Earth Matters

Matters of Scale, and Why They Matter

March 25th, 2013 by jallen

Recently, we published a data visualization showing tropospheric NO2 over the Indian Ocean. The effort got us to thinking about how we try to present data in a way that’s easy to interpret while staying true to the science.

The visualization below of satellite measurements of NO2 in the atmosphere revealed the location of shipping lanes in the Indian Ocean. Ships tend to pass consistently along the same paths — through the Red Sea, across the Arabian Sea, across the southern end of the Bay of Bengal, through the Malacca Straits — to major ports in eastern Asia. On any given day, the exhaust fumes from a few ships do not provide a dramatic signal. But by making a long-term average (2005 through 2012) of data, the small day-to-day fluctuations add up to a discernible signal.

Global NO2 (2005-2012)
Global NO2 palette


One of the other things we did in building this visualization was to mask the land surfaces with light grey, in order to emphasize the NO2 over the oceans. But what happens if we take off that gray blanket over the land masses?

Global NO2 (with land mask removed)
Global NO2 palette

Oh my! Pretty much anywhere there are people, there’s a saturated pool of NO2. All of Europe looks like a putrid mass of polluted air, as does eastern China, the cities of the Middle East, the Himalayan regions of India and Pakistan. In fact, pretty much anywhere there are significant human populations, there is NO2 running right off the scale! You can still see the ship tracks, but it’s the deep, over-saturated brown-orange that grabs your attention.

If you want to show concentrations over land, you need a breath of fresh air, like this:

Global NO2 (no land mask; scaled 0 - 20E15 molecules/cm^2)
Global NO2 palette (0 - 20E15 molecules/cm^2)

This is a better way to show NO2 emissions over land.  Distinct signals show up around industrialized cities in Europe, the Middle East, and southern Asia, as well as fire emissions in equatorial Africa. Eastern China is still a saturated mess, as are some of the major industrial areas elsewhere in China. Heavy industrialization and an increase in automobiles for transportation has resulted in levels of atmospheric pollution in China not seen since the 1940s to 60s in the U.S. and Europe.

But this third map scarcely shows the NO2 emissions over the sea, and the ship track signals are hardly discernible, even though we are still using the same exact set of data in all three visualizations. So what is going on?

Look carefully at the color palette, or scale bar, below each map describing how different colors reflect different concentrations of NO2. The high end of the scale has been changed; in fact, it has been multiplied by a factor of ten in the last version. When compared to land-based sources of pollution, ship tracks are quite faint. As much as ships contribute to NO2 pollution, they can’t compare to land-based sources.

That makes sense, if you think about it. If a single ship emitted the same amount of NO2 each day as a small coal-fired power plant, you would expect the signals to match. But the ship is not sitting still; it is moving back and forth across thousands of miles of open ocean and its emissions are thinned out over long distances and time. It is only when there are  hundreds of similar ships traveling along the same route that the signal begins to build; and even then, the emissions are still spread across a vast area in a way that land-based sources are not.

So for our story on ship tracks, we made the visualization with tight limits on the NOconcentration in order to bring out the signal from the noise. Had we not masked out the land sources, the ship tracks would have been  lost.


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12 Responses to “Matters of Scale, and Why They Matter”

  1. Mark says:

    This is amazing and a clarification of what we are doing to ourselves and our children.

    The explanation of the visualization has clarified the data in a most horrifying way.

    Thank you again NASA.

  2. Louise Lacey says:

    It would be more than interesting to see the other continents and their production.

    • Jesse Allen says:

      Yes, it would be. We’re working with some scientists on some future visualizations of NO2 data from the OMI instrument based on their research. In this instance, we were focused on the ship tracks, which are most dramatic in the Indian Ocean, so chose projections centered on that area of interest. To talk about global land-based emissions, we’d give a global view, not just one hemisphere.

  3. Rahul Goswami says:

    Dear Jesse, very good entry. The explanations about the scale bar colour should be helpful to all those who similarly need to show relative change over time. I’d like to see EO do a comparison, using this NO2 data set, of industrialisation and respiratory health impacts. Perhaps The Lancet will be interested.

    • Jesse Allen says:

      Thank you! The Earth Observatory is a news agency, reporting on science rather than generating science, so we’d be reporting on that work when the research gets done rather than going out and actively seeking health research partners. That said, health officials do use these data and similar ground-based observations in their work.

  4. Richard Swann says:

    I wonder, is the concentration over eastern China due to weather conditions (no wind) or is that due to the sheer volume of emissions?

    • Jesse Allen says:

      NO2 breaks down pretty quickly (it’s highly reactive), so it doesn’t tend to drift from its source far before breaking down into something else. So it’s not (much of) a question of atmospheric circulation. Industrialization in China has lend to a huge increase in emissions. If we did a comparison from 2005 to 2012, you’d see quite a few changes, and the greatest increases have been in China.

  5. Azad Abul Kalam says:

    It’s a amazing documents for us and also for our next generation.
    This is right time to do something to protect our globe.
    I’m great full to Jesse.
    Thank you Jesse and thanks a lot to NASA

  6. Sergio A. Prado says:

    NO2 concentration is for sure alarming not only in Asia, Europe and AFrica but in Amerca and Oceania.

    My question regards all those scientific projects that invest milions to search water on Mars, but we do not invest THE SAME MONIES to protect all the physical forms of OUR WATER ON EARTH. It means TO TAKE CARE OF our oceans, the artic and antartic ices, our rivers and all sources of our drinkable water… Suppose we discover water on Mars and we loose water on Earth, how will be transported to the earth that water?
    This assimetry to protect our earth before to try do discover water on our galaxy reflects that our scientists and politicians maybe are a little bit BLIND when they are not able to PUT IN ACT NEW POLITICAL AND FINANCIAL drives to INVERT the tendence to loose the treasure that we still have!
    Best regards my blind friends!

    • Jesse Allen says:

      Well, working in the Earth Sciences myself, I must confess to a bias in favour of studying the Earth. NASA has been very active in research focused on studying and understand the Earth for decades now. It’s just not the only thing NASA does and does well… and there is a certain appeal that makes a satellite orbiting Mars making spectacular measurements while a remote control rover rolls around on the ground to provide highly detailed local measurements compared to, say, a Landsat observation of the Earth and a field geologist making highly detailed local measurements of the same area. Both seem really important to do and do well, within the fiscal realities of what can be done. Mars research is being done in conjunction with studies of the Earth, not at the expense of neglecting the Earth. Both deserve our attention.

  7. Wiebina Heesterman says:

    Just a week ago the Guardian newspaper published an article about pollution in UK cities.photochemical smog with a high NO2 content. The article warned about the harm to asthma sufferers and the damage to children’s lungs. That same day I happened to see long rows of cars waiting at the gate of the local school. Parents don’t realise that children inside cars are exposed to polluted air as much as children in the street.
    The UK government is being taken to the Supreme Court by ClientEarth, an organisation of activist environmental lawyers over its failure to meet European laws on nitrogen pollution. Jesse’s map makes it very visible. I would love to show it to those who think they protect their children by driving them everywhere. However, there was only a tiny strip of the UK visible on the map.

  8. Raymond Williams says:

    From Australia, we would say visualisation, and express the data per Capita. That is divide a spatial population layer into your air quality pollution spatial layer. Thanks Dr Allen.