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I would like to measure the air quality related to road traffic in a city for an IoT application.

The European Environmental Agency has defined quality indicators (good, bad, etc) according to the concentration of some gases in the air such as CO, NOx etc. I want to measure the concentration in the air of these gases.

I have come around the Waspmote board by Libellium which produces also some sensors boards. However the gas sensors, particularly the calibrated ones are really expensive. Does anyone know a more affordable solution or integrated product to measure air quality?

Francesco Boi
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  • A few weeks ago I got an email that https://smartcitizen.me is starting up again. See their forum for details about the new hardware: https://forum.smartcitizen.me/t/the-all-new-urban-sensor-board-in-the-smart-citizen-kit-2-0/960 I have not used it (but PM 2.5 seems insufficient for regular outdoor use). – Arjan May 11 '18 at 18:31

2 Answers2

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I think it's non-existent today.

I work at a Dutch municipality and, though things might have changed now, in December 2017 our environmental department concluded that even semi-professional gas or dust sensors were just not good enough to determine the low levels needed for outdoor measurements. (Including everything Libelium offered at that time for CO, NOx and particulate matter.)

It's not my cup of tea, but the reasoning of our environmental department:

  • For particulate matter Libelium claimed to count PM 1. But outdoor measurements would only make sense for PM 0.1 (particulates smaller than 0.1 micrometer). Also, the weight of the counted particulates is not a constant, so converting the counts into some μg/m3 value, to compare to government regulations, is difficult if not impossible.

  • For NO2 Libelium claimed an accuracy of 0.1 ppm (parts per million) in ideal conditions. With, for NO2, 1 pbb (parts per billion) being equal to† about 1.88 μg/m3, a claimed accuracy of 0.1 ppm = 100 ppb = about 188 μg/m3. But outdoor hourly averages of NO2 concentrations vary between 1 μg/m3 and 60 μg/m3, and the Dutch government indicators define an upper limit for the annual average of 40 μg/m3. So, a claimed accuracy of 188 μg/m3 does not suffice.

  • Any other measurements other than (ultra) fine dust and NO2 were deemed not interesting for air quality.

Of course, combining a lot of low-cost measurements and comparing those to the results of (very expensive) professional measurements might still yield useful indicators.

† µg/m3 = ppb × 12.187 × M / T, where µg/m3 is micrograms of gaseous pollutant per cubic meter of ambient air, ppb is parts per billion by volume (i.e., volume of gaseous pollutant per 109 volumes of ambient air), M is the molecular weight of the gaseous pollutant, and T is the temperature in Kelvin. An atmospheric pressure of 1 atmosphere is assumed.

So, for a temperature of 25 °C: 1 ppb SO2 = 2.62 μg/m3; NO2 = 1.88 μg/m3; NO = 1.25 μg/m3; O3 = 2.00 μg/m3; CO = 1.145 μg/m3; Benzene = 3.19 μg/m3.

Source: DCE - Danish Centre For Environment And Energy.

Arjan
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    Did your municipality produced any public document about this conclusions? Do you know if they might work as threshold alarms at least(when they exceed the legal limit for example)? – Francesco Boi May 11 '18 at 14:27
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    @Francesco, unfortunately they did not, but I edited my answer to add the reasoning I found in an email. – Arjan May 11 '18 at 14:40
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    (I removed a link to a presentation from the TTN LoRaWAN Conference in early 2018, where Joost Wesseling of the Dutch RIVM presented about measuring huge pollutions. Nice, but not related. At 4'37 some graphs of some sensors can be seen, but I'm sure you'll find similar results elsewhere on the internet.) – Arjan May 11 '18 at 14:56
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    Sorry it came to my mind another question. Do you know the devices Waspmote Plug & Sense! by Libelium again? I am not sure but I think they use the same sensors: aren't they professional and precise devices? – Francesco Boi May 11 '18 at 17:10
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    Mid 2016, we used Waspmote PRO v1.2 Plug & Sense, Smart Environment, with the "regular" Gas Sensor Board v2.0. These were terrible, but I hope things have changed. It did not even give reliable temperature readings (taking subsequent samples would show large differences), but such can hardly be caused by poor sensors? For the gas sensors, many variables needed to be set, for which our supplier was just told "it depends of the concentration you are going to measure". Read through that forum before ordering anything... – Arjan May 11 '18 at 17:34
  • I thought they were good devices also because they are quite expensive. – Francesco Boi May 11 '18 at 17:45
  • Again, things might have changed since 2016. (Browsing through their forum might tell you if that's the case.) But in December 2017 we still concluded that the NO2 and particle sensors are not good enough for outdoor measurements. We cancelled the project. That's all I know. :-) – Arjan May 11 '18 at 17:54
  • For NO2, there are interesting electrochemical sensor by SPEC (https://www.spec-sensors.com/wp-content/uploads/2016/10/3SP_NO2_5F-C-Package-110-508.pdf) that are accessible (<20€) yet still offer some interesting precision (tens of ppb) compared to the French "alert threshold" of 400 µg/m3 (~213ppb). Interesting for fine-grain mapping of pollution that more accurate, and expensive, sensors don't allow. Nice stuff by Alphasense also (http://www.alphasense.com/WEB1213/wp-content/uploads/2017/07/NO2B43F.pdf) but more expensive. – Sylvain May 28 '18 at 19:33
  • For particles, Alphasense has release a new sensor the OPC-R1 (http://www.alphasense.com/WEB1213/wp-content/uploads/2018/02/OPC-R1.pdf) that does particle counting and binning down to 0.4µm. I'm evaluating it right now. It's said to be as accurate as the OPC-N2 (you'll find lots of evaluations of this one) yet should be available for a price divided by 4 or 5. Nothing is this price range come this close to counting PM0.1. – Sylvain May 28 '18 at 19:41
  • Nice, @Sylvain, curious about your results! – Arjan May 28 '18 at 19:53
  • Hi, @Sylvain, did you ever get good results with those? – Arjan Nov 04 '19 at 09:59
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Adafruit has a Volatile Organic Componds (VOC) / CO2 sensor built to monitor air quality (https://www.adafruit.com/product/3566 $19.95 USD) which sounds like would fit your requirements. Its not as fancy as the other ones you are looking at, but the price is right. Adafruit does have a number of gas sensors to choose from: https://www.adafruit.com/category/897

JD Allen
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    It seems more like for indoor applications. – Francesco Boi May 11 '18 at 13:24
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    @FrancescoBoi Yes, they are. I have mine outside in my Weather sensors box, and they work just fine. Its a trade off: cheaper/less weather proof, weather proof/more expensive ;) – JD Allen May 11 '18 at 15:54
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    sparkfun has that sensor too. https://www.sparkfun.com/products/14193 – Sam May 11 '18 at 16:08
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    If I understand correctly, the difference between indoor and outdoor is about the lowest levels the sensors can or should measure. Indoor levels are much higher, hence much easier to detect reliably. For outdoor use, much more sensitive sensors are needed? – Arjan May 11 '18 at 16:38