The Copernicus Sentinel 5 Precursor (S5P) satellite is more than one year operational and in its first year it has delivered many remarkable successes. Especially the high spatial resolution data (3.5 km x 7 km at nadir) of the S5P payload TROPOMI (TROPOspheric Monitoring Instrument) provides a wealth of information on emissions sources and the transport of pollutants. In addition, the combination of different atmospheric species observed by TROPOMI allows for a much better understanding of the processes in the troposphere. Already during its first year in orbit, the TROPOMI data have attracted new data users outside of the traditional user groups, such as NGOs that are interested in environmental data.

Whereas TROPOMI provides the users with superior data compared to previous missions, it also puts new challenges and opportunities to significantly improve the data products. This is partly a technical challenge, to further improve our description of the atmosphere and its underlying surface, for example by bringing in high spatial resolution surface reflectance information and atmospheric model data. On the other hand, new products should be defined by working closely with new user groups to implement their needs. For example, by defining new regional data products to fulfill needs from specific regions, or by working with policy makers to generate data products that they can directly interpret for verification of environmental policies. Working with such new user groups is challenging because it takes time and effort to match the user needs with the technical capabilities. Satellite data is often one piece of the information that should be incorporated with existing data streams.

In this contribution we will present cases that highlight the improved capability of TROPOMI compared to its predecessors. Using these case studies, we will discuss the strengths and the limitations of the current global data product and discuss potential new opportunities for exploiting the TROPOMI data.

Satellite observations of air pollutants have been used to estimate or constrain emissions for over twenty-years, primarily with the use of three-dimensional chemical transport models. More recently, alternative approaches have been developed that merge observations from nadir-viewing satellites with wind information from meteorological reanalyses. One such method for detection and quantification from larger point sources will be presented and applied to observations of nitrogen dioxide (NO₂), sulphur dioxide (SO₂), carbon monoxide, ammonia (NH3), and methane. This method utilizes a wind rotation procedure which enables observations from many overpasses combined with a customized plume function to which the satellite observations are fit. Observations from several different sensors, including the Ozone Monitoring Instrument (OMI), the Tropospheric Monitoring Instrument (TROPOMI), the Cross-track Infrared Sounder (CrIS), and the Infrared Atmospheric Sounding Interferometer (IASI), are utilized. An extension of the method to area or multiple point sources will also be presented.
A particular focus will be on the new TROPOMI sensor, which, owing to its superior spatial resolution, is able to map individual plumes and hence enable emissions to be derived from single-overpasses and to separate and quantify emissions from facilities in close proximity. Examples from urban areas and the oil and gas sector will be presented, which may also include emissions of methane (pending data availability).
Additionally, a brief survey of results will be presented from other global, regional, and individual case studies of natural and anthropogenic sources. These include (i) mysterious inconsistencies between OMI-derived and reported SO₂ emissions from the Canadian oil sands, (ii) a multi-sensor study of CO, NO₂, and NH₃ emissions from wildfires and comparisons with estimates from fire radiative power and fuel-consumed approaches, (iii) NOx emissions from global power plants and inferred emissions of CO₂, and (iv) point-source and area emissions of NH₃ derived from CrIS and IASI and compared with leading emissions inventories.

Prior to the Neolithic Revolution around 12 000 Yr BP, the Earth’s population was around 4 million hunter gatherers. Population grew to around 1 Billion at the beginning of the Industrial Revolution in the UK (1750-1800). Thereafter, its growth began to accelerate and population is now over 7.6 Billion with more than 50% living in megacities or urban agglomerations, known as Major Population Centres. The energy to fuel this growth has been in large produced from fossil fuel combustion. It has resulted in improved longevity, a rapidly increasing standard of living, supplied by an evolving industrial sectors. However, this has resulted in ever increasing and evolving emissions of pollutants and global land use change. This is impacting on air and water quality, human health, ecosystem services and climate change. Pollution extends from the local to global scale. The world has now entered a new geological epoch the Anthropocene. To assess accurately the impact of man in the evolving Anthropocene both space based and airborne measurement of atmospheric composition are a prerequisite.

The Effect of Megacities on the transport and transformation of pollutants on the Regional and Global scales, EMeRGe, project the impact of transport and transformation of plumes from MPC. It exploits the unique capabilities of the new High Altitude and LOng Range (HALO) aircraft research platform coupled with ground based and satellite data products (GOME-2, Sentinel 3 and Sentinel 5 Precursor). The impact of MPC emissions on air pollution at local, regional and hemispheric scales was investigated.

EMeRGe aims to improve our knowledge and prediction of the transport and transformation patterns of European and Asian MPC pollutant outflows. The short lived climate pollutants i.e. reactive gases, temporary reservoirs, and aerosol particles. The European Campaign took place in July of 2017 and the Asian campaign took place in the inter-monsoon period. In total around 400 scientists have been involved in the two campaigns.

The European Campaign took place from Oberpfaffenhofen, HALO was joined by the UK NERC FAAM aircraft and the CNR light aircraft. The aircraft sorties targeted the outflows from a) the MPC North of the Alps (e.g. London, Benelux, Paris Ruhr, Frankfurt and MPC) and b) the MPC South of the Alps (e.g. Po Valley, Rome, Barcelona and the western Mediterranean as pool). The impact of Biomass burning produced south of the Alps and that transported from North America were clearly observable in different layers. In addition, some dust events observed.

The EMeRGe Asian Campaign took place in March and April 2018. HALO was based in Taiwan and supported by scientists from Taiwan, Japan, South Korea, China, Thailand and the Philippines. The outflow from the MPC in China, Korea, Japan, the Philippines and Thailand were well observed. The impact of biomass burning from South East was significant in the first part of the campaign.
This presentation will focus on aspects of the EMeRGe Campaign which combined the aircraft data and the satellites data products, coupled with models to investigate our understanding of the emission, transport and transformation of MPC Pollution.

Nitrogen dioxide (NO₂) is a regulated air pollutant that is of particular concern in many cities, where concentrations are high. Emissions of nitrogen oxides to the atmosphere lead to the formation of ozone and particulate matter, with adverse impacts on human health and ecosystems. The effects of emissions are often assessed through modeling based on inventories relying on indirect information, that is often outdated or incomplete. Here we use NO₂ measurements from the new TROPOMI satellite sensor to directly determine the strength and distribution of emissions within European cities, and Paris in particular. From the observed build-up of NO₂ pollution over the city, we find highest emissions on cold weekdays in February 2018, and lowest emissions on warm weekend days in spring. We show that the new TROPOMI measurements allow quantification of emissions on a daily basis, and provide information on the spatial distribution of emissions within a large city, which helps to monitor and verify the effectiveness in meeting emission reduction targets.

The tropospheric NO₂ data from TROPOMI provide the opportunity to estimate the NO₂ emissions directly from the build-up of NO₂ observed over a city on a single clear-sky day, without the need for complex inversions with a chemistry-transport model. We have developed a new method (a variation of the well-known method presented by Beirle et al. [2011]) that allows inferring the NOx emissions from the observed NO₂ in air advected over the city, provided that wind speed and wind direction are known to good accuracy. Under non-stagnant conditions, the chemical decay of NO₂ in the boundary layer is of minor importance given the short time it takes for an air parcel to cross a city relative to the chemical lifetime. The quality of the NO₂ retrievals from TROPOMI is such, that it is not necessary to reduce noise by averaging satellite NO₂ distributions for a particular wind direction sector first. This avoids errors associated with interpreting average patterns based on an ensemble of individual plumes from different days with different wind directions and wind speeds. Instead, we can directly analyse the build-up of NO₂ pollution over the city from an individual TROPOMI orbit, thereby achieving one or sometimes even two estimates of city-wide NOx emissions on a particular day.

The method we developed converts the observed tropospheric NO₂ columns into one-dimensional line densities along the wind direction over the city. Using mean boundary layer wind speed at 12 UTC (13:00 hrs local time) from ECMWF data, and a loose constraint on boundary-layer mean OH concentrations (NOx lifetime) from the CAMS model as inputs, we constructed a simple model that simulates the NO₂ line density as a function of the along-wind distance over the city. This model simulates the build-up of NO₂ in the air column as a superposition of line densities caused by the emissions in sub-city scale cells. The resulting build-up reflects the underlying emission strength and pattern and is influenced by the first order chemical loss and wind speed over the city. Evaluating a large number of model realizations against the observed line density, identifies that function and its driving parameters NOx emission strength and loss rate constant that minimizes the differences between the modelled and the observed line density. We find that the observed along-wind line densities are described well by the modelled function giving the smallest residuals. Because of the TROPOMI overpass of approximately 13:40 hrs local time, the inferred NOx emissions are generally representative for the hours just prior to the TROPOMI overpass, i.e. noontime.

We compare the daily TROPOMI NOx emissions that we find over cities on clear-sky days to the TNO-MACC-III emissions projected for 2018. For Paris, our results indicate more pronounced weekend reductions and lower NOx emissions compared to the TNO-MACC-III estimates. This indicates that emissions from cars have likely decreased substantially over Paris, in line with many policies developed over the last decade to reign in the environmental burden of vehicles. The high-resolution TROPOMI measurements confirm that Parisian emissions are highest within the area enveloped by the A86 ring road. Our work demonstrates the potential of TROPOMI and follow-up geostationary sensors to monitor emissions in ever greater spatial and temporal detail, not just for nitrogen oxides, but also for other air pollutants such as carbon monoxide, sulfur dioxide, and formaldehyde. TROPOMI data holds a strong promise for the detection of emissions also from smaller cities and point sources, and is especially interesting for as of yet uncharted sources.

Since two decades, satellite instruments like GOME(-2), SCIAMACHY, and OMI allow the global retrieval of tropospheric NO2, with increasing spatial resolution. By temporal averaging, spatial patterns of anthropogenic sources can be clearly identified. In these averaged maps, the effective NOx lifetimes and emissions can even be quantified from the NO2 decay downwind from large NOx sources.

TROPOMI offers a high spatial resolution of 3.5 km times 7 km, which is more than 10 times better than OMI (nadir). This allows to detect also weaker sources and to partly resolve the spatial distribution of NOx sources within a megacity or urban areas. In addition, TROPOMI offers a high signal to noise ratio, leading to low noise in daily maps of tropospheric NO2, such that emission plumes from cities and large power plants are clearly visible. Thus, methods that have been applied to long-term means in the past can now be applied to daily measurements as well.

Single day measurements generally provide much higher contrast compared to temporal means, as upwind columns are lower than the mean, while downwind columns are far higher than the mean. From the upwind measurements, constraints on spatial extent of sources can be derived. From the downwind decay, a combined lifetime/emission fit can be performed on daily basis rather than for temporal means as done in previous studies. From multiple days, thus statistical analysis of the mean and variability of both NOx emissions and lifetime can be performed, allowing to investigate the consistency of the methodology and to infer temporal changes of emissions and potentially also the NO2 lifetime on daily basis.

Here we present first results of the analysis of lifetime/emission fits on daily basis, focussing on the Saudi-Arabian capital Riyadh.

To better exploit the high resolution NO2 observations of the new Sentinel-5p instrument TROPOMI, we have adapted our inverse algorithm DECSO (Daily Emission estimates Constrained by Satellite Observations) for NOx emissions to perform at a higher spatial resolution. While DECSO used to derive monthly regional NOx emissions on a 0.25 x 0.25 degree based on OMI observations, we have upgraded the resolution to 0.125 x 0.125 degree for the DECSO-TROPOMI version.
DECSO is an inverse modelling method to update daily emissions of NOx based on an extended Kalman Filter. NOx emissions are constrained by combining simulated NO2 column concentrations of a regional Chemical Transport Model (CTM) with satellite observations. The Eulerian regional off-line CTM CHIMERE v2013 is used to obtain simulated NO2 concentrations. A detailed description of DECSO can be found in Mijling and van der A (2012), while the latest improvements are described in Ding et al. (2017, 2018).
The new DECSO-TROPOMI version is using ECMWF operational data on a 0.125 degree resolution and TROPOMI NO2 concentrations (algorithm version 1.2) as input. New error estimates are made for the emission model and the combined observation-, CTM- and representation-errors. Because of the higher spatial resolution the model time step resolution had to be increased and the domain region had to be decreased to maintain a reasonable calculation time. Our first region of choice is the Iberian peninsula because of the various types of NOx sources in this region. However, the algorithm can without restriction be applied to other regions in the world.
The results will be demonstrated for emissions over the Iberian peninsula in the year 2018. Special attention will be given to the time series of weekly averaged emissions over several big cities like Madrid and the emissions of ships in the Strait of Gibraltar.

References:
Ding, J., van der A, R. J., Mijling, B., Jalkanen, J.-P., Johansson, L. and Levelt, P. F., Maritime NOx emissions over Chinese seas derived from satellite observations. Geophysical Research Letters, 45. https://doi.org/10.1002/2017GL076788, 2018
Ding, J., R.J. van der A, B. Mijling and P.F. Levelt, Space-based NOx emission estimates over remote regions improved in DECSO, Atmospheric Measurement Techniques, 2017, 10, 925-938, doi:10.5194/amt-10-925-2017.
Mijling, B. and R.J. van der A, Using daily satellite observations to estimate emissions of short-lived air pollutants on a mesoscopic scale. J. Geophys. Res., 117, 2012, doi:10.1029/2012JD017817.