A test of inexpensive LLS fine particle sensors

November 9, 2018

1. Intro

Fine particle measurements are hip, and unbelievable articles about the death-toll they cause abound. Usually traffic (and especially Diesel cars, the new villain of the block) are given as the main culprit. This is completely nonsense, as only about 25% of fine particles come from car engines; a big chunk has a natural origin, and a real big part comes from wood burning. Nevertheless, measuring these very small particles is important, but not easy. Here we speak about sizes less than 2.5um or less than 10 um (1 um = 1 micrometer = one thousands part of a millimeter). “Official” measurement devices are costly, typically in the 20000 Euro range and measure directly the mass per cubic-meter of dry air (in ug/m3). They are based either on the attenuation of radioactive beta radiation (BAM) or on a direct mass measurement: either by weighing a filter exposed to the dust (for integral measurements over longer periods) or by detecting the changes of the oscillation frequency of an oscillator on which the particles attach themselves. A real problem is humidity: many type of particles (e.g. salt dust) absorb water vapor and increase in size and mass, and so give faulty results. So professional sensors first dry the incoming air flow, which must be kept rigorously constant and be influenced by changes in atmospheric pressure or wind conditions.

Laser light scattering (LLS) sensors are completely different. A laser beam from a solid-state sensor is more or less diffracted by the number of particles in a black chamber, and the diffracted light is measured by a photosensor and analyzed by a micro-controller. Actually what is measured is the count of particles per volume, and from this count, by many assumptions and proprietary algorithms, a mass per volume is calculated. In the inexpensive sensors like the Nova SDS011 or the Plantower series the air is pushed into the measurement chamber by a small fan; so there is no drying, and the airflow is under the influence of changes of atmospheric pressure and wind. Clearly, this type of sensor cannot rival the professional ones, but they are far from useless. Many grass-root movements of “citizen science” use these sensors which often are in a price range between 15 and 50 Euros; nearly all are made in China or in Japan.

2. The meteoLCD test setup

I started working on these sensors in June by buying several SDS011 and a Airmaster AM7, which combines PM2.5 and PM10 with T (temperature), RH (relative humidity) and CO2 measurements. All these sensors do not store their measurements, but either give a binary stream (SDS011) or a stream of ASCII lines. So I added a Raspberry Pi nano-computer running a Python script to make a standalone device, which logged its data on the SD card. A third type of sensor acquired was the Airvisual Pro from the Swiss company IQAir. This is a real stylish device costing 259 Euro which has its own storage and WiFi communication facility. The picture below shows the Airmaster AM7 with the Rapspberry Pi mounted in a Stevenson hut on the meteoLCD terrace in a previous test:

The next test (which gave the data for the paper) had a SDS011 and an Airvisual Pro in the hut:

The black box on the SDS011 is the fan, and the shining case with the letter A is the measuring chamber. The serial output is read by the Raspberry Pi through a serial-to-USB converter.

3. Test results

The test ran from 25th Sep. to 22 Oct. 2018, and the full days 26/09 to 21/10 were used to compare the results to the of the nearest official station (Beidweiler). The full text of the report is here.

Let me just give the figure which shows that the inexpensive sensors were able to reproduce the variations of the average daily PM10 concentrations, and also the peak event on the 18th October:

BEIDWLR = Beidweiler station, AV = Airvisual Pro, AM = Airmaster. Left scale in ug/m3.

The test period was rather dry, with RH not exceeding 75%, a number which is given by some authors as the threshold above which measurement become strongly impacted by humidity. So a test during wet and foggy days will be repeated in the near future.

4. Conclusion

The test shows that these inexpensive LLS sensors are more than a useless gimmick. Sure, they lack the bell and whistles of the professional sensors costing 400 times more, and certainly should not be used as a basis for legal action. But they are able to give a nice picture of the ambient fine particle concentration, and its variation. If you want to impress, strike a match (or light a candle) and blow out the flame in front of a sensor. You will be surprised by spectacular peak values!

Plume Labs Flow: a round-trip record

October 14, 2018

In this blog I report a first round-trip made with the flow-sensor attached to my belt and my iphone recording the flow’s readings.

The picture shows the FLOW in its charging cradle and its packaging. The leather buckle is to attach the device to a backpack, a belt or otherwise. The dimensions of the noiseless, silent FLOW are height = 90mm (or 140mm with the leather buckle), base 40mm x 25mm, weight = 70 g. There seems to be a miniature, inaudible fan inside. More info here and here.

  1. Introductory comments

The trip was made the 13th October 2018 afternoon in fine weather, blue sky, moderate temperature of 27°C, low wind, low humidity. I started at the parking near the entrance of the Echternach lake, and returned to the same point. Here a first screen-shot made at home later in the afternoon from the iphone displaying the Flow app:

The trip started at approx. 2:45 local time (12:45 UTC); the green line left to the red start point corresponds to the trip in the car from my home to the lake. The AQI of 32 represents the average of the total move (so probably including the green part at the left). Let me remember that the AQI from a set of pollutants is the specific AQI of the pollutant having the highest AQI (it is NOT an average or weighted sum of all the sub-AQI’s!). If you are confused, please re-read my 5 part series on AQI .

Dwelling deep into Plume Labs poor explanations of their AQI (which sometimes they label PAQI), I presume that they use more or less the AQI breakpoints defined by EPA, i.e. the EAQI. So if you want to know the concentration of a relevant pollutant, you must use the following EPA table in reverse, going from AQI to concentration:

The Plume Labs plot uses the same 7 categories and the same colors as EPA does.

2. The extraordinary PM10 peaks at the start.

The next picture shows the 2 very high PM10 peaks recorded at the start of the trip, i.e. not far away from the parking lot:

First notice that NO2 and VOC levels are zero or close to zero (which has to be expected!). The AQI peak at the cursor corresponds to a PM10 fine particle concentration of about 380 ug/m3 when using the EAQI table to get the concentration from the AQI, and a “Very Unhealthy” situation. This is a very high and unexpected PM concentration for a semi-rural location without any industry. There is a relatively busy road about 250m away (main road from Luxembourg-City to Echternach), but I do not expect it being such an important source of PM10s (and the low NO2 seems to confirm that traffic does not play a role here). The surrounding of the lake is mostly meadow and forest on the West side (left), and meadows on the East side (right); the wind was more or less blowing from the West. There is a small road (label 378) on the East side of the lake with practically zero traffic. So we have here something close to a mystery, as some comparisons made at home with 3 other sensors do not shows an exaggeration in PM10’s by the Flow ? Are the PM10’s of natural origin, as dust blown over by some wind gusts from the dry meadows?

3. Unsuspected NO2 peak

The next picture shows a curious NO2 peak at the last third part of the trip:

The cursor corresponds to a position close to a small parking lot and a picnic area with 3 public barbecues, one of them being active (and smelly!). My best guess is that the plumes from this barbecue caused the higher NO2 concentration of about 56 ug/m3. The small peak right of the cursor at 3:15 (15:15) is caused by higher PM2.5 (AQI 118, about 38 ug/m3), not NO2, possibly also related to barbecuing.

So it seems plausible that the Flow sensor correctly recorded a footprint of the nuisances caused by an open barbecue burning charcoal.

4. Conclusions

The Flow app is a nice feature, but I do have some serious complains:

1. why does the app not show in addition to the AQIs the concentrations of the pollutants ? There are so many different AQIs (EAQI, Chinese CAQI, European EAQI etc..) that simply stating that the AQI is a qualifier of the pollution is not enough.

2. why does the app not allow to download the data, as they must be stored in some part in the smartphone’s memory?

3. why does Plume Labs not give technical details on the sensors used in the device? How are its readings influenced by humidity and temperature?

There is an avalanche of low-cost sensors on the market, and one should not expect too much regarding accuracy. I made some short comparisons with three other PM sensors in my office (two SDS011, and a SNDWAY SW-825). Using the EPA table for the Flow AQI’s, a typical situation for PM2.5 concentration is ug/m3 is: FLOW = 0.6, SW-825 = 3, SDS011 = 3.6 and 2.6.

The FLOW PM2.5 AQI was 2.5 which would correspond to 0.6 ug/m3.

These big relative differences are not shocking, as the absolute differences are small, and one should not expect identical measurements even from a batch of same model sensors. But as a general rule I suggest to take the readings of all these sensors with some healthy skepticism. That being said, the FLOW is a very sexy device and easy to use.

Plume Labs Flow sensor… first impressions.

October 11, 2018

I just received Plume Labs FLOW portable air quality sensor, which measures PM2.5, PM10, NO2 and VOC’s. An IOS or Android app is used to read out the measurements on a smart-phone or tablet, which are given as a Plume Labs specific AQI.

The definition of the Plumelabs AQI’s is hidden in deep mystery: writing that it is based on WHO guidelines, EPA and EU AQI’s does not add to the clarity, as all these AQI’s have different thresholds to define the different ranges (you might read my blog about “AQI air quality confusion: https://meteolcd.wordpress.com/2018/05/20/aqi-air-quality-confusion-1/). So it would be nice to get the concentrations of the different pollutants (e.g. [NO2] in ug/m3) as well as the relevant AQI’s.

A quick first check at home shows an impossible high NO2 AQI, but let’s wait for a week as the “AI” firmware (hm!!!) seems to need some deep learning….

Another unrelated question which waits for an answer: is it possible to log the readings into a data-file?


My first impression: nice design, but I hold back my judgment …Is the Flow more than a gimmick riding the air quality trend?



UVI and total ozone column: 3 days in September 2018

September 27, 2018

I wrote many times on the relationship between the total thickness of the ozone layer(TOC) and the UVB radiation at ground level (see paper, and comments here, here. Roughly speaking, the ozone column is a filter for UVB, so when this filter becomes thinner (the measure of TOC gives decreasing Dobson units), the UVB irradiance should increase. One common relationship to quantify this increase is the radiation amplification factor RAF defined by RAF = -[ln(DU1/DU0)]/[ln(TOC1/TOC0)].

Today and the two previous days (27, 26, 25 Sep.2018) gives a nice illustration.

First note that the maximum of the total solar irradiance was more or less constant, whereas the UVB irradiance increases markedly:


The next plot shows the variation of the UVI (UV index, here UVI = 0.00278*UVB when UVB is given in mMED/h):


The TOC numbers are given in DU below the x-axis: they decrease from 267 to 227. A quick X/Y graph shows a possible linear relation-ship:

From the 25th to the 27th September, a TOC decrease by 17% causes an increase of the UVI by 14% (please note that the relationship is logarithmic, so this line should seen as a linear approximated segment of the “real” curve).

Using the RAF formula given above for the second and last days and replacing the DU readings by DU/cos(SZA) as stated in the paper yields an average RAF of 1.086, close to the values found in the paper

Conclusion: our measurement clearly show the inverse relation-ship between TOC and UVB irradiance.



29 Sep 2018:
changed calculation of RAF’s by using “slanted” formula given in the paper (DU replaced by DU/cos(SZA)) and calculating the RAF’s for the second and third day by taking the 25th September as base-line (index 0 in the formula)


Stop this energy transition!

September 23, 2018

Laurent Alexandre, a well known urologist and company founder (former director of Doctissimo) has a very interesting and clear article in the French weekly L’Express, one of the oldest and most read French news weeklies. L’Express usually is left and green leaning, so this article (“Arrêtons cette transition énergétique“) which speaks clearly against the official French energy politics is remarkable.

L. Alexandre writes that the ecologists work (often through ignorance) to increase CO2 and fine particle emissions. The French greens, as most of their European partners and the multi-billion lobby Greenpeace, are strongly anti-nuclear, and stubbornly ignore that France is a world-champion in low-CO2 electricity production. The following picture from https://www.electricitymap.org/ shows the live CO2 intensity of electricity production for a large part of the EU (the picture corresponds to 23 Sep. 2018, 11:300 UTC):


Clearly France with its more than 75% nuclear electricity is the winner, by a very large margin. The 32 g must be compared to the “über-grün” Germany situation of 339 g. Germany’s part of renewables is 42%, France’s is 27%. Germany has a breathtaking 105000 MW wind and solar installed capacity, France’s nuclear capacity is only 63000 MW.  This much lower nuclear capacity produces 400 TWh CO2 free and reliable electricity a year, whereas Germany’s 167% higher renewables (at a cost > 200 billion €) do not generate more than 140 TWh intermittent energy a year.

Alexandre has this biting sentence: “L’affreuse réalité, c’est que, aujourd’hui encore, les énergies intermittentes ne sont vertes que quelques heures par jour et sont indirectement des énergies noires la plupart du temps..” which translates into “the frightful reality is that today the intermittent energy sources are green only for a couple of hours per day, and indirectly black for most of the time..”

An he concludes by “Vous serez fascinés par notre bêtise collective… alimentée par une stratégie de communication mali(g)ne des industriels “verts”. Les ayatollahs verts veulent nous amener dans l’impasse allemande…” (you should be fascinated by our collective imbecility…which is fostered by the evil communication of the green industrial lobby. The green ayatollahs would like to push us into the German dead-end…”).

Is this comment published in the Express a sign that some EU media do not sheepishly echo anymore anything written in green?


h.t. to René T. for pointing me to this article.


CO2 storage in superfast magnesite reaction?

August 18, 2018

Magnesite (or magnesium carbonate MgCO3) is an abundant mineral that stores huge quantities of atmospheric CO2 in its crystal structure. One ton of rock can fix half a ton of CO2, but this well known geological process is not fast: it takes hundred of thousands of years to do so at the Earth’s surface under normal temperatures and pressures. So a presentation by Ian M. Power, professor at the Trent University, Ontario at the Goldschmidt Geochemistry Conference in Boston made quite a splash: he and his co-authors claim to have found a way to speed the reaction enormously, going down to about 72 days. They use micro-particles of polystyrene as a catalyst according to a ScienceDaily article.

It seems to good to believe, and some critics object that professor I. Power is “seriously over-optimistic”. CCS (carbon storage and sequestration) has been overhyped for a long time as the solution to continue using fossil fuels without problems from CO2 emissions into the atmosphere. But pushing CO2 in liquid form into the underground is energy-intensive; a subterranean lake of liquid CO2 waiting to escape makes many people nervous. If this CO2 could be definitively stored away in solid rock (neglecting weathering), the whole CCS problem would become much more acceptable.

In the abstract the authors explain that they found that magnesite forms by direct precipitation from aqueous solutions at low temperatures (3 – 10°C). Using carboxylated polystyrene microspheres this precipitation was found to happen in 72 days. The process does not need any energy input and is several magnitudes faster than natural magnesite formation.

Will this be the coming way to carbon sequestration, or even, as some speculate, to direct CO2 removal from the atmosphere, making all hugely expensive and complicated former CCS trials crash into a dead-end?

AQI: air quality confusion (5, last part)

May 29, 2018

This is part 5 (the last part).

Click on the links below for the other parts.

Part 1,  2,  3,  4.




8. The Luxembourg Meng Loft AQI (LuxAQI)

8.1. An introduction to the LuxAQI

The 7th May 2018 our minister of Environment presented a new app called “Meng Loft” (which means My Air) for smart-phones (IOS and Android). The app has been developed by the company 4sfera Innova (from Catalonia) together with the Environment Agency.

4sfera Innova specializes in air quality related software and created an Europe Air app in 2013 (it’s website seems to be dormant, the last update is from May2014).

The AQI for Luxembourg (let’s call it LuxAQI) is quite different from the EAQI: there are 10 quality levels from “Excellent” to “Miserable” (in French from “Excellent” to “Exécrable”) and also 10 color shadings.

The core pollutants used are O3, NO2 and PM10. Only 4 stations (2 in Luxembourg-City, Esch-Alzette and Beidweiler) measure PM10’s, so that for instance the whole Northern part of the country of Luxembourg has no PM sensor.

The app is nicely designed; the following picture is a screenshot from an Android tablet used at Bettendorf, 28 May, 17:34 local time (15:34 UTC):

The pointer “My location” shows that the LuxAQI has been computed for the region of Bettendorf; if one launches the app being outside of Luxembourg (or with locations services unavailable) the screen gives an average for the whole country (a nice feature!). Some other features of the app will be introduced later.

The next picture shows the naming of the quality bands and break-points used:

If I understand correctly, the 1h values are used for O3 and NO2, and 24h values for PM10’s. As with all other AQI’s seen, the sub-AQI with the highest index defines the Meng Loft LuxAQI.

My first reflection seeing this new app was “boy, 10 levels, 10 colors, is that not a bit too much?” Digging deeper I actually wonder why tiny Luxembourg has stubbornly ignored the harmonization efforts of Brussels EU Environmental Agency and has not simply chosen to use the EAQI. So we have here a situation which instead of clarifying will in my opinion add to the overall confusion. Is a deeper cause that 4sfera Innova had this app lying in a drawer and was good in salesmanship?

The next three pictures compare the LuxAQI with the corresponding EAQI for O3, NO2 and PM10. Note how clear and uncluttered the EAQI is w.r. to its new sibling!

Note that The EAQI “Moderate” category extends for all 3 pollutants over the LuxAQI “Très médiocre” which translate approximatively to “poor”!

The LuxAQI uses a “geostatistical” model to calculate the AQI from the surrounding readings up to a cell of 1km2 area. I could not find any further information on the model used, and I am highly suspicious on the ability to milk the PM10 data of the 4 stations to deliver a modelled value for any location in the Northern part of Luxembourg.


8.2. The problem with rural and city stations and the “highest index trumps all” strategy

In Luxembourg Vianden is the archetype of a rural station: the measuring station is not located in the small town of Vianden, but much higher up on the border of the basin of the SEO pumping storage facility. There is only minimal traffic, air is very pure (practically no NO2 and PM10) and solar UVB radiation is unfiltered by aerosols. As the region contains large forests (deciduous and not) there is during warm summer weather a healthy natural out-gasing of terpenes and isoprenes from the trees. Both volatile gases are O3 precursors. So it does not come as a surprise that “natural” O3 levels are high and do not diminish rapidly during nighttime due to the absence of O3 destroying NO. In Luxembourg-City the situation is exactly opposite: PM10 and NO2 levels a relatively high, O3 levels lower and fall off during nighttime in the absence of UVB but presence of O3 scavenging NO. The next 2 pictures show this for Vianden and Luxembourg Pl. Winston, a high traffic area:

At Vianden O3 values do not go lower at night than about 50 ug/m3 and than rise again,  wheres at Pl. Winston the lower values of 30ug/m3 linger for a longer time. The usual pattern is normally much more smooth at Vianden than at Pl. Winston. (for a still clearer picture please look at the addendum).

But look carefully at the legends: Vianden’s air quality is POOR (index 7, deep orange) whereas that of Pl. Winston is relatively good (“assez bon”, light green).

Let’s take a look at the NO2 and PM10 situation for both stations. The next two pictures show the NO2 pattern:

At Vianden No2 levels never exceed 10 ug/m3 whereas at Pl.Winston the oscillate around 45 ug/m3, with a peak value at least 6 times higher than that at Vianden.

Now look at the daily PM10 levels for the last week:

The PM10 levels at Vianden are very low, never exceeding10ug/m3; at Pl. Winston they are constantly two to three times higher!

So let us summarize the situation:

Vianden  LuxAQI= POOR

O3max = 150  NO2max < 10  PM10max < 7

PlaceWinston (Luxembourg-City) LuxAQI = Relatively GOOD

O3max = 60  NO2max = 60  PM10max = 22

The 2.5 times higher natural O3 levels put Vianden into the POOR category, whereas at Pl. Winston the much more dangerous and mostly anthropogenic PM10 and NO2 levels (3 and 6 times higher) are simply ignored and do not change the “relatively Good” classification!


In my opinion the LuxAQI classification and break-point table introduced by “Meng Loft” is a bad choice: instead adopting a European standard, Luxembourg’s Environmental Agency has chosen for reasons unknown to go it’s own way, adding confusion to an already complicated matter. The Meng Loft app is visually satisfying and easy to use, but it suggests a precision that is more of a virtual character that of a physical reality.


9. Overall conclusion

The title of this 5 part comment was “air quality confusion”. I guess that the enumeration of the different AQI’s is a strong argument for the validity of the “confusion” qualifier. What all AQI’s wrongly point at is that a complicated situation can be well defined by a single number. The AQI’s suggests a scientific precision that is and can not exist. Increasing the number of the different levels as does the LuxAQI is simply an exercise in futility which adds to the confusion.

All AQI’s take the highest sub-index as the relevant parameter to define the air quality of the moment. I am not comfortable with that choice (even if everybody seems to accept) as it leads to a mis-qualification of clean air when natural O3 levels are higher.


(end of the 5th and last part).

Addendum 1:

A much clearer picture showing the different O3 pattern for rural Vianden (i.e. Mont Saint-Nicolas) and the urban traffic location at Luxembourg-Bonnevoie is given below (screenshots taken the 30May2018, 15:35 local time): the nightly minimum at Vianden is about 60 ug/m3, whereas it is lower than 10 ug/m3 at Bonnevoie.



Addendum 2:

4sfera Innova also has its own app called EuropeAir (Android version checked) which is easy to use and gives the standard EAQI . Here a screen-shot of today (30May2018); zooming in gives all the stations and the actual measurement data, but no time-graph of past values. The filter allows to select one pollutant (e.g. O3) only.


AQI: air quality confusion (4)

May 27, 2018

This is part 4.

Click on the links below for the other parts.

Part 1, 23, 5.



7. The EU AQI’s

As everything in Europe is complicated, so are the AQI’s. The oldest index is the CAQI  which has different quality bands names, color scheme and break-points than the newest EAQI.

7.1. The CAQI

The  CAQI index was introduced by the CiteAir (Common Information to European Air) project in 2006. It was revised in 2012 and is meant as an index for the air in the cities. It uses the concentrations (measured in ug/m3) of 3 core pollutants (O3, NO2, PM10) with PM2.5, SO2 and CO as optional. The different sub-indexes run from 0 to 100 as defined in the following table (link):

The table shows that the index break-points are not proportional to the concentrations for all pollutants as for instance for NO2 the “Very low” range [0..50] covers 0…50 ug/m3 whereas the “High” range [75…100] covers 200…400 ug/m3.

The names of the different categories relate to the magnitude of the index, and not the quality of the air. As customary, the highest sub-index defines the CAGI.  Note that the sampling period usually is 1 hour. The CAQI can be viewed on-line at http://www.airqualitynow.eu/

Clicking a city gives more detailed information, as shown for Paris:

Clearly the CAQI is heavily “French” leaning, as the vast majorities of cities are in France.

This is probably the reason that the DG Environment commanded a research to Ricardo Energy & Environment which delivered in 2013 a paper defining an harmonized EAQI. The proposal was very similar to the EPA AQI, with an identical number of pollutants and an index running from 0 to 500; the names of the different categories were different. This project seems to have been a dead-end (due to it’s US similarities?) and never has been officially applied.

7.2 The EAQI

In November 2017 the EU introduced the EAQI = European Air Quality Index with a web-site showing the live-data. It is very difficult to find precise literature for this index which is somewhat similar to the CAQI, albeit with a different color scheme, a renaming of the categories and most important, different break-points. So most of the following information has been extracted from the excellent airindex.eea.europa.eu

First the naming of the 5 categories is now relevant to the air quality and not the magnitude of the index, the colors go from turquoise to brown and the indices from 0 to 100 relate to concentrations in ug/m3 as shown:

The numerical ranges of the indices are here (often the upper bound is given as equal to the lower bound of the following category which does not easy a decision for qualifying if by chance the data fall on a boundary):


I wrongly presumed that the break-points for each category are defined as they are for the CAQI.

The graph for the EAQI in Epinal (France) shows that the category is POOR as the PM10 concentration exceeds 71 ug/m3.  As this is an 1h concentration, the EAQI break-point for PM10-POOR seems to be lower than in the CAQI table.

Finally after quite some detective-investigation, I found legends at the provisional web-site
which confirm those of the table at the start of chapter 7.2. See that here upper and lower bound values coincide!

Note how different these break-points are  from those of the CAQI, which adds one more level to the overall confusion!



Hopefully the EAQI will be the definitive step in harmonizing an AQI. But I have some doubt that these break-points will be stable for the coming years. As the trend goes to an ever tightening of the tolerated levels, the chances for future stability seem poor.

The web site airindex.eea.europa.eu betters that of the CAQI  enormously as it includes measuring data all over Europe and shows the time series for the different pollutants.


The last part 5 to be followed asap concludes this series with a discussion of the new Luxembourg AQI introduced a couple of weeks ago with a smart phone app called “Meng Loft” , and which shows that adding confusion is not a privilege of big countries!

(go to part 5)

AQI: air quality confusion (3)

May 23, 2018

This is part 3.
Click on the links below
for the other parts.

Part 12, 4,  5.

6. The UK revised DAQI

In the UK the Department for Environment, Food and Rural Affairs (DEFRA) publishes since 2013 the revised Daily Air Quality Index (DAQI); the first DAQI was introduced in 2012. The DAQI has 4 quality levels (from best to worst: LOW, MODERATE, HIGH, VERY HIGH) and the index runs from 1 to 10. It is based on concentrations of O3, NO2, SO2 and PM’s (PM10 and PM2.5) measured in ug/m3 (CO has been removed in the revised DAQI).

The following table gives the break-points (link):

The first comment should be that the “band” qualifiers correspond to the numerical index, and that “Low” means low index = good air quality conditions. The break-points are not proportional to the concentration: note that for O3 index 2 spans over 33 ug/m3, whereas that index 4 extends over 20 ug/m3. The pollutant with the highest index defines the published DAQI.

Ozone concentrations are only taken as 8h running mean. The color levels used for the different bands and subdivisions are different from those of EPA and China, and the health messages attached reflect the differences between individuals at risk and the general population:

There are at least two methods to view live DAQI data:

1. use a Google Earth KMZ file (link)

2. go to the interactive map (link):

Clicking on a station gives very detailed information, a shown below (all lines are active links):


  • Care should be taken to not confuse a “LOW” index with “low air quality”.
  • Not further subdividing the “Very high” category is a good decision.
  • 10 different color-shades are in my opinion way too many: it is difficult to distinguish neighboring colors when the map is shown in a poorer resolution.


7. The French ATMO index

The “Fédération des Associations de la Surveillance de la Qualité de l’Air” (link) defines two indices:
– the ATMO is based on 4 pollutants (O3, NO2, SO2, PM10) and applicable for cities of more than 100000 inhabitants
– the IQA (Indice de Qualité de l’Air simplifié) is based on a subset of these 4 pollutants and used for cities with less than 100000 inhabitants

In the following, I will consider only the ATMO which extends from 1 to 10, uses 6 quality levels and only 3 different colors (GREEN, ORANGE, RED) as defined in the relevant “arrêté” (link). The break-points are based on 1h concentrations measured in ug/m3 (except PM10 given as the 24h mean); if several stations cover a geographic zone, the average is used. As for all previous indices, the highest sub-index defines the ATMO (link):

I did not find an interactive map covering France, but you may start here with the map of the regions (link) and click on a region to get more details, as shown for the Eastern Region of France (link):

The details in the regional maps vary from region to region; in the sub-map above clicking on a station gives further specific indices for the individual pollutants.


  • using only 3 colors makes an overall view easier
  • the format for the individual regions is not exactly the same, which is slightly annoying. Besides the ATMO France often uses an AQI called CiteAir, which is based on a EU convention. The EU AQI’s will be discussed in the upcoming part 4.

(go to part 4)


AQI: air quality confusion (2)

May 21, 2018

This is part 2.
Click on the links below
for the other parts.

Part 1, 3, 4, 5.



In the first part of this blog on AQI I finished with the definition of the US EPA AQI, which has 6 quality levels and AQI numbers of to 500. Before going to Europe, let us begin this part with the Chinese AQI.


5. The PRC Chinese AQI

China uses for defining air quality the same 6 pollutants as does the US EPA (see here): O3, NO2, PM10 and PM2.5, CO and SO2. It also uses 6 quality levels albeit with different wording: EXCELLENT to SEVERELY POLLUTED. The Chinese sub-AQI’s (called IAQI = individual AQI) run from 0 to 500, as do the EPA sub-AQI’s. And the highest IAQI defines the published AQI. Most of the times PM2.5 is the primary pollutant with the highest IAQI, but during summer time O3 may have the highest IAQI.

The break-points for PM’s differ from those of EPA (link, attention: this blog has a completely wrong table comparing US and China other AQI’s):

The qualifiers (“Description”) in this table are the Chinese ones, and obviously China is much more tolerant for PM2.5 as is the EPA (notice that the first Chinese break-points are considerably higher). The same remark holds for some of the other pollutants as shown in this comparison table which uses 1996 EPA break-points (link):

First one should note that China uses ug/m3 as concentration unit. Second it calculates NO2 pollution only using a 24h average, whereas EPA uses 1h values, which makes comparison impossible. Third where comparison can be made, the break-point numbers are very close for O3, CO and PM10 but differ for SO2 (24h), NO2 (24h) and PM2.5 (24h).

Chinese AQI’s are often calculated as the average from readings of multiple stations around a city, whereas US EPA AQI’s allways come from a single station.

The US Embassies in China have their own measuring stations which use the EPA standard: see here.

There are a couple of smartphone apps to visualize real-time Chinese AQI’s, but I did not find an official Chinese live map on the web based on Chinese AQI standards. The waqi.info web site shows Chinese air quality using EPA standards; the same holds for the website aqicn.org.

The next picture shows the situation today 21 May 2018 at 14:10 UTC:

Clicking on a label gives more information, as for instance for the City of Yulin:

We see that PM2.5 and PM10 situation is particularly bad, whereas O3 and NO2 levels are GOOD.

Comparing AQI’s over China with those of other parts of the world clearly shows that bad air quality (mostly PM’s) is a serious issue in many parts of mainland China.


Be careful when reading AQI’s for China, as often it is not clear on what standard they are based. The following paragraph (link) summarizes this well:

(go to part 3)