtained emission information can be used to improve available emissions inventories, since some of the sources seen by OMI are not included in the inventories.  SO2 measurements by two other satellite sensors, SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), and Global Ozone Monitoring Experiment -2 (GOME-2), are also discussed. Quantitatively, the mean amount of SO2 in the vicinity of the sources, estimated from the three instruments is in general agreement. However the better spatial resolution of OMI makes it possible for this instrument to detect smaller sources and with more details compared to the other two instruments.

The EU FP7 Monitoring and Assessment of Regional air quality in China using space Observations, Project Of Long-term sino-european co-Operation, MarcoPolo, project focuses on deriving emission estimates from space. Long term satellite observations of suphur dioxide, SO₂, over the greater China area from the SCIAMACHY/Envisat, GOME2/MetopA and OMI/Aura missions are compared and their relative strong points and limitations are discussed. Rigorous spatiotemporal statistical analysis based on novel analysis techniques [Fioletov et al., 2011; 2013] is performed for each data set in order to reduce noise and biases and enhance pollution signals in satellite datasets. Furthermore, identification of point sources such as power plants, smelters and urban agglomerations, as well as definition of their relative contribution to the regional SO₂ levels, form the main findings of this investigation. Comparison of different satellite datasets and their post-processed products with ground based MaxDOAS SO₂ measurements in Xianghe, China, located at ∼ 50 km southeast of downtown Beijing, helps validate the satellite datasets.

We present a new retrieval algorithm developed at NASA Goddard Space Flight Center for global observations of SO₂ and HCHO using spaceborne UV-Vis spectrometers. By applying the principal component analysis (PCA) technique to measured satellite reflectance spectra, we can extract spectral features (principal components or PCs) that explain the variance of the spectra. Since SO₂ and HCHO typically has very small signals outside of major source regions, the leading PCs (that explain the most variance) extracted from clean background areas can be used to represent atmospheric processes (e.g., ozone absorption, rotational Raman scattering) and measurement details (e.g., wavelength shift) other than the absorption by SO₂ or HCHO. We can then estimate the loading of SO₂ or HCHO, by fitting the PCs and pre-computed SO₂ or HCHO Jacobians to the measured radiance spectra. We will present PCA retrievals of SO₂ and HCHO using Aura/Ozone Monitoring Instrument (OMI) and NASA/NOAA S-NPP/Ozone Mapping and Profiler Suite (OMPS). We will also compare the PCA retrievals to other algorithms. For example, comparison with the previous generation OMI standard planetary boundary layer (PBL) SO₂ product indicates that the PCA algorithm reduces the retrieval noise by a factor of two and greatly reduces retrieval artifacts. This allows the detection of weaker anthropogenic SO₂ sources from space. Finally, we will discuss the most recent progress in our algorithm development, as well as the contributions that our group can make to the Sentinel-5P (S5P) mission as a member of both the ESA S5P validation team and the NASA Earth Science U.S. Participating Investigator program.