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Interesting. But why would you monitor CO2 outdoors? Even the NDIR ones just return "400" when they detect outdoor air and use it as calibration.

Though i believe you can turn off the auto calibration which is probably a good idea in busy city areas where I imagine outdoor CO2 is slightly higher than 400. And manually calibrate them with an actual known source.

Outdoor CO2 levels can go up to 700-800ppm in rural areas due to plants creating CO2 [1]. I was initially surprised it's that high but it was verified with reference instruments.

Why measure CO2 outdoors?

We are able to detect emission sources pretty well now with our outdoor monitors [2] that have the SenseAir S8 NDIR sensor built in. You can see some real data in the blog post I wrote about launching our global CO2 map [3].

So setting up a dense network of these sensors in a city would allow to measure if for example the introduction of low-emission zones or switching to electric buses etc. would work.

Another use case is to check for leakage in underground CO2 storage facilities.

All-in-all it's still experimental (to some extent) but we know the accuracy is there and we can see more and more use cases as outlined in my blog post. So we now work hard to get more and more of these sensors out there to get more data to identify additional use cases.

[1] https://www.airgradient.com/blog/performance-of-low-cost-co2...

[2] https://www.airgradient.com/outdoor/

[3] https://www.airgradient.com/blog/airgradient-global-co2-map/

> Outdoor CO2 levels can go up to 700-800ppm in rural areas due to plants creating CO2 [1].

We need to get those rural voters more oxygen, STAT!

Em, do you know that plants are actually a net source of oxygen ? And a sink of CO2 ?

Net, sure, but it's actually really really complex when you look at point in time numbers.

> Carbon dioxide is not released during photosynthesis, but small amounts of that gas are emitted both day and night as a by-product of cellular respiration. It is worth noting that the majority of plants absorb carbon dioxide during the day for photosynthesis and do so in greater amounts than they release for cellular respiration.

So depending on the time plants could easily be pushing out more CO2 than they are pulling it at that time, which could result in CO2 concentrations increasing during that time.

Time dependent, plants give off O2 in the evening. The co2 is released by the micro organisms which are bacterial and fungal and breathe O2 just like us.

Plants are overall a sink of CO2, as it goes into building their mass.

Surely this refers to decomposition and not living plants. Your link doesn't appear to finger plants as the culprit at all.

Plants produce CO2 as well, photosynthesis is only used to produce sugars, normal respiration (I.e. sugars + O2 = energy + CO2), is used by plants to consume those sugars.

Only a plant cells containing chlorophyll and exposed to sunlight will photosynthesise. Notably at night there isn’t much sunlight, so normal respiration dominates a plants gas conversion processes, and results in them producing net CO2.

Based on the graphs in the blog post, I assume GP is placing the CO2 production on plants because it the CO2 levels peak at night, then return to normal every morning.

Yep, I've noticed this as well with my cannabis grow tent. At an 18/6 lighting schedule, it's noticeable that CO2 grows up to 800ppm in the tent at the end of the fake-night cycle.

It seems like ozone is the missing piece. The only ones that measure outdoor ozone are governments, and the numbers keep increasing.

Isn't the increasing numbers good here at this point? Or not at this altitude?

No it is not. Ozone is not good for breathing.

Who do you calibrate these CO2 sensors in areas where the background level is so high?

Not sure what you mean with calibration here. Above is not so much about calibration but correlation. Basically comparing the monitors to the reference data.

Sorry, should've been "how". My understanding is that the optical sensors work by recalibrating every week or so, assuming that the lowest CO2 level they've seen is equivalent to the baseline "outdoor" ppm that was set in the factory (which can be adjusted over time to account for climate change).

If the sensors are in an environment where the CO2 is always elevated, how do you keep it properly calibrated (eg if the CO2 never goes under 700ppm, how do you stop it from recalibrating so that it returns "400" when it should actually return "700") I know you can just turn calibration off but won't the sensors' accuracy decrease over time?

I think he meant How?, meaning normally a calibration is placing indoor monitors outdoors for 20 mins.

If you’re using reference grade instruments, I would assume they’re calibrated in labs using well controlled environments.

I.e. put sensor in vacuum chamber, remove all gases, then introduce know amount of gases to produce a known target environment.

Given their reference instruments, I assume they’re also capable of maintaining their calibration for a long period of known time, before they need to be re-calibrated. They would never rely on something as inaccurate as “20 mins outdoors” to calibrate themselves.

> Interesting. But why would you monitor CO2 outdoors?

For example, to know that it's futile to try to turn on ventilation to lower the indoor CO2 concentration.

Yup. In my city, on windy winter days the CO2 is as low as it gets, in the low 400's.

But on a hot windless summer day in a temperature inversion, my meter will read 700 or even 800 outdoors, as the CO2 from vehicles and maybe even power plants just sits around without going anywhere.

In the winter, if my indoor reading is 800 I open the window a crack to improve my concentration. On certain days in the summer, I just accept it's going to be 1,000 indoors and there's nothing I can do about it.

Interesting. I never gave this whole thing much of a thought, but always assumed co2 levels would be higher during winter as there's no green outside to absorb it, plus everyone turning on the central heating systems.

Optical linewidths are influenced by temperature and pressure. Outdoors, humidity can be a factor as well. When I did photoacoustic spectroscopy, we used two or more wavelengths in the measurement; one on resonance, one off resonance to help with background discrimination. I am not sure whether these ultracompact instruments have that capability.

Additionally, contaminants on any surface the light strikes will generate spurious signals. Lock-in (synchronous) detection at the optical source modulation frequency can help get rid of a lot of noise but if the noise appears at that frequency, it will corrupt the measurement.

This classic paper from 1986 covers a lot of what can be done with the technique and how to do it correctly. Reviews of Modern Physics is behind a paywall but there's a copy on the web at the URL below.

  Applications of photoacoustic sensing techniques
  Andrew C. Tam
  Rev. Mod. Phys. 58, 381 – Published 1 April 1986

http://users.df.uba.ar/dkunik/photothermal/review/tam86.pdf

If money is not a problem (let's say, up to $1,000), what's the best CO2 indoor sensors you would suggest today?

If you have time, could you write a bit more about the humming phenomenon?

Photoacoustic is usually done with a chopped or pulsed light source at a few hundred Hz (ie. audio frequencies). If the molecules don't absorb at that wavelength or if the molecules are not present, the light just goes through. If light is absorbed, there will be slight heating when the beam is on resulting in a sound wave at the chopping/pulsing frequency. This is detected by the microphone. For weak absorptions, the intensity of the sound wave will be linear in the concentration.

I think this is what is meant by "humming": There is a sound that is generated when the target molecule is present.

The Infineon press release talks about indoor monitoring only, and it is even present in the title, but I have noticed that you guys consider HN to be a very important marketing channel, so I get it.

If you take the time and actually read the blog post I linked to, you will see that mostly it is about indoor CO2.

However, to understand the performance of these different sensor technologies it is important to point out that they fail under certain environmental conditions, e.g. outdoors.