Improving aerosol correction of OMI tropospheric NO2 over China, based on a CALIOP climatology – Happy to co-author the work of Liu et al. (2019)!

screenshot 2019-01-03 at 22.25.42
Seasonal spatial distribution of tropospheric NO2 VCD in 2012 for (a) POMINO v1.1, (b) POMINO, and (c) their relative difference.

A very nice work by Mengyao Liu et al. (2019), from Peking University in Beijing, China, has just been published in the Atmospheric Measurement Techniques (AMT) journal, on which I am co-author!

This study evaluated the possibility to improve OMI tropospheric NO2 retrieval over China by creating and exploiting an aerosol vertical profile climatology database from 9 years of CALIOP observations. Among other elements, it shows the potential benefits to use satellite observations in a synergistic way (OMIMODIS-CALIOP) and how to constrain better aerosol models in view of correcting aerosol scattering and absorption effects in UV-vis satellite measurements.

This notably leads to an update of the POMINO dataset from Lin et al. (2014, 2015).


More information?

Retrieving aerosol height and tropospheric NO2 from OMI – Ebook Thesis online!

Research Thesis book cover – Julien Chimot – July 2018

Almost 3 months ago, on 2018.09.10, I had the privilege to defend my research thesis at Delft University of Technology. A big moment after 4 years of intensive collaboration with my colleagues of the Geoscience & Remote Sensing Department and KNMI and in the presence of several friends and relatives. An very strict protocol to follow according to the Dutch rules and tradition.

My thesis book is now available online as an Ebook here! Feel free to have a look if you are interested by aerosol layer height retrieval, UV-Vis satellite measurements such as OMI, tropospheric NO2, air quality and climate observations. My main papers are concatenated there.

Enjoy the reading!

More than 20 years of satellite NO2 observations!

More than 20 years of Tropospheric NO2 satellite observations that we have now thanks to the great investments of Europe and all scientists. An example with the month of March GOME on ERS-2 (from the European Space Agency ESA) in 1998 & TROPOMI on Sentinel 5-Precursor (from Copernicus) in 2018. Maps are from TEMIS website.

Analyses of the trends is possible but overall a challenging and sensitive task. Over 20 years, very different sensor techniques, instrument specificities & degradations, variable pixel sizes, cloud detection possibilities etc… A lot of works to harmonise these data!


More information?

  • TEMIS website here
  • The European Copernicus program here
  • The European Space Agency here
  • NO2 – Nitrogen Dioxide WebPage here

Just another Day on Aerosol Earth by NASA – A well-done picture highlighting the very diverse aerosol types and their heterogeneous distributions!

Just Another Day on Aerosol Earth
NASA Earth Observatory images by Joshua Stevens, using GEOS data from the Global Modeling and Assimilation Office at NASA’s Goddard Space Flight Center (Credit NASA,


Have you seen this very recent visualisation aerosol mapping, released by NASA and made by Joshua Stevens? This visualisation very nicely highlights the different aerosol types and their complex distribution on Earth for a single day, 23rd August 2018, based on the GEOS FP model output. Some satellite observations were assimilated to constrain the modelling of atmospheric transport and physical processes.

Atmospheric aerosol are particles suspended in the air. Their sources are very mixed. Aerosol can be man-made or natural: e.g. smoke, desert dust, sea spray, nitrates and sulfates. The aerosol effects on the sunlight modify the shortwave radiation field in the atmosphere. This directly impacts the climate and the satellite observations devoted to ocean surface, land surface, vegetation, and atmospheric gases. Furthermore, heavy load of aerosols affects our air quality.

In spite of many progresses during the last 10-20 years, aerosol observations from space-borne instruments remain incredibly complex. One of the main reasons is their heterogeneity: aerosols are everywhere, but with very variable quantities  spatially (horizontally and vertically!), and temporally. And, as highlighted by this NASA picture, aerosol types are also very heterogeneous! Retrieving all these parameters from single satellite measurements, without ambiguity with respect to surface characteristics and clouds, is the difficult task of the scientists working with atmospheric satellite measurements. Many works to continue to do…


More information

  • NASA WebPage “Just another Day on Aerosol Earth” here
  • Aerosol WebPage here

A new paper submitted – Minimizing aerosol effects on the OMI tropospheric NO2 retrieval – An improved use of the 477 nm O2-O2 band and an estimation of the aerosol correction uncertainty

We recently submitted a new paper in the Atmospheric Measurement Techniques (AMT) journal. This work relies on the activities achieved during the last months with my colleagues of the Geoscience and Remote Sensing (GRS) department of TU Delft and KNMI: Dr. J. Pepijn Veefkind, Dr. Johan de Haan, Dr. Piet Stammess, and Prof. Dr. Pieternel  F. Levelt.

This paper is based on the last developments we published during 2016, 2017, and 2018. During these years, not only the OMI cloud algorithm was improved (Veefkind et al., 2016), but also an OMI aerosol layer height (and optical thickness) neural network algorithm was developed (Chimot et al., 2017, 2018). This time, we directly evaluate the impacts of these developments to correct of aerosol absorption and scattering effects in the visible spectral range in view of retrieving tropospheric NO2, an important trace gas affecting air quality in urban and industrialised areas.

Gotten curious? See more information here.

I greatly thank my co-authors from the Netherlands for this very interesting work! This paper closes the loop of my whole research work achieved during the last 4 years with the Geoscience and Remote Sensing (GRS) department of TU Delft and KNMI.

Statistics of relative tropospheric NO2 VCD changes in (%) in 2006-2007, due to differences between the different explicit aerosol corrections and the implicit aerosol correction based on OMCLDO2-New: (a), and (b): China summertime (June-July-August), (c), and (d): China wintertime (December-January-February), (e), and (f): South America biomass burning season (August-September).

A new paper published on OMI aerosol layer height retrieval from the O2-O2 visible band and neural networks – Comparison with CALIOP aerosol spatial patterns

I am very glad to have a new paper recently published in the Atmospheric Measurement Techniques (AMT) journal. This paper relies on a research work achieved during the last months with my colleagues of the Geoscience and Remote Sensing (GRS) department of TU Delft and KNMI: Dr. J. Pepijn Veefkind, Dr. Tim Vlemmix, and Prof. Dr. Pieternel  F. Levelt.

This paper is based on the work of 2017, in which a neural network algorithm was developed for retrieving aerosol layer height (ALH) from the OMI O2-O2 visible measurements. This time, we directly compare our retrievals with CALIOP aerosol observations and evaluate the spatial patterns on several remarkable case studies including urban pollution, biomass burning events and a Saharan dust outbreak!

Gotten curious? See more information here.

I greatly thank my co-authors from the Netherlands for this very interesting work!

No borders for particles! – Red Sun, Saharan dust and smoke over whole Europe

What happened on Monday 16th October 2017? From diverse countries (France, United Kingdom, the Netherlands, Finland), people reported to have observed a “red Sun”. This is a direct result of natural events, although issued from different sources and locations, which combined together over the whole Europe.

During a couple of days, between 16th and 18th October 2017, northern Portugal and Spain were victims of violent wildfires ravaging parts of their territory. These fires released large amounts of fine black particles, also named aerosols, that can absorb the Sun light.

Aerosol optical depth (AOD) (550 nm) simulated by CAMS forecasts of aerosol optical depth (AOD) show enhanced AOD extending from the Iberian peninsula CAMS) on 2017.10.16 (Source:

The Copernicus Atmosphere Monitoring Service (CAMS) forecasts of aerosol optical depth (AOD) show high values extending from the Iberian peninsula to the British Isles.

In addition to these fires, a tropical storm, named Ophelia, appeared 1300 km south-west of the Azores islands and some 2000 km off the African coast. Originally classified as a tropical storm, it was upgraded to a hurricane. The storm moved north-easterly, towards Spain and Britain, collecting sand from the Sahara desert. The related dust particles were then mixed with the black carbon from Portugal fires. Their scattering properties with the solar light led to this red Sun observed by many Europeans, instead of its natural yellow colour in clear sky or white/milky in presence of thin cloud.

The Ophelia storm as observed by the Sentinel-3 A satellite. Left: image on 11.102017, when Hurricane Ophelia was about 1300 km south-west of the Azores islands and some 2000 km off the African coast. Right: Brightness temperature of the clouds at the top of the storm, some 12–15 km above the ocean, range from about –50°C near the eye of the storm to about 15°C at the edges on17.10.2017. Copyright ESA (Source:

The dust particles can be observed through the visible colour composite image from the MODIS instrument, on-board Aqua, on 16.10.2017: some yellow colours are mixed with some thin clouds.

MODIS-Aqua visible image on 16.10.2017 (left) and 17.10.2017 (right) acquired in early afternoon: dust particles in yellow over France, united Kingdom and the Netherlands (Source:

It remains, overall, challenging for satellite measurements acquired in the visible spectrum to easily distinguish dust particles from transparent clouds or cirrus.

Aerosol Absorbing Index (AAI) from GOME-2 sensor on-board MetOp-B platform on 16-17-18.10.2017. Copyright KNMI, EUMETSAT and the Atmospheric Composition (AC) Satellite Applicattion facilities (SAF) (Source:

GOME-2 and OMI satellite sensors also reveal through the AAI index, with high values in red, the presence of black absorbing particles (i.e. smoke from biomass fires in Portugal) in large quantity. These particles were released in the North of Portugal before being transported to the North (UK) and then East (the Netherlands, Finland and Russia).

Aerosol Absorbing Index (AAI) from OMI sensor on-board the Aura platform on 16-17.10.2017. Copyright KNMI, FMI and NASA. (Source:

Although the aerosol particles were the visible part of the pollutant transport, IASI sensor revealed the additional presence of CO – Carbon monoxide, a toxic gas issued from incomplete biomass combustion by the fires. CO is a gas pollutant that cannot be visible in the eyes. It can only be measured in the shortwave or thermal infrared spectrum such as the IASI measurement.

Total CO – Carbon monoxide column observed by the IASI sensor on-board MetOp platforms. Copyright LATMOS-ULB & AC SAF (Source:

This impressive pollutant transport is quite unique due the combination of heterogeneous sources at different locations. But, similar episodes of particle and gas pollution were observed last Summer in Canada as well, as described in our last post Canada wildfires from space and ground – Seeing beyond the flames: a series of observations.

All these worldwide satellite pictures very well illustrate that, although emissions can be national, mix of pollutants and their transport are not contained within the limits of borders. This shows how much pollution and their scientific and societal challenges are an international concern!

More information?

  • CAMS, Saharan dust and smoke over France and UK here
  • GOME-2 AAI index by KNMI, EUMETSAT AC SAF & TEMIS here
  • OMI AAI maps on the TEMIS website here
  • The Ophelia storm as seen by the ESA Sentinel-3 mission here
  • OMI sensor here & IASI sensor here
  • Canada wildfires from space and ground – Seeing beyond the flames: a series of observations WebPage here