Forum Sensor.Community

Why measurements are in mk g/m3 and not in PPM3 (particles per m3)?

Measurement stations used by sensor.community output values of PM in mkg per m3.
But can sensors used measure the weight of the particles?!

I think that what they actually measure is particles per m3.

Because µg/m³ is a standard unit used for PM concentratio in air pollution monitoring.

You may think that. But this is also a huge over simplification.

TL;DR;
Laser light scattering sensors (like SDS011, SPS30, PMSx003) don’t measure mass of particles or even particle count. This sensors measure light impulses and convert them to mass using algorithms and heavily depend on proper calibration of sensor (usually done in factory as last step of testing).

Long version - I’m sure I already posted that on this forums:

Laser dust sensors use laser light scattering method to determine amount of dust in air. That means there is a small laser (usually red, because it’s cost efficient a.k.a. “cheap”) inside the sensor. This laser shines a steady beam of light trough dark measurement chamber. The beam of light ends up in light trap - small cavity covered with non-reflective pure black paint or similar surface - so light is never reflected back to measuring chamber. At the botom of measurement chamber there is a photosensitive element (photodiode, CMOS detector, etc) perpendicular to light’s pathway. Small fan build in sensor is responsible for steady airflow with known rate. In clean environment light travels across the chamber and have no medium to scatter on. So basically no dust in air means no light is registered by photosensitive element. Sensor is able to register light impulses. Weak, small, strong… Impulses of any kind. That is what sensor measure - how much light is scattered in period of time. How many and how strong impulses are. The rest is calculation and approximation. The real “magic” behind sensor is converting light impulses to PM values in µg/m³. And that is tricky part. That depends on many factors.

As you already know Particulate Matter indexes like PM100, PM10, PM4.0, PM2.5, PM1.0 doesn’t mention light impulses or even particle number. They inform how much dust weights in given volume of air. Number after PM describes diameter of measured particles. So PM10 include all particles 10µm and smaller. So inside PM10 we got PM4.0, PM2.5, PM1.0, nano particles, etc. Reference method for measuring dust concentration use ultra-precise electrostatic scales for weighting carefully crafted filters before and after 24h measurement. It’s not cost effective. it’s labor intensive, slow and far from perfect. This reference method show how much dry fraction of dust weights, but completely ignores substances which can evaporate during filter conditioning before weighting. To proper measure mass of dust you have to use a method equivalent to the reference method which basically means your measurement must be mass dependent (light impulses are not mass). The only real time equivalent method which works in filed is beta radiation absorption. You need radioactive source and detector. More mass of dust means more beta radiation absorbed by dust, less radiation on detector.

Laser light scattering method is not mass dependent - so it can’t be equivalent method to reference method. It comes with many problems, but this method is fast, cheap and “good enough”.

To some point dust particles in lighter “color” will reflect more light. Darker “colors” absorb more light. “Color” isn’t very good word here, but it’s great analogy. But as you may suspect world isn’t that simple… Dust size is comparable in size with light wavelength we use to conduct measurements. Red laser have wavelength about 650nm (0.65µm) so it is comparable in size with many measured particles. And because of that we got some strange effects heavily based on laser light wavelength, reflectivity and scattering angle of given type size and type. Long story short sometimes smaller particles shine more that bigger ones. So what can we do? We assume that dust in environment isn’t homogenous. Typically there are many sources of pollution, and we got mix of different types, shapes, sizes and “colors”.

But this may not be adequate in situation where there is one primary dust source, eg: flour powder in grain mill, dust from some kind of industrial milling or grinding operations or as in your case nearby cement factory. In that situation there most of dust have same characteristics. All laser dust sensors are calibrated to work with some kind of universal reference dust. And it will be different of any other dust on planet. So if you have a huge budget you may want conduct some reference measurements (gravimetric or beta radiation absorption) to check is there are any differentes between. But most probably it won’t matter that much, unless you are conducting some kind of important scientific experiment. In homogeneous environments it may be a good idea to use sensors which can also report number of particles in size bin. Sensirion SPS30 is a good example. This sensor allows you to collect PM data with some insight on what exactly was seen by sensors detector.

In high humidity environments you may observe fenomena called condensation nuclei. Tiny droplets of water (0.05µm - 1.0µm) stick to surface of dust. From sensors perspective this kind of particle looks bigger, because it scatter light more that dust itself. And it’s huge problem because typically we will have thousands small (<0.2µm) and hundreds large (0.2µm - 1.0µm) condensation nuclei in 1 cm³. Amount of condensation nuclei in air changes with humidity, temperature and pressure. General rule of thumb - higher relative humidity, more condensation nuclei and larger size. Some dust particles are hygroscopic and attract more water. If relative humidity reach certain threshold you will observe fog. Fog consist of droplets with size bigger than 10µm. Typically in 1 cm³ we got 300 droplets, but it may vary from tens to even thousands droplets. Condensation nuclei and fog are huge problems for laser light scattering method. Water aerosole may influence reading in major may (2-10 times).

In my opinion counting fog or condensation nuclei as dust is not a valid way of doing Citizen Scence. It’s true that in water droplets may be dissolved harmful substances like NOx. But water itself isn’t part of reference measurement of particulate matter. High humidity in Poland is a major problem. From October we got RH70%+ non stop. So all measurements taken in autumn and winter would be affected.

What I mean is that you will have at least double
To simplify it is like that:

  1. The sensor measures the light reflected from particles
  2. Transfers the light to electric signal, amplifies it
  3. Analyzes the signal and converts it by some algorithm to mk/m3

What I want to say is that more massive particle (lets assume it like a micro black hole) can potentially not reflect any light. And light particle (lets call it super mirror) can reflect a lot of light!

Also particles can reflect the same amount of light but can be dramatically different in sizes.

Thus measuring PPM is more accurate then measuring mkg/m3.

I wounder what could be the difference of an error between PPM and mkg/m3?

In short: All official limits are in µg/m3
There is no limit in particles per m3. So either you “convert” the limits to particles per m3 with “average dust”. Or you convert the particles per m3 to µ/m3.

Blockquote
Thus measuring PPM is more accurate then measuring mkg/m3.

Blockquote

We can measure mV of maximum response on dust particle it is more accurate, but not useful in real life also number of particles not useful to compare with common value of ug/m3

No it is not. Different particle size means also different volume. Different volume means different mass per particle.

Like I mentioned:

That means we assume that dust is made of particles in many different sizes and “colors”. We use approximation, which is “good enough” for most applications.

:+1:

Some sensors could use more precision methods and algorithms, and comparing results in ug/m3 is better way.